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C. twr bevtr,Uiest fAioa 3 ot Cher North 933 Arizona, of University Observatory, Steward eatmnod srnma nvria eCie Casilla Chile, de Universidad Astronomía, de Departamento eateto hsc,Uiest fHlik,Gsa Häll Gustaf Helsinki, of University Physics, of Department eateto hsc n srnm,Uiest fVictori of University Astronomy, and Physics of Department uoenSaeAtooyCnr,EA ilnead aCaña la de Villanueva ESA, Centre, Astronomy Space European colo hsc n srnm,Uiest fBirmingham, of University Astronomy, and Physics of School ednOsraoy ednUiest,P o 53 NL-230 9513, Box PO University, Leiden Observatory, Leiden etrfrSaeSineTcnlg,Uiest fMarylan of University Technology, Science Space for Center eateto hsclSine iohm nvriy 1-3 University, Hiroshima Science, Physical of Department D oessrcuefrainocr hierarchi- occurs formation structure models CDM OUS H LWQECIGO TRFRAINI LSE GALA CLUSTER IN FORMATION STAR OF QUENCHING SLOW THE LOCUSS: mtsna srpyia bevtr,6 adnStreet, Garden 60 Observatory, Astrophysical Smithsonian lse-eti ai,wietervlct distribution velocity their while radii, cluster-centric r ossetwt trfrigcutrglxe en an being galaxies cluster ∼ star-forming with consistent are Fsdcieepnnilyo unhn iesae f1 of time-scales quenching on Millenni exponentially the decline from SFRs predictions with diagram caustic stacked epeetasuyo h pta itiuinadkinematic and distribution spatial the 0 of at study ters a present We omn aaisatal elnssedl ihrdu,f radius, with steadily declines actually galaxies forming Keywords: rtpepoesdwti aaygop.Dsieteincre the Despite groups. galaxy within pre-processed first trfrigglxe ihncutr.Tevlct dispe 1.44 of velocity peak The sharp a shows clusters. within spiral galaxies accreted recently star-forming within survive to star-formation eti ais nraigfieodb 2 by fivefold increasing ( radius, fraction centric The members. cluster of spectroscopy fsa omto tlrerdicno erpoue ymode by within reproduced pass they be once cannot only radii galaxies large field falling at formation star of .– y eodpsigwithin passing beyond Gyr 0.5–2 AINES O . 1,2,3 15 A CTOBER T E tl mltajv 12/16/11 v. emulateapj style X < aais cie—glxe:cutr:gnrl—galaxies: — general clusters: galaxies: — active galaxies: .J P J. M. , 1. z < INTRODUCTION 0 ,2018 8, . 0 obnn wide-field combining 30, EREIRA σ 2 .P S P. G. , ν t0 at .D R D. T. . 3 r MITH 200 r 500 r ycmaigteosre itiuino trfrigga star-forming of distribution observed the comparing By . 200 n s1-5 ihrta hto h nciecutrmembe cluster inactive the of that than higher 10-35% is and , AWLE 3 n u ean elblwfil ausee t3 at even values field below well remains but , .E E. , rf eso coe ,2018 8, October version Draft PRE-PROCESSING Spitzer 2,8 Balti- d ,3800 a, strömin “red e views yAv- ry a E- da, 36-D, GAMI RA 0 Cam- f .O N. , Edg- ABSTRACT rip- SF r 200 a- -1 i- fsa-omn lse aaisrsssedl ihclus with steadily rises galaxies cluster star-forming of ) s s l hw a,tphtpol within profile top-hat flat, a shows 2 24 ftecutr u scnitn ihsm fte being them of some with consistent is but cluster, the of .B A. , KABE µ o – y obidu h paetoe-est of over-density apparent the up build to Gyr 2–3 for s and m ncutr ntrso aaiso hi rtifl nothe into infall first their on galaxies of terms cluster. in gala the clusters star-forming reproduce of in to gradients al. able population et were sup- Diaferio radial (2001) observed early al. (2000), et gained Ellingson Morris approach and & Navarro (2001) This Balogh, tim when over galax- 2010). cluster port the the al. et into of Mamon accreted histories are (e.g. merger which sub-halos and or orbits ies cluster massive the more following or and one containing simulations logical a enue ocntanteknmtc n ac- and populations, kinematics galaxy the cluster constrain different of to epochs used cretion been has akmte D)hlshsigindividual accrete hosting continually halos they nodes (DM) web, the matter at filamentary also residing dark complex are Preferentially al. the and et 2012). Boylan-Kolchin of latest al. (e.g. et form Gao immature 2009; clusters dynamically galaxy most structure collapsed the Universe, massive the most the in as that meaning cally, (M n hscnrbto aissrnl ihpoetdclust projected with ( strongly radius varies centric r contribution virial actually this the are outside and located members physically cluster project galaxies spectroscopic consider infalling to many a vital as clusters also effects the is it into Moreover, accreted transformed. continually being fundamental star-formi are thus whereby context galaxies is cosmological it this in properties, them place galaxy cluster in trends 2009). otecutrrdhf ( redshift cluster the to otmsiecutr aeo vrg obe nms since mass in doubled average z on have clusters massive most oa lseshv enacee since accreted been have clusters local . alling so rfieo h trfrigglx population galaxy star-forming the of profile rsion naln ouain u n htms losurvive also must that one but population, infalling 73 ∼ msmlto,w banabs-tmdli which in model best-fit a obtain we simulation, um asing 9 ABUL h asi iga,wihplots which diagram, caustic The ocretyitrrtteosre vltoayadradia and evolutionary observed the interpret correctly To .M M M. S. , 0 DM ± . fsa-omn aaisi 0msieclus- massive 30 in galaxies star-forming of s Bya-oci ta.20) hl afo aaisin galaxies of half while 2009), al. et (Boylan-Kolchin 5 0 ∼ si hc trfraini unhdi in- in quenched is formation star which in ls GALEX . ∼ 4 5Gruo crto notecluster. the into accretion upon Gyr 25 f 10 .F A. , SF 15 12 rdu rn,tesraedniyo star- of density surface the trend, –radius × ORAN M vlto aais tla content stellar galaxies: — evolution INOGUENOV rmtecr o2 to core the from ⊙ r U mgn ihhighly-complete with imaging NUV r j pro rglx rus(M groups galaxy or ) 10 n ieo-ih LS eoiyrelative velocity (LOS) line-of-sight and ) v los 5,6 ISADTENE FOR NEED THE AND XIES − .Z F. , h r v 500 i r .Ti eursuigcosmo- using requires This ). 200 l fteeresults these of All . IPARO r hssuppression This . 200 hsrequires This . 3 DM .L M L. S. , v aisi the in laxies los z ∼ ∼ − 10 0 h sa all at rs . v Brire al. et (Berrier 4 13 i ∼ C − versus ter- 14 G L ∗ M EE ⊙ galaxies 3,7 .The ). adius, , r xies r j pro ion er- nd ng to s, e s l , 2 HAINESETAL. as well as the constrain the masses, density profiles population gradients of star-forming galaxies ( fSF –radius re- and dynamical states of the clusters themselves (e.g. lation) out to 3r200, where we find a shortfall of star-forming Moss & Dickens 1977; Binggeli et al. 1987; Biviano et al. galaxies in comparison to the coeval field population that can- 1997, 2013; Biviano & Katgert 2004; Gill, Knebe & Gibson not be easily explained by purely cluster-related quenching 2005; Mahajan et al. 2011; Hernández-Fernández et al. 2014; mechanisms, indicating a need for galaxies being first pre- Muzzin et al. 2014; Jaffé et al. 2015). processed within infalling galaxy groups. This progress has permitted recent attempts to constrain In §2 we present our observational data, and in §3 the main the time-scales required to halt star formation in recently ac- results. In §4 we follow the infall and orbits of galaxies in the creted cluster spirals, with results supporting gentle physi- vicinity of massive galaxy clusters from the Millennium simu- cal mechanisms (e.g. starvation) that slowly quench star- lation, to predict their spatial distributions and kinematicsas a formation over a period of several Gyrs (Wolf et al. 2009; function of accretion epoch. These model predictions are then von der Linden et al. 2010; De Lucia et al. 2012; Wetzel et al. compared to observations in §5. We discuss the resultant con- 2013), rather than more violent processes (e.g. merg- straints on the time-scales required to quench star formation ers) that rapidly terminate star formation (although see in recently accreted galaxies and the need for pre-processing e.g. Balogh et al. 2004; McGee et al. 2011; Wijesinghe et al. in §6 and summarize in §7. Throughout we assume ΩM=0.3, −1 −1 2012). ΩΛ=0.7 andH0=70kms Mpc . In Haines et al. (2009) we estimated the composite radial population gradients in the fraction of star-forming galaxies 2. DATA ( fSF ) in 22 massive clusters at 0.15redshifts to ters were selected from the parent sample simply on the basis identify cluster galaxies, such that we had to statistically ac- of being observable by Subaru on the nights allocated to us count for the contamination for field galaxy interlopers when (Okabe et al. 2010), and should therefore not suffer any gross estimating the fSF (r). biases towards (for example) cool core clusters, merging clus- We have since completed ACReS (Arizona Cluster Red- ters etc. Indeed, Okabe et al. (2010) show that the sample is shift Survey12) which provides highly-complete spectroscopy statistically indistinguishable from a volume-limited sample. 13 of cluster members for all 30 clusters from LoCuSS with 2.1. Chandra/XMM X-ray imaging wide-field Spitzer/MIPS data. With this data, Haines et al. 10 (2013) found the specific-SFRs of massive ( &10 M⊙) All but two (Abell 291, Abell 2345) of the 30 clusters M star-forming cluster galaxies within r200 to be systematically have available deep Chandra data (texp=9–120ksec). De- 28% lower than their counterparts in the field at fixed stellar projected dark matter densities, gas densities and gas tem- mass and redshift, a difference significant at the 8.7σ level. perature profiles for each cluster were derived by fitting This is the unambiguous signature of star formation in most the phenomenological cluster models of Ascasibar & Diego (and possibly all) massive star-forming galaxies being slowly (2008) to a series of annular spectra extracted for each cluster quenched upon accretion into clusters, and was best fit by (Sanderson & Ponman 2010). The best-fitting cluster models models in which their star formation rates decline exponen- were then used to estimate r500, the radius enclosing a mean tially on quenching time-scales in the range 0.7–2.0Gyr. overdensity of 500 with respect to the critical density of the In this article we analyse the spatial distribution and kine- Universe at the cluster redshift (Sanderson et al. 2009). matics of star-forming galaxies within the same set of 30 The r500 value for Abell 689 is taken from Gilesetal. clusters, and by comparing with predictions from cosmolog- (2012), as they separated the extended cluster X-ray emission ical simulations, draw further independent constraints on the from the central BL Lac, and for the clusters lacking Chan- quenching time-scale. In particular, we determine the radial dra data, the r500 values are taken from the XMM analysis of surface density profile, Σ(r), of star-forming cluster galaxies Martino et al. (2014). For the 23 clusters in common with the and show that it declines steadily with radius, falling 15 joint Chandra–XMM analysis of the LoCuSS high-LX cluster ∼ × from the core to 2r200. We show that this simple obser- sample of Martino et al. (2014), there is good consistency of vation provides powerful constraints for how long massive the cluster radii with r500,Haines/r500,Martino =1.003 0.034. h i ± star-forming galaxies are able to continue forming stars once The Chandra data were also used to identify X-ray AGN they are accreted into rich clusters, quickly ruling out models as described in Haines et al. (2012). The survey limit of six in which star-formation is rapidly halted in infalling spirals broad (0.3–7keV) X-ray counts results in on-axis sensitivity 42 −1 when they pass within r200. We also re-examine the radial limits of LX 1.0 10 ergs for X-ray AGN at the cluster redshift for all≤ 28× systems (Table 1 from Haines et al. 2012). 11 http://www.sr.bham.ac.uk/locuss/ Deep XMM data was available for 23 systems, allow- 12 http://herschel.as.arizona.edu/acres/acres.html ing other groups and clusters in the region to be identi- 13 http://herschel.as.arizona.edu/locuss/locuss.html fied. Each 0.5–2keV image is decomposed into unresolved THE SF–RADIUS RELATION, SLOWQUENCHINGANDPRE-PROCESSING 3

(r /r ) 0 1 proj 200 2 3 primarily through the Cycle 4 (GI4-090; PI. G.P. Smith) and 1.0 Cycle 6 (GI6-046; PI. S. Moran) Guest Investigator Pro- 0.9 grams. The Cycle 4 program provided far-ultraviolet (FUV) 0.8 and near-ultraviolet (NUV) imaging for 14 clusters (texp=3.2– 0.7 13.6ksec), while comparable FUV+NUV data were obtained 0.6 for 7 more clusters (texp=3.4–29.0ksec) from the GALEX sci- ence archive. Sixteen clusters were observed in Cycle 6, 0.5 including 8 systems not previously observed, but this pro- 0.4 0.25magnitude limits, after correcting for Galactic extinc- proj 500 tion as in Wyder et al. (2007), are shown in Table 1. Figure 1. Radial coverage of the Spitzer/MIPS 24µm data. Each curve shows ′ ′ Optical photometry in the ugriz bands were taken from the fraction of cluster members covered by our 24µm images (25 ×25 field- the Sloan Digital Sky Survey, using the dereddened SDSS of-view) as a function of cluster-centric radius in units of r500, for clusters in the following redshift ranges: 0.15

Cluster hzi Nz (lit) r500 M500 σν texp(NUV) mAB(NUV) 14 −1 Name (Mpc) (10 M⊙) (km s ) (sec) (S/N=5) +89 Abell 68 0.2510 194 (0) 0.955 3.193 1186−88 6731 22.90 +72 Abell 115 0.1919 213 (36) 1.304 7.628 1219−71 16217 23.36 c +65 Abell 209 0.2092 393 (49) 1.230 6.519 1369−67 11616 23.76 a +52 Abell 267 0.2289 230 (139) 0.994 3.515 1045−52 11268 23.32 e +80 Abell 291 0.1955 126 (0) 0.868 2.259 704−82 6059 23.16 d +64 Abell 383 0.1887 266 (92) 1.049 3.958 950−63 6117 23.16 +55 Abell 586 0.1707 247 (21) 1.150 5.117 933−54 — — +67 Abell 611 0.2864 297 (7) 1.372 9.847 1039−67 9928 23.01 +59 Abell 665 0.1827 359 (31) 1.381 8.975 1227−59 9503 23.26 a f +57 Abell 689 0.2776 338 (153) 1.126 5.390 721−58 — — a +57 Abell 697 0.2821 486 (141) 1.505 12.93 1268−58 36858 23.89 a +49 Abell 963 0.2043 466 (50) 1.275 7.226 1119−49 29043 23.96 a +46 Abell 1689 0.1851 857 (416) 1.501 11.55 1541−46 7716 23.34 a +64 Abell 1758 0.2775 471 (50) 1.376 9.835 1442−63 20977 23.88 a +52 Abell 1763 0.2323 423 (126) 1.220 6.522 1358−53 13589 23.80 a +35 Abell 1835 0.2524 1083 (608) 1.589 14.73 1485−35 21827 23.50 a +44 Abell 1914 0.1671 454 (65) 1.560 12.73 1055−44 3804 23.17 +42 Abell 2218 0.1733 342 (49) 1.258 6.716 1245−42 14617 23.73 a +59 Abell 2219 0.2257 628 (297) 1.494 11.90 1332−58 9886 23.42 e +43 Abell 2345 0.1781 405 (39) 1.249 6.607 1000−43 9864 23.49 +63 Abell 2390 0.2291 517 (140) 1.503 12.16 1372−62 6819 22.35 +54 Abell 2485 0.2476 196 (0) 0.830 2.088 799−54 — — +66 RXJ0142.0+2131 0.2771 204 (15) 1.136 5.531 1123−63 — — b +37 RXJ1720.1+2638 0.1599 473 (114) 1.530 11.92 938−36 3074 22.67 a +82 RXJ2129.6+0005 0.2337 334 (78) 1.227 6.648 879−82 36323 23.72 e +74 ZwCl0104.4+0048 (Z348) 0.2526 185 (1) 0.760 1.613 806−73 14435 23.46 +50 ZwCl0823.2+0425 (Z1693) 0.2261 337 (4) 1.050 4.130 671−48 14314 23.41 +89 ZwCl0839.9+2937 (Z1883) 0.1931 173 (3) 1.107 4.674 834−87 21250 23.39 +79 ZwCl0857.9+2107 (Z2089) 0.2344 147 (0) 1.024 3.866 815−80 20225 23.68 +84 ZwCl1454.8+2233 (Z7160) 0.2565 157 (1) 1.128 5.294 988−84 19850 23.58

Table 1 The cluster sample. Column (1) Cluster name; col. (2) Mean redshift of cluster members; col. (3) Total number of spectroscopic cluster members (contribution taken from the literature, including new redshifts from Rines et al. (2013)a, Owers et al. (2011)b, Mercurio et al. (2003)c and Geller et al. (2014)d ); col. (4) e f 14 radius r500 in Mpc. From Martino et al. (2014), From Giles et al. (2012); col. (5) Cluster mass M500 in 10 M⊙ ; col. (6) Velocity dispersion of cluster members within r200; col. (7) Total exposure time of GALEX NUV images; col. (8) NUV magnitude limit for SNR = 5 structure (Dünner et al. 2007). For most systems, there is IMF. Where SDSS photometry was unavailable we classified a strong contrast in phase-space density from inside to out- the galaxy as being either star-forming or passive according side these caustics (Rines & Diaferio 2006), making their vi- to whether it was 24µm or NUV detected or not, and adopted sual identification relatively simple. The central redshift z appropriate M/L ratios. and velocity dispersion σν of each cluster (Table 1) are iter-h i For each 24µm-detected galaxy with known redshift, its in- atively measured for member galaxies within r200 (estimated trinsic bolometric luminosity (LTIR) and rest-frame 24µm lu- as in Finn et al. 2008), using the biweightscale estimator (SBI; minosity is estimated by comparison of its 24µm flux to the Beers, Flynn & Gebhardt 1990), with uncertainties estimated luminosity-dependenttemplate infrared spectral energy distri- using bootstrap resampling. butions of Rieke et al. (2009). The latter is then converted to The field galaxy sample was taken from the same dataset an obscured SFR using the calibration of Rieke et al. (2009) as the primary cluster galaxy sample, but were located in nar- −1 −10 row redshift ranges on either side of the cluster, for which SFRIR(M⊙yr )=7.8 10 L(24µm,L⊙) (1) our spectroscopic survey ACReS should still be complete to × ∗ + which is valid for either a Kroupa (2002) or Chabrier (2003) MK 1.5 (for full details see Haines et al. 2013). Overall, ∗ + IMF. Our Spitzer data should be sensitive to galaxies with 1398 coeval (0.15

(rproj /r 200 ) 0 1 2 Cluster galaxies rproj < r 500 0.4 1000 All Infall regions r proj > r 500 Field galaxies at 0.15

−1 0.3 100

0.2 Galaxies SFR > 2 M yr−1 10 IR SF IR

f ( SFR > 2.0 M yr ) 0.1

30 clusters 1 0.0 0 1 2 3 −27 −26 −25 −24 −23 clustercentric radius (r /r ) proj 500 K−band absolute magnitude Figure 2. Radial population gradients for MIR-selected star forming galax- Figure 3. K-band luminosity functions (LFs) for cluster galaxies within r ies from our stacked sample of 30 clusters. Red symbols show the fraction of 500 10 (red solid curve), galaxies from the infall regions (rproj>r500; green solid massive (M > 2.0×10 M⊙) cluster galaxies with obscured star-formation curve) and the coeval field population (light-blue solid curve). The latter two −1 at rates SFRIR > 2.0M⊙ yr as a function of projected cluster-centric radius are normalized to contain the same total number of MK <−23.1 galaxies as the (rproj/r500). The error bars indicate the uncertainties derived from binomial cluster galaxy population, and the points slightly shifted horizontally, to per- statistics calculated using the formulae of Gehrels (1986). Each radial bin mit easier comparison of the LFs. Error bars indicate Poissonian uncertaintes contains 400 cluster galaxies. The blue horizontal line indicates the corre- based on Gehrels (1986). The corresponding K-band LFs for star-forming 10 −1 sponding fraction of field galaxies (M>2.0×10 M⊙; 0.152.0M⊙yr are indicated by dashed curves. −1 SFRIR > 2.0M⊙ yr and its 1σ confidence limits (shaded region). lar populations including emission from dusty circumstel- dust heated by the active nucleus rather than star formation lar envelopes peak at 10–20µm, but then drop rapidly (e.g. Xu et al. 2015). BCGs are also excluded due to their at longer wavelengths (Piovanet al. 2003), and so galax- unique evolutions (Lin & Mohr 2004), and the direct link be- ies whose 24µm emission is due to TP-AGB stars should tween BCG activity and the presence of cooling flows within not be detected in our Herschel/PACS data, unlike nor- clusters (Smith et al. 2010a; Rawle et al. 2012). mal star-forming galaxies whose infrared SEDs peak at 70– The fraction of obscured star-forming galaxies increases 170 m (e.g. Daleetal. 2012). For the 11 nearest clus- steadily with cluster-centric radius from fSF 0.04 in the clus- µ ∼ ters (0 15 z 0 20) in our sample, 98% of galaxies with ter core to fSF 0.23 at 3.0r500 (1.9r200). However even at . < < . > ∼ −1 these large radii the fSF remains well ( 1/3) below that seen SFRIR>2.0M⊙yr and Herschel/PACS coverage were also ∼ in coeval field galaxies ( fSF =0.33 0.01; blue line). A simple detected at 100µm, while >99% show clear Hα emission ± in our ACReS MMT/Hectospec spectra, indicating that their linear extrapolation of the observed trend suggests that the fSF should reach that of the field galaxy population at 4.5r500. 24µm emission is indeed due to ongoing star formation. ∼ In a comparable Spitzer/MIPS analysis of 814 galaxies However the limited extents of our Spitzer 24µm maps mean in the Shapley supercluster at z=0.048, sensitive to much that we cannot establish whether this occurs or not. −1 The fraction of star-forming cluster galaxies evolves very lower obscured star formation rates (SFRIR 0.05M⊙yr ) 7.6±1.1 ∼ rapidly at these redshifts, with fSF (1+z) (Haines et al. Haines et al. (2011b) did find a significant population of qui- ∝ escent (basedupona lack of Hα emission) cluster galaxies de- 2013). It is thus vital to ensure that this shortfall in star- tected at 24µm, but none with 24µm luminosities that would forming cluster galaxies at large radii with respect to field −1 convert to an obscured SFR above 0.5M⊙yr . They also ob- valuesis not producedby a redshift bias between the two sam- tained a tight correlation (0.22dex) between the 24µm and the ples. This is certainly not the case here, as the mean redshifts 1.4GHz radio luminosities for star-forming cluster galaxies, of each radialbin for thecluster populationsall lie in therange −1 0.217–0.241,while for the field population z =0.225. There down to SFRIR 1.0M⊙yr , consistent with both the mid- h i infrared and radio∼ emission being due to star formation. is a marginal redshift bias within our cluster sample, as the outer two radial bins have z =0.241, while the remaining h i 3. RESULTS bins all have mean redshifts in the range 0.217–0.231. This is due to our Spitzer data providing wider radial coverage (in 3.1. Radial population gradients terms of r500) for the higher redshift systems, but it is likely 10 Figure 2 shows the fraction of massive ( >2.0 10 M⊙) only a marginal effect, artificially increasing the outer two fSF cluster galaxies with obscured star formationM occurring× at by . 10% (or 0.02 in the figure). −1 ∼ rates SFRIR > 2.0M⊙ yr ( fSF ) as a function of projected A second possible explanation for the lower fSF among cluster-centric radius in units of r500, across the full sample cluster galaxies would be if they were more massive on aver- of 30 clusters. The fractions only consider cluster members age than the field galaxy comparison sample, as fSF is known covered by the Spitzer 24µm maps, and exclude X-ray AGN to decline with increasing stellar mass at fixed galaxy density and QSOs, as their 24µm emission is usually dominated by (e.g. Haines et al. 2007). However, the mean stellar masses of 6 HAINESETAL.

(rproj /r 200 ) (rproj /r 200 ) 0 1 2 3 0.1 1.0 1000 0.4 All MK < −23.10 gals 0.6 Field galaxies ( NUV − r ) 0.0 < 4.5 −1 −1 SFRs > 2.0 M yr 100 0.5 0.3 −1

0.4 0.0 −2 500

r 10 0.2 0.3 galaxies (r ) cluster

0.2 Σ SF

SF IR 1.0 f ( (NUV − ) < 4.5

0.1 f ( SFR > 2.0 M yr ) 0.1 GALEX Spitzer 0.1 1.0 0.0 0.0 clustercentric radius (rproj /r 500 ) 0 1 2 3 4 5 Figure 5. Composite galaxy surface density profile for all M 2.0×10 M⊙) cluster galaxies with blue sponding observed surface density (r) of MK 2.0M⊙yr from the same ensem- scured star-formation. Each radial bin contains 300 cluster galaxies. The ble cluster. The magenta squares show the radial profile for unobscured star- magenta dashed line indicates the corresponding fraction of field galaxies forming galaxies with (NUV −r)0.0<4.5 from the 22 clusters that have SDSS 10 (M>2.0×10 M⊙; 0.15 2.0M⊙ yr taken from Fig. 2, scaled via the right-hand axis, such that the fraction of field galaxies with surface density profiles for all “spectroscopic” member galaxies of the 75 −1 most massive clusters from the Millennium simulation, scaled to fit our ob- SFR > 2.0M⊙ yr (blue dashed line) coincides with the corresponding IR served ensemble surface density profile of all galaxies. fraction selected in the NUV. the cluster core to fSF 0.44at 4–5 r500 ( 3r200). The fraction ∼ ∼ cluster galaxies for each radial bin in Fig. 2 are always within of star-forming cluster galaxies remains significantly below 0.04dex of that of the coeval field population. Moreover, the that seen in coeval field galaxies ( fSF =0.53 0.01; magenta ± K-band luminosity functions of cluster galaxies within r dashed line), even out at 5r500, the trend appearing to flat- 500 ∼ (excluding BCGs), those in the infall regions (rpro j>r500), and ten off rather than continue upwards to field values. Again the the field galaxy samples are all indistinguishable (Fig. 3). We two samples are confirmed to be coeval: each radial bin for −1 also note that due to the fixed SFR lower limit of 2.0 M⊙ yr the cluster population has a mean redshift in the range 0.217– 0.236, while for field galaxies z =0.226. used to define a star-forming galaxy, the fSF do not vary much h i with stellar mass (K-band luminosity). We therefore exclude To allow comparison between the NUV-based and 24µm- secular quenching due to increased stellar masses among the based SF–radius relations, the latter (gray points) is replotted cluster galaxy population as being responsible for their lower from Fig. 2 after adjusting its vertical scale (right hand axis) to ensure that fractions of IR-selected and UV-selected star- fSF (at all radii in Fig. 2) with respect to the field. The pro- gressive suppression in star-formation in moving from field forming field galaxies coincide on the plot. While both re- to infall regions, and on to cluster environments is seen at all lations show the same steadily increasing trends with radius, stellar masses (compare dashed curves in Fig. 3). the NUV-based SF–radius relation consistently lies above the The GALEX NUV data providesa complementary means of re-scaled IR-based relation. identifying star forming galaxies from their ultraviolet emis- Looking at the IR-based SF-radius relation, it is tempting sion, and an opportunityto measure the SF–radius relation out to suggest that the fraction of star-forming galaxies even falls to larger cluster-centric radii. Figure 4 shows the composite to zero at the cluster core. However, we confirm that this is radial population gradient (magenta squares) in the fraction not the case. Further splitting each radial bin into four, the of cluster galaxies having blue rest-frame UV–optical col- fSF,IR never fall below a floor value of 3–5% in the cluster core. Similarly, the NUV-based SF-radius∼ relation never falls ors (NUV−r)0.0<4.5, for the 23 clusters with deep GALEX − below 8–15% in the cluster core, when the radial bins are NUV data and SDSS ugriz photometry. The (NUV r)0.0=4.5 ∼ color limit lies in the middle of the UV–optical “green valley” further sub-divided. A residual population of cluster galaxies (Wyder et al. 2007), and allows passively-evolving galaxies with ongoing star formation exists at all radii. to be efficiently excluded without losing dusty star-forming galaxies due to reddening (Haines et al. 2008). Optically- 3.2. Radial surface density profiles quiescent early-type galaxies with residual (or “rejuvenated”) The spatial distribution of galaxies within clusters provide extended star formation in the form of rings or spiral arms key constraints on the primary epoch at which they were ac- should also still be recovered (Salim & Rich 2010). creted into the system, as well as the effects of continual clus- As before, the fraction of star-forming cluster galaxies in- ter mass growth on their orbital parameters and tidal and ram- creases steadily with cluster-centric radius from fSF 0.11 in pressure stripping on their stellar masses. Figure 5 shows the ∼ THE SF–RADIUS RELATION, SLOWQUENCHINGANDPRE-PROCESSING 7

0.0−0.5 r 0.5−1.0 r 3 500 500 0.10 Std. Dev Kurtosis 0.10 Std. Dev Kurtosis 1.074σν −0.32 0.09 0.873σν −0.05 0.23 0.08 1.376σν −0.76 0.15 0.08 1.102σν −0.85 0.10 σ σ ν/σ 1.368 ν −0.75 0.14 1.091 ν −0.87 0.10 2 0.06 0.06

Fraction 0.04 0.04 1 0.02 0.02 0.00 0.00 −3 −2 −1 0 1 2 3 −3 −2 −1 0 1 2 3 ( ∆ν ) / σν ( ∆ν ) / σν

0 1.0−2.0 r500 2.0−4.0 r500 0.14 Std. Dev Kurtosis 0.14 Std. Dev Kurtosis σ σ 0.12 0.754 ν −0.01 0.11 0.12 0.733 ν +0.29 0.16 0.943σν −0.66 0.09 0.799σν −0.07 0.17 0.10 σ 0.10 σ −1 0.930 ν −0.49 0.14 0.807 ν −0.35 0.10 0.08 0.08 0.06 0.06 Fraction −2 0.04 0.04 0.02 0.02 0.00 0.00

Relative line−of−sight velocity ( ) −3 −2 −1 0 1 2 3 −3 −2 −1 0 1 2 3 ( ∆ν ) / σν ( ∆ν ) / σν −3 Figure 7. Stacked relative LOS velocity distributions of inactive galaxies 0 1 2 3 (red solid histograms), 24µm-detected (blue striped histogram) and UV- clustercentric radius (r /r ) selected (NUV − r < 4.5; magenta striped histogram) star-forming galaxies proj 500 from our ensemble cluster, in four bins of projected radius. The standard de- viations (in units of σν ) and kurtosis values of each distribution are indicated. −1 ter galaxies with obscured SFRIR>2.0M⊙yr (blue points) −11.0 −10.8 −10.6 −10.4 −10.2 −10.0 −9.8 declines steadily with radius from 7 galaxies r−2 cluster−1 log specific−SFR (/yr) ∼ 500 10 in the cluster cores to 0.4 by 3r500. These Σ(r) include ∼ ∼ Figure 6. Stacked observed phase-space diagram, (νlos−hνi)/σν versus corrections for spectroscopic incompleteness (§ 2.4) and the rproj/r500, of galaxies combining all 30 clusters in our sample. Each gray radial variation in coverageby the 24µm images (§ 2.2). Even solid dot represents a spectroscopic cluster member, while open points indi- cate non-cluster galaxies. Only those galaxies covered by our Spitzer data are though the fraction of star-forming galaxies is falling to close plotted. Star-forming galaxies detected at 24µm are indicated by larger sym- to zero in cluster cores, clusters mark over-densities in the bols, colored according to their specific-SFRs from red (sSFR<10−11 yr−1) spatial distribution of star-forming galaxies in the plane of to blue (sSFR>10−10 yr−1). The blue curve and error bars indicates the 1-σ the sky. This is simply due to the cuspy density profile of velocity dispersion profile of the 24µm-detected cluster members. The un- the global cluster galaxy population more than compensat- certainty in each σ(r) value is estimated by bootstrap resampling the galaxies ing for the steady decrease in fSF when approaching the clus- in that radial bin. The black curve shows the corresponding radial profile for ∗ + the remaining inactive cluster members. ter core. The surface density of MK

than their passive counterparts at the same cluster-centric ra- los r200 dius, a result significant at the 8.0σ (10.7σ) level. σ / 1.10 The same trends are obtained when selecting star-forming inactive cluster galaxies according to their rest-frame NUV − r color. Their velocity dispersion remains 10–35% higher than that of 1.05 inactive cluster galaxies out to 2.5–3 r500, although at larger radii they become indistinguishable. 1.00 The relative LOS velocity distributions of star-forming 10.47 (both 24µm-detected and UV-selected) and inactive cluster 10.25 10.52 galaxies for the stacked LoCuSS cluster sample are shown in 0.95 10.50 10.62 Figure 7, in four bins of projected cluster-centric radius. The Velocity dispersion ( ) 10.75 10.96 LOS velocity distributions of inactive galaxies (not detected 11.05 10.65 0.90 at 24µm and having (NUV −r)0.0 > 4.5) can be approximately described as a Gaussian at all radii. The distributions of star- 0.8 0.9 1.0 1.1 1.2 forming galaxies in the two inner radial bins (rpro j / r500 stead appear more consistent with a flat, top-hat profile than a Figure 8. Variation in the spatial distribution and kinematics of cluster galax- Gaussian, including an relative excess of star-forming galax- ies as a function of their specific-SFR and stellar mass. Colored crosses ies at ∆ν/σν > 1.2 in comparison to the inactive popula- indicate the mean cluster-centric radii (hr /r i) and LOS velocity dis- | | proj 500 tion. At 0.5–1.0 r500 there is even marginal evidence a cen- persions (σlos(rproj<2.0r500)/σν ) of star-forming cluster galaxies split into tral dip in the LOS velocity distribution of star-forming galax- four bins of specific-SFR. Error ellipses show the 1σ uncertainties in both values. The black cross indicates the corresponding values for inactive clus- 1 N − ¯ 4 1 N − ¯ 2 2 − ies. The kurtosis, γ2 =[ N Pi=1(νi ν) /( N Pi=1(νi ν) ) ] 3 ter galaxies not detected with Spitzer. Only cluster galaxies within 2r500 are of the LOS velocity distributions of star-forming galaxies included within each sub-sample. The mean stellar mass, hlog(M/M⊙)i, of each sub-sample is indicated in the lower-right corner. Thin dashed el- (γ2 − 0.81) is significantly lower than that expected for a ∼ lipses indicate the results after applying additional stellar mass cuts to each Gaussian distribution (γ2=0.0) at >5σ level in both inner ra- specific-SFR bin to equalize their mean stellar masses to that of the inac- dial bins, and closer to expectations for a uniform top-hat tive population. Grayscale symbols and error-bars indicate the mean cluster- centric radii and LOS velocity dispersions of cluster galaxies (including both distribution (γ2= − 1.2). At large cluster-centric radii (2.0– 4.0 r ), the velocity distribution of star-forming galaxies is star-forming and inactive sub-populations) split into four bins of stellar mass: 500 log(M/M⊙)<10.4 (light gray circle); 10.4< logM<10.6 (gray triangle); consistent with a Gaussian function, and is almost indistin- 10.610.85 (dark-gray square). guishable from that of the inactive population, albeit with a ulation into bins of stellar mass (grayscale symbols), much marginally ( 9%) higher dispersion. ∼ less variation in the mean radii and LOS velocity dispersions To further delineate the connection between the kinematics is seen between stellar mass bins in comparison to those split of cluster galaxies and their ability to form stars, the sample by specific-SFR, and no overall trend with stellar mass is vis- of 24µm-detected cluster galaxies within 2.0r500 is split into ible. Moreover, the kinematic segregation by specific-SFR four bins of specific-SFR and their mean cluster-centric radii persists even if additional stellar mass cuts are applied to each rpro j/r500 , and LOS velocity dispersions σlos(<2r500)/σν , h i specific-SFR bin to equalize their mean stellar masses (thin compared in Figure 8. Kinematic segregation of galaxies with dashed ellipses). diverse specific-SFRs is apparent. Galaxies with the highest − − specific-SFRs (>10 9.9 yr 1; blue cross) have the highest ve- 4. MAPPING THE CONTINUAL ACCRETION OF GALAXIES ONTO locity dispersion as a population, and a higher mean cluster- MASSIVE CLUSTERS IN THE MILLENNIUM SIMULATION centric radius, than those star-forming galaxies with the low- To correctly interpret the previous observed trends in clus- est specific-SFRs (<10−10.5 yr−1; red cross). There is a general ter galaxy properties, clusters must be placed in the cosmo- progression towards lower mean radii and LOS velocity dis- logical context of continually accreting galaxies and groups persions with decreasing specific-SFR. This progression con- from their surroundings. With this aim, we have examined tinues to the inactive galaxy cluster galaxy population (not de- the spatial distributions and orbits of galaxies in the vicinity tected at 24µm; black cross), which has a significantly lower of the 75 most massive clusters from the Millennium simula- mean radius and velocity dispersion than any of the four sub- tion (Springel et al. 2005), a cosmological dark matter simu- populations of star-forming galaxies. lation covering a (500h−1Mpc)3 volume. These clusters have 14 −1 The mean stellar masses of the star-forming cluster galax- present day virial masses in the range 4.0–23.6 10 h M⊙, ies increases by 0.5dex from the highest specific-SFR bin to velocity dispersions of 630–1540kms−1, and a median× forma- those in the lowest one, reflecting the systematic decline in − tion epoch, z =0 59. We have extracted 20 20 140h 3 Mpc3 specific-SFR with stellar mass for star-forming cluster galax- f . volumes centered on each cluster. These× volumes× are ex- ies (Haines et al. 2013). We may thus be concerned that these tended in the z-direction so that, for a distant observer view- kinematical differences are in fact due to mass segregation ing along this axis, all galaxies with LOS velocities within rather than a sequence in declining specific-SFR. −1 There is no evidence for mass segregation within 5000kms of the cluster redshift are included, enabling pro- our cluster galaxy population however, consistent with jection effects to be fully account for and quantified. von der Linden et al. (2010). Splitting the cluster galaxy pop- There exists a full database of properties for each galaxy in the simulation, including positions, peculiar velocities, ab- THE SF–RADIUS RELATION, SLOWQUENCHINGANDPRE-PROCESSING 9 −10 Redshift 4 3 2.0 1.5 1.0 0.7 0.5 0.4 0.3 0.2 0.1 0.0 18

17 −5 16 15

14

Mpc 0 13 −1

h 12 −1

h 11 5 10 9

8

10 7 −10 −5 0 5 10 h−1 Mpc 6 Clustercentric radius ( Mpc) 5

0.00 0.02 0.040.09 0.14 0.21 0.28 0.36 0.51 0.69 0.91 4 Redshift at which galaxy is accreted by cluster 3 Figure 9. The orbits of galaxies about the 10th most massive cluster in the Millennium simulation, as viewed by a distant observer along the z-axis. The 2 10 orbit of each galaxy with M>2×10 M⊙ at z=0 is shown by a colored curve, tracing its movement from z=0.76 (snapshot 44) to z=0.0 (snapshot 1 63). The final location of the galaxy is marked by a dot, while the cross indi- cates its location at z=0.41 (snapshot 50). Each curve is color coded accord- 0 ing to the epoch at which the galaxy is accreted into the cluster, as indicated 12 11 10 9 8 7 6 5 4 3 2 1 0 at the bottom of the plot. This accretion epoch is defined as the snapshot at Look−back time (Gyr) which the galaxy passes within r200(z) for the first time. Galaxies yet to pass Figure 10. The infall of galaxies onto a massive cluster. Each galaxy with within r200 have mid-blue colors, while those accreted earliest into the clus- 10 ter (zacc>0.51) have red colors. The dashed black circle indicates the present M>2×10 M⊙ at z=0 is shown by a colored curve tracing its comoving day r200 radius of the cluster. Only galaxies which would be spectroscopi- cluster-centric distance as a function of look-back time. The curves are color cally identified as cluster members by the observer are shown. coded according to the epoch at which the galaxy is accreted into the clus- ter, as in Figure 9. The first pericenter and apocenter of each galaxy’s orbit solute magnitudes, stellar masses etc., based upon the GAL- about the cluster are marked with dots. The black dashed curve indicates the FORM semi-analytic models (SAMs) of Bower et al. (2006) evolution of the cluster radius r200 with time. Only galaxies which would be at 63 snapshots throughout the life-time of the Universe to spectroscopically identified as cluster members by the observer are shown. z=0. Similarly, another database provides the positions, veloc- devoid of galaxies. Galaxies tend to be drawn first into the ities, masses (M200) and radii (r200) of each DM halo at each filaments from the surrounding field, then flow along the fil- snapshot. For each galaxy and halo in a given snapshot, the aments into the cluster. While many galaxies are infalling as database provides links to identify its most massive progenitor individual objects from the field, others are arriving into the in the preceding snapshot, and so on, all the way back to its cluster within galaxy groups (e.g. the tangle of green curves formation, allowing its mass growth and full merger history to coming in from the right-hand side). be mapped in detail (see e.g. De Lucia & Blaizot 2007). This The full extent of the cluster’s gravitational sphere of influ- process also allows the orbit of each galaxy with respect to ence is revealed in Fig. 10, with all galaxies within 18h−1 Mpc the cluster to be followed from formation to the present day, of the cluster falling steadily inwards. For all 75 clusters, the enabling us to determine its epoch of accretion (zacc) into the boundarybetween the infall regions and beyond, where galax- cluster, defined here as the redshift at which the galaxy passes ies remain attached to the Hubble flow, is found at a comoving within r (z) for the first time. − 200 distance 10–20h 1 Mpc from the cluster. The infall∼ of galaxies into the cluster is highly coherent, at 4.1. The infall of galaxies onto clusters least for z.1: the radial velocities of infalling galaxies at the Figure 9 shows the orbits of galaxies about the tenth most same cluster-centric radius at a given epoch are all roughly the 14 −1 massive cluster (M200=9.9 10 h M⊙ at z=0) in the Millen- same, and the future trajectories and accretion epochs of an in- nium simulation, color coded× according to accretion epoch. falling galaxy can be accurately estimated simply on the basis Almost all the galaxies over the 20 20h−2 Mpc2 field-of-view of their current cluster-centric distance. For example, almost are falling steadily into the cluster× or have already been ac- all galaxies which were 5h−1 Mpc from the cluster 4 billion creted. The complex large-scale structure around the cluster is years ago, are due to be accreted into the cluster at z 0.1, as apparent, including clear preferential directions for the galax- indicated by the parallel diagonal green colored curves.∼ ies to flow into the cluster, while other regions appear largely After accretion, galaxies remain bound to the cluster, but 10 HAINESETAL. 4000 4000 backsplash galaxies σ 3000 3000 z = 0.00 = 837.1 km/s

2000 2000

1000 1000

0 0

−1000 −1000

Radial velocity (km/s) −2000 infalling galaxies −2000

−3000 14 −3000 M 200 = 9.93x10 M Line−of−sight velocity offset (km/s)

−4000 −4000 0 1 2 3 4 0 1 2 3 clustercentric radius (r/r ) 200 clustercentric radius (r/r200 )

0.00 0.02 0.04 0.09 0.14 0.21 0.28 0.36 0.51 0.69 0.91 0.00 0.02 0.04 0.09 0.14 0.21 0.28 0.36 0.51 0.69 0.91 Redshift at which galaxy is accreted by cluster Redshift at which galaxy is accreted by cluster Figure 11. Radial phase-space diagram (νradial vs r/r200) for galaxies with ∆ 10 9 Figure 12. The caustic diagram ( νlos vs rproj/r200) for M>2×10 M⊙ M>5×10 M⊙ orbiting around the same massive cluster as in Figs 9 and 10. galaxies orbiting around the same massive cluster, as viewed by distant ob- Galaxies are color coded according to their accretion epoch as indicated. server along the z-axis. Galaxies are color coded according to their accretion −1 many have orbits which take them outside r (black dashed epoch as in Figs. 9–11. Galaxies within 20h Mpc of the cluster center at 200 z=0.0 are indicated by solid symbols, while those at comoving cluster-centric curve), including some which bounce out as far as 2.5r200. −1 ∼ radii r>20h Mpc are shown by open symbols. The velocity dispersion of These galaxies, rebounding out of the cluster after their first galaxies within a projected cluster-centric radius rproj

3 3 3 ν/σ ν/σ ν/σ 2 2 2

1 1 1

0 0 0

−1 −1 −1

−2 −2 −2

Relative line−of−sight velocity ( ) −3 (a) All cluster galaxies Relative line−of−sight velocity ( ) −3 (b) Galaxies yet to be accreted Relative line−of−sight velocity ( ) −3 (c) Recently accreted and still infalling

0 1 2 3 0 1 2 3 0 1 2 3 clustercentric radius (rproj /r200 ) clustercentric radius (rproj /r200 ) clustercentric radius (rproj /r200 )

0.00 0.02 0.04 0.09 0.14 0.21 0.28 0.36 0.51 0.69 0.91 3 3 Redshift at which galaxy is accreted by cluster ν/σ ν/σ 2 2

1.6 Galaxies yet to be accreted 1 1 1.4 Recently accreted and still infalling Backsplash galaxies los r200 1.2 σ / Virialized population 0 0 1.0 All cluster galaxies 0.8 −1 −1 0.6 −2 −2 0.4

0.2 (d) Relative line−of−sight velocity ( ) −3 (e) Backsplash galaxies Relative line−of−sight velocity ( ) −3 (f) Virialized population Velocity dispersion ( ) 0.0 0 1 2 3 0 1 2 3 0 1 2 3 clustercentric radius (rproj /r200 ) clustercentric radius (rproj /r200 ) clustercentric radius (rproj /r200 )

Figure 13. Stacked phase-space diagram (∆νlos/σν vs. rproj/r200) for the 75 most massive clusters in the Millennium simulation at z=0.0 (panel a). Each point 10 indicates an M>2×10 M⊙ galaxy from the Bower et al. (2006) semi-analytic model catalog, colored according to when it was accreted onto the cluster. The four panels on the right split these cluster galaxies into dynamical sub-populations: (b) just those galaxies yet to pass within r200 and be accreted into the cluster; (c) infalling galaxies (νradial<0) which have passed within r200, but have yet to reach pericenter; (e) back-splash galaxies which have passed through pericenter and are on their way back out of the cluster (νradial >0) and are yet to reach apocenter; (f) the virialized cluster population which have passed through apocenter. The lower left panel (d) displays the velocity dispersion profiles (σlos(r)/σν ) of each sub-population as well as that of the overall cluster population. −1 clear fore/background objects are 19 3h Mpc (13 2r200). To demonstrate how the distribution of galaxy populations Unlike the radial phase-space diagram,± it is not possible± to in the caustic plot provides information about their dynamical select individual galaxies from a specific region of the caustic evolution and accretion history, the right-hand panels show diagram and then identify it as an infalling, recently accreted the caustic diagrams after splitting the cluster population into or virialized galaxy. However, several trends within the dis- four dynamical sub-populations, while panel (d) plots each tribution of galaxies in the caustic diagram can be seen. First, of their velocity dispersion profiles σlos(r)/σν alongside that those galaxies accreted earliest (zacc&0.4; red points) are spa- for the overall cluster population. First, galaxies yet to pass tially localized in the cluster core with typical LOS veloci- within r200 and be accreted (panel b) are found at all cluster- −1 ties .1000kms . Second, beyond rpro j&1.8r200 the bulk of centric radii, becoming increasingly dominant with radius. galaxies have yet to be accreted into the cluster while the re- The fall in numberstowards the cluster core (rpro j.0.6r200) is mainder all appear to be back-splash galaxies accreted 3Gyr due to the area of sky covered in any given narrow radial slice ago. Finally, many of the galaxies with the largest LOS∼ ve- scaling as r, rather than a physical decline in the surface den- locities (& 1000kms−1) have only recently arrived into the sity of such galaxies at low radii (see Fig. 17). The velocity cluster (green points). Although these general trends hold for dispersion of these galaxies remains relatively constant with the vast majority of the clusters in our sample, there is signifi- radius, as they have yet to approach the cluster core cant cluster-to-cluster variation in the spatial distributions and Panel (c) shows those infalling galaxies which have recently relative contributions of galaxies accreted at different epochs, passed within r200 for the first time, but have yet to reach as expected given their dynamical immaturity. pericenter. These objects are all found within r200 by de- fault. They have the highest LOS velocities of all our sub- 4.3. Stacking the clusters populations (σlos(r)&1.4σν ), being fully accelerated as they fall deep into the gravitational potential well of the cluster To account for the cluster-to-cluster scatter in a statistical core, and at late epochs when the clusters are much moremas- way, the caustic diagrams for all 75 clusters are stacked to sive than at any previous point in their history. produce Fig. 13a. The cluster-centric radius of each cluster Panel (e) shows the back-splash population of cluster galax- member is scaled by the r200 of that cluster, and the LOS ve- ies which have passed through pericenter, and are now head- locities are scaled in units of σν (rpro j0), but have dispersion of all “spectroscopic” members within r200. 12 HAINESETAL.

1.0 1.0 1.0

r500 r200 2 r200 0.91 0.8 0.8 0.8 0.69 0.51 0.36 0.6 0.6 0.6 0.28 0.21

0.4 0.4 0.4 0.14 0.09 0.04 0.2 0.2 0.2 0.02 0.00 Redshift at which galaxy is accreted by cluster 0.0 0.0 0.0 0 1 2 3 0 1 2 3 0 1 2 3 Fraction of galaxies already accreted by redshift Fraction of galaxies already accreted by redshift clustercentric radius (rproj /r 200 ) clustercentric radius (rproj /r 200 ) Fraction of galaxies already accreted by redshift clustercentric radius (rproj /r 200 ) Figure 14. Radial population gradients in clusters. Each curve shows the fraction of “spectroscopic” cluster galaxies which have been accreted by a given redshift as a function of projected cluster-centric radius (normalized by r200) averaged over our stacked sample of 75 massive clusters observed at z=0.0, color coded according to accretion redshift as indicated on the far right of the plot. The three panels indicate the trends produced when considering three definitions for describing when a galaxy is accreted into the cluster, taken to be the snapshot when the galaxy passes within r500 (left panel), r200 (middle panel) or 2r200 (right panel). The vertical dashed line in each panel indicates the corresponding radius at which the accretion epoch for the curves in that panel are defined. yet to reach apocenter. These galaxies were typically accreted bulk of the “back-splash” population found beyond r200 were 1–4Gyr ago. This population shows a triangular distribution accreted between z=0.14 and z=0.51. in the caustic diagram, with high LOS-velocities (σlos 1.3– The remaining panels show the effect of changing the nom- 1.5σν ) in the cluster core, having recently completed∼ their inal radius for identifying when a galaxy has been accreted first infall, which steadily fall as the galaxies rebound out into a cluster. Reducing the accretion radius from r200 to r500 of the cluster, slowing down as they attempt to climb back (left panel) simply squeezes the curves inwards by a compa- out of its potential well. The back-splash population ex- rable amount. Pushing the accretion radius outwards to 2r200 tends as far out as 2–3 r200, where they can be differentiated (right panel) increases the prominence of the bump produced from the infalling population by their characteristically low by recently accreted galaxies, but greatly reduces the contri- LOS-velocity dispersions (σlos(r) 0.4–0.55σν), some 35% bution from back-splash galaxies, as so few galaxies which ∼ ∼ lower than the overall cluster population at the same cluster- pass within 2r200 rebound from the cluster beyond this radius. centric distances (see Gill, Knebe & Gibson 2005). The key aspect of all these curves is that irrespective of ac- Finally, the virialized population(panel f) are those galaxies cretion epoch, zacc, or the precise radius used to define ac- which were accreted at early epochs (zacc&0.4) and have all cretion (within reasonable limits), the resulting radial popu- passed through apocenter of their first orbit. This population lation gradient drops from 100% in the cluster core to zero ∼ also presents a triangular distribution in the caustic diagram, by 3r200. This steep gradient is primarily due to the com- but is much more concentrated towards the cluster core (par- plementary∼ increase with radius in the fraction of “interloper” ticularly at .0.2r200) than the back-splash galaxies, and has galaxies that have yet to be accreted into the cluster among lower LOS-velocity dispersions at all radii. We identify this the “spectroscopic” cluster population, from zero in the clus- population with those galaxies that either formed locally or ter core to 100% by 3r200. A second contribution to the were accreted when the cluster’s core was being assembled. steepness comes from∼ the correlations between the epoch of Their low velocities reflect the fact that the system they fell accretion of satellite galaxies and their present cluster-centric into was much less massive than the present-day cluster. distance (De Lucia et al. 2012), with those satellite galaxies close to the cluster core having been accreted significantly earlier on average than those located close to the virial ra- 4.4. Radial population gradients dius (both physically or in projection). Gao et al. (2004) find Figure 14 shows the radial population gradients obtained the same radial trend for sub-halos, with the median accretion when stacking the 75 massive clusters as observed at z=0.0. redshift of sub-halos in massive cluster halos decreasing from The thick blue curve in the middle panel shows how the frac- zacc 1.0 in the cluster core to zacc 0.4 at r200 (their Fig. 15). tion of “spectroscopic” cluster members which have been ac- ∼ ∼ creted varies as a function of projected cluster-centric radius, 5. CONSTRAINING STAR FORMATION QUENCHING MODELS BY COMPARISON TO THE OBSERVED SF–RADIUS TRENDS rpro j/r200. This fractiondropsslowly from 100% at the cluster core to 90% at 0.4r200, before falling at an ever increasing Figures 2–4 showed that the fraction of massive cluster ∼ −1 rate down to 50% by r200, and approaching zero by 3r200. galaxies with SFRIR>2.0M⊙ yr or blue UV–optical colors ∼ ∼ For rpro j>r200, this contribution represents the “back-splash” increases steadily with cluster-centric radius, but at the largest population galaxies which have previously been accreted into radii probed, the fSF remained significantly (20–30%) below the cluster, but now have bounced back out beyond r200. that seen in coeval field galaxies in both cases. The remaining curves show the effects on this fraction by In Figure 15 we attempt to reproduce these two SF-radius progressively excluding galaxies which were accreted aftera trends by a simple toy model in which the star formation of given redshift. The first three curves (zacc<0.09) only diverge infalling field galaxies is instantaneously quenched at the mo- from the top curve at rpro j2 10 M⊙ galaxies from the M × THE SF–RADIUS RELATION, SLOWQUENCHINGANDPRE-PROCESSING 13

(rproj /r 200 ) (rproj /r 200 ) 0 1 2 3 0 1 2 3 0.4 0.4 0.6 0.6 Field galaxies Field galaxies

−1 0.3 0.5 −1 0.3 0.5 19% drop in fSF 0.4 0.4 0.0 0.0 r r 0.2 0.2 0.3 0.3

0.2 0.2 SF SF SF IR SF IR f ( (NUV − ) < 4.5 f ( (NUV − ) < 4.5

f ( SFR > 2.0 M yr ) 0.1 f ( SFR > 2.0 M yr ) 0.1 0.1 0.1 0.15 < z < 0.30 0.15 < z < 0.30 0.0 0.0 0.0 0.0 0 1 2 3 4 5 0 1 2 3 4 5 clustercentric radius (rproj /r 500 ) clustercentric radius (rproj /r 500 )

0.00 0.35 0.70 1.06 1.43 2.17 2.93 3.68 4.42 5.48 6.47 0.00 0.35 0.70 1.06 1.43 2.17 2.93 3.68 4.42 5.48 6.47 Delay between accretion into cluster and SF quenching (Gyr) Delay between accretion into cluster and SF quenching (Gyr) Figure 15. Comparison of the observed radial population gradients with Figure 16. Radial population gradients. The red and magenta points show predictions from cosmological simulations. The red points show the frac- the observed IR- and UV-based SF–radius relations as in Figure 15. The col- 10 −1 tions of M>2×10 M⊙ cluster galaxies with obscured SFR>2.0M⊙ yr ored curves are the same as those in Figure 15, except that they now assume from Fig. 2, while the magenta squares show the corresponding SF-radius that the infalling galaxies have a fraction fSF which has been reduced by 19% relation for UV-selected star-forming galaxies (Fig. 4). The blue horizontal with respect to that seen in the general field (blue horizontal line). The black 10 line indicates the corresponding fraction of field galaxies (M>2×10 M⊙; dot-dashed curve indicates the predicted SF–radius relation in the case that −1 star formation is instantaneously quenched when galaxies reach pericenter. 0.15 2.0M⊙ yr , while the blue shaded region in- dicates the 1σ confidence levels. The thick blue diagonal curve shows the predicted SF–radius relation obtained from our “observations” of the 75 mas- (2013). The “excess” obscured star-formation (red points)ob- sive clusters in the Millennium simulation, assuming that infalling galaxies served in the cluster core can then be reasonably reproduced have the same fSF as our observed field galaxy sample, and star-formation is immediately quenched upon being accreted into the cluster, i.e. passing by a model with a short quenching delay of the order 0.3– within r200 for the first time. The colored curves show the effects of delaying 1.0Gyr (light-blue/green curves). Much longer quenching this quenching by a time ∆t after the galaxy is accreted into the cluster, as time-delays (∆t&3Gyr) are clearly excluded, as they leave indicated by the color scale below the plot. The dashed blue curve shows too many star-forming galaxies at rpro j 1 − 2r500. the effect on the predicted SF–radius relation of changing the cluster-centric One possible way to reconcile the model∼ predictions with radius at which quenching is initiated from r200 out to 2r200. our data at 3–5r500 would be to initiate the quenching pro- same 75 massive clusters in the Millennium simulation are cess at larger∼ radii. The dashed curve shows the radial popu- reproduced, as they would appear if observed at z=0.21, to lation gradient produced when star-formation in all infalling best match the redshifts of the LoCuSS sample. The model galaxies is immediately quenched when they pass within galaxy positions and velocities relative to the cluster halo are 2r200 for the first time (rather than r200 as before). While the now measured as they stood at z=0.21, while the clusters are fraction of star-forming galaxies has now been reduced suf- stacked using their r200 and σν values measured at z=0.21. ficiently at 2r500, the gradient of the model trend remains At z=0.21 these clusters have M200 masses in the range 2.6– much steeper∼ than that observed, and vastly under-predicts the 14 −1 14 −1 21.7 10 h M⊙, with a median M200 of 5.0 10 h M⊙. fraction of star-forming galaxies at rpro j.1.5r500. The× predicted SF–radius relation in the case× that star for- The model curves provide little or no leeway to reproduce mation in all infalling galaxies is instantaneously quenched the low fSF observed over 3–5r500 in the NUV-based SF– upon accretion (thick blue diagonal curve) is qualitatively radius relation (magenta squares), as they all approach the similar in form to the observed trends, and consistent with field value at 3.5–4.0 r500 ( 2.5–3.0r200), the radius at which ∼ the data points at 1 − 2r500, suggesting that this is to the fraction of back-splash galaxies falls to zero (Fig. 14), ir- first order a reasonable∼ assumption, as found previously by respective of the radius at which quenching is initiated. Balogh, Navarro & Morris (2000) and Haines et al. (2009). The only way to improve the model fits to these observa- The model radial gradient is too steep however, resulting in tions would be to allow the fraction of star-forming galax- predicted values of fSF that are much higher than our data ies among the infalling population to be lower than that ob- points in the range 1.8–3.0r500, and too low in the cluster core served among the general coeval field population. We con- (rpro j.0.8r500). sider the simplest approach in Figure 16, that of simply re- The remaining colored curves show the effects of delay- ducing the fSF of cluster galaxies by a single fixed amount ing the moment at which quenching occurs, by terminating (19%) from the value observed in the field, at all radii, to star-formation only in those galaxies accreted into the clus- model the impact of whatever physical process is reducing ter more than ∆t Gyr prior to observation, corresponding to star formation among galaxies in the infall regions of clus- the “delayed-then-rapid” quenching scenario of Wetzel et al. ters. One feasible mechanism to achieve this is via the “pre- 14 HAINESETAL. processing” of galaxies within galaxy groupswhich are subse- sity profiles observed within cluster halos (Fig. 17), which quently accreted into clusters. The cosmological simulations are much steeper than the relatively flat radial distribu- of Gabor & Davé (2014) suggestthat 40% of satellite galax- tions of DM sub-halos within 0.3r500 (Budzynski et al. 2012; ies within clusters are pre-processed,∼ the fraction decreasing Vogelsberger et al. 2014). These SAMs resort to ad hoc pre- weakly with cluster-centric radius. The overall IR-based SF– scriptions for mass-stripping and adjustment of the orbits of radius relation can now be reproduced at all radii, via a model these “orphan” galaxies within the cluster halos, resulting in in which star-formation is quenched in galaxies 0.7–1.5Gyr strong variations in the radial satellite number density profiles after being accreted into the cluster. This occurs∼ on average within cluster cores (Budzynski et al. 2012), and the fractions after the galaxy has passed through the cluster core, as the ob- of “orphans” in the cluster satellite population, from 25% in servedtrend lies abovethat predictedfor the case in which star the Guo et al. (2011) model to 50% in the Bower et al. (2006) formation is quenched in galaxies at the moment they reach model (Gifford et al. 2013). This likely explains the inabil- pericenter (black dot-dashed curve). ity of the predicted radial profile of model cluster galaxies The NUV-based SF–radius relation is systematically above from the Bower et al. (2006) SAM to match the observed ra- that of the IR-based inside 2.5r500, andis best-fitby a model dial profile at rpro j.0.1r500, overestimating the number den- is which star-formation is quenched∼ in galaxies 2.1–3.6Gyr sity of cluster galaxies by a factor 2–3 (Fig. 5). after accretion, or slightly later than that suggested∼ by the IR- We should therefore be cautious about using model curves, based relation. such as those in Figs. 15 and 16, that depend upon the radial For all these model curves, to use the terminology of distribution and numbers of those galaxies accreted earliest Peng et al. (2010, 2012), we assume that the environmental into the clusters, and which have suffered repeated interac- quenching process is 100% efficient (ǫsat=1), i.e. all star- tions over multiple orbits within the ever growing cluster halo. forming galaxies are quenched ∆t Gyr after accretion into the All the model curves in these two figures do this, even those cluster. The result of decreasing ǫsat for clusters would leave referring to galaxies yet to be accreted into cluster, as they de- the model curves unchanged at large radii (rpro j&3r200), but pend upon the relative contributions of those accreted aftera squeeze the curves upwards in the core, effectively reducing given epoch, with all those accreted before the same epoch. the radial population gradient by a factor (1 − ǫsat). The fact that the observed gradients are steep, with fSF increasing five- 5.2. Radial galaxy surface density profiles fold over 0–3 r , necessitates a high environmental quench- 200 Figure 17 compares the radial galaxy surface density pro- ing efficiency, ǫsat&0.8, otherwise there would be evidence of Σ a residual population of primordial cluster galaxies with on- files, (r), of star-forming galaxies with model surface den- sity profiles considering just those galaxies accreted within going star-formation in cluster cores (r .0.1r ). pro j 200 the last ∆t Gyr or which have not yet been accreted. This plot 5.1. Uncertainties in the model trends largely resolves the above issues, by focusing solely on star- One concern about attempting to fit the SF–radius relation forming galaxies, which are most likely to still have surviving with model curves, is that the latter require the semi-analytic parent sub-halos, and comparison model curves that contain models to reliably follow the orbits of cluster galaxies long just the most recent arrivals into clusters, and hence minimiz- ing the uncertain contribution from “orphan” galaxies. after their accretion, and accurately model the long-term evo- −1 lution of their stellar masses once they become satellites. The The surface density of star-forming (SFRIR>2.0M⊙yr ) Millennium simulation itself considers only the dark matter cluster galaxies from our ensemble of 30 clusters (blue points) declines steadily with radius out to 3r500, with no evidence component, the baryons bolted on afterwards via SAMs. ∼ When galaxies are accreted into massive clusters, their par- of flattening off inside r500. This immediately rules out mod- ent DM halos become sub-halos of the cluster halo, losing els in which star formation is instantaneously quenched when mass continuously through tidal stripping, until in many cases galaxies are accreted into clusters (thick blue curve), as these they fall below the resolution limit of the simulation, dissolv- produce radial profiles which are essentially flat within 2r500. ing into the parent halo or are completely disrupted (Gao et al. As recently accreted galaxies are progressively included, the 2004; Weinmann et al. 2010). The galaxies hosted by these radial density profile steadily builds up and steepens within sub-halos are much more compact and tightly bound than the 2r500. The best fitting curve to the Spitzer data is obtained dark matter, and as long as some of the surrounding sub- by considering those model cluster galaxies accreted within ∆ +0.8 2 halo survives, are not expected to suffer significant stellar the last t=2.1−0.7 Gyr, with a χ value of 21.15 for 23 data mass loss, although their diffuse hot gas halo is likely to be points and two degrees of freedom (φ and ∆t). The uncertain- lost at a rate commensurate with that of the parent sub-halo ties in ∆t are derived as the values for which the χ2 value has (Guo et al. 2011). Prior to their accretion into the clusters, increased by 2.30 from the minimum value. 10 model galaxies at our lower stellar mass limit of 2 10 M⊙ Outside of 0.3r500, the shape of the Σ(r) profile 11×−1 typically have parent DM halos of masses 6 10 h M⊙ in for unobscured star-forming galaxies (magenta squares; 10 the Millennium simulation, comprising ∼730× DM particles, >2 10 M⊙,(NUV − r)0.0 < 4.5), coincides well with and hence must suffer > 95% stripping∼ before their parent Mthat of× the obscured star-forming population. In contrast, the sub-halo falls below the mass resolution limit of 20 particles Σ(r) profile for the UV-selected star-forming galaxies steep- (Springel et al. 2005), a process which typically takes 5Gyr ens more rapidly in the cluster core to form a cusp, parallel- (Weinmann et al. 2010). Even after their parent sub-halo∼ has ing that seen for the overall cluster population (red points). been entirely disrupted, they are expected to survive as “or- The best fitting curve to the NUV data has ∆t=3.2 0.4Gyr phan” galaxies, although now they will be likely subject to (χ2=29.57 for 32 data points), implying that the NUV± emis- significant stellar mass loss via tidal stripping and may be sion takes 1.1Gyr longer after accretion to be shut down than completely disrupted. the 24µm emission. The inclusion and treatment of “orphan” galaxies by SAMs Figure 18 replots the Σ(r) profile for obscured star-forming is required to explain the cuspy radial satellite number den- cluster galaxies (blue points), but now compares it with slow THE SF–RADIUS RELATION, SLOWQUENCHINGANDPRE-PROCESSING 15

(rproj /r200 ) (rproj /r 200 ) 0 1 2 0 1 2

All M* > 2x1010 M gals All M* > 2x1010 M gals ( NUV − r ) 0.0 < 4.5 100 −1 −1 −1 100 SFRs > 2.0 M yr −1 SFRs > 2.0 M yr −2 −2 500 500 10 10 galaxies (r ) cluster galaxies (r ) cluster Σ Σ 1.0 1.0

0 1 2 3 0 1 2 3 clustercentric radius (r /r ) proj 500 clustercentric radius (rproj /r 500 )

0.0 0.35 0.7 1.4 2.2 2.9 3.7 4.4 5.5 6.5 7.4 (Gyr) 0.0 0.25 0.5 1.0 1.5 2.0 2.5 3.0 4.0 5.0 7.0 10 Maximum time since galaxy was accreted by cluster Quenching time−scale (Gyr)

10 10 Figure 17. Blue symbols show the Σ(r) profile of M>2×10 M⊙ galax- Figure 18. Blue symbols show the Σ(r) profile of M>2×10 M⊙ galaxies −1 −1 ies with obscured star formation at rates SFRIR>2.0M⊙yr averaged over with obscured star formation at rates SFRIR>2.0M⊙yr . Red symbols in- our 30 clusters. Magenta squares show the Σ(r) profile for unobscured star- dicate the corresponding Σ(r) profile for all M>2×1010 M⊙ galaxies. The forming galaxies with (NUV−r)0.0<4.5, normalized to match the profile of thick blue curve shows the predicted surface density profile of infalling galax- obscured star-forming galaxies at rproj&1.5r500. The thick blue curve shows ies yet to pass within r200, normalized to fit the observed surface density the predicted surface density profile of infalling galaxies yet to pass within profile of star-forming galaxies at large radii. The remaining colored curves r200, obtained from our “observations” of the 75 massive clusters in the Mil- show the predicted surface density profiles for star-forming galaxies accreted lennium simulation at z=0.21, normalized to fit the observed radial profiles of into the clusters and subsequently have their star-formation rates decline ex- star-forming galaxies at large radii. The remaining colored curves show the ponentially with a range of quenching time-scales (tQ=0.25–10 Gyr). predicted radial profiles produced by including also those galaxies accreted into the clusters within the last ∆t Gyr as indicated by the color scale. The compares the observed VDPs of 24µm-detected star-forming black curve shows the predicted surface density profile for all “spectroscopic” member galaxies of the same 75 clusters, scaled to best match the observed galaxies (blue points) with the predicted VDPs of model clus- 10 ensemble Σ(r) profile of all M>2×10 M⊙ cluster galaxies (red points). ter galaxies, selected according to their accretion epoch. quenching models. In this scenario model star-forming galax- The VDP for star-forming cluster galaxies shows a high, narrow peak of 1.44σν at rpro j 0.3r500, before dropping ies are accreted into the cluster and subsequently gradually ∼ quenched, their SFRs declining exponentially on a quench- to the innermost radial bin, and a steady decline outwards to 0.8σν at large radii. This profile shape is best reproduced ing time-scale tQ until their SFRs fall below our nominal − by∼ model cluster populations combining infalling galaxies yet limit of 2.0M yr 1. These model star-forming galaxies are ⊙ to be accreted and the most recent arrivals into the cluster. given initial SFRs taken at random from our observed sam- −1 The progressive inclusion of these recently accreted galax- ple of coeval field star-forming galaxies (SFR>2M⊙ yr ; ies causes the velocity dispersion within r500 to rise rapidly, 0.15

0.0−0.5 r500 0.5−1.0 r500 (rproj /r 200 ) 0 1 2 0.10 Std. Dev Kurtosis 0.10 Std. Dev Kurtosis 0.983σν +0.14 0.867σν −0.15 0.08 1.246σν −0.92 0.08 1.245σν −0.82 1.6 1.280σν −0.74 1.182σν −0.80 0.06 0.06 1.5 Fraction 0.04 0.04 1.4 0.02 0.02 0.00 0.00 −3 −2 −1 0 1 2 3 −3 −2 −1 0 1 2 3

los r200 ∆ν σ ∆ν σ 1.3 ( ) / ν ( ) / ν σ /

1.0−2.0 r500 2.0−4.0 r500 0.18 0.18 1.2 0.16 Std. Dev Kurtosis 0.16 Std. Dev Kurtosis 0.664σν +0.19 0.480σν +1.22 0.14 0.14 1.017σν −0.09 0.764σν +1.62 1.1 0.12 1.018σν −0.59 0.12 0.889σν +0.38 0.10 0.10 0.08 0.08

1.0 Fraction 0.06 0.06 0.04 0.04 0.9 0.02 0.02 0.00 0.00 −3 −2 −1 0 1 2 3 −3 −2 −1 0 1 2 3 ( ∆ν ) / σν ( ∆ν ) / σν Velocity dispersion ( ) 0.8 Figure 20. Stacked relative LOS velocity distributions of “spectroscopic” 0.7 cluster galaxies for the 75 most massive clusters in the Millennium sim- ulation at z=0.21 split into three dynamic sub-populations: (i) infalling 0 1 2 3 “spectroscopic” cluster members yet to reach pericenter (blue striped his- clustercentric radius (r /r ) togram); (ii) back-splash galaxies which have passed through pericenter, are proj 500 on their way back out of the cluster νradial>0, and are yet to reach apocenter (green striped histogram); and (iii) the virialized cluster population which has passed through apocenter (red solid histogram). Each panel represents a different bin in cluster-centric radius as in Fig. 7. The standard deviations (in 0.0 0.35 0.7 1.4 2.2 2.9 3.7 4.4 5.5 6.5 7.4 (Gyr) units of σν ) and kurtosis values of each distribution are indicated. Maximum time since galaxy was accreted by cluster that of the overall cluster population at all radii, along with Figure 19. Predicted velocity dispersion profiles as a function of accretion the apparent sharp peak in the VDP at 0.3r500, unambigu- epoch for model galaxies in our stacked sample of 75 clusters from the Mil- ously identifies the star-forming cluster∼ galaxy population as lennium simulation at z=0.21. The thick blue curve shows the predicted LOS velocity dispersion profile of “spectroscopic” cluster members yet to pass recent arrivals. Considering a simple kinematical treatment of within r200 and be accreted into the cluster DM halo. The remaining solid infalling and virialized cluster galaxies in a cluster-scale grav- curves show the impact on the velocity dispersion of progressively includ- ∆ itational potential well leads to T/V 1 for infalling galax- ing those galaxies accreted into the clusters within the last t Gyr as indi- ies and T/V 1/2 for the virialized| population,|≈ where T and cated by the color scale. The black solid curve shows the velocity dispersion | |≈ profile obtained by including all cluster members. Blue symbols with er- V are the kinematic and potential energies. Thus, the veloc- ror bars show the observed velocity dispersion profile of the 24µm-detected ity dispersions of the two populations are naively related by cluster members from our stacked sample of 30 clusters. The black dashed 10 σin f all √2σvirial (Colless & Dunn 1996). curve shows the corresponding profile considering all M>2×10 M⊙ clus- ≈ ter members. From the first dynamical studies of cluster galaxies, the velocity dispersions of spiral galaxies have been found to cluster galaxies peaks at 0.2r500 and drops off sharply to- be systematically higher than early types (Tammann 1972; wards the cluster core (dark∼ red curve), the observed VDP Moss& Dickens 1977). Based on much larger samples, shows no corresponding dip in the cluster core. The σlos(r) the stacked velocity dispersions of blue/emission-line galax- instead declines steadily from its peak value 1.18σν in the ies were found to be 20% higher than the remaining in- innermost radial bin, falling to values of 0.75σν over the active galaxies (Biviano et al. 1997; Aguerri et al. 2007). ∼ range 1.6–2.8r500, significantly below the velocity dispersions Biviano & Katgert (2004) showed that if early-type galax- predicted by simulations at these radii. Our observed VDP is ies are assumed to have isotropic orbits within clusters, as qualitatively similar to that obtained by Rines et al. (2003) by supported by their Gaussian velocity distributions, the kine- stacking the member galaxies of eight z<0.05 X-ray luminous matic properties of late-type spirals are inconsistent with be- clusters: their σlos(r) also drops from 1.1σν in the cluster core ing isotropic at the > 99% level. Instead they indicate that to 0.8σν by r200, albeit with marginal evidence for a decline spirals and emission-line galaxies follow radial orbits in clus- within 0.1r200. At larger radii (&2r500), their σlos(r) drops to ters, pointing towards many of them being on their first cluster values of 0.5σν , which is even lower than ours and hence infall. poses further∼ problems for the simulations. Figure 7 showed that the LOS velocity distribution of star- One possible explanation is that the mis-match is linked to forming cluster galaxies within r500 to have a rather flat, top- the prediction of too many model galaxies in the cluster core hat profile, a high LOS velocity dispersion and a negative (rpro j . 0.1r500; Fig. 5), which assuming that the excess pop- kurtosis (γ2 − 0.81) strongly inconsistent with the Gaussian ulation were all accreted early, could artificially increase the distribution∼ typical of a virialized cluster population. This contribution from low-velocity virialized cluster members, re- flat-topped distribution is well reproduced by model cluster ducing the σν estimates for each cluster, and pushing the re- galaxy populations that are either infalling into the cluster for sultant model curves upwards. the first time, or back-splash galaxies which are currently re- Irrespective of the difficulties in reproducing the observed bounding out of the cluster and are yet to reach apocenter VDP of all cluster members, the key finding that the veloc- (Fig. 20). The velocity distributions of these two dynami- ity dispersion of star-forming galaxies is 10–35% higher than cal sub-populations appear indistinguishable within r500, both THE SF–RADIUS RELATION, SLOWQUENCHINGANDPRE-PROCESSING 17

1.0 3

0.1 ν/σ 2 0.01

−3 1 10

Probability (/Gyr) 10−4 0

10−5 0 1 2 3 4 −1 Quenching time−scale (Gyr)

Figure 21. Likelihood function of the quenching time-scale, tQ, based on fitting the distribution of star-forming galaxies in the caustic diagram, nor- −2 malized so that R L(tQ) = 1 with tQ in units of Gyr. The best-fit value of tQ is indicated by the vertical dot-dashed line, while the shaded regions indicate the Relative line−of−sight velocity ( ) 1, 2 and 3-σ confidence limits in tQ. The magenta dashed line and hashed re- −3 gion indicates the best-fit tQ value and 1-σ confidence limits by Haines et al. (2013) based on the systematically low specific-SFRs of star-forming cluster 0 1 2 3 galaxies within r200. clustercentric radius (rproj /r 500 ) having velocity dispersions 1.2σν and negative kurtosis val- ∼ ues γ2 −0.8. At 1–2r500 the velocity distribution of star- ∼ forming galaxies becomes more rounded, albeit still with a 0.0 0.350.7 1.4 2.2 2.9 3.7 4.4 5.5 6.5 7.4 (Gyr) negative kurtosis (γ2 − 0.6), which again is well reproduced Maximum time since galaxy was accreted by cluster ∼ by the model infalling galaxy population (γ2= − 0.59). At Figure 22. Stacked phase-space diagram, ∆νz/σν versus rproj/r500, for these radii, the back-splash population is expected to show model star-forming galaxies around the 75 most massive clusters in the Mil- a more Gaussian-like distribution with γ2= − 0.09, inconsis- lennium simulation at z=0.21 for the best-fit quenching model, in which star tent with observations. However, there are only expected formation declines exponentially on a time-scale tQ=1.73 Gyr in galaxies from the moment they pass within r200 of the cluster for the first time. Each to be 40% as many back-splash galaxies in this radial bin 10 −1 point marks a M>2×10 M⊙ star-forming galaxy with SFR>2.0M⊙yr , as infalling ones, and so we cannot rule out the possibility colored according to time elapsed since it was accreted into the cluster (mid- these star-forming galaxies represent a mixture of infalling blue if it is yet to be accreted). The grayscale contours indicate the resulting and back-splash populations. probability distribution function of the model star-forming galaxies in phase- space, P(rproj/r500,∆νz/σν ). Each contour indicates a factor 0.2 dex change in phase-space density of model star-forming galaxies. The thick contours 5.4. Distribution of galaxies in the caustic diagram indicate a factor 10 change in phase-space density. The information gained from the galaxy surface density profiles and VDPs can be combined by comparing the ob- the model galaxies in phase-space, and σ0 is the initial ker- served spatial distribution of star-forming galaxies in the nel width 0.2. To remove the effects of the discontinuity at stacked caustic diagram (Fig. 6) with those obtained from the rpro j=0, the phase-space distribution is mirrored about both Millennium simulation (Fig. 13). Using the stacked sample of velocity and radial axes. The distribution is normalized to modelgalaxies from the 75 clusters extractedfrom the Millen- unity when summed over the region 0.0<(rpro j/r500)<3.2, so nium simulation, the distribution of model star-forming galax- that it can be considered a probability distribution function, ∆ ies in the phase-space diagram νlos/σν versus rpro j/r500 was P(rpro j/r500, ∆νlos/σν ). determined using the adaptive kernel estimator, for a range of The probability distributions of model star-forming galax- quenching time-scales from 0–10Gyr. ies for each value of tQ are then compared with the ob- As in Section 5.2 model star-forming cluster galaxies are served distribution of star-forming galaxies in the stacked given initial SFRs taken at random from our observed sam- phase-space diagram (Fig. 6). The best-fitting model clus- −1 ple of coeval field star-forming galaxies (SFR>2M⊙ yr ; ter population is identified using a maximum-likelihood anal- 0.153σ be identified as star forming. For each quenching time-scale, level, primarily due to the radial distribution of star-forming tQ, the spatial distribution of star-forming model galaxies in galaxies observed in our clusters (Fig. 18). phase-space, ρ(rpro j/r500,∆νlos/σν ), is determined using the Figure 22 shows the corresponding stacked caustic diagram adaptive Gaussian kernel method. Each model galaxy i is of model star-forming cluster galaxies for tQ=1.73Gyr. Each −1/2 represented by a 2D Gaussian kernel of width σ0(ρi/ρ) , cluster galaxy is color coded according to its accretion epoch, where ρ is the geometric mean of the local densities ρi of and the overall phase-space density distribution is shown 18 HAINESETAL. by the grayscale contours. The model star-forming galax- quenching model by finding kinematic segregation between ies are most frequently found at large cluster-centric radii star-forming cluster galaxies with normal or enhanced star (rpro j&1.5r500) and low velocity offsets ( ∆νlos .1.1σν ). The formation, and those with reduced star-formation, indicative phase-space density of star-forming galaxies| drops| by a fac- of ongoing quenching. This suggests that the process of tor 3 as rpro j/r500 0, for ∆νlos .1.4σν , producing the quenching occurs over a sufficiently long time-scale that the vertical∼ × contours in the→ left-center| of the| plot. kinematics and cluster-centric radii of quenching star-forming Averaging over the 75 simulated clusters, 42% of the model galaxies to have evolved significantly. More definitively, in star-forming galaxies with rpro j3 Gyr prior to observation. With biguous signature of star formation in most (and possibly all) the model SFRs declining exponentially upon accretion on a star-forming galaxies being slowly quenched upon their ar- time-scale of 1.73Gyr, overall the model SFRs and specific- rival into massive clusters. Assuming a model in which the SFRs of the star-forming cluster galaxies (rpro j4.7 10 L⊙ increases steadily with cluster-centric marks the end of continual gas replenishment of the ISM, sig- × radius, but remains well below the field value even at 3r200. nalling the beginning of the end for star formation in the host ∼ galaxy (Sánchez Almeida et al. 2014). von der Linden et al. (2010) found that suppression of star- formation in cluster galaxies could be traced out to 4r200. As these galaxies continue their journey towards the clus- ∼ ter core, the ICM they encounter becomes increasingly Wetzel et al. (2012) found their fSF (r) suppressed with re- 2 spect to field values as far out as 10 r200 around cluster-mass dense and the resultant ram pressures (Pram ρICMν ) become 14 strong enough to progressively strip their gas∝ disks from the halos (M200>10 M⊙), but interestingly saw this suppression outside in (Brüggen & de Lucia 2008), producing truncated entirely limited to satellite galaxies, with no evidence of sup- pression among centrals found beyond 1–2 r . HI, H2 gas and Hα disks (e.g. Koopmann & Kenney 2004; 200 Boselli et al. 2006, 2014), and outer regions showing recently The best way to reproduce the observed SF–radius trends quenched stellar populations (Crowl & Kenney 2008). Hy- appears to require a certain fraction of infalling galaxies drodynamical simulations predict that the moderate ram pres- to arrive onto the clusters having already been quenched sures acting on infalling massive spirals as they pass within (Fig. 16). One commonly identified mechanism by which galaxies may be transformed at large distances from the clus- r500 − r200 are sufficient to strip half their gas contents on 500–1000Myr∼ time-scales (Roediger & Hensler 2005), while ter center is through “pre-processing” in infalling galaxy observations confirm that ram pressure stripping is effective at groups (e.g. Kodama et al. 2001; Fujita 2004; Berrier et al. 2009; McGee et al. 2009; Dressler et al. 2013). Star forma- removing gas and quenching star formation as far out as r500 (Chung et al. 2007; Merluzzi et al. 2013). The effective time- tion is suppressed in group galaxies, with fSF values inter- scale for quenching via ram-pressure stripping then becomes mediate between those of field and cluster galaxy popula- the 1Gyr required for galaxies to travel from the cluster out- tions (Wilman et al. 2005; Bai et al. 2010; McGee et al. 2011; ∼ Rasmussen et al. 2012) up to at least z 1 (Balogh et al. 2011; skirts to the pericenter where the peak in ram pressure occurs ∼ (Roediger & Hensler 2005). The complete absence of HI- Ziparo et al. 2014). The fraction of star-forming galaxies within groups declines steadily with increasing group mass normal spirals within the r500 radius of Virgo and Coma clus- ters (Boselli & Gavazzi 2006), and the 0.5–0.8dex HI defi- (at fixed stellar mass and group-centric distance) and prox- ciencies of Virgo spirals at fixed stellar∼ mass, NUV − r color imity to the group center (Weinmann et al. 2006; Wetzel et al. and stellar mass surface density, cannot be reproduced by 2012; Woo et al. 2013). The fractions and specific-SFRs of predictions from chemospectrophotometric models involving star-forming galaxies within groups both show accelerated declines since z 1 with respect to the coeval field population starvation alone and can only be explained by ram-pressure ∼ stripping actively removing the gas (Cortese et al. 2011). (Popesso et al. 2014) and also “filament-like” environments In typical spirals, the dust-to-gas ratio and internal extinc- with comparably high galaxy densities but no X-ray emis- sion (Ziparo et al. 2014), indicative of ongoing slow quench- tion (AFUV ) decline steadily with radius (Muñoz-Mateos et al. 2009), meaning that for ram pressure stripping events where ing within groups (Balogh et al. 2011), comparable to that gas is removed from the outside in, the more extended un- seen in our clusters (Haines et al. 2013). Galaxies in groups 13 obscured star formation component should be preferentially with masses &10 M⊙ are HI-deficient at fixed stellar mass quenched prior to the more concentrated and bound obscured and NUV − r color, suggesting that ram-pressure stripping ∗ SF. This is supported by the finding of a significant cluster can remove atomic gas from L spirals in such groups ∼ population of 24µm-detected spirals with reduced SFRs, in- (Catinella et al. 2013). dicative of ongoing slow quenching, reddened by dust onto Galaxy groups are ubiquitous, and host 50% of galaxies ∼ the optical red sequence (Wolf et al. 2009). This appears in- in the local Universe (McGee et al. 2009). The impact of en- consistent with our finding that the quenching time-scale for vironmental quenching in groups will be manifest also in a UV-selected star-forming cluster galaxies is 1Gyr longer reduced global fraction of star-forming galaxies in our coeval than that for 24µm-selected galaxies. We suggest∼ that the field sample. Hence, in order to reproducethe observationthat longer tQ for the NUV-selected star-forming galaxies is due the fraction of star-forming galaxies in the infall regions of to the 1–1.5Gyr time-scale required for recently quenched clusters (2–3 r200) remains significantly lower than that seen in coeval field galaxies, a specific form of “pre-processing” 20 HAINESETAL.

images over the rest of the 0.15–0.30 redshift range. halo 12.0 The observed ratio of 5 X-ray groups associated with the Mhalo = 10 M z = 0.21 12.5 clusters for every field X-ray∼ group in the remainder of the 1.0 Mhalo = 10 M 13.0 0.15 M rproj / r 200 in high-density regions are 4 likelier to not be accreting ∼ × Figure 23. History bias for galaxies falling into massive clusters. The solid any mass (or even losing it) at late epochs than those in low- curves show the fraction of model galaxies surrounding massive clusters density regions. 14.2 (Mcl >10 M⊙) at z=0.21 which already reside in halos over a given mass as a function of projected cluster-centric radius. Only those galaxies yet to 7. SUMMARY pass within r200 and be accreted into the cluster are included. The dashed curves show the corresponding cosmic fractions for field galaxies over the We present an analysis of the radial distribution and whole Millennium simulation. kinematics of star-forming galaxies in 30 massive clusters at 0.1510 M⊙) in the Millennium simulation at ∼ × z=0.21 (see Lu et al. 2012, for details), excluding those galax- to 2r200. This simple observation requires star for- ies already accreted into the clusters. Figure 23 shows the re- mation to survive within recently accreted spirals for sulting fractions of these model galaxies which reside in halos 2–3Gyr to build up the apparent over-density of star- over a given mass as a function of projected cluster-centric forming galaxies within clusters. radius (solid curves) for five values of minimum halo mass 12 14 2. The velocity dispersion profile of the star-forming clus- from 10 M⊙ to 10 M⊙. The dashed lines show the cor- ter galaxy population shows a sharp peak of 1.44 σν at responding fractions for coeval field galaxies over the whole 0.3 r500, and is 10–35% higher than that of the inac- Millennium simulation. This confirms that galaxies infalling tive cluster members at all cluster-centric radii, while into clusters are indeed more likely to already reside within their velocity distribution shows a flat, top-hat profile DM halos of a given mass than coeval field galaxies, for each 12 within r500. All of these results are consistent with star- halo mass range. While the effect is marginal for 10 M⊙ forming cluster galaxies being an infalling population, mass halos, galaxies infalling into massive clusters are 35% but one that must also survive 0.5–2Gyr beyond pass- more likely to already be residing in group-mass halos∼ with ∼ 13 ing within r200 to achieve the high observed velocities. Mhalo>10 M⊙ and twice as likely to already be residing in 14 Mhalo>10 M⊙ halos than their counterparts in the field. 3. The distribution of star-forming galaxies in the stacked We find numerous potential sites where infalling galaxies caustic diagram are best-fit by models in which their can be pre-processed before being accreted into the LoCuSS SFRs decline exponentially on quenching time-scales clusters (e.g. Pereira et al. 2010). Across the 23 clusters with tQ=1.73 0.25Gyr upon accretion into the cluster. The XMM imaging, a total of 30 X-ray groups (with extended X- above results,± and the observed kinematic segregation ray emission detected at >4σ levels) have been identified with of star-forming galaxies according to their specific- redshifts placing them inside the cluster caustics, indicating SFRs, support the conclusion from Haines et al. (2013) that they are most likely infalling into the primary cluster that star formation in most (and possibly all) high-mass (Haines et al. 2015, in preparation). Only six further “iso- star-forming galaxies is slowly quenched on accretion lated” X-ray groups were detected (at >4σ) in the same XMM into rich clusters on 0.7–2.0Gyr time-scales THE SF–RADIUS RELATION, SLOWQUENCHINGANDPRE-PROCESSING 21

4. The fraction ( fSF ) of star-forming cluster galaxies rises De Lucia G., Weinmann S., Poggianti B. M., Aragón-Salamanca A., steadily with cluster-centric radius, increasing five-fold Zaritsky D., 2012, MNRAS, 423, 1277 by 2r , but remains well below field values even at Diaferio A., Kauffmann G., Balogh M. L., White S. D. M., Schade D., 200 Ellingson E., 2001, MNRAS, 323, 999 3r200. Pre-processing in infalling galaxy groups ap- Dressler A., Oemler A. Jr., Fritz J., Poggianti B. M., Gladdders M. D., pears the most likely explanation for this suppression Abramson L., Vulcani B., 2013, ApJ, 770, 62 of star-formation at large distances from the cluster. Dünner R., Reisenegger A., Meza A., Araya P. A., Quintana H., 2007, MNRAS, 376, 1577 ACKNOWLEDGEMENTS Ellingson E., Lin H., Yee H. K. C., Carlberg R. G., 2001, ApJ, 547, 609 Faltenbacher A., Finoguenov A., Drory N., 2010, ApJ, 712, 484 CPH was funded by CONICYT Anillo project ACT-1122. Finn R. A., Balogh M. L., Zarizsky D., Miller C. 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