arXiv:1204.1052v1 [astro-ph.CO] 4 Apr 2012 fteglxe,mr ealdosrain r required processes. are dominant observations the properties detailed understand play star-forming more to may the , merger the shaping the major that in of a appears role of significant it phase a while passage Thus, the core in has vicinity. intense formation its star 2004) the in on al. galaxies effect et “Bul- appreciable Markevitch the an in had front (1ES0657-558 not context, shock this the cluster that In find let” (2009) activity. al. merger et which severe Chung fronts of shock should observed regions processes of mark vicinity these the for in Evidence itself formation reveal while 1999). star al. Couch further 2010) ter- et & halting (Fujita medium al. (Bekki formation, interstellar gi- et GMC the Bekki formation of minates of 2008; star stripping collapse subsequent al. of the the leading et burst Kronberger , in a initial 2003; result within an passage may (GMCs) to core clouds merger envi- the molecular pressure a ant during high of encounters the phase galaxy that 2004; a suggest ronment remains Simulations al. responsi- it et both are although Poggianti mechanisms 2010), ble. in al. merger-specific al. et 1997; which important et Ma unclear (Caldwell 2009; be Lee Rose & starbursts & may Hwang Caldwell pas- truncating merger core 1993; the 2010). a and that al. of suggested et triggering Bekki been phase of has 1996; sage it rates al. pericen- et example, galaxies abnormal to (Roettiger For close drive subject passage times can at can tric particularly that mergers evolution, environment galaxy major an These galaxy to of ap- pairs of between clusters. observations mergers mass by equal revealed proximately spectacularly is mation tet abig,M 23,USA 02138, MA Cambridge, Street, Australia 1710, NSW m Australia; 3122, [email protected] VIC Hawthorn, Technology, of University nti en h egn lse bl 74(z=0.3064, 2744 Abell cluster merging the vein, this In for- structure large-scale of nature hierarchical The rpittpstuigL using 2018 typeset 6, Preprint June version Draft 4 3 2 1 avr mtsna etrfrAtohsc,6 Garden 60 Astrophysics, for Center Fellow Smithsonian Science Harvard Super Council Research Australian Epping, 296, Box PO Observatory, Astronomical Australian Swinburne Supercomputing, and Astrophysics for Center egrevrnetmyb epnil o h trfraini th in d headings: merger. formation galaxies Subject cluster of star major transformation the a rapid for of the for responsible evidence be observational in may detected post environment front asso starburst, features shock merger ICM reveal its near and lie which indicatin galaxies rap subcluster jellyfish spectra, kn galaxies, for the of optical evidence parent star-forming three Further intriguingly, their the of from environment. from trails the stripped comes harbor by tails transformed galaxies gas being These in in-situ formed 2744. Abell cluster eietf orrr jlys”glxe nHbl pc eecp ima Telescope Space Hubble in galaxies “jellyfish” rare four identify We atS Owers S. Matt 1. A INTRODUCTION T E tl mltajv 5/2/11 v. emulateapj style X aais lses niiul(bl 74 -as aais clust galaxies: X-rays: — 2744) (Abell individual clusters: galaxies: HCIGTISI H AO EGRAEL2744. ABELL MERGER MAJOR THE IN TAILS SHOCKING 1,2,3 arc .Couch J. Warrick , rf eso ue6 2018 6, June version Draft Chandra ABSTRACT ow- 1 alEJ Nulsen E.J. Paul , -a mgs esgetta h ihpressure high the that suggest We images. X-ray ogl ot-ot xsat axis north-south roughly onig ahhvn xouetmso 64sfrthe south for and s 6624 north of F814W times of exposure and consist having F606W observations each F435W, The pointings the using filters. October 2009 signifi- is turmoil’. A2744 in ‘galaxies of these environment influencing suggest merger We cantly major S10). hereafter the 2010, al. that et (Smith in- galaxy ent formed are stars 2010; filaments young al. hot, and in-situ of et knots manifestation Smith the These 2007; 2010; as al. terpreted 2012). al. et et al. Sun Yagi al. et also Yoshida et 2008; see Owen al. (2007, fila- by et al. and Yoshida et noticed Cortese knots first and blue those (2006) of extremely reminiscent of ments trails one-sided hibit archival revealed in scope discovered region galaxies merger-affected liar the a by between resident incidence the affects environment understand- for extreme major target galaxies. this the prime a how of it ing properties make the A2744 The in of indicate merger remnant subcluster. 1) the massive is (Figure more subcluster member northern-most fea- brightest the lensing subclus- that its strong massive and around less dispersion the tures velocity of higher Its core remnant ter. The the by 2011). driven al. et Merten dra also to group (see infalling northwest an with the along passage, core past shortly oci ta.20)adidct htahg velocity high a that indicate and 2004; ( 2006) David of & refinement al. (Kempner the a et scenarios allowed Boschin of data merger dynamics new previous The the the A2744. constrain in merger to observations combined O11 optical Recently, merg- galaxies. dra major an cluster of is effects on O11) the ers hereafter testing 2011, for candidate al. excellent et Owers A2744; hereafter ∼ S/C bevtoso 24 eetknduring taken were A2744 of observations HST/ACS co- spatial suggestive a present we Letter this In 70k s km 4750 aarva vdnefra“ultlk”sokfront shock “Bullet-like” a for evidence reveal data -a n nl utainTlsoeAAOmega Telescope Australian Anglo and X-ray HT mgr.Tee“elfih aaisex- galaxies “jellyfish” These imagery. (HST) ihngswihhsbe tipdfo h par- the from stripped been has which gas within Chandra trus n G etrs Most features. AGN and starburst rn h iln oepsaephase passage core violent the uring dtasomto nteegalaxies these in transformation id − itdwt egn “Bullet-like” merging a with ciated 1 4 ,na edo,mjrmre ln a along merger major on, head near ), ct .Randall W. Scott , aeu al.Ti provides This tails. gaseous e t n lmnswihhave which filaments and ots nlsso 1 n eea pecu- several and O11 of analysis 2. hyaei h rcs of process the in are they g OBSERVATIONS eyo h ao merger major the of gery ∼ 27 ers ◦ oorln fsgtis sight of line our to ubeSaeTele- Space Hubble 4 Chan- Chan- 2 Owers et al.

F606W and F814W filters and 8081s for the F435W AAOmega spectrum confirms Couch & Sharples’ post- filter. The data were retrieved from the archive and starburst classification—there is strong Balmer absorp- the pointings were combined using the multidrizzle task tion (EW Hδ ∼ 6) with no measurable OII or Hα emis- (Koekemoer et al. 2002). The RGB color image is shown sion. The fiber aperture covers only a portion of F0237, in Figure 1, where the F814W image is used for the leaving the extent of its poststarburst emission undeter- red channel, F606W green and F435W blue. Overlaid mined. However, the lack of blue star-forming regions are contours from our background-subtracted, exposure- within the system indicates a dearth of ongoing star for- corrected Chandra X-ray image (O11). Plotted as a ma- mation. ∼ ∗ genta arc is the position of the shock edge described in The faint ( 0.3LR), blue galaxy F1228 is the western- (O11). We also make use of our AAOmega spectra. most of the highlighted galaxies in Figure 1. It hosts the least spectacular trail of blue knots, the most dis- 3. RESULTS AND ANALYSIS tant of which is ∼ 11 kpc from F1228’s main component. 3.1. HST/ACS imaging The morphology is similar to the “tadpole” galaxies seen in the Hubble Ultra-Deep Field (Straughn et al. 2006; Close inspection of the HST images revealed four Elmegreen & Elmegreen 2010) with a head that contains galaxies (marked by red boxes in Figure 1) with distinct a number of bright knots of star formation, a blue ridge trails of extremely blue knots and filaments, most con- on its western side, and a tail pointing to the SE con- spicuous in the bluest F435W band and having magni- taining the blue knots. The spectrum (Figure 2) reveals tudes 24.7 < F435W < 28.5. This band corresponds strong Hα emission with EW ∼ −52A˚. The emission to the Sloan u-band in the cluster rest frame, which is line EW ratios of [NII]/Hα=-0.64 and [OIII]/Hβ=-0.04 known to be sensitive to light from young, hot OB stars, are consistent with a star-forming origin (Baldwin et al. hence active regions of star formation (Hopkins et al. 1981). 2003). The top panels of Figure 2 show images of The jellyfish galaxy closest to the Bullet-like subcluster these four galaxies, revealing their “jellyfish” morphol- (right-most panel of Figure 2) has a disk-like morphology, ogy (nomenclature of S10), which is due to trails and with no discernible bulge or spiral arms. The trail of filaments of bright star-forming regions. This interpreta- extremely blue knots points to the NW and extends to a tion is consistent with star-forming signatures in the op- radius of ∼ 26 kpc. This galaxy is below the brightness tical spectra discussed below for three of the four galax- limit used in O11 and thus does not have a spectrum or, ies. All 3 galaxies are spectroscopically confirmed cluster therefore, a . members (O11). ∼ ∗ 3.2. Galaxy F0083 is a luminous ( 3LR), nearly face-on Colors late-type spiral with numerous blue star-forming regions We estimate the ages of the knots and filaments by in its disk and an unresolved bright nucleus. The disk comparing their colors to those predicted by the solar structure is irregular with an asymmetry on the eastern models of Maraston (2005). The models used side connecting to a faint tidal feature which connects to ∼ correspond to the stellar populations produced by two a nearby faint galaxy (F606W mag 22.6), indicating extremal star formation histories – a single burst, and an interaction has taken place. There is a bright blue rim an exponentially decaying star formation with a decay ∼ 4kpc north of the galaxy center. A trail of blue knots ∼ timescale of 20Gyrs. The colors of the knots and fila- and filaments extends 35kpc from the galaxy center ments and the color evolution tracks derived from the to the southwest – these knots and filaments are not as- models are shown in Figure 3. There are two caveats. sociated with the disk. The spectrum (Figure 2) reveals First, the contribution of emission lines is ignored, al- Balmer lines with narrow and broad components which though this is exepected to be minimal within the broad are well fitted by two Gaussians having rest-frame ve- ± −1 ± −1 bands of the filters (e.g., see Figure 9 Cortese et al. 2007). locity dispersions of 1078 13kms (1155 40kms ) Second, we do not account for reddening. However, dust ± −1 ± −1 and 124 2kms (133 4kms ), respectively, for Hα obscuration tends to make stellar populations appear (Hβ). The OIII doublet is also well fitted by broad and older, thus our age estimates are upper limits. From narrow components with dispersions 336 ± 5kms−1 and − Figure 3 it can be seen that in the majority of cases the 93 ± 1kms 1, respectively. The combination of broad knots/filaments are bluer than predicted by the 100 Myr and narrow lines indicate that F0083 hosts a Seyfert 1 nu- populations, thus we take this to be the upper limit on cleus. However, the ratios of the equivalent widths (EW) the times since the onset of star formation. of the narrow components of log([NII]/Hα)=-0.51 and 4. log([OIII]/Hβ)=0.67 lie close to the AGN/star-forming DISCUSSION AND INTERPRETATION boundary, indicating that a fraction of the emission may We have presented HST observations which reveal four be due to star formation (Baldwin et al. 1981). A bright “jellyfish” galaxies in the merging cluster A2744. Low X-ray point source is associated with this galaxy (Fig- redshift jellyfish analogues are observed in nearby clus- ure 1). ters (Sun et al. 2007; Yoshida et al. 2008; Smith et al. F0237 (originally CN104; Couch & Sharples 1987) is a 2010; Hester et al. 2010). However, at compa- ∼ ∗ major merger involving two 0.7LR late-type galaxies rable to A2744, only three have been observed, all in (classified by Couch et al. 1998, as Sbc and Sab) the cen- separate clusters (Owen et al. 2006; Cortese et al. 2007). ters of which are separated by ∼ 5kpc. The multi-color Indeed, Cortese et al. (2007) found only two examples in image (Figure 2) reveals a faint tidal feature extending a survey of 13 intermediate redshift clusters. The rar- ∼ 30 kpc to the southeast, and a trail of ∼ 6 blue knots ity of jellyfish galaxies in intermediate redshift clusters extending ∼ 21 kpc to the southwest. Unlike F0083, emphasizes the significance of the observations presented there are no blue knots within the galaxy disks. The here; we observe four such systems within A2744. While Shocking Tails in 3

Figure 1. HST/ACS RGB image of A2744. The green contours show Chandra X-ray surface brightness. The red boxes highlight the “jellyfish” galaxies which are shown in more detail in Figure 2. From east to west, the four galaxies are F0083, F0237, the “central” jellyfish and F1228. The magenta dashed curve shows the shock edge reported in O11. the statistics at intermediate redshifts are sparse, S10 dicates that the picture outlined by S10 is unlikely in compiled a sample of 13 galaxies which this case for the jellyfish observed in A2744. Thus, there harbor ultraviolet asymmetries/tails. The orientation of appears to be a relatively larger number of jellyfish in the tails, which generally point away from the cluster A2744 compared to other intermediate redshift clusters center, suggested that these galaxies formed from an in- and these jellyfish appear to be a different, brighter ver- falling population experiencing the cluster environment sion of their low redshift counterparts. for the first time (S10). However, there are two key dis- Is the major merger in A2744 driving the formation of tinctions to be made when comparing the low redshift an excess of these bright jellyfish galaxies? There is a jellyfish population with that of A2744. First, the knots strong spatial correlation between the jellyfish galaxies in A2744 are bright (−13 . MI . −17) when compared and features associated with the high speed Bullet-like to prominent examples in Coma (GMP4060/RB199, subcluster (Figure 1 and O11): the proximity of (i) F0083 MI > −13 S10; Yoshida et al. 2012) and Virgo (IC3418, and F0237 to the portion of the Bullet-driven shock front MI > −11 Hester et al. 2010; Fumagalli et al. 2011) revealed as an edge in the Chandra observations, and (ii) while only ESO 137-001 in the merging cluster A3627 the central jellyfish to the X-ray peak associated with the has knots with comparable brightness (Sun et al. 2007; remnant gas core of the Bullet-like subcluster. While we Woudt et al. 2008). Thus, the bright knots seen in A2744 cannot know the exact 3D locations of the jellyfish galax- are rare in low redshift clusters. Second, the orientation ies with respect to the ICM structures, the small pro- of the tails in A2744 are less well ordered than those jected distances suggest that the jellyfish galaxies may seen in Coma. This does not preclude the existence of a have recently been overrun by the shock front and/or the faint, Coma-like infalling population of jellyfish in A2744, Bullet-like subcluster gas. This indicates that a mecha- which may be revealed by deeper observations, but in- nism related to an interaction with these ICM features 4 Owers et al. residuals. Bottom panel: above the background for an image σ 1 ∼ the AAOmega fiber aperture size. ˚ A seen in the F0237 and F1228 spectra is due to sky subtraction 8650 ∼ e 1. The green contours show the surface brightness at ng the SWarp tool (Bertin et al. 2002). The white circle shows s. We note that the broad feature at Close up views of the jellyfish galaxies highlighted in Figur Top panel: generated by co-adding theAAOmega F435W, spectra F606W (where and available) for F814W the images jellyfish usi galaxie Figure 2. Shocking Tails in Abell 2744 5

Figure 3. Comparison of the colors of the knots and filaments in the jellyfish tails (black symbols with error bars) to those predicted by models of stellar populations produced by single burst (solid blue line) and exponentially declining (red dashed line) star formation histories. 6 Owers et al. may be responsible for either the stripping of the gas so the interaction is likely too fast (∼ 10 Myr) for the leading to the tails or the triggering of the star forma- shock’s to be responsible for stripping the tion in the tails, or both. This assertion is supported by gas leading to the observed tails. This is supported the young ages of the stellar populations in the knots by the southwesterly orientation of the trails of mate- and filaments, which suggest that the star formation rial seen in F0083 and F0237. If ram pressure from the was triggered . 100Myr ago (Section 3.2). On these shock was responsible, the tails should point southeast timescales, a galaxy with velocity ∼ 1000kms−1 travels toward the near part of the shock front. This high pres- . 100kpc, so we would expect there to still be a strong sure shock-triggering scenario is consistent with the sim- spatial coincidence between the jellyfish and the puta- ulations of Tonnesen & Bryan (2012) and Kapferer et al. tive ICM features responsible for triggering the star for- (2009) who found that the star formation rates in ram- mation. Furthermore, consideration of the peculiar ve- pressure stripped gaseous tails is largely driven by the −1 locity of the Bullet-like subcluster (vpec ≃ 2500kms ; ICM pressure. However, simulations specific to the sce- O11) and of the three jellyfish galaxies with measured nario outlined above are required for confirmation. −1 redshifts (vpec = −729, −2277 and − 2528kms ; lower On the question of whether the increased ram pres- panels, Figure 2) indicates that the jellyfish galaxies are sure which occurs in mergers enhances or suppresses not members of the Bullet-like subcluster, and that if the star formation in cluster members (Fujita et al. they have interacted with the shock or the Bullet-like 1999; Bekki & Couch 2003; Kronberger et al. 2008; subcluster’s ICM, then the relative velocity of the in- Bekki et al. 2010), the observations presented here show teraction was high – of the order of the merger velocity that both effects may be in play. Considering F0083, a ∼ 4750kms−1. Similarly, Owen et al. (2006) suggested number of blue knots are seen across the galaxy indi- that the jellyfish-like galaxy C153 in A2125 may be a re- cating that there is disk-wide star formation. There is sult of enhanced ram pressure stripping caused by a high evidence for an interaction with a smaller galaxy (Fig- velocity encounter with the ICM due to a cluster merger, ure 2), which may have driven gas towards the galaxy while S10 find hints that some of their jellyfish galaxies center, triggering the AGN activity. However, such a are associated with merger-related enhancements in the minor interaction is unlikely to trigger disk-wide star ICM density. formation. Thus, we suggest that the high ICM pres- The proximity of the jellyfish to merger-related ICM sure has compressed the GMCs and triggered disk-wide features in this high speed merger suggests that the star formation (Bekki & Couch 2003). The asymmet- merger is responsible for the increased fraction and more ric distribution in the star formation—particularly the extreme nature of the jellyfish in A2744. A high-speed, arc-shaped star-forming region ∼ 4 kpc to the north of head-on merger creates much greater ram pressure and the galaxy center on the side opposing the tail of star- a powerful shock which can significantly enhance galaxy- forming regions—is consistent with that expected due ICM processes when compared with those felt by a to the compression from ram pressure as a galaxy moves galaxy falling into a relaxed cluster. Enhanced ram pres- edge-on through the ICM (Kronberger et al. 2008). Con- sure may strip gas more efficiently leading to a higher in- versely, the spectrum of F0237 (Figure 2) shows strong cidence of gaseous tails, and therefore a higher likelihood Balmer absorption and an absence of emission lines in- of forming the observed jellyfish phenomena. This is con- dicating a starburst has recently (< 1Gyr ago) been sistent with the simulations of Domainko et al. (2006), abruptly truncated. Here, the major merger of two spiral who find the mass lost from a galaxy due to ram pres- galaxies is the most likely trigger for the initial starburst sure during a major merger is substantially increased, (Mihos & Hernquist 1996; Couch et al. 1998; Bekki 2001; and also with Vollmer et al. (2006) who suggest the en- Owers et al. 2007). However, in the absence of external hanced ram pressure stripping due to an interaction with mechanisms the poststarburst phase is expected to oc- the infalling M49 group explains the strong ram pres- cur at the later stages of the major merger after the sure stripping observed in NGC 4522. Furthermore, the galaxies have coalesced (Bekki et al. 2001; Blake et al. increased ICM pressure due to the merger shock may 2004; Snyder et al. 2011). The premature truncation of promote star formation in the tails and may also drive the starburst may be due to the enhanced ram pressure higher star formation rates, hence higher surface bright- felt by a galaxy during a cluster merger, which strips ness features, compared with jellyfish galaxies in lower gas more rapidly. Furthermore, the high pressure clus- pressure environments. The shock has a Mach number ter merger environment may increase the star formation M ≃ 3 (O11) meaning the pressure jumps by a factor leading to a more rapid consumption of the gas. The of ∼ 11 with respect to the pressure of the surrounding difference in the star-forming properties of F0083 and 5 3 unshocked ICM, PICM/kB ∼ 10 K/cm . Thus, the static F0237 may be attributed to the different mass and dy- 6 3 namical states of the two systems. Given its luminosity, pressure due to the shock is PShock/kB ∼ 10 K/cm which is an order of magnitude higher than the thresh- F0083 is likely more massive than F0237, meaning it is old pressure required to trigger the collapse of GMCs less susceptible to complete stripping of gas due to ram leading to star formation (Elmegreen & Efremov 1997; pressure. Further, strong tidal forces due to the major Bekki et al. 2010). Furthermore, there is a rapid increase merger in F0237 may unbind gas, making it more suscep- in the ram-pressure the galaxy feels as it is overrun by the tible to complete ram pressure stripping (Vollmer 2003; shock due to the high relative velocity and the factor of Kapferer et al. 2008) thereby halting star formation in 3 increase in ICM density for a Mach 3 shock. If, like the the disks. Bullet cluster (Markevitch & Vikhlinin 2007), the shock 5. 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