arXiv:0706.0019v2 [astro-ph] 14 Sep 2007 od abig B H,UK 0HA, CB3 Cambridge Road, icn vdnefrdr atr n aecharacter- have and 10 matter, of luminosities dark istic for evidence vincing 06 eouo ta.20;Iwne l 07,btalso but 2007), al. et al. Irwin et 2007; Zucker found al. 2005; Recent only et al. not Belokurov Sloan et have 2006; (Willman 2000). the objects al. dwarf as halo et many York Galactic such (SDSS; for surveys searches mostly Survey years, CCD Sky few large-area Digital last the on in based rapidly increased has Way great of undoubtely is 2001). properties globular interest. al. extreme of et and study with Odenkirchen and 1997) clusters Pal5, identification King context, in & this re- Sosin Within (as internal M15, disruption for in tidal evidence (as clear processes is for shaping observed there survive could reflects cases, the that individual mainly that objects clusters of sense subset globular a 1997). the Galactic been Heggie of & long Meylan population has 1997; there Ostriker Indeed, & ensu- (see, Gnedin collapse and core friction; e.g., ini- and evaporation, dynamical the segregation, de- processes, and mass be ing tides external processes: galactic to subsequent as astrophysical appears orbit; such of it and wide. as structure set quite interest, tial proper- a is great structural by concentration) of of termined and is range size, range observed mass, This the Yet, (e.g. ties pc. 3 of edleg Germany; Heidelberg, nstemjrt tlwlttdsi h ne inner [ the “old” in sally latitudes low at ( census majority current the the and finds extensively studied been has Way R 2 rpittpstuigL using ApJ typeset Preprint in publication for Accepted 1 Milky the of outskirts the at objects of census Our h ouaino lblrcutr rudteMilky the around clusters globular of population The GC .Koposov S. nttt fAtooy nvriyo abig,Madingle Cambridge, of University Astronomy, of Institute a lnkIsiuefrAtooy ¨ngth 7 69117, K¨onigstuhl 17, Astronomy, for Institute Planck Max H ICVR FTOETEEYLWLMNST IK A GLOB WAY MILKY LUMINOSITY LOW EXTREMELY TWO OF DISCOVERY THE < hi bevdszsof sizes observed Their hs bet eedtce nteSonDgtlSySre aaR Data Kopos Survey clusters, Sky The Digital Observatory. Alto Sloan Calar the the at in at imaging detected deeper were objects These nemc agrpplto fgoua clusters. globular tim survival of headings: remaining population Subject in short larger their clusters much through globular once add, a of also space They parameter explored. structural the that show rnho h aitru tem h afms eaaintmso times a relaxation 1 half-mass Koposov The extreme. t stream. most are Sagittarius the clusters the 2 new Koposov of these with 1, branch Way, Whiting Milky and the 4 orbiting AM 1, Palomar with Together evr ocnrtd hi vprto ieclsmyb slwa low segregatio as mass be may drastic timescales undergone evaporation have their they concentrated, that very seem be would it so and 0kc.Goua lsesaeams univer- almost are clusters Globular kpc). 20 erpr h icvr ftoeteeylwlmnst lblrclust globular luminosity low extremely two of discovery the report We ∼ t ∼ 40 age 1,2 5Mrrsetvl 2odr fmgiuesotrta h g o age the than shorter magnitude of orders (2 respectively Myr 55 − ...d Jong de J.T.A. , 1. ≈ koposov,[email protected] 0kcadapa ohv l tla ouain n uioiiso on of luminosities and populations stellar old have to appear and kpc 50 INTRODUCTION 0 A T . 5 5 E L tl mltajv 02/07/07 v. emulateapj style X − − ⊙ aayhl aaygoua clusters Galaxy:globular – Galaxy:halo M 1 V × − ∼ ∼ t 1 Hubble .Belokurov V. , caewl ihnteepce ia ii of limit tidal expected the within well are pc 3 )adtpclsizes typical and 8) ,so ocon- no show ], ∼ cetdfrpbiaini ApJ in publication for Accepted t Hubble Irwin 2 .W Rix H.-W. , ABSTRACT In . 2 ..Bell E.F. , y ftefits aatcgoua nw odt,A 4 AM date, to known globular Galactic ( faintest the of uhbihe;at brighter; much epriaiga aa lo h oa uioiyof luminosity be total to The appears Alto. 2 with Calar Koposov confirmed at subsequently imaging SDSS and in deeper (DR5) detected 5 first 2, Release Koposov Data and 1 Koposov named ayn aatcbcgon htaelkl ob ei- be signif- to all likely detect are slowly to that the was background above search Way Galactic overdensities the Milky varying stellar of the small-scale aim icant in The systematic substructure our of halo. small-scale course for the in search candidates other among the of substantial halo a outer that the mean in Way. may lurks Milky sky clusters We such the of of globulars 1). population 1/5 low-mass Table two than these (see less of struc- in objects discovery the the new that of the argue estimates of give parameters and tural discoveries the magni- firming of order an have also are 1 larger. but Segue tude and luminosities, 1 Willman low Whiting and 2002). extremely 1, al. Palomar et 4, (Whiting AM 1 sizes: small and minosity xrml an n opc ( compact and on faint all. doubt extremely at metallic- some cluster and globular casting a matter 2007), is al. dark it seems et whether for (Martin 1 evidence spread Willman ity some show disruption. to perhaps tidal isopleths, and ongoing irregular 1 distorted indicating have Willman both newcomers, Milky 1, The be Segue may clusters. that globular objects Way extended and faint two added oetlmnst.I oa,ol u ftepreviously the of out 3 only known total, In luminosity. lowest − h w e lblrcutr eeoiial selected originally were clusters globular new two The con- data follow-up deep the describe we paper, this In ee eanuc h icvr ftonw distant, new, two of discovery the announce we Here, 1 1 ..Zucker D.B. , . a,Imn&Cre 97.Kpsv1i not is 1 Koposov 1987). Carney & Inman mag, 4 1 ∼ 2. 6 aatccutr,hv oprbylwlu- low comparably have clusters, Galactic 160 ICVR N OBSERVATIONS AND DISCOVERY h ik a aoi o e fully yet not is halo Way Milky the ooo n r only are 2 and 1 Koposov f s s infiatdrc vdnefor evidence direct significant es, v1adKpsv2 r located are 2, Koposov and 1 ov .Snete ontapa to appear not do they Since n. ∼ 2 elws uioiyglobulars luminosity lowest he ..Evans N.W. , lae5adcnre with confirmed and 5 elease M 0 past i ls odistant to close lie to ppears . r nteMlyWyHalo. Way Milky the in ers 1 V,tot h tla populations), stellar the f t ∼ Hubble 0p tta distance. that at pc 10 − ∼ − ∼ hs discoveries These . ly ∼ a,i a h third- the has it mag, 2 2 a,lwrta that than lower mag, 1 LRCLUSTERS ULAR .Gilmore G. , M c lblrclusters, globular pc) 3 V − ∼ 1 mag ∼ 70 2 M.J. , . 2 Koposov et al.

Fig. 1.— 3′ × 3′ SDSS cutout images of and 2. The bright in the center of Koposov 1 is a foreground star with V ∼ 14.5m −1 and large proper motion (µα, µδ) ∼ (−32, −12)µas yr , according to the USNO-B1 catalog (Monet et al. 2003). The bright extended object near the center of Koposov 2 is a background galaxy.

ing to their statistical significance. Figure 1 shows the TABLE 1 SDSS images, and Figure 2 shows the spatial distribu- Properties of Koposov 1 and Koposov 2 tion of extracted sources, where central concentrations Parameter Koposov 1 Koposov 2 of are clearly visible. These concentrations are de- tected at high level of significance. The areas of 1′ radius Coordinates (J2000) 11:59:18.5 +12:15:36 07:58:17.00 +26:15:18 marked by circles centered on Koposov 1 and 2 plotted in Coordinates (ℓ, b) (260.98◦, 70.75◦) (195.11◦, 25.55◦) Distance ∼ 50 kpc ∼ 40 kpc Figure 2 contain 22 objects and 23 objects, respectively, Size ∼ 3 pc ∼ 3 pc while mean density of g-r< 0.6, r> 20mag stars should m m MV ∼−2 ∼−1 produce approximately 2.5 objects, which implies a high ∼ ∼ Relaxation Time 70 Myrs 55 Myrs statistical significance of the overdensities; for pure Pois- Tidal radius ∼ 11 pc ∼ 9 pc son distribution of objects, the probability to find such group of stars in all DR5 is around 10−9. The differential Hess diagrams for stars within 2′.5 ra- ther dwarf spheroidal galaxies or globular clusters. A dius centered on the objects are shown in Figure 3. There detailed description of the algorithm and its efficiency is a clear excess of blue stars (g − r< 0.5), which we in- will be provided in a future paper, and we only present terpret as main-sequence turnoff stars at r ∼ 22, which here a brief outline of the method. The algorithm is roughly corresponds to distances of ∼ 50kpc. based on the so-called Difference of Gaussians method, To confirm the nature of discovered candidates and first developed in Computer Vision (Babaud et al. 1986; quantify their structural and population properties, we Lindenberg 1998). Starting from a flux-limited cata- acquired follow-up GTO observations in 2007 January log of stellar positions, the number-counts map in (α, δ) on the 2.2m telescope at Calar Alto using the CAFOS camera. This camera has a 2k × 2k CCD with a 16′ × 16′ plane is convolved with a filter optimized for the detec- ′′ − tion of overdensities, namely the difference of two two- field of view and a pixel scale of 0 .5 pixel 1. We ob- dimensional Gaussians (Koposov et al. 2007). Having served each object for a total of 2 hr in Johnson B zero integral, the kernel guarantees that the convolution and 1.5 hr in Cousins R. The integrations were split with a constant (or slowly varying) background will re- into five individual dithered exposures for cosmic ray sult in zero signal. When the data contain an overdensity and bad pixel rejection. The observations were car- ried out in good photometric conditions with a seeing of with a size comparable to the size of the inner Gaussian, ′′ ′′ the filter will be close to optimal. 1 −1.3 . The data were bias-subtracted and flat-fielded. We applied this filtering procedure to the entire stellar The individual frames were WCS-aligned, drizzled, and subset of the DR5 source catalog with r < 22m,g − r < median-combined using our software and the SCAMP 1.2m. In our analysis we used the photometry cleaned and SWARP programs (Bertin 2006). The combined B by switching on quality flags as described in SDSS SQL band images of the objects are shown in Figure 4. pages 3 This minimizes the influence of various artefacts The central stellar overdensities are clearly corrobo- including those caused by proximity of very bright or rated by the Calar Alto photometry, which is nearly 2 extended objects. In the resulting map that had been mag deeper than the original SDSS data. While the convolved with a 2′ kernel, we found two very compact follow-up data are quite deep, the stars are subject to sig- objects among other overdensities ranked highly accord- nificant crowding, due to the compactness of the clusters. Therefore, for the purposes of robust source detection 3 http://cas.sdss.org/dr5/en/help/docs/realquery.asp#flags and photometry, we used the DAOPHOT/ALLSTAR The discovery of two globular clusters 3

Fig. 2.— The spatial distribution of the objects in the area of Koposov 1 and Koposov 2. All objects classified as stars with colors (g-r)< 0.6m and r> 20m in the area 0.3◦ × 0.3◦ are shown. The circles with 1′ radii centered on the objects are overplotted.

Fig. 3.— The residual g − r versus g Hess diagrams of the clusters from the SDSS data. In each case the residual Hess diagram is costructed by subtracting the normalized background Hess diagram from the Hess diagram of stars lying within 2′.5 radius from the centers of objects software (Stetson 1987). To get the absolute calibra- mag. The CMDs clearly show the presence of the main tion of the photometry from each frame, we cross- sequences near the centers of the objects, while they are matched the DAOPHOT sources with the SDSS cata- absent in the the comparison fields. The statistical sig- log using the Virtual Observatory resource SAI CAS 4 nificance of the overdensities is also clearly supported by (Koposov & Bartunov 2006). To convert the Sloan g and the new data. The CMD of objects within 2′ from the r magnitudes into the Johnson-Cousins photometric sys- center of Koposov 1 contain 96 objects, while the back- tem, we used the conversion coefficients from Smith et al. ground density inferred from the comparison field should (2002). The resulting B versus B − R color-magnitude give around 23 objects, which gives a 15 σ deviation. For diagrams (CMDs) of the central regions of the objects Koposov 2 , the number of objects within 1.2′ is 92, while together with the CMDs of the comparison fields, ex- the background density from the comparison field should tending to B ∼ 23.5m − 24m, are shown in the Figure 5. produce around 24 objects, which gives a 14 sigma devi- The median photometric accuracy of the data is 0.05-0.1 ation. In the next section we will discuss the properties of the objects which can be derived from the follow-up 4 http://vo.astronet.ru/cas data. 4 Koposov et al.

Fig. 4.— B band Calar Alto view of Koposov 1 and Koposov 2. The 2′ × 2′ images are centered on the clusters (north is up, east is left).

Fig. 5.— Left panel, left half : B vs. B −R CMDs derived from the Calar Alto data for stars lying within 2′ of Koposov 1 with 8 Gyr and [Fe/H]=-2 Girardi et al. (2000) isochrones overplotted. Left panel, right half: For comparison, the CMDs of stars in the annulus centered on Koposov 1 defined by radii 3.2′ and 3.7′are plotted. Right panel, left half: B vs. B − R CMDs of stars lying within 1.′2 of Koposov 2 with 8 Gyr and [Fe/H]=-2 Girardi et al. (2000) isochrones overplotted.Right panel, right half: for comparison, the CMDs of stars in the annulus centered on Koposov 2 defined by radii 2′ and 2.3′are plotted.

3. PROPERTIES estimate of −1 & MV & −2 is based on the absence The CMDs of the objects from the Calar Alto data of the giants in these clusters and the visible similarity (Fig. 5) clearly show a distribution of stars which can be of the CMDs to that of the lowest luminosity globular attributed quite convincingly to an old main sequence. cluster AM4 (MV = −1.6, Inman & Carney 1987). We In the case of Koposov 1, the main-sequence turnoff is checked that estimate by a simple Monte Carlo exper- clear-cut, while for the second cluster it is not so well iment: using the Salpeter IMF and Girardi isochrones defined. To estimate the distances to the objects, we we simulated fake clusters and deduced that the clusters overplot in Figure 5 the 8 Gyr [Fe/H]=-2 isochrones with −1 & MV & −2 have a number of stars within 1.5-2 from Girardi et al. (2000). For Koposov 1, this gives mag below the turnoff is close to the observed number of a distance of 50 kpc. For Koposov 2, the estimate is stars (50-70) in our objects. We must say also that due 40±5 kpc, but it is not well constrained due to a lack of to the intrinsic faintness of the clusters and low num- main-sequence turnoff stars. The angular diameters of ber of stars in them the estimates of the total luminosity the clusters are < 0.5′, which translates into a physical and especially the age have large uncertainties. However, size of r ∼ 5pc. Unfortunately, the number of stars de- with the existing data we cannot do any better. Much tected in the central regions is not enough to precisely deeper and more accurate photometry may be required measure half-light radii of the objects; our best estimate to get precise age/luminosity measures. The spectro- scopic observations would be interesting in constraining is rh ∼ 3pc. For Koposov 1, we subtracted the bright foreground star near the center, integrated the light of the metallicity of these objects, which is currently com- the whole cluster in apertures and fitted it to a Plummer pletely unknown. profile with r = 3pc. For Koposov 2, we performed a We note that the CMD of Koposov 1 shows several h stars brighter and bluer than the tentative main-sequence maximum likelihood fit with rh ∼ 3pc. Moreover, the minuscule number of stars in both clusters does not al- turnoff, which we interpret as blue stragglers. This hy- low us to firmly establish their total luminosities. Our pothesis is not implausible considering the low luminos- ity of the cluster and taking into account the observed The discovery of two globular clusters 5

Fig. 6.— Right ascension vs. distance for the A and C branches Fig. 7.— Size vs. absolute magnitude plot for Galactic globular of the (see Belokurov et al. 2006). The position clusters. The data from the Harris (1996) catalog are plotted with of Koposov 1 is marked by a star. diamonds. Squares mark the locations of the recently discovered globular clusters , and Whiting 1. Koposov 1 anti-correlation between the frequency of blue stragglers and 2 are shown as stars. and the luminosity of the (Piotto et al. 2004). The expected tidal truncation of these clusters occurs The distance and the position of Koposov 1 suggest at (see, e.g., Innanen et al. 1983) that this cluster may be related to the Sagittarius tidal 1/3 stream. Its location is a good match to the distant Mcluster rt =0.43 × Rperi =11 and 9 pc tidal arm discovered in Belokurov et al. (2006). Figure 6  MMW  shows the arms of the Sagittarius stream in the DR5 slice around δ ∼ 10◦ and the position of Koposov 1. where we have assumed an orbital eccentricity of 0.5, and that the clusters are now near apocenter (hence 4. DISCUSSION Rperi ≈ 16 kpc), a circular speed of 190 −1 Figure 7 shows Koposov 1 and 2 on the size-luminosity kms at 16 kpc and a cluster (stellar) mass of 600 and plane along with other Galactic globular clusters. This 300 M⊙ for Koposov 1 and 2, respectively. Hence, the illustrates how unusual Koposov 1 and 2 are in their detectable extent of the globular clusters (3 pc) falls well structural properties. It appears that the detection of within the tidal limit. From this argument, the clusters these clusters contributes to growing evidence for a large are under no threat of destruction by tidal forces. Al- population of small and extremely faint objects (includ- though formal profile fits are not feasible with so few ing , AM 4, E3 and Whiting 1). There is a stars, the stellar distributions (see Figs. 1 and 4) are clear indication as well that this sub-population of glob- well localized, but not centrally concentrated by globu- ular clusters may have significantly younger ages than lar cluster standards; a core to tidal radius ratio of the classical globular clusters: Palomar 1 (Sarajedini et al. observed stellar distribution of 4 seems reasonable, im- 2006) and Whiting 1 (Carraro et al. 2007) have ages be- plying a concentration parameter of c ≡ log(rt/rc) ≈ 0.5. tween 4 and 6 Gyrs. The current estimate for the age For such low concentrations, the evaporation timescale of Koposov 1 is ≈ 8 Gyr, and the age of E3 globular tev, which is the time-scale over which two-body re- cluster is ≈ 10 Gyr. This group of clusters is also quite laxation drives stars to beyond the escape velocity, is apparent on the galactocentric distance versus luminos- tev ≈ 1.5tcc ≈ 12trh(where tcc is core collapse time) (Fig- ity plane shown in Figure 8. At least 2 out of these 5 ure 17 and 19 in Gnedin et al. 1999)For Koposov 1 and unusual clusters (Whiting 1 and Koposov 1) seem to be 2, this implies evaporation time-scales of 0.7 and 1.1 Gyr, associated with the Sagittarius . Two quan- respectively. This estimate of tev ∼ 0.1tHubble may be an tities that are crucial for the long term evolution and underestimate, if the brightest stars which we observe survival of Koposov 1 and 2 are the relaxation time and are more concentrated than the faint stars due to mass the expected tidal radius. For the half-mass relaxation segregation; then the total mass and half-mass radius time, we find using equation (2-63) of Spitzer (1987) or can be larger. Nonetheless, this estimate makes it clear equation (72) of Meylan & Heggie (1997), that the present structural and dynamical state cannot have prevailed, even approximately, for a time-span of 1/2 3/2 ∼ 10 Gyr. The above arguments hold irrespective of Mtot Rhl trh =0.14 = 70 and 55 Myr whether Koposov 1 and Koposov 2 were once part of a hm∗iG1/2 ln(Λ) satellite galaxy, because they are mostly derived from in- respectively for Koposov 1 and Koposov 2. Here, we ternal evolution factors. This discrepancy of time-scales have assumed L ≈ 200L⊙, M/L ≈ 1.5, hm∗i ≈ 0.6M⊙ is more pronounced in Koposov 1 and 2, because their and N=500 for Koposov 2, while for Koposov 1, we have relaxation time-scales are shorter than those of Palomar assumed twice as many stars, using the observational 1 and Whiting 1, which in any case have accurate pho- estimates of § 3. This means that both clusters have ex- tometry suggesting younger ages of ∼ 4 − 6 Gyr. tremely short relaxation times, less than 1% of tHubble At face value, Koposov 1 and 2 have survival times in and trh ≈ 0.01tage,∗. The most immediate effect of two- their current state of ∼ 0.1tHubble, and were found in a body relaxation is mass segregation, which should be search of 20% of the whole sky (SDSS DR5). The naive quite drastic given the apparent stellar population age. multiplication of these factors points to a large parent 6 Koposov et al.

Based on observations collected at the Centro As- tron´omico Hispano Alem´an (CAHA) at Calar Alto, oper- ated jointly by the Max-Planck Institut f¨ur Astronomie and the Instituto de Astrof´ısica de Andaluc´ıa (CSIC). Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium for the Participat- ing Institutions. The Participating Institutions are the Fig. 8.— Galactocentric distance vs. magnitude plot for Galactic American Museum of Natural History, Astrophysical In- globular clusters. Symbols are as in Fig. 7. stitute Potsdam, University of Basel, Cambridge Uni- versity, Case Western Reserve University, University of population of ∼ 100 objects. The most likely reservoir Chicago, Drexel University, Fermilab, the Institute for for this parent population is the globular clusters, and Advanced Study, the Japan Participation Group, Johns possibly even old open clusters, in satellite galaxies that Hopkins University, the Joint Institute for Nuclear As- have been accreted, like the Sagittarius. In objects like trophysics, the Kavli Institute for Particle Astrophysics Koposov 1 and 2, it is clear that the very short relaxation and Cosmology, the Korean Scientist Group, the Chi- and evaporation times must lead to drastic mass segrega- nese Academy of Sciences (LAMOST), Los Alamos Na- tion and the expulsion of basically all low-mass stars (this tional Laboratory, the Max-Planck-Institute for Astron- line of reasoning lead us to the modest M/L ≈ 1.5). This omy (MPIA), the Max-Planck-Institute for Astrophysics gives new life to the view that truly many of the accreted (MPA), NewMexico State University, Ohio State Univer- globular clusters must have been destroyed. Yet, it is also sity, University of Pittsburgh, University of Portsmouth, clear that the actual dynamical prehistory and future of Princeton University, the United States Naval Observa- these clusters requires much more careful modelling. The tory, and the University of Washington. This research small number of stars makes them ideal subjects of direct has made use of the SAI Catalog Access Services, Stern- N -body calculations. But regardless of their dynamical berg Astronomical Institute, Moscow, Russia. S. Ko- evolution, these clusters manifestly demonstrate that the posov is supported by the DFG through SFB 439 and by parameter space of globular clusters in the Milky Way is a EARA-EST Marie Curie Visiting fellowship. not yet fully explored.

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