arXiv:astro-ph/0502192v1 9 Feb 2005 ed se o xml iae l(03) iae al.(2003 et Bica (2004)). al.(2003a), al. et et Bica lat Kobulnicky galactic example low for in general, (see, in h fields or were disc, that Galactic the systems into stellar al- den several has of Spitzer) past recognition (2MASS, the the imaging lowed infrared in extinctio by high precedent provided into no insight gions the has stel example, For new that decades. of structures three cataloguing and and surroundings, discoveries systems its of lar burst and a Galaxy to the lead has of the regions of explored swaths poorly large exploring di surveys at sky modern of advent The Introduction 1. uiu aeo tla ytmw icvrdi h back- the in discovered Tucanae. 47 we cluster globular the system nearby the on stellar of report a ground (s of briefly galaxies case we dwarf curious Here or re- globular Irwin(2000)). body clusters nearby & their star Hau some behind Whiting, as hide as systems may sky, stellar M31, solved the instance for in or angles clusters solid large ing aartivdfo h ESO the Im from Field observi Wide retrieved the on data based under Also Imager 64.L-0439. Field and 62.L-0354 Wide grammes the with Chile, Silla, La edo Send ⋆ later) hand by inserted be will Astrophysics (DOI: & Astronomy icvr faselrsse ntebcgon f4 Tucanae 47 of background the in system stellar a of Discovery lo elkonhg-ufc-rgtesojcssubtend objects high-surface-brightness known well Also, ae ndt olce tteErpa otenObservator Southern European the at collected data on Based ff fteLclGopcno opeeyb ue u yteprese words. the Key by out ruled be completely cannot Group Local the of i ntebcgon f4 u.Atog eslkl,tepos the likely, less Although ag Tuc. 47 intermediate-old of an background is the it in that lie suggest sky the in position vrest fbu tr ( stars blue of overdensity tr nteaalbeCDiae,rahn iiigmagni limiting a reaching images, CCD available the on stars 1 2 Abstract. accepted ; Received 0) oae 14 located 104), rn eussto requests print ff NF-Osraoi srnmc iBlgaVaRnai1 I- 1, Ranzani Via e-mail: Bologna di Astronomico Osservatorio - INAF nvri` iBlga iatmnod srnma i Ra e-mail: Via Astronomia, di Dipartimento Bologna, Universit`a di rn aeegt,ta rvdddee nih into insight deeper provided that wavelenghts, erent ihl.elzii [email protected] michele.bellazzini, [email protected] erpr ntedsoeyo tla ytmi h backgrou the in system stellar a of discovery the on report We lblrcutr:idvda:B A Bo individual: clusters: globular . 8 .Bellazzini M. : ′ ot-eto h lse etr h bet hs apparen whose object, The center. cluster the of North-West / TEFSineAcieFacility. Archive Science ST-ECF e lse fteSalMglai Cloud? Magellanic Small the of cluster new A aucitn.final no. manuscript B − V < .Bellazzini M. 0 . ) h oo antd iga ftesse,ischaracter its system, the of diagram magnitude color The 7). 1 .Pancino E. , gpro- ng itude re- n ager id- b), lse eogn oteSalMglai lu,woeout whose Cloud, Magellanic Small the to belonging cluster e ee y, iiiyta h beti nukondafglx nteout the in galaxy dwarf unknown an is object the that sibility - - ueof tude zn ,I417 oon,Italy Bologna, I-40127, 1, nzani tdata. nt of h pia etro imtyis simmetry of center optical the scnb oie rmFg ,teoealsaeo oAis A Bo of shape overall is the diameter 1, apparent the Fig. and from roundish noticed be can As A Bo of nature The 3. Tuc, 47 A). cluster (Bo nearby A Bologna more named much we brevity the for syst which of stellar resolved background re- partially 1, the a Fig. in found in have shown we image that combined vealing the (ch images obtained B we seeing only), best 4 three the averaging o By concentration light. visible solved barely a clusteri over spot. placed anomalous bright stars, an unusual faint of an of presence frames, the V revealed and look B closer the A of 4 chip on noted, n nadtoa aae a lortivdfo h ESO the from retrieved (WFI) also Imager was Field dataset Wide additional the an with 1999 and July at in telescope Silla) ESO-MPI (La m ESO 2.2 the at obtained were Observations Observations 2. − cl f0 of scale (2004). a al. et b Bellazzini and can (2004) dataset al. this et Ferraro from in obtained found results other reduc data and the of procedure Details center Tucanae. filter) V 47 each cluster B, in globular exposures the several 60s on seven of or consisted (six images set I final and The Archive. Science ECF 02,Blga Italy Bologna, 40127, 71 1 ≃ h F samsi fegt2048 eight of mosaic a is WFI The ◦ V n ..Ferraro F.R. and 56 34 ∼ do h aatcgoua lse 7Tcne(NGC Tucanae 47 cluster globular Galactic the of nd ′ ′ 7 22 × ′′ . ,adi eetda infiat(oeta 5 than (more significant a as detected is and 5, 33 eunx20.) esace h E,SIMBAD NED, the searched We 2000.0). (equinox . 238 imtris diameter t ′ uigtevsa npcino h mgswe images the of inspection visual the During . ′′ pixel 2 − D 1 h hl oaccvr narea an covers mosaic whole The . ≃ 30 ′′ si rjce ieadits and size projected istic sprilyrsle into resolved partially is , RA = × ∼ 0 h 06pxlciswith chips pixel 4096 30 21 ′′ m h oiinof position The . 30 . aur ,2019 4, January 5 s skirts skirts and σ ) unre- f Dec ⋆ / tion em, ST- ng ed ip = e 2 Bellazzini et al.: Discovery of a stellar system in the background of 47 Tucanae

Fig. 2. Projected map of all the confirmed SMC clusters from the catalogue by Bica & Dutra (2000) (open circles). The cross marks the position of the center of 47 Tuc, the circle has the radius equal to its tidal radius. Bo A is represented by the filled square.

Fig. 1. Upper panel: Combined B image of 2.7′ × 2.7′ centered on the newly discovered stellar system. North is up and East is left. Lower panel: The same region zoomed in by a factor of ∼ 2. The overplotted circles have a diameter of 40′′. Virtually all the bright stars present in the image are members of the of Bo A can explain why it has never been identified before, globular cluster 47 Tucanae. lying only just ≃ 14.8′ away from the center of this cluster. The previous record of proximity to 47 Tuc was hold by HW5 (Hodge & Wright(1974)), located 34.6′ away from the center. and Vizier databases as well as other databases (as for instance the catalogue of SMC clusters by Bica & Dutra (2000), here- after BD00) and we found no known objectwithin 2′ of this po- 3.2. Color Magnitude Diagram sition, except for a few bright stars belonging to 47 Tuc. Bo A is clearly visible in all of our B and V images but is hard to Since the previous published photometric analysis of this recognise in I images, suggesting that it is dominated by blue dataset was aimed at obtaining the cleanest possibile sample of stars. It is also (barely) visible in the DSS-2 images. stars in 47 Tuc (Bellazzini et al. (2004), Ferraro et al. (2004)), In the following we discuss the properties and nature of only stars that were detected in at least three images per pass- Bo A as can be deduced from our dataset. band were included in the final catalogues. Here we noted that the stellar population of Bo A appears faint and blue, therefore it is not clearly visible in the shallow exposures, and it is barely 3.1. Position in the sky visible on the I band images. Therefore,we took the best seeing B and V images of chip 4 (FWHM = 0.8′′) and performed a It is well known that 47 Tuc is projected onto the outskirts new photometry with less restrictive requirements, to reach as of the Small Magellanic Cloud (SMC), a galaxy which has deep as possible. a rich cluster system (469 confirmed members, according to BD00). Fig. 2 shows the sky-projected map of the con- The resulting Color Magnitude Diagram (CMD) of Bo A is firmed SMC clusters from the catalogue of BD00 (open cir- shown in Fig. 3 (left panel), where we plotted only stars within cles). The center (Djorgovski & Meylan(1993)) and tidal ra- 1′ from the center of Bo A (small dots). Stars lying within the dius (Trager, Djorgovski & King (1993)) of 47 Tuc are marked apparentradius ofBo A (r < 15′′) are highlighted as large filled by a cross and a large circle, respectively. Bo A is represented circles. The CMD is dominated by the Main Sequence (MS) by the filled square: clearly, its position is fully compatible of 47 Tuc (going approximately from V ∼ 17, B − V ≃ 0.8 with that of a SMC cluster. Hence, the most natural hypothesis to V ∼ 22, B − V ≃ 1.6), while most of the stars within the that can indeed be made is that Bo A is a previously unknown apparent body of Bo A appear quite faint (V > 20.0) and blue SMC cluster, projected by chance near 47 Tuc. The postition (B − V < 0.8). Bellazzini et al.: Discovery of a stellar system in the background of 47 Tucanae 3

Fig. 3. Left panel: CMD of all the stars within r = 1′ from Fig. 4. Upper Left Panel: Stars counted in 10000 randomly the center of Bo A (tiny dots). Stars with r ≤ 15′′ are high- placed circles of radius 15′′ in the field around Bo A, having lighted (large filled circles). The thin sequence of stars going V > 20.0. The number of stars effectively counted in the circle from V ∼ 17, B − V ≃ 0.8 to V ∼ 22, B − V ≃ 1.6 is the MS centered on Bo A (NF = 38) is marked by the arrow. Upper of 47 Tuc. The ridge lines of the upper MS and of the RGB of Right: Same as above, but only for blue stars, having V > 20.0 OGLE-SMC0003 (Pietrzynski et al. (1998)) are overplotted on and B−V ≤ 0.7. The numbercountedin the circle aroundBo A both panels. Right panel: CMD of the Small Magellanic Cloud (NB = 27) is marked by the arrow. Lower Panel: radial density cluster OGLE-SMC0003 from Pietrzynski et al. (1998). profile of Bo A, obtained using faint blue stars (V > 20.0 and B − V ≤ 0.7) and 6′′ bins. The profile flattens outside r ∼ 20′′ and remains flat over the whole chip 4 field (not shown here for 3.3. Bo A as an overdensity of blue stars reasons of scale). We now use the CMD of Fig. 3 to assess the statistical sig- nificance of Bo A as an overdensity of resolved stars with re- spect to the underlying field of 47 Tuc. There are 38 stars with V > 20.0 lying within r ≤ 15′′ from the center of Bo A. To measured for Bo A is significantly different, at the 5σ level. study the probability of a chance occurrence of such a cluster- The result is illustrated in the top right panel of Fig. 4, where it can be noted that the maximum value obtained for NB is only ing, we counted the total number of faint (V > 20.0) stars (NF ) in 10000 circles, having r = 15′′, randomly placed on chip 4. 8. We added the further condition that their centers avoid a region Finally, if we consider the ratio of faint blue stars to faint ′ of radius r = 2 from the center of Bo A1. The average over red stars NB/NR (where faint red stars are those having B − V > ± the 10000 random extractions is < NF >= 9.7 ± 3.1 (Poisson 0.7 and V > 20.0) we obtain < NB/NR >= 0.1 1.3 over statistics), therefore NF = 38 measured in Bo A is significantly 10000 random extractions, to compare with the observed value different at the 5.5σ level. This is graphically illustrated in the of NB/NR = 2.45 ± 0.88 for Bo A. Therefore, we can conclude top left panel of Fig. 4, where it can also be appreciatedthat NF not only that Bo A is a statistically significant aggregation of values equal to or higher than 38 happen in 0.1% of the cases resolved stars in the field of 47 Tuc, but also that its stellar pop- only. Note that the adopted Poisson statistics provides an error ulation differs from that encountered anywhere else in chip 4, which is fully compatible with the observed distribution. being significantly bluer. Moreover, as we already noted, Bo A appears bluer than the surrounding field, with 27 out of 38 faint stars bluer than 3.4. Shape, Size and Density Profile B − V ≤ 0.7. We therefore repeated the experiment, this time counting the number of faint (V > 20.0)and blue (B − V ≤ 0.7) The density contours map of blue stars has been computed on ′′ stars (NB) in 10000 circles of r = 15 , randomly placed on circles of radius r = 10′′, placed on a grid spaced by 5′′. The chip 4 as above. The average over 10000 random extractions only structure emerging above the 5σ level on the background is < NB >= 1.7 ± 1.3 (Poisson statistics), where the NB = 27 is Bo A, shownin Fig. 5, and the peakdensity is morethan 16σ 1 We recall here that the field covered by chip 4 is ≃ 8.1′ × 16.2′ higher than the background. A radial density profile has also ′′ and that it samples the outer halo of 47 Tuc, hence the crowding is been computed in 6 bins, using the faint blue stars as tracers moderate all over the chip, and finally Bo A is not located where the of Bo A (Fig. 4, lower panel). It can be readily appreciated that density of stars of 47 Tuc is the highest. the density of these stars becomes essentially flat outside ∼ 20′′ 4 Bellazzini et al.: Discovery of a stellar system in the background of 47 Tucanae

The lower luminosity of the bright end of the MS could imply that the age of Bo A is larger than that of OGLE- SMC00032, although small statistics prevent us from reaching a firm conclusion on this point. The presence of a fully devel- oped RGB and of a well defined Red Clump would imply that OGLE-SMC0003 is older than 2-3 Gyr, which can provide a tentative lower limit to the age of Bo A as well. However, it has to be noted that the putative RGB stars of Bo A are still fully compatible in color, magnitude and number with being MSstars of 47Tuc.Thereare in fact8 starswithin 15′′ from the center ofBo A with 0.4 < B−V < 1.1 and V < 20.5.Inasetof 10000 random extractions as those described above, 8 or more stars with these characteristics are found in 20% of the cases, hence there is no significant overdensity of RGB stars around Bo A. A fainter MS with respect to OGLE-SMC0003, coupled with the lack of a corresponding RGB would point against the hypothesis that Bo A is an ordinary SMC cluster and may in- stead suggest that is a much farther system (this possibility is briefly considered in Sect. 4, below). In any case, low number statistics prevent us from making any firmer statement, at the Fig. 5. 2′ × 2′ cutout of the density map of faint blue stars (V > present stage. 20.0 and B − V < 0.7) on chip 4. The map is computed by It is clear that a better characterization of the stellar content counting stars in circles of r = 10′′, placed on a grid spaced of Bo A requires a much deeper and higher resolution photom- by 5′′. The average density (computed over the whole chip) etry. We therefore searched various scientific archives for better has been subtracted and the remaining over-density has been images of a field includingBo A, but the onlypromisingdataset divided by the standard deviation of the background, hence it (a set of t = 900 s images taken with the ESO-NTT) resulted is expressed in units of σ. Only around Bo A the density of exp useless because of the very bad seeing under which they were stars reaches a level of 5σ above the background, while the acquired (FWHM ≥ 2.5′′). Hence, dedicated observations are peak density is more than 16σ. needed for a deeper insight. from the center of the system and remains flat out to the edges 4. Conclusions of the considered field. We have identified a previously unknown stellar system (Bo A) Summarizing, Bo A appears as a quite concentrated, ap- in the background of the galactic globular cluster 47 Tucanae, proximately circular overdensity, with a characteristic apparent located ∼ 15′ North-West of its center. The system, which we diameter of D ≃ 30′′ (see Figs. 1 and 4). We note that accord- call Bo A, is partially resolved into stars and is clearly detected ing to BD00 the typical diameters of confirmed SMC clusters as a statistically significant overdensity of faint and blue stars range from ≥ 12′′ to ∼ 4′, where 25% of the whole sample in the considered ≃ 8.1′ × 16.2′ field (chip 4), located in the has D ≤ 30′′. Hence, the characteristic size of Bo A is fully outskirts of 47 Tuc. The appearance, the characteristic size, the compatible with the hypothesis that it is a cluster of the SMC CMD and the position in the sky of Bo A indicate that it is most galaxy. In this hypothesis, and assuming a distance modulus of probably a stellar cluster belonging to the Small Magellanic (m − M) = 18.82 (Mateo(1998)) for the SMC, the linear size 0 Cloud. The proximity of Bo A to 47 Tuc is the most likely of Bo A would therefore be of approximately 8.4 pc. explanation to why the system has not been discovered before. While the identification of Bo A as a SMC cluster is 3.5. Stellar content most likely, the available data (and, in particular, the shal- low CMD) leave formally open (at least) another interpreta- Now that we have established the statistical significance of tion (see Sect. 3.4, above). A dwarf irregular galaxy located Bo A as an aggregate of resolved stars, we can use the CDM at ∼ 1 Mpc or more from us (like, for instance, Sag DIG of Fig. 3 to study its stellar content. The ridge lines of the up- Lee & Kim (2000)) may appear similar to Bo A if seen in the − per MS (the blue plume at B V < 0.6) and of the Red Giant background of 47 Tuc, whose diffuse brightness may hide the ∼ − ∼ ∼ Branch (RGB, going from V 21 and B V 0.8 to V 17 unresolved body of the galaxy. Its CMD would appear very − ∼ and B V 1.4) of OGLE-SMC0003 are overplotted to both similar to that of Bo A, once the same limiting magnitude CMDs in Fig. 3. We can concludefrom this comparisonthat the (V ∼ 22.5) is attained (see, Fig. 2 of Lee& Kim(2000)). CMD of Bo A is fully compatible with that of a sparse cluster Hence, accurate photometry down to V ∼ 24 is required to belonging to the SMC. In this case the plume of blue stars, ly- definitely rule out this less likely, but still viable, hypothesis. ing around the extrapolation of the template MS ridge line at faint magnitudes, should be interpreted as the upper MS of the 2 In the hypothesis that the two clusters have approximately the cluster, while most of the redder stars may belong to its RGB. same disctance. Bellazzini et al.: Discovery of a stellar system in the background of 47 Tucanae 5

Acknowledgements. Financial support to this research has been pro- vided by the Agenzia Spaziale Italiana (ASI) and the Ministero dell’Istruzione, dell’Universit`ae della Ricerca (MIUR). We are grate- ful to an anonymous Referee for her/his detailed and useful sugges- tions.

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