A&A 533, A37 (2011) Astronomy DOI: 10.1051/0004-6361/201117275 & c ESO 2011 Astrophysics HST-ACS photometry of the isolated dwarf galaxy VV124=UGC 4879 Detection of the blue horizontal branch and identification of two young star clusters, M. Bellazzini1,S.Perina1, S. Galleti1, and T. Oosterloo2 1 INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: [email protected] 2 Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands Received 17 May 2011 / Accepted 9 July 2011 ABSTRACT We present deep V and I photometry of the isolated dwarf galaxy VV124=UGC 4879, obtained from archival images taken with the Hubble Space Telescope – Advanced Camera for Surveys. In the color-magnitude diagrams of stars at distances greater than 40 from the center of the galaxy, we clearly identify a well-populated, old horizontal branch (HB) for the first time. We show that the distribution of these stars is more extended than that of red clump stars. This implies that very old and metal-poor populations 4 dominate in the outskirts of VV124. We also identify a massive (M = 1.2 ± 0.2 × 10 M) young (age = 250 ± 50 Myr) star cluster 3 (C1), as well as another of younger age (C2, ∼<30 ± 10 Myr) with a mass similar to classical open clusters (M ≤ 3.3 ± 0.5 × 10 M). Both clusters lie at projected distances less than 100 pc from the center of the galaxy. Key words. galaxies: dwarf – Local Group – galaxies: structure – galaxies: stellar content – galaxies: individual: UGC 4879 – galaxies: ISM 1. Introduction particular, they studied the stellar content in the innermost 40 (see their Fig. 1), deriving the star formation history (SFH) in = The dwarf galaxy VV124 UGC 4879 has been recently recog- that region by means of the synthetic color-magnitude diagram nized to lie at the outer fringes of the Local Group (LG), near the (CMD) technique (see, for example Rizzi et al. 2002; Cignoni turn-around radius (Kopylov et al. 2008; Tikhonov et al. 2010). & Tosi 2010, and references therein). J11 conclude that most It is extremely isolated, being at 1.3 Mpc from the nearest giant of the SF (93 percent of the total, in mass) occurred at the galaxy (the Milky Way) and at 0.7 Mpc from the nearest dwarf earliest epochs between 14 and 10 Gyr ago. After this, the SF (Leo A). ceased until 1 Gyr ago, when 6 percent was formed between In Bellazzini et al. (2011, Paper I hereafter) we presented 1 and 0.5 Gyr ago and nearly 1 percent in the last 0.5 Gyr. The new deep wide-field photometry of VV124, which allowed us derived SFH is similar to that observed in other nearby dwarf to confirm and refine the distance estimate by Kopylov et al. irregular/transition-type galaxies (see, for example Weisz et al. (2008), to study in some detail the stellar content of the galaxy 2011). The prevalence of old stars seems to increase at greater and, in particular, its structure out to large radii, revealing the distances from the center (see also Paper I). From the morphol- presence of low surface brightness structures in the outskirts. ogy of the blue-loop region, J11 infer that the metallicity of the We also presented the first detection of H i in VV124, ob- youngest stars is [M/H] = −0.7. tained with the Westerbork Synthesis Radio Telescope (WSRT). In the present contribution, we use the same observational 6 10 M of H i were found, with a maximum column density of − material of J11 to address three issues raised in Paper I and not ∼>50 × 1019 cm 2. It is worth noting that the distribution of H i is considered by J11. These are, in particular: less extended than the stellar body of the galaxy. Jacobs et al. (2011, J11 hereafter) present Hubble Space – The tentative detection of an extended horizontal branch Telescope (HST) – Advanced Camera for Surveys (ACS) ob- (HB), including stars in the color range of RR Lyrae vari- servations of VV124, resolving the stars very well down to ables and blue HB stars, in the CMD of the less-crowded F814W 27, also in the most central region of the galaxy. In outer regions of the galaxy (see Fig. 9 of Paper I). While J11 report that a population of HB stars reaching V − I ∼ 0.4 / is likely present in their CMD, its clean identification and Based on observations made with the NASA ESA Hubble Space quantification is hampered by the overlap with the young Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, main sequence (MS) in the central region they consider. It would be interesting to search for the HB in the outer por- Inc., under NASA contract NAS 5-26555. These observations are asso- ciated with program GO-11584 [P.I.: K. Chiboucas]. tion (Rp > 40 ) of the ACS field, where young stars should Photometry catalogue is only available at the CDS via anonymous be quite rare and the old component should be dominant. ftp to cdsarc.u-strasbg.fr (130.79.128.5)orvia – A possible correlation was noted between the asymmetric http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/533/A37 distributions of the young stars and of the neutral hydrogen. Article published by EDP Sciences A37, page 1 of 10 A&A 533, A37 (2011) Percival et al. 2009), in the native ACS- Wide Field Channel (WFC) VEGAMAG system1. The plan of the paper is as follows. In Sect. 2 we describe the observational material and the data reduction process, and in Sect. 3 we present the clear detection of the extended HB in the external regions of the ACS field and discuss the radial distri- bution of the various stellar populations of VV124. Section 4 is devoted to a brief discussion of the correlation between the spa- tial distribution of the H i and of the young stars, and in Sect. 5 we study the candidate luminous star clusters identified here and in Paper I. Finally we briefly summarize and discuss our main results in Sect. 6. 2. Observations and data reduction The observational material consists of two pairs of F606W and F814W ACS-WFC2 images obtained within the program GO-11584, which we retrieved from the HST archive. The to- tal exposure time of the stacked drizzled images is 1140 s for the F606W and 1150 s for the F814W image. The WFC is a mosaic Fig. 1. Upper panel: stars detected in the ACS images (dark dots) are of two 4096 × 2048 px2 CCDs, with a pixel scale of 0.05 arc- superposed to the stars from the LBT sample of Paper I (gray dots) sec px−1. The center of VV124 is placed at the center of one of in a projected coordinate system centered on the center of VV124 and the mosaic chips (chip 1). rotated to have the X axis coincide with the major axis of the galaxy (see Paper I). Only stars brighter than I = 26.5 from the ACS sample have Positions and photometry of individual stars were obtained been plotted, for clarity. Lower panel: zoomed view of the central part of with the ACS module of the point spread function (PSF) – fit- 3 the ACS sample in the same coordinate system. Two concentric ellipses ting package DOLPHOT v.1.1 (Dolphin 2000), as described, of semi-major axis a = 1.0 and a = 1.5, with the same orientation and for example, in Galleti et al. (2006). The package identifies the ellipticity of the galaxy isophotes (see Paper I) are overplotted (in white sources on a reference image (in this case the drizzled F814W and in black, respectively). The white circle has a radius Rp = 40 , image, as in Mackey et al. 2006) and performs the photometry enclosing the region studied by J11. Blue stars lying on the young main on individual frames, also considering all the information about sequence (YMS; −0.4 ≤ V −I ≤ 0.0andI < 25.5, see Fig. 3) are plotted image cosmetics and cosmic-ray hits that is attached to the ob- ≤ as red filled circles, the brightest ones (having I 23.0) are encircled servational material. We adopted a threshold of 3σ above the in orange. background for the source detection. DOLPHOT automatically provides magnitudes both in the WFC VEGAMAG system and in the Johnson-Kron-Cousins system (Dolphin 2000). As done Since the young population is concentrated in the innermost in Galleti et al. (2006)andPerina et al. (2009, 2010), we se- part of the galaxy, it can be traced in much deeper detail lected the sources according to the quality parameters provided with the ACS data. This, in turn, prompts a more stringent by DOLPHOT, to clean the sample from spurious or badly mea- i comparison with the H distribution. sured entries. In particular, we retained only stars having quality – We identified a candidate cluster located near the center of flag=1 in the final catalog (i.e. best-measured stars), chi < 2.0, the galaxy and within the sheet of young stars (Cluster 1, C1 |sharp| < 0.5, crowding parameter crowd < 0.34, and photo- hereafter). The integrated spectrum of the source, shown in metric error <0.3 mag in both filters (see Dolphin 2000,for Paper I, shows strong Hβ in absorption, suggesting a young details on the meaning of the various parameters).