Volume 566, June 2014
|Number of page(s)||16|
|Published online||06 June 2014|
Colour magnitude diagrams from HST photometry provide the quality benchmark for crowded stellar systems like the distant (but resolved) galaxies we are considering within this project. Since deep HST photometry for Sex A and Sex B is publicly available from the ANGST database (Dalcanton et al. 2009), a direct comparison is worth performing.
In Fig. A.1 we show the CMDs obtained from the two datasets for the stars in common. Clearly our ground-based photometry cannot rival the top-level quality achievable from space, especially in the highly crowded central regions sampled by the HST data. Still, it is remarkable that all the features that can be identified in the LBC CMDs (e.g. the young MS and the parallel sequence of Blue Loop stars, the RSG plume) have a clear counterpart in the HST CMDs, thus indicating that the precision of our photometry is sufficient to reliably discriminate stars in different evolutionary phases.
Comparison between HST photometry (left panels, from Dalcanton et al. 2009) and LBC photometry (right panels) for the stars in common between the two datasets.
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Clusters and candidate clusters.
It has been observed that dIrr galaxies of luminosity similar to Sex A and Sex B may host sizable globular clusters systems, with up to ~10 members (Georgiev et al. 2010). These clusters can provide very interesting insights into the early formation history of their parent galaxies (see, e.g. Hwang et al. 2011; Huxor et al. 2013, and references therein). The only reference to a systematic search for star clusters in Sex A and Sex B available in the literature is by Pedreros & Gallart (2002); this was a brief contribution to the proceedings of a meeting and no candidate list was provided. On the other hand, Sharina et al. (2007) identified a relatively bright (MV = −7.8) and compact (rh ≃ 4 pc)
cluster in Sex B on HST images where it is marginally resolved into stars (Sex B-C1, hereafter). A low-resolution spectrum provided further confirmation of the membership of the cluster to Sex B as well as an estimate of the age (≃2 Gyr) and metallicity ([Z/ H] = −1.35 ± 0.3, Sharina et al. 2007).
In the present contribution we focus our attention on compact clusters, i.e, objects that can resemble globular clusters, and old extended clusters as defined by Huxor et al. (2005), neglecting young associations, whose brightest stars are resolved in our images. We searched our image for compact star clusters by visual inspection and looking for sources more extended than the PSF in source catalogs obtained with Sextractor, as described in Galleti et al. (2007). Our search was not intended to be exhaustive, but to single out the best candidates. The images of the four objects we finally identified as the most reliable candidates are shown in Fig. B.1; their main properties are listed in Table B.1.
Stamp images of the cluster and candidate clusters listed in Table B.1. The circles around the objects have a radius of 10 arcsec; north is up, east to the left.
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We re-identified Sex B-C1 and we found two additional candidates in Sex B and one in Sex A. We got the positions, integrated magnitudes, and measured FWHM from aperture photometry that we performed with Sextractor on the g,r pair of deep images with the best seeing. All the candidates are clearly non-stellar objects, i.e, their light profile is significantly more extended than the PSF. It is reassuring that our estimate of the integrated V magnitude for Sex B-C1 is in excellent agreement with Sharina et al. (2007), that report V0 = 17.90 ± 0.02. The case of Sex B-C1 shows that compact clusters cannot be unequivocally identified in our images (Fig. B.1). None of the presented candidates is particularly convincing. The hint of a diffuse halo and the relatively red colour suggest that Sex B-C2 may in fact be a roundish elliptical galaxy in the background (see Peacock et al. 2011). The fuzzy appearance of Sex B-C3 and Sex A-C1 is more akin to extended clusters (Huxor et al. 2005, 2013) than to classical globulars. Spectroscopic and/or high-spatial-resolution imaging follow up is clearly needed to establish the actual nature of these objects.
© ESO, 2014
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