A study of the old galactic star cluster Berkeley 32

We present new CCD photometry of the distant old open star cluster Berkeley 32 in Johnson V and Cousins I passbands. A total of about 3200 stars have been observed in a field of 13X13 arcmin**2. The colour-magnitude diagram in V, (V-I) has been generated down to V = 22 mag. A broad but well defined main sequence is clearly visible. Some blue stragglers, a well developed subgiant branch and a Red Clump are also seen. By fitting isochrones to this CMD as well as to other CMDs available in the literature, and using the Red Clump location, the reddening, distance and age of the star cluster have been determined. The cluster has a distance of 3.3 kpc, its radius is about 2.4 pc; the reddening E(B-V) is 0.08 mag and the age is 6.3 Gyr. By comparison with theoretical isochrones, a metallicity of [Fe/H]= -0.2 dex has been estimated. We find a much flatter mass function than what has been found for young clusters. If the mass function is a power law dN/dm = const.*m**alpha, then we get alpha = -0.5+-0.3 in the mass range 0.6-1.1 solar masses. This may be seen as a signature of the highly evolved dynamical state of the cluster

The curious case of the companion: evidence for cold accretion onto a dwarf satellite near the isolated elliptical NGC 7796

The isolated elliptical (IE) NGC 7796 is accompanied by an interesting early-type dwarf galaxy, named NGC7796-DW1. It exhibits a tidal tail, very boxy isophotes, and multiple nuclei or regions (A, B, and C) that are bluer than the bulk population of the galaxy, indicating a younger age. These properties are suggestive of a dwarf-dwarf merger remnant. We use the Multi-Unit Spectroscopic Explorer (MUSE) at the VLT to investigate NGC 7796-DW1. We extract characteristic spectra to which we apply the STARLIGHT population synthesis software to obtain ages and metallicities of the various population components of the galaxy. The galaxy's main body is old and metal-poor. A surprising result is the extended line emission in the galaxy, forming a ring-like structure with a projected diameter of 2.2 kpc. The line ratios fall into the regime of HII-regions, although OB-stellar populations cannot be identified by spectral signatures. Nucleus A is a relatively old (7 Gyr or older) and metal-poor super star cluster, most probably the nucleus of the dwarf, now displaced. The star-forming regions B and C show younger and distinctly more metal-rich components. The emission line ratios of regions B and C indicate an almost solar oxygen abundance, if compared with radiation models of HII regions. NGC7796-DW1 occupies a particular role in the group of transition-type galaxies with respect to its origin and current evolutionary state, being the companion of an IE. The dwarf-dwarf merger scenario is excluded because of the missing metal-rich merger component. A viable alternative is gas accretion from a reservoir of cold, metal-rich gas. NGC7796 has to provide this gas within its X-ray bright halo. As illustrated by NGC7796-DW1, cold accretion may be a general solution to the problem of extended star formation histories in transition dwarf galaxies. (abridged)Comment: comments: 13 pages, 8 figures, accepted for publication in Astronomy & Astrophysic

The baryonic halos of isolated elliptical galaxies

Without the interference of a number of events, galaxies may suffer in crowded environments (e.g., stripping, harassment, strangulation); isolated elliptical galaxies provide a control sample for the study of galaxy formation. We present the study of a sample of isolated ellipticals using imaging from a variety of telescopes, focusing on their globular cluster systems as tracers of their stellar halos. Our main findings are: (a) GC color bimodality is common even in the most isolated systems; (b) the specific frequency of GCs is fairly constant with galaxy mass, without showing an increase towards high-mass systems like in the case of cluster ellipticals; (c) on the other hand, the red fraction of GCs follows the same inverted V shape trend with mass as seen in cluster ellipticals; and (d) the stellar halos show low S\'ersic indices which are consistent with a major merger origin.Comment: 5 pages. Published in the special issue of Galaxies, "On the Origin (and Evolution) of Baryonic Galaxy Halos

Dark Matter Deprivation in Field Elliptical Galaxy NGC 7507

Previous studies have shown that the kinematics of the field elliptical galaxy NGC 7507 do not necessarily require dark matter. This is troubling because, in the context of LCDM cosmologies, all galaxies should have a large dark matter component. We use penalised pixel fitting software to extract velocities and velocity dispersions from GMOS slit mask spectra. Using Jeans and MONDian modelling we produce best fit models to the velocity dispersion. We find that NGC 7507 has a two component stellar halo, with the outer halo and inner haloes counter rotating. The velocity dispersion profile exhibits an increase at ~70" (~7.9 kpc), reminiscent of several other elliptical galaxies. Our best fit models are those under mild anisotropy which include ~100 times less dark matter than predicted by LCDM, although mildly anisotropic models that are completely dark matter free fit almost equally well. Our MONDian models, both isotropic and anisotropic, systematically fail to reproduce the measured velocity dispersions at almost all radii. The counter rotating outer halo implies a merger remnant, as does the increase in velocity dispersion at ~70". From simulations it seems plausible that the merger that caused the increase in velocity dispersion was a spiral-spiral merger. Our Jeans models are completely consistent with a no dark matter scenario, however, some dark matter can be accommodated, although at much lower concentrations that predicted by LCDM simulations. This indicates that NGC 7507 may be a dark matter free elliptical galaxy. Whether NGC 7507 is completely dark matter free or very dark matter poor, this is at odds with predictions from current LCDM cosmological simulations. It may be possible that the observed velocity dispersions could be reproduced if the galaxy is significantly flattened along the line of sight (e.g. due to rotation), however, invoking this flattening is problematic.Comment: 9 pages, 7 figures. Accepted for publication in A&

The Chemistry of Extragalactic Globular Clusters

We present preliminary results of VLT/FORS spectroscopy of globular clusters in nearby early-type galaxies. Our project aims at studying the chemistry and determine the ages of globular cluster (sub-)populations. First results indicate that the different galaxies host from little to significant intermediate-age populations, and that the latter have alpha-element over iron ratios closer to solar than the old population that show an alpha-element enhancement similar to the diffuse stellar light.Comment: 4 pages (incl 2 figures) to appear in the proceedings of "Extragalactic Globular Cluster Systems", ed.M.Kissler-Patig, Springer; see also related contributions by T.H.Puzia and M.Hempel et a

What do globular clusters tell us about isolated ellipticals? The case of NGC 6411

We summarize the results from a study of the globular cluster (GC) system of the isolated elliptical galaxy NGC 6411, based on Gemini/GMOS g´, r´, i´ photometry. The extent of the globular cluster system is about 70 kpc. It contains ≍700 members. The colour distribution and luminosity function are typical of old GC systems. An excess of bright GCs with intermediate colours might evidence an intermediate-age merger.Fil: Caso, Juan Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Bassino, Lilia Patricia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Richtler, T.. Universidad de Concepción; ChileFil: Salinas, R.. Gemini Observatory; ChileInternational Astronomical Union SymposiumBolognaItaliaInternational Astronomical Unio