Category Archives: extragalactic

The intermediate-age cluster NGC 1783 in the LMC

Extragalactic GCs :: new preprint

The intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud:

Authors: A. Mucciarelli, L. Origlia , F. R. Ferraro

Abstract: We present Hubble Space Telescope ACS deep photometry of the intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud. By using this photometric dataset, we have determined the degree of ellipticity of the cluster (0.14 +/- 0.03) and the radial density profile. This profile is well reproduced by a standard King model with an extended core (rc=24.5”) and a low concentration (c=1.16), indicating that the cluster has not experienced the collapse of the core. We also derived the cluster age, by using the Pisa Evolutionary Library (PEL) isochrones, with three different amount of overshooting (namely, Lambda = 0.0, 0.10 and 0.25). From the comparison of the observed Color-Magnitude Diagram (CMD) and Main Sequence (MS) Luminosity Function (LF) with the theoretical isochrones and LFs, we find that only models with the inclusion of some overshooting (Lambda(os)=0.10-0.25) are able to reproduce the observables. By using the magnitude difference Delta V(SGB- He-Cl)=0.90 between the mean level of the He-clump and the flat region of the SGB, we derive an age equal to 1.4 +/- 0.2 Gyr.


Three Subpopulations of Globular Clusters in AM 0139-655

Extragalactic GCs :: new preprint

Evidence for Three Subpopulations of Globular Clusters in the Early-Type Post-Starburst Shell Galaxy AM 0139-655:

 Authors: A. Maybhate , P. Goudfrooij , F. Schweizer, T. Puzia, D. Carter

Abstract: We present deep HST ACS images of the post-starburt shell galaxy AM 0139-655. We find evidence for the presence of three distinct globular cluster subpopulations associated with this galaxy: a centrally concentrated young population (~ 0.4 Gyr), an intermediate age population (~ 1 Gyr) and an old, metal-poor population similar to that seen around normal galaxies (…)

Newly discovered bright and remote clusters in M31

Extragalactic globular clusters :: preprint

[0705.4037] An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters:

An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters Authors: S. Galleti, M. Bellazzini, L. Federici, A. Buzzoni, F. Fusi Pecci (Submitted on 28 May 2007) Abstract: We present the first results of a large spectroscopic survey of candidate globular clusters located in the extreme outskirts of the nearby M31 galaxy. We obtained low resolution spectra of 48 targets selected from the XSC of 2MASS, as in Galleti et al. (2005). The observed candidates have been robustly classified according to their radial velocity and by verifying their extended/point-source nature from ground-based optical images. Among the 48 observed candidates clusters we found 5 genuine remote globular clusters…

Spectroscopy of GCs out to Large Radius in the Sombrero Galaxy

External Galaxies :: Preprint

Spectroscopy of Globular Clusters out to Large Radius in the Sombrero Galaxy

Authors: Terry Bridges , Katherine Rhode , Steve Zepf , Ken Freeman
Comments: 40 pages, 7 figures, accepted for publication in ApJ

We present new velocities for 62 globular clusters in M104 (NGC 4594, the Sombrero Galaxy), 56 from 2dF on the AAT and 6 from Hydra on WIYN. Combined with previous data, we have a total sample of 108 M104 globular cluster velocities, extending to 20 arcmin radius (~60 kpc), along with BVR photometry for each of these. We use this wide-field dataset to study the globular cluster kinematics and dark matter content of M104 out to 10 arcmin radius (30 kpc). We find no rotation in the globular cluster system. The edge-on nature of M104 makes it unlikely that there is strong rotation which is face-on and hence unobserved; thus, the absence of rotation over our large radial range appears to be an intrinsic feature of the globular cluster system in M104. We discuss ways to explain this low rotation, including the possibility that angular momentum has been transferred to even larger radii through galaxy mergers. The cluster velocity dispersion is ~230 km/s within several arcmin of the galaxy center, and drops to ~150 km/s at ~10 arcmin radius. We derive the mass profile of M104 using our velocity dispersion profile, together with the Jeans equation under the assumptions of spherical symmetry and isotropy, and find excellent agreement with the mass inferred from the stellar and gas rotation curve within 3 arcmin radius. The M/L_V increases from ~4 near the galaxy center to ~17 at 7 arcmin radius (~20 kpc, or 4 R_e), thus giving strong support for the presence of a dark matter halo in M104. More globular cluster velocities at larger radii are needed to further study the low rotation in the globular cluster system, and to see if the dark matter halo in M104 extends beyond a radius of 30 kpc.

A Comparison Between the GCs in NGC 5128 and the Galaxy

Globular Cluster System :: new preprint

Sidney van den Bergh
Comments: To be published in the Astronomical Journal

A Comparison Between the Globular Clusters in NGC 5128 and the Galaxy

Some of the properties of the globular clusters in NGC 5128 are compared to those of Galactic globular clusters. Assuming the color- metallicity relations that hold for Galactic globular clusters then the metal-poor clusters in NGC 5128 that have [Fe/H] < -1.80 are significantly fainter than are the more metal-rich globulars in that galaxy. No such metallicity dependent luminosity difference is observed among the globular clusters associated with the Milky Way. Furthermore the NGC 5128 cluster sample contains two objects that, on the basis of their observed colors, appear to be super metal-poor. It is speculated that many of these apparently faint and metal-poor clusters in NGC 5128 are actually objects resembling intermediate-age Galactic open clusters. It is also found that large clusters with FWHM > 10 pc are typically less luminous in NGC 5128 than are their more more compact counterparts. In this respect the NGC 5128 cluster system is similar to the Galactic globular cluster system. Finally, the present data may hint at the possibility that the NGC 5128 cluster system differs from that surrounding the Milky Way, in that the NGC 5128 objects do not seem to exhibit a clear cut gap between the regions of the FWHM vs M_v plane that are occupied by globular clusters and dwarf spheroidal galaxies.

Preprints: W. Miller, F. Schweizer


Globular Clusters in Dwarf Galaxies

Author: Bryan W. Miller (Gemini Observatory)

Recent work on globular cluster systems in dwarf galaxies outside the Local Group is reviewed. Recent large imaging surveys with the Hubble Space Telescope and follow-up spectroscopy with 8-m class telescopes now allow us to compare the properties of massive star clusters in a wide range of galaxy types and environments. This body of work provides important constraints for theories of galaxy and star cluster formation and evolution.

Globular Cluster Formation in Mergers
Author: Francois Schweizer (Carnegie Observatories, Pasadena)

Mergers of gas-rich galaxies lead to gravitationally driven increases in gas pressure that can trigger intense bursts of star and cluster formation. Although star formation itself is clustered, most newborn stellar aggregates are unbound associations and disperse. Gravitationally bound star clusters that survive for at least 10-20 internal crossing times (~20-40 Myr) are relatively rare and seem to contain