Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy

HIRES on the Keck I telescope has been used to measure the first radial velocities for stars belonging to eleven, heavily-reddened globular clusters in the direction of the inner Galaxy. The question of kinematic substructuring among the Galactic globular cluster system is investigated using an updated catalog of globular cluster distances, metallicities and velocities. It is found that the population of metal-rich globular clusters shows significant rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc of the Galactic center, the measured rotation velocity and line-of-sight velocity dispersion are similar to those of bulge field stars. We investigate claims that the metal-rich clusters are associated with the central Galactic bar by comparing the kinematics of the innermost clusters to that of the atomic hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich and metal-poor clusters bears a remarkable similarity to that of the gas, including the same non-circular motions which have traditionally been interpreted as evidence for a Galactic bar, or, alternatively, a non-axisymmetric bulge. However, uncertainties in the existing three-dimensional Galactocentric positions for most of the clusters do not yet allow an unambiguous discrimination between the competing scenarios of membership in a rigidly rotating bar, or in a bulge which is an oblate isotropic rotator. We conclude that the majority of metal-rich clusters within the central 4 kpc of the Galaxy are probably associated with the bulge/bar, and not the thick disk. (ABRIDGED)Comment: 18 pages, including 7 of 13 postscript figures. Figures 1-6 available at http://astro.caltech.edu/~pc. Accepted for publication in the Astronomical Journa

An Age Difference of 2 Gyr between a Metal-Rich and a Metal-Poor Globular Cluster

Globular clusters trace the formation history of the spheroidal components of both our Galaxy and others, which represent the bulk of star formation over the history of the universe. They also exhibit a range of metallicities, with metal-poor clusters dominating the stellar halo of the Galaxy, and higher metallicity clusters found within the inner Galaxy, associated with the stellar bulge, or the thick disk. Age differences between these clusters can indicate the sequence in which the components of the Galaxy formed, and in particular which clusters were formed outside the Galaxy and later swallowed along with their original host galaxies, and which were formed in situ. Here we present an age determination of the metal-rich globular cluster 47 Tucanae by fitting the properties of the cluster white dwarf population, which implies an absolute age of 9.9 (0.7) Gyr at 95% confidence. This is about 2.0 Gyr younger than inferred for the metal-poor cluster NGC 6397 from the same models, and provides quantitative evidence that metal-rich clusters like 47 Tucanae formed later than the metal-poor halo clusters like NGC 6397.Comment: Main Article: 10 pages, 4 figures; Supplementary Info 15 pages, 5 figures. Nature, Aug 1, 201

Dynamics of metal clusters in rare gas clusters

We investigate the dynamics of Na clusters embedded in Ar matrices. We use a hierarchical approach, accounting microscopically for the cluster's degrees of freedom and more coarsely for the matrix. The dynamical polarizability of the Ar atoms and the strong Pauli-repulsion exerted by the Ar-electrons are taken into account. We discuss the impact of the matrix on the cluster gross properties and on its optical response. We then consider a realistic case of irradiation by a moderately intense laser and discuss the impact of the matrix on the hindrance of the explosion, as well as a possible pump probe scenario for analyzing dynamical responses.Comment: Proceedings of the 30th International Workshop on Condensed Matter Theories, Dresden, June 05 - 10, 2006, World Scientific. 3 figure

Near-Infrared Properties of Metal-poor Globular Clusters in the Galactic Bulge Direction

Aims. J, H, and K' images obtained from the near-infrared imager CFHTIR on the Canada-France-Hawaii Telescope are used to derive the morphological parameters of the red giant branch (RGB) in the near-infrared color-magnitude diagrams for 12 metal-poor globular clusters in the Galactic bulge direction. Using the compiled data set of the RGB parameters for the observed 12 clusters, in addition to the previously studied 5 clusters, we discuss the properties of the RGB morphology for the clusters and compare them with the calibration relations for the metal-rich bulge clusters and the metal-poor halo clusters. Methods. The photometric RGB shape indices such as colors at fixed magnitudes of MK = MH = (-5.5, -5, -4, and -3), magnitudes at fixed colors of (J - K)o = (J - H)o = 0.7, and the RGB slope are measured from the fiducial normal points defined in the near- infrared color-magnitude diagrams for each cluster. The magnitudes of RGB bump and tip are also estimated from the differential and cumulative luminosity functions of the selected RGB stars. The derived RGB parameters have been used to examine the overall behaviors of the RGB morphology as a function of cluster metallicity. Results. The correlations between the near-infrared photometric RGB shape indices and the cluster metallicity for the programme clusters compare favorably with the previous observational calibration relations for metal-rich clusters in the Galactic bulge and the metal-poor halo clusters. The observed near-infrared magnitudes of the RGB bump and tip for the investigated clusters are also in accordance with the previous calibration relations for the Galactic bulge clusters.Comment: 12 pages, 9 figures, accepted for publication in Astronomy & Astrophysic

Extragalactic Globular Clusters in the Near Infrared I: A comparison between M87 and NGC 4478

We compare optical and near infrared colours of globular clusters in M87, the central giant elliptical in Virgo, and NGC 4478, an intermediate luminosity galaxy in Virgo, close in projection to M87. Combining V and I photometry obtained with the WFPC2 on HST and Ks photometry obtained with the NIRC on Keck 1, we find the broad range in colour and previously detected bi-modality in M87. We confirm that NGC 4478 only hosts a blue sub-population of globular clusters and now show that these clusters' V-I and V-K colours are very similar to those of the halo globular clusters in Milky Way and M31. Most likely, a metal-rich sub-population never formed around this galaxy (rather than having formed and been destroyed later), probably because its metal-rich gas was stripped during its passage through the centre of the Virgo cluster. The V-I, V-K colours are close to the predicted colours from SSP models for old populations. However, M87 hosts a few red clusters that are best explained by intermediate ages (a few Gyr). Generally, there is evidence that the red, metal-rich sub-population has a complex colour structure and is itself composed of clusters spanning a large metallicity and, potentially, age range. This contrasts with the blue, metal-poor population which appears very homogeneous in all galaxies observed so far.Comment: accepted in A&A, 13 pages using the A&A macr

Multivariate analysis of the globular clusters in M87

An objective classification of 147 globular clusters in the inner region of the giant elliptical galaxy M87 is carried out with the help of two methods of multivariate analysis. First independent component analysis is used to determine a set of independent variables that are linear combinations of various observed parameters (mostly Lick indices) of the globular clusters. Next K-means cluster analysis is applied on the independent components, to find the optimum number of homogeneous groups having an underlying structure. The properties of the four groups of globular clusters thus uncovered are used to explain the formation mechanism of the host galaxy. It is suggested that M87 formed in two successive phases. First a monolithic collapse, which gave rise to an inner group of metal-rich clusters with little systematic rotation and an outer group of metal-poor clusters in eccentric orbits. In a second phase, the galaxy accreted low-mass satellites in a dissipationless fashion, from the gas of which the two other groups of globular clusters formed. Evidence is given {\bf for a blue stellar population in the more metal rich clusters, which we interpret by Helium enrichment.} Finally, it is found that the clusters of M87 differ in some of their chemical properties (NaD, TiO1, light element abundances) from globular clusters in our Galaxy and M31.Comment: 19 pages, 10 figures,Accepted in Publications of The Astronomical Society of Australi

Image states in metal clusters

The existence of image states in small clusters is shown, using a quantum-mechanical many-body approach. We present image state energies and wave functions for spherical jellium clusters up to 186 atoms, calculated in the GW approximation, where G is the Green's function and W is the dynamically screened Coulomb interaction, which by construction contains the dynamic long-range correlation effects that give rise to image effects. In addition, we find that image states are also subject to quantum confinement. To extrapolate our investigations to clusters in the mesoscopic size range, we propose a semiclassical model potential, which we test against our full GW results

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

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. The g-I color distribution of the clusters is bimodal with peaks at 0.85 and 1.35. The redder peak at g-I=1.35 is consistent with the predicted color for an old metal-poor population. The clusters associated with the peak at g-I=0.85 are centrally concentrated and interpreted as a younger and more metal-rich population. We suggest that these clusters have an age of ~ 0.4 Gyr and solar metallicity based on a comparison with population synthesis models. The luminosity function of these "blue" clusters is well represented by a power law. Interestingly, the brightest shell associated with the galaxy harbors some of the youngest clusters observed. This seems to indicate that the same merger event was responsible for the formation of both the shells and the young clusters. The red part of the color distribution contains several very bright clusters, which are not expected for an old, metal-poor population. Furthermore, the luminosity function of the "red" GCs cannot be fit well by either a single gaussian or a single power law. A composite (gaussian + power law) fit to the LF of the red clusters yields both a low rms and very plausible properties for an old population plus an intermediate-age population of GCs. Hence, we suggest that the red clusters in AM 0139-655 consist of two distinct GC subpopulations, one being an old, metal-poor population as seen in normal galaxies and one having formed during a recent dissipative galaxy merger.Comment: 35 pages, 12 figures, accepted for publication in A