Duncan A. Forbes

On the Origin of Globular Clusters in Elliptical and cD Galaxies

Perhaps the most noteworthy of recent findings in extragalactic globular cluster (GC) research are the multimodal GC metallicity distributions seen in massive early–type galaxies. We explore the origin of these distinct GC populations, the implications for galaxy formation and evolution, and identify several new properties of GC systems. First, when we separate the metal–rich and metal–poor subpopulations, in galaxies with bimodal GC metallicity distributions, we find that the mean metallicity of the metal–rich GCs correlates well with parent galaxy luminosity but the mean metallicity of the metal–poor ones does not. This indicates that the metal–rich GCs are closely coupled to the galaxy and

A catalogue and analysis of X-ray luminosities of early-type galaxies

ABSTRA C T We present a catalogue of X-ray luminosities for 401 early-type galaxies, of which 136 are based on newly analysed ROSAT PSPC pointed observations. The remaining luminosities are taken from the literature and converted to a common energy band, spectral model and distance scale. Using this sample we fit the LX : LB relation for early-type galaxies and find a best-fit slope for the catalogue of ,2.2. We demonstrate the influence of group-dominant galaxies on the fit and present evidence that the relation is not well modelled by a single power-law fit. We also derive estimates of the contribution to galaxy X-ray luminosities from discrete-sources and conclude that they provide Ldscr /L B . 29:5 erg s 21 L 21 B( . We compare this result with luminosities from our catalogue. Lastly, we examine the influence of environment on galaxy X-ray luminosity and on the form of the LX : LB relation. We conclude that although environment undoubtedly affects the X-ray properties of individual galaxies, particularly those in the centres of groups and clusters, it does not change the nature of whole

Optical Rotation Curves of Distant Field Galaxies: Keck Results at Redshifts to z ~ 1

Spatially resolved velocity profiles are presented for nine faint field galaxies in the redshift range 0.1 z 1, based on moderate-resolution spectroscopy obtained with the Keck 10 m telescope. These data were augmented with high-resolution Hubble Space Telescope images from WFPC2, which provided V and I photometry, galaxy type, orientation, and inclination. The effects of seeing, slit width, and slit misalignment with respect to galaxy major axis were modeled along with inclination for each source, in order to derive a maximum circular velocity from the observed rotation curve. The lowest redshift galaxy, though highly elongated, shows a distorted low-amplitude rotation curve that suggests a merger in progress seen perpendicular to the collision path. The remaining rotation curves appear similar to those of local galaxies in both form and amplitude, implying that some massive disks were in place at z ~ 1. The key result is that the kinematics of these distant galaxies show evidence for only a modest increase in luminosity (ΔMB 0.6) compared to velocity-luminosity (Tully-Fisher) relations for local galaxies.

Keck Spectroscopy of Globular Clusters around NGC 1399

We report moderate-resolution, high signal-to-noise ratio spectroscopy of globular clusters around NGC 1399, the central cD galaxy in the Fornax Cluster. We address issues as diverse as elemental abundances of globular clusters versus stellar populations in elliptical galaxies, blue horizontal branches in metal-rich globular clusters, broadband colors as metallicity tracers, possible overestimation of the age-metallicity degeneracy in globular clusters, and dark matter in the halo of NGC 1399. We obtained spectra for 21 globular cluster candidates with multislit spectroscopy using the Low Resolution Imaging Spectrograph on the Keck I Telescope. Our sample turned out to include 18 globular clusters, one star, and two low-redshift late-type galaxies (z 0.3). The mean velocity of our globular cluster sample is 1293 ± 71 km s-1, and its velocity dispersion is 302 ± 51 km s-1. Both are slightly lower than, but in agreement with, previously derived values. We derive a mass of (1–5) × 1012 M☉ within 28 kpc for the galaxy, and an M/LB ratio of 36 ± 20 or 76 ± 40 M☉/L☉B, depending on the mass estimator. Both estimates indicate that dark matter dominates the potential at 6reff. The derived elemental abundances for the globular clusters span the entire range observed in the Milky Way and M31, with a mean metallicity of our sample around [Fe/H] -0.8 dex. This implies that the two major subpopulations reported from photometry could have formed by the same processes that formed halo and disk/bulge globular clusters in the Local Group spiral galaxies. Two globular clusters, which we associate with a group of very red globular clusters, representing about 5% of the total system, clearly stand out and exhibit metal abundances as high as those observed for stellar populations in giant elliptical galaxies. In addition, they display surprisingly high Hβ and Hγ indices that are not explained by any age/metallicity combination of existing models. The high Mg and Hβ values in these clusters could, however, be explained by the presence of blue horizontal branches. Finally, we find that V - I and metallicity are well correlated in the globular cluster system, but also that the slope of the relation is twice as flat at high metallicities as an extrapolation from the relation for Milky Way globular clusters. This implies that the mean metallicities of globular cluster systems in elliptical galaxies are lower, and cover a smaller range, than ones previously derived from broadband V - I colors.

ON THE RELATIONSHIP BETWEEN AGE AND DYNAMICS IN ELLIPTICAL GALAXIES

Here, using a sample of 88 galaxies, we examine the scatter about the Faber-Jackson (F-J) relation, and find that a galaxys position relative to this relation depends on its age. We attempt to reproduce the observed trend of the F-J residuals with age using two simple models. The first assumes that galaxy age is tracing the last major star formation event in an elliptical galaxy. The fading of this burst changes the M_B component of the F-J residuals, with time. Such a model was very successful at reproducing the B-V and Mg_2 evolution reported in our previous paper, but is unable to reproduce the strength of the F-J trend. A second model is required to describe age-correlated changes in galaxy dynamics. Following expectations from cosmological simulations, we assume that the velocity dispersion, for a galaxy of a given mass, scales with the epoch of galaxy formation, i.e. with the mean density of the Universe. Hence recently formed ellipticals have systematically lower velocity dispersions than old ellipticals. We find that a combination of these two models provides a good match to the change in F-J residuals with galaxy age. This suggests that young ellipticals will have subtly different dynamical properties to old ellipticals. We also find that there is not a strong relationship between a galaxys age and its luminosity for our sample. This suggests that the tilt of the fundamental plane is not totally driven by age.

HST imaging of the globular clusters in the Fornax Cluster: NGC 1399 and NGC 1404

A B STR A CT The Fornax cluster galaxies NGC 1399 and NGC 1404 are ideal for studying the effects of a cluster environment on globular cluster systems. Here we present new optical imaging of these two galaxies from both the Hubble Space Telescope’s Wide Field and Planetary Camera 2 and the Cerro Tololo Inter-American Observatory 1.5-m telescope. The combination of both data sets provides a unique insight on the spatial and colour distribution of globular clusters. From B—I colours, we find that both galaxies have a broad globular cluster metallicity distribution that is inconsistent with a single population. Two Gaussians provide a reasonable representation of the metallicity distribution in each galaxy. The metal-rich subpopulation is more centrally concentrated than the metal-poor one. We show that the radial metallicity gradient can be explained by the changing relative mix of the two globular cluster subpopulations. We derive globular cluster surface density profiles, and find that they are flatter (i.e., more extended) than the underlying starlight. The total number of globular clusters and specific frequency are calculated to be N\5700„500, SN\11.5„1.0 for NGC 1399, and N\725„145, SN\2.0„0.5 for NGC 1404. Our results are compared with the expectations of globular cluster formation scenarios.

HST Imaging of the Globular Clusters in the Formax Cluster: Color and Luminosity Distributions

We examine the luminosity and B-I color distribution of globular clusters for three early-type galaxies in the Fornax Cluster using imaging data from the Wide Field Planetary Camera 2 on the Hubble Space Telescope. The luminosity functions we derive are in most cases better than 50% complete down to B = 26.6. We find that the color distributions of globular clusters in the central region of NGC 1399 and its nearby neighbor NGC 1404 are bimodal and statistically indistinguishable. The metallicity spread, as inferred from the color distributions in these two galaxies, is very similar to that of M87. NGC 1399s luminosity function is also very similar to that of M87, and comparing their respective peak magnitudes indicates that the Fornax Cluster is at very nearly the same distance from the Local Group as is the Virgo Cluster. From this we derive H0 = 82 ± 8 km s-1 Mpc-1, where the uncertainty reflects only the effects of random errors. The number of unresolved objects we find at a projected distance of 440 kpc from NGC 1399 is consistent with nothing more than compact background galaxies, though the small field of view of the WFPC2 does not allow us to put strong constraints on the number of intergalactic globular clusters. The luminosity function of objects detected around NGC 1316 is more nearly exponential than lognormal, and both the color and size distribution of these objects distinguish them from the clusters surrounding NGC 1399. We suggest that these objects are more akin to old open clusters in the Galaxy than they are to globular clusters in typical early-type galaxies.

The X-ray emission in post-merger ellipticals

The evolution in X–ray properties of early–type galaxies is largely unconstrained. In particular, little is known about how, and if, remnants of mergers generate hot gas halos. Here we examine the relationship between X–ray luminosity and galaxy age for a sample of early–type galaxies. Comparing normalized X–ray luminosity to three different age indicators we find that LX/LB increases with age, suggesting an increase in X–ray halo mass with time after a galaxy’s last major star-formation episode. The long-term nature of this trend, which appears to continue across the full age range of our sample, poses a challenge for many models of hot halo formation. We conclude that models involving a declining rate of type Ia supernovae, and a transition from outflow to inflow of the gas originally lost by galactic stars, offers the most promising explanation for the observed evolution in X-ray luminosity.

Cold gas and star formation in a merging galaxy sequence

We explore the evolution of the cold gas (molecular and neutral hydrogen) and star formation activity during galaxy interactions, using a merging galaxy sequence comprising both pre- and post-merger candidates. Data for this study come from the literature, but are supplemented by some new radio observations presented here. First, we confirm that the ratio of far-infrared luminosity to molecular hydrogen mass (LFIRM(H2); star formation efficiency) increases close to nuclear coalescence. After the merging of the two nuclei there is evidence that the star formation efficiency declines again to values typical of ellipticals. This trend can be attributed to M(H2) depletion arising from interaction induced star formation. However, there is significant scatter, likely to arise from differences in the interaction details (e.g., disc-to-bulge ratio, geometry) of individual systems. Secondly, we find that the central molecular hydrogen surface density, ΣH2, increases close to the final stages of the merging of the two nuclei. Such a trend, indicating gas inflows caused by gravitational instabilities during the interaction, is also predicted by numerical simulations. Furthermore, there is evidence for a decreasing fraction of cold gas mass from early interacting systems to merger remnants, attributed to neutral hydrogen conversion into other forms (e.g., stars, hot gas) and molecular hydrogen depletion resulting from ongoing star formation. The evolution of the total-radio to blue-band luminosity ratio, reflecting the total (disc and nucleus) star formation activity, is also investigated. Although this ratio is on average higher than that for isolated spirals, we find a marginal increase along the merging sequence, attributed to the relative insensitivity of disc star formation to interactions. However, a similar result is also obtained for the nuclear radio emission, although galaxy interactions are believed to significantly affect the activity (star formation, AGN) in the central galaxy regions. Nevertheless, the nuclear-radio to blue-band luminosity ratio is significantly elevated compared with that for isolated spirals. Finally, we find that the FIR–radio flux ratio distribution of interacting galaxies is consistent with star formation being the main energizing source.

Galaxy Disruption in a Halo of Dark Matter

The relics of disrupted satellite galaxies have been found around the Milky Way and Andromeda, but direct evidence of a satellite galaxy in the early stages of disruption has remained elusive. We have discovered a dwarf satellite galaxy in the process of being torn apart by gravitational tidal forces as it merges with a larger galaxys dark matter halo. Our results illustrate the morphological transformation of dwarf galaxies by tidal interaction and the continued buildup of galaxy halos.