Bulge Globular Clusters in Spiral Galaxies

There is now strong evidence that the metal-rich globular clusters (GC) near the center of our Galaxy are associated with the Galactic bulge rather than the disk as previously thought. Here we extend the concept of bulge GCs to the GC systems of nearby spiral galaxies. In particular, the kinematic and metallicity properties of the GC systems favor a bulge rather than a disk origin. The number of metal-rich GCs normalized by the bulge luminosity is roughly constant (i.e. bulge S_N ~ 1) in nearby spirals, and this value is similar to that for field ellipticals when only the red (metal--rich) GCs are considered. We argue that the metallicity distributions of GCs in spiral and elliptical galaxies are remarkably similar, and that they obey the same correlation of mean GC metallicity with host galaxy mass. We further suggest that the metal-rich GCs in spirals are the direct analogs of the red GCs seen in ellipticals. The formation of a bulge/spheroidal stellar system is accompanied by the formation of metal-rich GCs. The similarities between GC systems in spiral and elliptical galaxies appear to be greater than the differences.Comment: 5 pages, Latex, 2 figures, 1 table, Accepted for publication in ApJ Letter

Revisiting the Globular Cluster System of the Merger Remnant Elliptical NGC 3610

We have obtained Keck spectra of six candidate globular clusters (GCs) in the dynamically young elliptical galaxy NGC 3610, supplementing our previous Keck spectra of eight confirmed GCs (Strader et al. 2003). Five of our new candidates are confirmed to be GCs. Of the thirteen GCs, eleven are located within the K band effective radius of 7 kpc. Two of these thirteen clusters are found to be young (~ 2 Gyr) and very metal-rich ([Z/H] ~ +0.5), three are old and metal-poor, and the remaining eight clusters are old and metal-rich. The ages of the young clusters are consistent with a recent spectroscopic age estimate of 1.6+/-0.5 Gyr for the galaxy itself (Denicolo et al. 2003) and suggest that these clusters formed in the disk-disk merger which likely created NGC 3610. Intriguingly, both young GCs have [alpha/Fe] ~ +0.3, while the majority of the old clusters are not alpha-enhanced, in contrast to Galactic and M31 GCs, and contrary to predictions of nucleosynthetic calculations. The two old subpopulations of GCs can be attributed to the merger progenitors. The relative numbers of old and new metal-rich GCs are poorly constrained because of the expected differences in radial distributions of the two subpopulations. However, based on our spectroscopic results and a comparison of the Hubble Space Telescope color magnitude diagram (Whitmore et al. 2002) with stellar population models, we argue that more than half of the metal-rich GCs are likely to be old.Comment: 21 pages, 6 figures, accepted for Jan 2004 A

Damp Mergers: Recent Gaseous Mergers without Significant Globular Cluster Formation?

Here we test the idea that new globular clusters (GCs) are formed in the same gaseous ("wet") mergers or interactions that give rise to the young stellar populations seen in the central regions of many early-type galaxies. We compare mean GC colors with the age of the central galaxy starburst. The red GC subpopulation reveals remarkably constant mean colors independent of galaxy age. A scenario in which the red GC subpopulation is a combination of old and new GCs (formed in the same event as the central galaxy starburst) can not be ruled out; although this would require an age-metallicity relation for the newly formed GCs that is steeper than the Galactic relation. However, the data are also well described by a scenario in which most red GCs are old, and few, if any, are formed in recent gaseous mergers. This is consistent with the old ages inferred from some spectroscopic studies of GCs in external systems. The event that induced the central galaxy starburst may have therefore involved insufficient gas mass for significant GC formation. We term such gas-poor events "damp" mergers.Comment: 17 pages, 5 figures, ApJ accepte

Ellipticals with Kinematically--Distinct Cores: WFPC2 Imaging of Globular Clusters

Here we present HST WFPC2 imaging of 14 kinematically--distinct core ellipticals to examine their globular cluster systems. In particular, we probe the galaxy central regions, for which we might expect to see the strongest signatures of some formation and destruction processes. These data substantially increase the number of extragalactic globular cluster systems studied to date. We find that the globular cluster luminosity, and color, vary only weakly, if at all, with galactocentric distance. The mean color of globular clusters are constant with globular cluster magnitude. Several clear trends are also present. Firstly, globular cluster colors are bluer (more metal poor by ~ 0.5 dex) than the underlying galaxy starlight at any given galactocentric distance. Second, we find a correlation over roughly ten magnitudes between the mean globular cluster metallicity and parent galaxy luminosity of the form Z goes as L^0.4. This relationship includes dwarf ellipticals, spiral galaxy bulges and giant ellipticals. Third, we find that globular cluster surface density distribution can be described by a core model, for which the core radius correlates with galaxy luminosity. Our results favor scenarios in which ellipticals form from massive, gas rich progenitors at early epochs. Detailed simulations of the formation of globular cluster systems would be valuable to draw firmer conclusions.Comment: text only is given here, 13 figures are available at http://www.ucolick.org/~forbes/home.html Paper accepted for publication in the Astrophysical Journa

Metal-Poor Globular Clusters and Galaxy Formation

We demonstrate a significant (> 5-sigma) correlation between the mean color of metal-poor globular cluster (GC) systems and parent galaxy luminosity. A Bayesian Markov Chain Monte Carlo method is introduced to find the mean color, and is easily generalizable to quantify multimodality in other astronomical datasets. We derive a GC color--galaxy luminosity relation of the form Z ~ L^ (0.15 +/- 0.03). When combined with evidence against a single primordial GC metallicity--galaxy luminosity relation for protogalactic fragments, the existence of such a correlation is evidence against both accretion and major merger scenarios as an explanation of the entire metal-poor GC systems of luminous galaxies. However, our relation arises naturally in an in situ picture of GC formation, and is consistent with the truncation of metal-poor GC formation by reionization. A further implication is that the ages of metal-poor GCs in dwarf galaxies constrain the main epoch of galaxy formation in hierarchical models. If the ages of old metal-poor GCs in Local Group dwarfs (> 11 Gyr) are typical of those in dwarfs elsewhere, then the bulk of galaxy assembly (at least in clusters and groups) must have occurred at z > 2.5, contrary to the predictions of some structure formation models.Comment: 13 pages, 1 figure, accepted for A

Keck Spectroscopy and HST Imaging of Field Galaxies at Moderate Redshift

We present 18 spectra, obtained with the Keck 10m telescope, of faint field galaxies (19 $<$ I $<$ 22, 0.2 $<$ z $<$ 0.84) previously imaged by {\it HST's} WFPC2. Though small, our sample appears to be representative of field spirals with a magnitude--limit of I $\le$ 22. Combining the results from the spectral and imaging data, we have derived various quantitative parameters for the galaxies, including colors, inclinations, emission line equivalent widths, redshifts, luminosities, internal velocity information and physical scale lengths. In particular, disk scale lengths (with sizes ranging from $\sim$1--5 kpc) have been measured from fits to the surface brightness profiles. We have also measured internal velocities with a rest frame resolution of $\sigma$ = 55 to 80 km s$^{-1}$ by fitting to the emission lines. The luminosity--disk size and luminosity--internal velocity (Tully--Fisher) relations for our moderate redshift galaxies are similar to the scaling relations seen for local galaxies, albeit with a modest brightening of $\sim$1 magnitude. The one bulge--dominated galaxy in our sample (at z = 0.324) has a relatively blue color, reveals weak emission lines and is $\sim$0.5 magnitude brighter in the rest frame than expected for a passive local elliptical. Our data suggest that galaxies at about half the age of the Universe have undergone mild luminosity evolution to the present epoch, but are otherwise quantitatively similar to galaxies seen locally.Comment: replaced short version of paper. 31 pages, figures and tables available at http://www.ucolick.org/~forbes/home.htm

Extragalactic Globular Clusters: Old Spectroscopic Ages and New Views on Their Formation

We present the results of a meta-analysis of Keck spectra of extragalactic globular clusters (GCs) in a sample of eight galaxies, ranging from dwarfs to massive ellipticals. We infer ages for the metal-poor and metal-rich GCs in these galaxies through comparisons to Galactic GCs. Both subpopulations appear to be no younger than their Galactic counterparts, with ages ~> 10 Gyr. This is the largest sample of galaxies for which ages have been constrained spectroscopically. Our results support the formation of most GCs in massive galaxies at high redshift. We propose a scenario for the formation of GC subpopulations that synthesizes aspects of both accretion and in situ approaches in the context of galaxy formation through hierarchical merging.Comment: 21 pages, 1 figure. Accepted to A

Stellar populations of Globular Clusters in the Elliptical galaxy NGC1407

We present high-quality, Keck spectroscopic data for a sample of 20 globular clusters (GCs) in the massive E0 galaxy NGC1407. A subset of twenty line-strength indices of the Lick/IDS system have been measured for both the GC system and the central integrated star-light of the galaxy. Ages, metallicities and [alpha/Fe] ratios have been derived using several different approaches. The majority GCs in NGC1407 studied are old, follow a tight metallicity sequence reaching values slightly above solar, and exhibit mean [alpha/Fe] ratios of ~ 0.3 dex. In addition, three GCs are formally derived to be young (~ 4 Gyr), but we argue that they are actually old GCs hosting blue horizontal branches. We report, for the first time, evidence for the existence of two chemically-distinct subpopulations of metal-rich (MR) GCs. We find some MR GCs exhibit significantly larger [Mg/Fe] and [C/Fe] ratios. Different star formation time-scales are proposed to explain the correlation between Mg and C abundances. We also find striking CN overabundances over the entire GC metallicity range. Interestingly, the behavior of C and N in metal-poor (MP) GCs clearly deviates from the one in MR GCs. In particular, for MR GCs, N increases dramatically while C essentially saturates. This may be interpreted as a consequence of the increasing importance of the CNO cycle with increasing metallicity.Comment: 53 pages, 11 figures, 5 tables. Accepted for publication in The Astronomical Journa

The early-type galaxies NGC 1407 and NGC 1400 - II: star formation and chemical evolutionary history

We present a possible star formation and chemical evolutionary history for two early-type galaxies NGC 1407 and NGC 1400. They are the two brightest galaxies of the NGC 1407 (or Eridanus-A) group, one of the 60 groups studied as part of the Group Evolution Multi-wavelength Study (GEMS). Our analysis is based on new high signal-to-noise spatially resolved integrated spectra obtained at the ESO 3.6m telescope, out to 0.6 (NGC 1407) and 1.3 (NGC 1400) effective radii. Using Lick/IDS indices we estimate luminosity-weighted ages, metallicities and $\alpha$-element abundance ratios. Colour radial distributions from HST/ACS and Subaru Suprime-Cam multi-band wide-field imaging are compared to colours predicted from spectroscopically determinated ages and metallicities using single stellar population models. The galaxies formed over half of their mass in a single short-lived burst of star formation (> 100 M(sun)/year) at redshift z>5. This likely involved an outside-in mechanism with supernova-driven galactic winds, as suggested by the flatness of the alpha-element radial profiles and the strong negative metallicity gradients. Our results support the predictions of the revised version of the monolithic collapse model for galaxy formation and evolution. We speculate that, since formation the galaxies have evolved quiescently and that we are witnessing the first infall of NGC 1400 in the group.Comment: 14 pages, 9 tables, 6 figures, Accepted for publication in MNRA

The early-type galaxies NGC 1407 and NGC 1400 - I: spatially resolved radial kinematics and surface photometry

This is the first paper of a series focused on investigating the star formation and evolutionary history of the two early-type galaxies NGC 1407 and NGC 1400. They are the two brightest galaxies of the NGC 1407 (or Eridanus-A) group, one of the 60 groups studied as part of the Group Evolution Multi-wavelength Study (GEMS). Here we present new high signal-to-noise long-slit spectroscopic data obtained at the ESO 3.6m telescope and high-resolution multi-band imaging data from the HST/ACS and wide-field imaging from Subaru Suprime-Cam. We spatially resolved integrated spectra out to 0.6 (NGC 1407) and 1.3 (NGC 1400) effective radii. The radial profiles of the kinematic parameters v(rot), sigma, h3 and h4 are measured. The surface brightness profiles are fitted to different galaxy light models and the colour distributions analysed. The multi-band images are modelled to derive isophotal shape parameters and residual galaxy images. The parameters from the surface brightness profile fitting are used to estimate the mass of the possible central supermassive black hole in NGC 1407. The galaxies are found to be rotationally supported and to have a flat core in the surface brightness profiles. Elliptical isophotes are observed at all radii and no fine structures are detected in the residual galaxy images. From our results we can also discard a possible interaction between NGC 1400, NGC 1407 and the group intergalactic medium. We estimate a mass of 1.03x10^9 M(sun) for the supermassive black hole in NGC 1407 galaxy.Comment: 11 pages, 6 tables, 6 figures, Accepted for publication in MNRA