Alpha-element Abundance Ratios Research Articles

Non-solar abundance ratios trends of dEs in the Fornax Cluster using newly defined high-resolution indices

ABSTRACT We perform a detailed study of the stellar populations in a sample of massive Fornax dwarf galaxies using a set of newly defined line indices. Using data from the Integral Field Spectroscopic data, we study abundance ratios of eight dEs with stellar mass ranging from 108 to 109.5 M⊙ in the Fornax Cluster. We present the definitions of a new set of high-resolution Lick-style indices to be used for stellar population studies of unresolved small stellar systems. We identify 23 absorption features and continuum regions, mainly dominated by 12 elements (Na, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Y, Ba, and Nd) in the wavelength range 4700–5400 Å and characterize them as a function of age, metallicity, and alpha element abundance ratios. We analyse eight dEs and interpret the line strengths, measured in our new high-resolution system of indices, with the aid of stellar population models with high enough spectral resolution. We obtain abundance ratio proxies for a number of elements that have never been studied before for dwarf ellipticals outside the Local Group. These proxies represent relative deviations from predicted index strengths of base stellar population models built-up following the abundance pattern of The Galaxy. The abundance proxy trend results are compared to abundance ratios from resolved stars in the Local Group, and indices from integrated light of larger early-type galaxies. We find that all our dwarfs show a pattern of abundance ratios consistent with the disc of the Milky Way, indicative of slow formation in comparison to their high-mass counterparts.

CHEMICAL ABUNDANCES OF PLANETARY NEBULAE IN THE SUBSTRUCTURES OF M31

We present deep spectroscopy of planetary nebulae (PNe) that are associated with the substructures of the Andromeda Galaxy (M31). The spectra were obtained with the OSIRIS spectrograph on the 10.4 m GTC. Seven targets were selected for the observations, three in the Northern Spur and four associated with the Giant Stream. The most distant target in our sample, with a rectified galactocentric distance >100 kpc, was the first PN discovered in the outer streams of M31. The [O III] 4363 auroral line was well detected in the spectra of all targets, enabling electron temperature determination. Ionic abundances are derived based on the [O III] temperatures, and elemental abundances of helium, nitrogen, oxygen, neon, sulfur, and argon are estimated. The relatively low N/O and He/H ratios as well as abundance ratios of alpha-elements indicate that our target PNe might belong to populations as old as ~2 Gyr. Our PN sample, including the current seven and the previous three observed by Fang et al., have rather homogeneous oxygen abundances. The study of abundances and the spatial and kinematical properties of our sample leads to the tempting conclusion that their progenitors might belong to the same stellar population, which hints at a possibility that the Northern Spur and the Giant Stream have the same origin. This may be explained by the stellar orbit proposed by Merrett et al. Judging from the position and kinematics, we emphasize that M32 might be responsible for the two substructures. Deep spectroscopy of PNe in M32 will help to assess this hypothesis.

Spatially resolved stellar population parameters in the BCGs of two fossil groups

We report the results of Gemini/GMOS long-slit spectroscopic observations along the major and minor axes of the central galaxies in two fossil groups, SDSS J073422.21+265133.9 and SDSS J075828.11+374711.8 (the NGC 2484 group). Spatially resolved kinematics and stellar population parameters (ages, metallicities and $\alpha$-element abundance ratios) derived using ~20 Lick indices are presented. Despite remarkable similarities in their morphologies, photometric properties (luminosity and colour) and kinematics, the two galaxies exhibit significantly different stellar population parameters. SDSS J073422.21+265133.9 exhibits a strong metallicity gradient (Delta[Z/H]/Delta R ~ -0.4) all the way into the centre of the galaxy. It also exhibits an age profile that suggest a relatively recent, centrally concentrated burst of star formation superimposed on an older, more spatially extended population. NGC 2484, a well known X-ray AGN, exhibits a flat core-like structure in its metallicity gradient, but no detectable age gradient. The alpha-element abundance ratio ([E/Fe]) profiles of the two galaxies are also significantly different. SDSS J073422.21+265133.9 exhibits a slightly positive gradient (Delta [E/H]/Delta R ~ 0.1), perhaps again suggesting a more recent central burst of star formation, while NGC 2484 shows a negative gradient (Delta [E/H]/Delta R ~-0.1), indicating that star formation may have happened "inside out". Our analysis of these two galaxies of similar mass, morphology and kinematics therefore suggests two different mechanisms to have been in action during their formation. Consequently, we conclude that the central galaxies of fossil groups can not be considered a homogeneous group with regard to their formation processes or star formation histories.

Chemical abundance analysis of the old, rich open cluster Trumpler 20

Trumpler 20 is an open cluster located at low Galactic longitude, just beyond the great Carina spiral arm, and whose metallicity and fundamental parameters were very poorly known until now. As it is most likely a rare example of an old, rich open cluster -- possibly a twin of NGC 7789 -- it is useful to characterize it. To this end, we determine here the abundance of several elements and their ratios in a sample of stars in the clump of Trumpler 20. The primary goal is to measure Trumpler 20 metallicity, so far very poorly constrained, and revise the cluster's fundamental parameters. We present high-resolution spectroscopy of eight clump stars. Based on their radial velocities, we identify six bona fide cluster members, and for five of them (the sixth being a fast rotator) we perform a detailed abundance analysis. We find that Trumpler 20 is slightly more metal-rich than the Sun, having [Fe/H]=+0.09$\pm$0.10. The abundance ratios of alpha-elements are generally solar. In line with recent studies of clusters as old as Trumpler 20, Ba is overabundant compared to the Sun. Our analysis of the iron-peak elements (Cr and Ni) does not reveal anything anomalous. Based on these results, we re-estimate the cluster age to be 1.5$^{+0.2}_{-0.1}$ Gyr. Its distance to the Galactic centre turns out to be 7.3 kpc. With this distance and metallicity, Trumpler 20 fits fairly well in the metallicity gradient for the galactic inner disc. With this new study, the characterization of Trumpler~20 is now on much more solid ground. Further studies should focus on the estimate of the binary fraction and on its main sequence membership.

SPECTROSCOPIC OBSERVATIONS OF PLANETARY NEBULAE IN THE NORTHERN SPUR OF M31

We present spectroscopy of three planetary nebulae (PNe) in the Northern Spur of the Andromeda Galaxy (M31) obtained with the Double Spectrograph on the 5.1 m Hale Telescope at the Palomar Observatory. The samples are selected from the observations of Merrett et al. Our purpose is to investigate formation of the substructures of M31 using PNe as a tracer of chemical abundances. The [O III] 4363 auroral line is detected in the spectra of two objects, enabling temperature determinations. Ionic abundances are derived from the observed collisionally excited lines, and elemental abundances of nitrogen, oxygen, and neon as well as sulphur and argon are estimated. Correlations between oxygen and the alpha-element abundance ratios are studied, using our sample and the M31 disk and bulge PNe from the literature. In one of the three PNe, we observed relatively higher oxygen abundance compared to the disk sample in M31 at similar galactocentric distances. The results of at least one of the three Northern Spur PNe might be in line with the proposed possible origin of the Northern Spur substructure of M31, i.e. the Northern Spur is connected to the Southern Stream and both substructures comprise the tidal debris of the satellite galaxies of M31.

A SHORT SCALE LENGTH FOR THE α-ENHANCED THICK DISK OF THE MILKY WAY: EVIDENCE FROM LOW-LATITUDE SEGUE DATA

We examine the \alpha-element abundance ratio, [\alpha/Fe], of 5620 stars, observed by the Sloan Extension for Galactic Understanding and Exploration survey in the region 6 kpc < R < 16 kpc, 0.15 kpc < |Z| < 1.5 kpc, as a function of Galactocentric radius R and distance from the Galactic plane |Z|. Our results show that the high-\alpha\ thick disk population has a short scale length (L_thick ~ 1.8 kpc) compared to the low-\alpha population, which is typically associated with the thin disk. We find that the fraction of high-\alpha\ stars in the inner disk increases at large |Z|, and that high-\alpha\ stars lag in rotation compared to low-\alpha\ stars. In contrast, the fraction of high-\alpha\ stars in the outer disk is low at all |Z|, and high- and low-\alpha\ stars have similar rotational velocities up to 1.5 kpc from the plane. We interpret these results to indicate that different processes were responsible for the high-\alpha\ populations in the inner and outer disk. The high-\alpha\ population in the inner disk has a short scale length and large scale height, consistent with a scenario in which the thick disk forms during an early gas-rich accretion phase. Stars far from the plane in the outer disk may have reached their current locations through heating by minor mergers. The lack of high-\alpha\ stars at large R and |Z| also places strict constraints on the strength of radial migration via transient spiral structure.

The stellar populations of early-type galaxies - II. The effects of environment and mass

The degree of influence that environment and mass have on the stellar populations of early-type galaxies is uncertain. In this paper we present the results of a spectroscopic analysis of the stellar populations of early-type galaxies aimed at addressing this question. The sample of galaxies is drawn from four clusters, with <z>=0.04, and their surrounding structure extending to ~10R_{vir}. We find that the distributions of the absorption-line strengths and the stellar population parameters age, metallicity and alpha-element abundance ratio do not differ significantly between the clusters and their outskirts, but the tight correlations found between these quantities and velocity dispersion within the clusters are weaker in their outskirts. All three stellar population parameters of cluster galaxies are positively correlated with velocity dispersion. Galaxies in clusters form a homogeneous class of objects that have similar distributions of line-strengths and stellar population parameters, and follow similar scaling relations regardless of cluster richness or morphology. We estimate the intrinsic scatter of the Gaussian distribution of metallicities to be 0.3 dex, while that of the alpha-element abundance ratio is 0.07 dex. The e-folding time of the exponential distribution of galaxy ages is estimated to be 900 Myr. The intrinsic scatters of the metallicity and alpha-element abundance ratio distributions can almost entirely be accounted for by the correlations with velocity dispersion and the intrinsic scatter about these relations. This implies that a galaxies mass plays the major role in determining its stellar population.

The stellar populations of early-type galaxies - I. Observations, line strengths and stellar population parameters

The influence of environment on the formation and evolution of early-type galaxies is, as yet, an unresolved issue. Constraints can be placed on models of early-type galaxy formation and evolution by examining their stellar populations as a function of environment. We present a catalogue of galaxies well suited to such an investigation. The magnitude-limited (b_J<19.45) sample was drawn from four clusters (Coma, A1139, A3558, and A930 at <z>=0.04) and their surrounds. The catalogue contains luminosities, redshifts, velocity dispersions and Lick line strengths for 416 galaxies, of which 245 are classified as early-types. Luminosity-weighted ages, metallicities, and alpha-element abundance ratios have been estimated for 219 of these early-types. We also outline the steps necessary for measuring fully-calibrated Lick indices and estimating the associated stellar population parameters using up-to-date methods and stellar population models. In a subsequent paper we perform a detailed study of the stellar populations of early-type galaxies in clusters and investigate the effects of environment.

The age-redshift relation for luminous red galaxies in the Sloan Digital Sky Survey

We present a detailed analysis of 17,852 quiescent, Luminous Red Galaxies (LRGs) selected from Sloan Digital Sky Survey (SDSS) Data Release Seven (DR7) spanning a redshift range of 0.0 < z < 0.4. These galaxies are co-added into four equal bins of velocity dispersion and luminosity to produce high signal-to-noise spectra (>100A^{-1}), thus facilitating accurate measurements of the standard Lick absorption-line indices. In particular, we have carefully corrected and calibrated these indices onto the commonly used Lick/IDS system, thus allowing us to compare these data with other measurements in the literature, and derive realistic ages, metallicities ([Z/H]) and alpha-element abundance ratios ([alpha/Fe]) for these galaxies using Simple Stellar Population (SSP) models. We use these data to study the relationship of these galaxy parameters with redshift, and find little evidence for evolution in metallicity or alpha-elements (especially for our intermediate mass samples). This demonstrates that our subsamples are consistent with pure passive evolving (i.e. no chemical evolution) and represent a homogeneous population over this redshift range. We also present the age-redshift relation for these LRGs and clearly see a decrease in their age with redshift (5 Gyrs over the redshift range studied here) which is fully consistent with the cosmological lookback times in a concordance Lambda CDM universe. We also see that our most massive sample of LRGs is the youngest compared to the lower mass galaxies. We provide these data now to help future cosmological and galaxy evolution studies of LRGs, and provide in the appendices of this paper the required methodology and information to calibrate SDSS spectra onto the Lick/IDS system.

Stellar populations of Virgo cluster early-type dwarf galaxies with and without discs: a dichotomy in age?

[Abridged] Using VLT/FORS2 spectroscopy, we have studied the properties of the central stellar populations of a sample of 38 nucleated early-type dwarf (dE) galaxies in the Virgo Cluster. We find that these galaxies do not exhibit the same average stellar population characteristics for different morphological subclasses. The nucleated galaxies without discs are older and more metal poor than the dEs with discs . The alpha-element abundance ratio appears consistent with the solar value for both morphological types. Besides a well-defined relation of metallicity and luminosity, we also find a clear anti-correlation between age and luminosity. More specifically, there appears to be a bimodality: brighter galaxies, including the discy ones, exhibit significantly younger ages than fainter dEs. Therefore, it appears less likely that fainter and brighter dEs have experienced the same evolutionary history, as the well-established trend of decreasing average stellar age when going from the most luminous ellipticals towards low-luminosity Es and bright dEs is broken here. The older and more metal-poor dEs could have had an early termination of star formation activity, possibly being "primordial" galaxies in the sense that they have formed along with the protocluster or experienced very early infall. By contrast, the younger and relatively metal-rich brighter dEs, most of which have discs, might have undergone structural transformation of infalling disc galaxies.

THE AGES, METALLICITIES, AND ALPHA ELEMENT ENHANCEMENTS OF GLOBULAR CLUSTERS IN THE ELLIPTICAL NGC 5128: A HOMOGENEOUS SPECTROSCOPIC STUDY WITH GEMINI/GEMINI MULTI-OBJECT SPECTROGRAPH

We present new integrated light spectroscopy of globular clusters (GCs) in NGC 5128, a nearby giant elliptical galaxy less than 4 Mpc away, in order to measure radial velocities and derive ages, metallicities, and alpha-element abundance ratios. Using the Gemini South 8 meter telescope with the instrument Gemini Multi-Object Spectrograph, we obtained spectroscopy in the range of ∼3400–5700 Å for 72 GCs with a signal-to-noise ratio greater than 30 Å−1; and we have also discovered 35 new GCs within NGC 5128 from our radial velocity measurements. We measured and compared the Lick indices from HδA through Fe5406 with the single stellar population models of Thomas et al. in order to derive age, metallicity, and [α/Fe] values. We also measure Lick indices for 41 Milky Way GCs from Puzia et al. and Schiavon et al. with the same methodology for direct comparison. Our results show that 68% of the NGC 5128 GCs have old ages (>8 Gyr), 14% have intermediate ages (5–8 Gyr), and 18% have young ages (<5 Gyr). However, when we look at the metallicity of the GCs as a function of age, we find 92% of metal-poor GCs and 56% of metal-rich GCs in NGC 5128 have ages >8 Gyr, indicating that the majority of both metallicity subpopulations of GCs formed earlier, with a significant population of young and metal-rich GCs forming later. Our metallicity distribution function generated directly from spectroscopic Lick indices is clearly bimodal, as is the color distribution of the same set of GCs. Thus, the metallicity bimodality is real and not an artifact of the color to metallicity conversion. However, the metallicity distribution function obtained from comparison with the single stellar population models is consistent with a unimodal, bimodal, or multimodal shape. The [α/Fe] values are supersolar with a mean value of 0.14 ± 0.04, indicating a fast formation timescale. However, the GCs in NGC 5128 are not as [α/Fe] enhanced as the Milky Way GCs also examined in this study. Our measured indices also indicate that the GCs in NGC 5128 may have a slight overabundance in nitrogen and a wider range of calcium strength compared to the Milky Way GCs. Our results support a rapid, early formation of the GC system in NGC 5128, with subsequent major accretion and/or GC and star-forming events in more recent times.

Ages and metallicities for quiescent galaxies in the Shapley supercluster: driving parameters of the stellar populations

We use high signal-to-noise spectroscopy for a sample of 232 quiescent galaxies in the Shapley Supercluster, to investigate how their stellar populations depend on velocity dispersion, luminosity and stellar mass. The sample spans a large range in velocity dispersion (sigma from 30-300 km/s) and in luminosity (M_R from -18.7 to -23.2). Estimates of age, total metallicity (Z/H) and alpha-element abundance ratio (a/Fe) were derived from absorption-line analysis, using single-burst models. Age, Z/H and a/Fe are all correlated positively with velocity dispersion, but we also find significant residual trends with luminosity: at given sigma, the brighter galaxies are younger, less alpha-enriched, and have higher Z/H. At face value, these results might suggest that the stellar populations depend on stellar mass as well as on velocity dispersion. However, we show that the observed trends can be reproduced by models in which the stellar populations depend systematically only on sigma, and are independent of stellar mass Mstel. For age, the observed luminosity correlation arises because young galaxies are brighter, at fixed Mstel. For metallicity, the observed luminosity dependence arises because metal-rich galaxies, at fixed mass, tend also to be younger, and hence brighter. We find a good match to the observed luminosity correlations with Age ~ sigma^0.40, Z/H ~ sigma^0.35, a/Fe ~ sigma^0.20, where the slopes are close to those found when fitting traditional scaling relations. We conclude that the star formation and enrichment histories of galaxies are determined primarily by the depth of their gravitational potential wells. The observed residual correlations with luminosity do not imply a corresponding dependence on stellar mass.

Globular cluster content and evolutionary history of NGC 147

Context. Globular clusters are representative of the oldest stellar populations. It is thus essential to have a complete census of these systems in dwarf galaxies, from which more massive galaxies are progressively formed in the hierarchical scenario. Aims. We present the results of spectroscopic observations of eight globular cluster candidates in NGC 147, a satellite dwarf elliptical galaxy of M 31. Our goal is to make a complete inventory of the globular cluster system of this galaxy, determine the properties of their stellar populations, and compare these properties with those of systems of globular clusters in other dwarf galaxies. Methods. The candidates were identified on Canada-France-Hawaii telescope photographic plates. Medium resolution spectra were obtained with the SCORPIO spectrograph at the prime focus of the 6 m telescope of the Russian Academy of Sciences. They were analyzed using predictions of stellar population synthesis models. Results. We were able to confirm the nature of all eight candidates, three of which (GC5, GC7, and GC10) are indeed globular clusters, and to estimate evolutionary parameters for the two brightest ones and for Hodge II. The bright clusters GC5 and GC7 appear to have metallicities ([Z/H] ∼− 1.5 ÷− 1.8) that are lower than the oldest stars in the galaxy. The fainter GC Hodge II has a metallicity [Z/H] = −1.1, similar to that of the oldest stars in the galaxy. The clusters GC5 and GC7 have low alpha-element abundance ratios. The mean age of the globular clusters in NGC 147 is 9 ± 1 Gyr. We also measured the radial velocities of Hodge II and IV, and derived a mass of NGC 147 in good agreement with the value from the literature. The frequency, S n = 6.4, and mass fraction, T = 14 of globular clusters in NGC 147 appear to be higher than those for NGC 185 and 205. Conclusions. Our results indicate that the bright clusters GC5, GC7, and Hodge III formed in the main star-forming period ∼8−10 Gyr ago, while the fainter clusters Hodge I and II formed together with the second generation of field stars.

A deep AAOmega survey of low-luminosity galaxies in the Shapley supercluster: stellar population trends

We present new optical spectroscopy for 342 R<18 galaxies in the Shapley Supercluster obtained with the AAOmega facility at the Anglo-Australian Telescope. We describe the observations and measurements of central velocity dispersion, emission line equivalent widths and absorption line indices. The distinguishing characteristic of the survey is its coverage of a very wide baseline in velocity dispersion (30-300 km/s), while achieving high signal-to-noise ratio throughout (median 60 per Ang). Significant emission at H-alpha was detected in ~20 per cent of red-sequence Shapley members. Using line-ratio diagnostics, we find that the emission is LINER-like at high luminosity, but driven by star-formation in low-luminosity galaxies. We use Lick indices to characterise the absorption spectra. We define a subset of galaxies with very low emission contamination, and fit the index-sigma relations for this subset. Comparing the index-sigma slopes against predictions from single-burst stellar population models, we infer the scaling relations of age, total metallicity, [Z/H], and alpha-element abundance ratio, [a/Fe]. To reproduce the observed index-sigma slopes, all three parameters must increase with increasing velocity dispersion. Specifically, we recover: Age \propto sigma^0.52+/-0.10, Z/H \propto sigma^0.34+/-0.07, and a/Fe \propto sigma^0.23+/-0.06 (error reflects systematic effects), derived over a decade baseline in velocity dispersion. The recovered age-sigma relation is shown to be consistent with the observed evolution in the giant-to-dwarf galaxy ratio in clusters at redshifts z=0.4-0.8. A companion paper will analyse the distribution of age, [Z/H] and [a/Fe] for individual galaxies. (Abridged.)

A globular cluster in the dwarf galaxy Sextans B

We present spectroscopic observations of a massive globular cluster in the dwarf irregular galaxy Sextans B, discovered by us on the Hubble Space Telescope Wide Field and Planetary Camera 2 (HST WFPC2) images. Long-slit spectra were obtained with the SCORPIO spectrograph on the 6-m telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences. We determine the age, metallicity and alpha-element abundance ratio for the globular cluster to be 2 ± 1 Gyr, −1.35 ±0.25 dex, and 0.1 ± 0.1 dex, respectively. Main photometric and structural parameters of it were determined using our surface photometry on the HST images. The mass (∼105 M⊙), luminosity and structural parameters appear to be typical of the globular clusters in our own Galaxy. Our findings shed new light on the evolutionary history of Sextans B.

The Fundamental Plane in RX J0142.0+2131: A Galaxy Cluster Merger at [FORMULA][F]z=0.28[/F][/FORMULA

We present the Fundamental Plane (FP) in the z = 0.28 cluster of galaxies RX J0142.0+2131. There is no evidence for a difference in the slope of the FP when compared with the Coma cluster, although the internal scatter is larger. On average, stellar populations in RX J0142.0+2131 have rest-frame V-band mass-to-light ratios (M/L_V) 0.29+-0.03 dex lower than in Coma. This is significantly lower than expected for a passively-evolving cluster formed at z_f=2. Lenticular galaxies have lower average M/L_V and a distribution of M/L_V with larger scatter than ellipticals. Lower mass-to-light ratios are not due to recent star formation: our previous spectroscopic observations of RX J0142.0+2131 E/S0 galaxies showed no evidence for significant star-formation within the past ~4 Gyr. However, cluster members have enhanced alpha-element abundance ratios, which may act to decrease M/L_V. The increased scatter in the RX J0142.0+2131 FP reflects a large scatter in M/L_V implying that galaxies have undergone bursts of star formation over a range of epochs. The seven easternmost cluster galaxies, including the second brightest member, have M/L_V consistent with passive evolution and z_f = 2. We speculate that RX J0142.0+2131 is a cluster-cluster merger where the galaxies to the east are yet to fall into the main cluster body or have not experienced star formation as a result of the merger.

Central stellar populations of early-type galaxies in low-density environments

We have investigated the properties of a volume and magnitude limited sample of nearby early type galaxies that were carefully selected from the AAO two degree field galaxy redshift survey. We used images from the DSS to confirm the E/S0 morphologies, and augmented this sample with field galaxies from Colbert et al. 2001. We present spectroscopic observations of 22 galaxies from the combined sample, from which central velocity dispersions and the Lick stellar population indices were measured. After carefully correcting the spectra for nebular emission we derived luminosity-weighted ages, metallicities, and alpha-element abundance ratios. We compare these isolated galaxies with samples of early-type galaxies in the Virgo and Coma clusters, and also with the sample of galaxies in low-density regions of Kuntschner et al. (2002). We find that galaxies in low-density environments are younger and have a greater spread of ages compared to cluster galaxies. They also show a wider range of metallicities at a given velocity dispersion than cluster galaxies, which display only super-solar metallicities. On average cluster, as well as, isolated galaxies show non-solar abundance ratios in alpha-elements, suggesting that, independent of galactic environment, star formation occurred on short time-scales. We reason that early-type galaxies in low-density environments experienced merging-induced star-formation episodes over a longer and more recent period of time compared to a cluster environment, and speculate that a considerable fraction of their stars formed out of low-metallicity halo gaseous material during the slow growth of a stellar disk between merging events.

Gemini/GMOS spectra of globular clusters in the Virgo giant elliptical NGC 4649

NGC 4649 (M60) is one of a handful of giant Virgo ellipticals. We have obtained Gemini/GMOS spectra for 38 GCs associated with this galaxy. Applying the multi-index chi^2 minimisation technique of Proctor & Sansom (2002) with the single stellar population models of Thomas, Maraston & Korn (2004) we derive ages, metallicities and alpha-element abundance ratios. We find several young (2--3 Gyr old) super-solar metallicity GCs, while the majority are old (>10 Gyrs), spanning a range of metallicities from solar to [Z/H]=-2. At least two of these young GCs are at large projected radii of 17-20 kpc. The galaxy itself shows no obvious signs of a recent starburst, interaction or merger. A trend of decreasing alpha-element ratio with increasing metallicity is found.

Gemini/GMOS spectra of globular clusters in the Leo group elliptical NGC 3379

We have obtained Gemini/GMOS spectra for 22 GCs associated with NGC 3379. We derive ages, metallicities and alpha-element abundance ratios from simple stellar population models using the multi-index chi^2 minimisation method of Proctor & Sansom (2002). All of these GCs are found to be consistent with old ages, i.e. >10 Gyr, with a wide range of metallicities. A trend of decreasing alpha-element abundance ratio with increasing metallicity is indicated. The projected velocity dispersion of the GC system is consistent with being constant with radius. Non-parametric, isotropic models require a significant increase in the mass-to-light ratio at large radii. This result is in contrast to that of Romanowsky et al. (2003) who find a decrease in the velocity dispersion profile as determined from planetary nebulae. Our constant dispersion requires a normal sized dark halo, although without anisotropic models we cannot rigorously determine the dark halo mass. A two-sided chi^2 test over all radii, gives a 2 sigma difference between the mass profile derived from our GCs compared to the PN-derived mass model of Romanowsky et al. (2003). However, if we restrict our analysis to radii beyond one effective radius and test if the GC velocity dispersion is consistently higher, we determine a >3 sigma difference between the mass models, and hence favor the conclusion that NGC 3379 does indeed have dark matter at large radii in its halo. (abridged)

AChandraX‐Ray Investigation of the Violent Interstellar Medium: From Dwarf Starbursts to Ultraluminous Infrared Galaxies

We have analyzed observations with the Chandra X-ray Observatory of the diffuse emission by hot gas in 7 dwarf starburst galaxies, 6 edge-on starburst galaxies, and 9 Ultra Luminous Infrared Galaxies. These systems cover ranges of \~ 10^4 in X-ray luminosity and several thousand in star formation rate and K-band luminosity (a proxy for stellar mass). Despite this range in fundamental parameters, we find that the properties of the diffuse X-ray emission are very similar in all three classes of starburst galaxies. The spectrum of the diffuse emission is well fit by thermal emission from gas with kT ~ 0.25 to 0.8 keV and with several-times-solar abundance ratios of alpha elements to Fe. The ratio of the thermal X-ray to far-infrared luminosity is roughly constant, as is the characteristic surface brightness of the diffuse X-ray emission. The size of the diffuse X-ray source increases systematically with both far-infrared and K-Band luminosity. All three classes show strong morphological relationships between the regions of hot gas probed by the diffuse X-ray emission and the warm gas probed by optical line emission. These findings suggest that the same physical mechanism is producing the diffuse X-ray emission in the three types of starbursts, and are consistent with that mechanism being shocks driven by a galactic superwind'' powered by the kinetic energy collectively supplied by stellar winds and supernovae in the starburst.