Thomas H. Puzia

The remnants of galaxy formation from a panoramic survey of the region around M31.

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than Mv = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

The NGC 404 Nucleus: Star Cluster and Possible Intermediate-mass Black Hole

We examine the nuclear morphology, kinematics, and stellar populations in nearby S0 galaxy NGC 404 using a combination of adaptive optics assisted near-IR integral-field spectroscopy, optical spectroscopy, and Hubble Space Telescope imaging. These observations enable study of the NGC 404 nucleus at a level of detail possible only in the nearest galaxies. The surface brightness profile suggests the presence of three components: a bulge, a nuclear star cluster (NSC), and a central light excess within the cluster at radii < 3 pc. These components have distinct kinematics with modest rotation seen in the NSC and counter-rotation seen in the central excess. Molecular hydrogen emission traces a disk with rotation nearly orthogonal to that of the stars. The stellar populations of the three components are also distinct, with half of the mass of the NSC having ages of ~1 Gyr (perhaps resulting from a galaxy merger), while the bulge is dominated by much older stars. Dynamical modeling of the stellar kinematics gives a total NSC mass of 1.1 × 107 M ☉. Dynamical detection of a possible intermediate-mass black hole (BH) is hindered by uncertainties in the central stellar mass profile. Assuming a constant mass-to-light ratio, the stellar dynamical modeling suggests a BH mass of <1 × 105 M ☉, while the molecular hydrogen gas kinematics are best fitted by a BH with a mass of 4.5+3.5 –2.0 × 105 M ☉. Unresolved and possibly variable dust emission in the near-infrared and active galactic nucleus-like molecular hydrogen emission-line ratios do suggest the presence of an accreting BH in this nearby LINER galaxy.

Extragalactic globular clusters in the near-infrared - II. The globular clusters systems of NGC 3115 and NGC 4365

We combine near-infrared photometry obtained with the VLT/ISAAC instrument and archival HST/WFPC2 optical images to determine VIK magnitudes and colours of globular clusters in two early-type galaxies, NGC 3115 and NGC 4365. The combination of near-IR and optical photometry provides a way to lift the age-metallicity degeneracy. For NGC 3115, the globular cluster colours reveal two major sub-populations, consistent with previous studies. By comparing the V I, V K colours of the NGC 3115 globular clusters with stellar populations models, we find that the colour dierence between the two >10 Gyr old major sub-populations is primarily due to a dierence in metallicity. We find(Fe/H) = 1:0 0: 3d ex and the blue (metal-poor) and red (metal-rich) globular cluster sub-populations being coeval within 3 Gyr. In contrast to the NGC 3115 globular clusters, the globular cluster system in NGC 4365 exhibits a more complex age and metallicity structure. We find a significant population of intermediate-age very metal-rich globular clusters along with an old population of both metal-rich and metal-poor clusters. Specifically, we observe a large population of globular clusters with red V K colours but intermediate V I colours, for which all current stellar population models give ages and metallicities in the range2 8G yr and0:5 Z 3 Z, respectively. After 10 Gyr of passive evolution, the intermediate-age globular clusters in NGC 4365 will have colours which are consistent with the very metal-rich population of globular clusters in giant elliptical galaxies, such as M 87. Our results for both globular cluster systems are consistent with previous age and metallicity studies of the diuse galactic light. In addition to the major globular cluster populations in NGC 3115 and NGC 4365 we report on the detection of objects with extremely red colours (V K> 3:8 mag), whose nature could not ultimately be revealed with the present data.

VLT spectroscopy of globular cluster systems - II. Spectroscopic ages, metallicities and [α/Fe] ratios of globular clusters in early-type galaxies

An analysis of ages, metallicities, and [α/Fe] ratios of globular cluster systems in early-type galaxies is presented, based on Lick index measurements summarized in Puzia et al. (2004, A&A, 415, 123, Paper I of this series). In the light of calibration and measurement uncertainties, age-metallicity degeneracy, and the relative dynamic range of Lick indices, as well as systematics introduced by abundance ratio variations (in particular variations of [α/Fe] ratios), we find that the most reliable age indicator for our dataset is a combination of the Lick Balmer-line indices HγA, Hβ, and HδA. [MgFe] is used as a spectroscopic metallicity indicator which is least affected by [α/Fe] variations. We introduce an interpolation routine to simultaneously derive ages, metallicities, and [α/Fe] ratios from diagnostic grids constructed from Lick indices. From a comparison of high-quality data with SSP model predictions, we find that ∼2/3 of the globular clusters in earlytype galaxies are older than 10 Gyr, up to 1/3 have ages in the range ∼5−10 Gyr, and only a few cluster are younger than ∼5 Gyr. Our sample of globular clusters covers metallicities from [Z/H] ≈ −1.3 up to ∼0.5 dex. We find that metal-rich globular clusters show on average a smaller mean age and a larger age scatter than their metal-poor counterparts. [α/Fe] diagnostic plots show that globular cluster systems in early-type galaxies have super-solar α/Fe abundance ratios with a mean [α/Fe] = 0.47 ± 0.06 dex and a dispersion of ∼0.3 dex. We find evidence for a correlation between [α/Fe] and metallicity, in the sense that more metal-rich clusters exhibit lower α-element enhancements. A discussion of systematics related to the Lick index system shows that the method suffers to some extent from uncertainties due to unknown horizontal branch morphologies at high metallicities. However, these systematics still allow us to make good qualitative statements. A detailed investigation of indices as a function of data quality reveals that the scatter in Balmer index values decreases for higher-quality data. In particular, extremely low Balmer index values that are lower than any SSP model prediction tend to disappear. Furthermore, we find that observed photometric colors are in good agreement with computed SSP colors using ages and metallicities as derived from the spectroscopic line indices.

Globular cluster systems in nearby dwarf galaxies – III. Formation efficiencies of old globular clusters★

We investigate the origin of the shape of the globular cluster (GC) system scaling parameters as a function of galaxy mass, i.e. specific frequency (SN), specific luminosity (SL), specific mass (SM) and specific number ( ˆ T ) of GCs. In the low-mass galaxy regime (MV −16 mag), our analysis is based on Hubble Space Telescope/Advanced Camera for Surveys observations of GC populations of faint, mainly late-type dwarf galaxies in low-density environments. In order to sample the entire range in galaxy mass (MV =− 11 to −23 mag =10 6 –1 0 11 L� ), environment and morphology we augment our sample with data of spiral and elliptical galaxies from the literature, in which old GCs are reliably detected. This large data set confirms (irrespective of the galaxy type) the increase in the specific frequencies of GCs above and below a galaxy magnitude of MV �− 20 mag. Over the full mass range, the SL value of early-type galaxies is, on average, twice that of late types. To investigate the observed trends, we derive theoretical predictions of GC system scaling parameters as a function of host galaxy mass based on the models of Dekel and Birnboim in which star formation processes are , �,

Integrated spectroscopy of bulge globular clusters and fields. II. Implications for population synthesis models and elliptical galaxies

An empirical calibration is presented for the synthetic Lick indices (e.g. Mg2, ‹Fe›, H s, etc.) of Simple Stellar Population (SSP) models that for the first time extends up to solar metallicity. This is accomplished by means of a sample of Milky Way globular clusters (GCs) whose metallicities range from ~ Zʘ /30 to Z ~ Zʘ, thanks to the inclusion of several metal rich clusters belonging to the Galactic bulge (e.g., NGC 6553 and NGC 6528). This metallicity range approaches the regime that is relevant for the interpretation of the integrated spectra of elliptical galaxies. It is shown that the spectra of both the globular clusters and the Galactic bulge follow the same correlation between magnesium and iron indices that extends to elliptical galaxies, showing weaker iron indices at given magnesium indices with respect to the predictions of models that assume solar-scaled abundances. This similarity provides robust empirical evidence for enhanced [ α/Fe] ratios in the stellar populations of elliptical galaxies, since the globular clusters are independently known to have enhanced [ α/Fe] ratios from spectroscopy of individual stars. We check the uniqueness of this α-overabundance solution by exploring the whole range of model ingredients and parameters, i.e. fitting functions, stellar tracks, and the initial mass function (IMF). We argue that the standard models (meant for solar abundance ratios) succeed in reproducing the Mg-Fe correlation at low metallicities ( [Z/H] ≲ -0.7) because the stellar templates used in the synthesis are Galactic halo stars that actually are

Integrated spectroscopy of bulge globular clusters and fields I. The data base and comparison of individual Lick indices in clusters and bulge

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The Age Difference between the Globular Cluster Subpopulations in NGC 4472

-enhanced. The same models, however, fail to predict the observed Mg-Fe pattern at higher metallicities ( [Z/H] ≳ -0.7 ) (i.e., for bulge clusters and ellipticals alike) because the high-metallicity templates are disk stars that are not α-enhanced. We show that the new set of SSP models which incorporates the dependence on the [ α/Fe] ratio (Thomas et al. 2003) is able to reproduce the Mg and Fe indices of GCs at all metallicities, with an α-enhancement α/Fe=+0.3, in agreement with the available spectroscopic determinations. The H β index and the higher-order Balmer indices are well calibrated, provided the appropriate morphology of the Horizontal Branch is taken into account. In particular, the Balmer line indices of the two metal rich clusters NGC 6388 and NGC 6441, which are known to exhibit a tail of warm Horizontal Branch stars, are well reproduced. Finally, we note that the Mg indices of very metal-poor ( [Z/H] ≲ -1.8 ) populations are dominated by the contribution of the lower Main Sequence, hence are strongly affected by the present-day mass function of individual globular clusters, which is known to vary from cluster to cluster due to dynamical effects.

New light on the formation and evolution of M 31 and its globular cluster system

We present a comprehensive spectroscopic study of the integrated light of metal-rich Galactic globular clusters and the stellar population in the Galactic bulge. We measure line indices which are defined by the Lick standard system and compare index strengths of the clusters and Galactic bulge. Both metal-rich globular clusters and the bulge are similar in most of the indices, except for the CN index. We find a significant enhancement in the CN/� Feindex ratio in metal-rich globular clusters compared with the Galactic bulge. The mean iron indexFeof the two metal-rich globular clusters NGC 6528 and NGC 6553 is comparable with the mean iron index of the bulge. Index ratios such as Mgb/� Fe� ,M g2/� Fe� , Ca 4227/� Fe� ,a nd TiO/� Fe� ,a re comparable in both stellar population indicating similar enhancements in individual elements which are traced by the indices. From the globular cluster data we fully empirically calibrate several metallicity-sensitive indices as a function of (Fe/H) and find tightest correlations for the Mg2 index and the composite (MgFe) index. We find that all indices show a similar behavior with galactocentric radius, except for the Balmer series, which show a large scatter at all radii. However, the scatter is entirely consistent with the cluster-to-cluster variations in the horizontal branch morphology.

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

The age difference between the two main globular cluster sub–populations in the Virgo giant elliptical galaxy, NGC 4472 (M 49), has been determined using HST WFPC2 images in the F555W and F814W filters. Accurate photometry has been obtained for several hundred globular clusters in each of the two main sub–populations, down to more than one magnitude below the turn–over of their luminosity functions. This allows precise determinations of both the mean colors and the turn–over magnitudes of the two main sub–populations. By comparing the data with various population synthesis models, the age–metallicity pairs that fit both the observed colors and magnitudes have been identified. The metal–poor and the metal–rich globular clusters are found to be coeval within the errors (∼ 3 Gyr). If one accepts the validity of our assumptions, these errors are dominated by model uncertainties. A systematic error of up to 4 Gyr could affect this result if the blue and the red clusters have significantly different mass distributions. However, that one sub–population is half as old as the other is excluded Feodor Lynen Fellow of the Alexander von Humboldt Foundation Guest Observer, Hubble Space Telescope, operated by the Association of Universities for Research in Astronomy for NASAThe age difference between the two main globular cluster subpopulations in the Virgo giant elliptical galaxy NGC 4472 (M49) has been determined using Hubble Space Telescope WFPC2 images in the F555W and F814W filters. Accurate photometry has been obtained for several hundred globular clusters in each of the two main subpopulations, down to more than 1 mag below the turnover of their luminosity functions. This allows precise determinations of both the mean colors and the turnover magnitudes of the two main subpopulations. By comparing the data with various population synthesis models, the age-metallicity pairs that fit both the observed colors and magnitudes have been identified. The metal-poor and the metal-rich globular clusters are found to be coeval within the errors (~3 Gyr). If one accepts the validity of our assumptions, these errors are dominated by model uncertainties. A systematic error of up to 4 Gyr could affect this result if the blue and the red clusters have significantly different mass distributions. However, that one subpopulation is half as old as the other is excluded at the 99% confidence level. The different globular cluster populations are assumed to trace the galaxys major star formation episodes. Consequently, the vast majority of globular clusters (and by implication the majority of stars) in NGC 4472 formed at high redshifts but by two distinct mechanisms or in two episodes. The distance to NGC 4472 is determined to be 15.8 ? 0.8 Mpc, which is in excellent agreement with six of the seven Cepheid distances to Virgo Cluster spiral galaxies. This implies that the spiral and elliptical galaxies in the main body of Virgo are at the same distance.