Metallicity Distribution Function Research Articles

Dwarf Galaxies Beyond Our Doorstep: the Centaurus Group

The study of dwarf galaxies in groups is a powerful tool for investigating galaxy evolution, chemical enrichment and environmental effects on these objects. Here we present results obtained for dwarf galaxies in the Centaurus A complex, a dense nearby (~4 Mpc) group that contains two giant galaxies and about 30 dwarf companions of different morphologies and stellar contents. We use archival optical (HST/ACS) and near-infrared (VLT/ISAAC) data to derive physical properties and evolutionary histories from the resolved stellar populations of these dwarf galaxies. In particular, for early-type dwarfs we are able to construct metallicity distribution functions, find population gradients and quantify the intermediate-age star formation episodes. For late-type dwarfs, we compute recent (~1 Gyr) star formation histories and study their stellar distribution. We then compare these results with properties of the dwarfs in our Milky Way and in other groups. Our work will ultimately lead to a better understanding of the evolution of dwarf galaxies.

A Chemical Evolution of Draco Dwarf Galaxy: Monolithic or Merger Scenario?

A chemical evolution of the Local Group dwarf galaxy Draco is considered. The stellar metallicity distribution function of Draco was calculated in the framework of both the monolithic and the merger scenario. The observed metallicity distribution isn’t reproduced quite well by monolithic collaps for this galaxy. The use of a merger of several fragments allows to obtain a better resemblance between modelled and observed stellar metallicity distributions.

Galactic astroarchaeology: reconstructing the bulge history by means of the newest data

The chemical abundances measured in stars of the Galactic bulge offer an unique opportunity to test galaxy formation models as well as impose strong constraints on the history of star formation and stellar nucleosynthesis. The aims of this paper are to compare abundance predictions from a detailed chemical evolution model for the bulge with the newest data. Some of the predictions have already appeared on previous paper (O, Mg, Si, S and Ca) but some other predictions are new (Ba, Cr and Ti). We compute several chemical evolution models by adopting different initial mass functions for the Galactic bulge and then compare the results to new data including both giants and dwarf stars in the bulge. In this way we can impose strong constraints on the star formation history of the bulge. We find that in order to reproduce at best the metallicity distribution function one should assume a flat IMF for the bulge not steeper than the Salpeter one. The initial mass function derived for the solar vicinity provides instead a very poor fit to the data. The [el/Fe] vs. [Fe/H] relations in the bulge are well reproduced by a very intense star formation rate and a flat IMF as in the case of the stellar metallicity distribution. Our model predicts that the bulge formed very quickly with the majority of stars formed inside the first 0.5 Gyr. Our results strongly suggest that the new data, and in particular the MDF of the bulge, confirm what concluded before and in particular that the bulge formed very fast, from gas shed by the halo, and that the initial mass function was flatter than in the solar vicinity and in the disk, although not so flat as previously thought. Finally, our model can also reproduce the decrease of the [O/Mg] ratio for [Mg/H] > 0 in the bulge, which is confirmed by the new data and interpreted as due to mass loss in massive stars.

Probing the Galactic thick disc vertical properties and interfaces

Aims. This work investigates the properties (metallicity and kinematics) and interfaces of the Galactic thick disc as a function of height above the Galactic plane. The main aim is to study the thick disc in a place where it is the main component of the sample. Methods. We take advantage of former astrometric work in two fields of several square degrees in which accurate proper motions were measured down to V-magnitudes of 18.5 in two directions, one near the north galactic pole and the other at a galactic latitude of 46 ◦ and galactic longitude near 0 ◦ . Spectroscopic observations have been acquired in these two fields for a total of about 400 stars down to magnitude 18.0, at spectral resolutions of 3.5 to 6.25 A. The spectra have been analysed with the code ETOILE, comparing the target stellar spectra with a grid of 1400 reference stellar spectra. This comparison allowed us to derive the parameters effective temperature, gravity, [Fe/H] and absolute magnitude for each target star. Results. The Metallicity Distribution Function (MDF) of the thin-thick-disc-halo system is derived for several height intervals between 0 and 5 kpc above the Galactic plane. The MDFs show a decrease of the ratio of the thin to thick disc stars between the first and second kilo-parsec. This is consistent with the classical modelling of the vertical density profile of the disc with 2 populations with different scale heights. A vertical metallicity gradient, ∂[Fe/H]/∂z = −0.068 ± 0.009 dex kpc −1 , is observed in the thick disc. It is discussed in terms of scenarios of formation of the thick disc.

The stellar content of the Hamburg/ESO survey

We determine the metallicity distribution function (MDF) of the Galactic halo based on metal-poor main-sequence turnoff-stars (MSTO) which were selected from the Hamburg/ESO objective-prism survey (HES) database. Corresponding follow-up moderateresolution observations (R ~ 2000) of some 682 stars (among which 617 were accepted program stars) were carried out with the 2.3m telescope at the Siding Spring Observatory (SSO). Corrections for the survey volume covered by the sample stars were quantitatively estimated and applied to the observed MDF. The corrections are quite small, when compared with those for a previously studied sample of metal-poor giants. The corrected observational MDF of the turnoff sample was then compared with that of the giants, as well as with a number of theoretical predictions of Galactic chemical evolution, including the mass-loss modified Simple Model. Although the survey-volume corrected MDFs of the metal-poor turnoff and the halo giants notably differ in the region of [Fe/H] > -2.0, below [Fe/H] ~ -2.0, (the region we scientifically focus on most) both MDFs show a sharp drop at [Fe/H] ~ -3.6 and present rather similar distributions in the low-metallicity tail. Theoretical models can fit some parts of the observed MDF, but none is found to simultaneously reproduce the peak as well as the features in the metal-poor region with [Fe/H] between -2.0 to -3.6. Among the tested models only the GAMETE model, when normalized to the tail of the observed MDF below [Fe/H] ~ -3.0, and with Z_{cr} = 10^{-3.4}Z_{\odot}, is able to predict the sharp drop at [Fe/H] ~ -3.6.

Dwarf spheroidals in the M 81 group – Metallicity distribution functions and population gradients

We study the dwarf spheroidal galaxies in the nearby M81 group in order to construct their photometric metallicity distributions and to investigate the potential presence of population gradients. We select all the dwarf spheroidals with available Hubble Space Telescope / Advanced Camera for Surveys archival observations, nine in total. We interpolate isochrones so as to assign a photometric metallicity to each star within a selection box in the color-magnitude diagram of each dwarf galaxy. We assume that the dwarf spheroidals contain mainly an old stellar population. In order to search for metallicity gradients, we examine the spatial distribution of two stellar populations that we separate according to their metallicities. As a result, we present the photometric metallicity distribution functions, the cumulative histograms and smoothed density maps of the metal-poor and metal-rich stars as well as of the intermediate-age stars. From our photometric data we find that all the dwarf spheroidals show a wide range in metallicities, with mean values that are typical for old and metal-poor systems, with the exception of one dwarf spheroidal, namely IKN. Some of our dwarf spheroidals exhibit characteristics of transition-type dwarfs. Compared to the Local Group transition type dwarfs, the M81 group ones appear to have mean metallicity values slightly more metal-rich at a given luminosity. All the dwarf spheroidals considered here appear to exhibit either population gradients or spatial variations in the centroids of their metal-poor and metal-rich population. In addition, there are luminous AGB stars detected in all of them with spatial distributions suggesting that they are well mixed with the old stars.

THE STELLAR POPULATION AND METALLICITY DISTRIBUTION OF THE SOMBRERO GALAXY

Hubble Space Telescope studies of the resolved stellar population of elliptical galaxies have shown that the galaxies form by steady accretion of gas which is all the while forming stars and evolving chemically to a metallicity distribution that is as high as solar composition in the most massive objects that have been analyzed, and much lower for low mass ellipticals. In this paper we study for the first time the stellar content of an early type spiral galaxy, the massive disk galaxy, the Sombrero, NGC 4594. We consider whether the metallicity distribution function (MDF) in the observed field matches that of elliptical galaxies of some luminosity, and what these data imply for the accretion and enrichment model that can be fitted to the MDF. The MDF of NGC 4594 is similar to that of the elliptical galaxy of similar luminosity, NGC 5128. The field we are probing is a combination of the galaxy's bulge and halo.

THE DUAL ORIGIN OF STELLAR HALOS. II. CHEMICAL ABUNDANCES AS TRACERS OF FORMATION HISTORY

Fully cosmological, high resolution N-Body + SPH simulations are used to investigate the chemical abundance trends of stars in simulated stellar halos as a function of their origin. These simulations employ a physically motivated supernova feedback recipe, as well as metal enrichment, metal cooling and metal diffusion. As presented in an earlier paper, the simulated galaxies in this study are surrounded by stellar halos whose inner regions contain both stars accreted from satellite galaxies and stars formed in situ in the central regions of the main galaxies and later displaced by mergers into their inner halos. The abundance patterns ([Fe/H] and [O/Fe]) of halo stars located within 10 kpc of a solar-like observer are analyzed. We find that for galaxies which have not experienced a recent major merger, in situ stars at the high [Fe/H] end of the metallicity distribution function are more [alpha/Fe]-rich than accreted stars at similar [Fe/H]. This dichotomy in the [O/Fe] of halo stars at a given [Fe/H] results from the different potential wells within which in situ and accreted halo stars form. These results qualitatively match recent observations of local Milky Way halo stars. It may thus be possible for observers to uncover the relative contribution of different physical processes to the formation of stellar halos by observing such trends in the halo populations of the Milky Way, and other local L* galaxies.

A close look at the Centaurus A group of galaxies

We study dwarf galaxies in the Centaurus A group to investigate their metallicity and possible environmental effects. The Centaurus A group (at ~4 Mpc from the Milky Way) contains about 50 known dwarf companions of different morphologies and stellar contents, thus making it a very interesting target to study how these galaxies evolve. Here we present results for the early-type dwarf galaxy population in this group. We use archival HST/ACS data to study the resolved stellar content of 6 galaxies, together with isochrones from the Dartmouth stellar evolutionary models. We derive photometric metallicity distribution functions of stars on the upper red giant branch via isochrone interpolation. The 6 galaxies are moderately metal-poor (<[Fe/H]>=-1.56 to -1.08), and metallicity spreads are observed (internal dispersions of sigma_[Fe/H]=0.10 to 0.41 dex). We also investigate the possible presence of intermediate-age stars, and discuss how these affect our results. The dwarfs exhibit flat to weak radial metallicity gradients. For the two most luminous, most metal-rich galaxies, we find statistically significant evidence for at least two stellar subpopulations: the more metal-rich stars are found in the center of the galaxies, while the metal-poor ones are more broadly distributed within the galaxies. We find no clear trend of the derived physical properties as a function of (present-day) galaxy position in the group, which may be due to the small sample we investigate. We compare our results to the early-type dwarf population of the Local Group, and find no outstanding differences, despite the fact that the Centaurus A group is a denser environment that is possibly in a more advanced dynamical stage.

THE SAGITTARIUS DWARF GALAXY: A MODEL FOR EVOLUTION IN A TRIAXIAL MILKY WAY HALO

We present a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (< 3 Gyr) tidal debris streams. In particular, this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years. While the model does not reproduce the apparent bifurcation observed in the leading debris stream, recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit. The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential which can be described in terms of a triaxial dark matter halo whose minor/major axis ratio (c/a)_Phi = 0.72 and intermediate/major axis ratio (b/a)_Phi = 0.99 at radii 20 < r < 60 kpc. The minor/intermediate/major axes of this halo lie along the directions (l, b) = (7, 0), (0, 90), and (97, 0) respectively, corresponding to a nearly-oblate ellipsoid whose minor axis is contained within the Galactic disk plane. We demonstrate that with simple assumptions about the star formation history of Sgr, tidal stripping models naturally give rise to gradients in the metallicity distribution function (MDF) along the stellar debris streams similar to those observed in recent studies. (Abridged).

STRUCTURE AND KINEMATICS OF THE STELLAR HALOS AND THICK DISKS OF THE MILKY WAY BASED ON CALIBRATION STARS FROM SLOAN DIGITAL SKY SURVEY DR7

The structure and kinematics of the recognized stellar components of the Milky Way are explored, based on well-determined atmospheric parameters and kinematic quantities for 32360 calibration stars from the Sloan Digital Sky Survey (SDSS) and its first extension, (SDSS-II), which included the sub-survey SEGUE: Sloan Extension for Galactic Understanding and Exploration. Full space motions for a sub-sample of 16920 stars, exploring a local volume within 4 kpc of the Sun, are used to derive velocity ellipsoids for the inner- and outer-halo components of the Galaxy, as well as for the canonical thick-disk and proposed metal-weak thick-disk populations. We first examine the question of whether the data require the presence of at least a two-component halo in order to account for the rotational behavior of likely halo stars in the local volume, and whether more than two components are needed. We also address the question of whether the proposed metal-weak thick disk is kinematically and chemically distinct from the canonical thick disk. In addition, we consider the fractions of each component required to understand the nature of the observed kinematic behavior of the stellar populations of the Galaxy as a function of distance from the plane. Scale lengths and scale heights for the thick-disk and metal-weak thick-disk components are determined. Spatial density profiles for the inner- and outer-halo populations are inferred from a Jeans Theorem analysis. The full set of calibration stars (including those outside the local volume) is used to test for the expected changes in the observed stellar metallicity distribution function with distance above the Galactic plane in-situ, due to the changing contributions from the underlying stellar populations. [abridged]

Abundance analysis of a sample of evolved stars in the outskirts of [formula omitted] Centauri

The globular cluster ω Centauri (NGC 5139) is a puzzling stellar system harboring several distinct stellar populations whose origin still represents a unique astrophysical challenge. Current scenarios range from primordial chemical inhomogeneities in the mother cloud to merging of different sub-units and/or subsequent generations of enriched stars – with a variety of different pollution sources – within the same potential well. In this paper we study the chemical abundance pattern in the outskirts of ω Centauri, half-way to the tidal radius (covering the range of 20–30 arcmin from the cluster center), and compare it with chemical trends in the inner cluster regions, in an attempt to explore whether the same population mix and chemical compositions trends routinely found in the more central regions is also present in the cluster periphery. We extract abundances of many elements from FLAMES/UVES spectra of 48 RGB stars using the equivalent width method and then analyze the metallicity distribution function and abundance ratios of the observed stars. We find, within the uncertainties of small number statistics and slightly different evolutionary phases, that the population mix in the outer regions cannot be distinguished from the more central regions, although it is clear that more data are necessary to obtain a firmer description of the situation. From the abundance analysis, we did not find obvious radial gradients in any of the measured elements.

Mining the Galactic halo for very metal-poor stars

Abstract We study the age and metallicity distribution function (MDF) of metal-poor stars in the Milky Way halo as a function of galactocentric radius by combining N-body simulations and semi-analytical methods. We find that the oldest stars populate the innermost region, while extremely metal-poor stars are more concentrated within r &amp;lt; 60 kpc. The MDF of [Fe/H]≤−2 stars varies only very weakly within the central 50 kpc, while the relative contribution of [Fe/H]≤−2 stars strongly increases with r, varying from 16 per cent within 7 &amp;lt; r &amp;lt; 20 kpc up to ≥40 per cent for r &amp;gt; 20 kpc. This is due to the faster descent of the spatial distribution (as seen from Earth) of the more enriched population. This implies that the outer halo &amp;lt; 40 kpc is the best region to search for very metal-poor stars. Beyond ∼60 kpc the density of [Fe/H]≤−2 stars is maximum within dwarf galaxies. All these features are imprinted by a combination of (i) the virialization epoch of the star-forming haloes, and (ii) the metal enrichment history of the Milky Way environment.

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.

Dwarf spheroidal galaxies in the M81 group of galaxies

AbstractWe study the properties of the dwarf galaxy population in the nearby interacting M81 group of galaxies. In particular, we are deriving the metallicity distribution functions, the ages and the star formation histories of these dwarf galaxies based on data from the Hubble Space Telescope's Advanced Camera for Surveys and the Sloan Digital Sky Survey. We are correlating this information with the dwarf galaxies' position in the group in order to derive the impact of interactions and environment on their evolution. Here we present preliminary results on the metallicity distribution functions of dwarf spheroidals in the M81 group (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Evolution of dwarf galaxies in the Centaurus A group

AbstractThe study of (dwarf) galaxies in nearby groups is one among the most powerful tools that can be used to investigate galaxy evolution, chemical enrichment and environmental effects on these objects. The Centaurus A group (at a distance of ∼4 Mpc) is dynamically evolved and contains about 30 dwarf companions of different morphologies and stellar contents. Here we present the first results for the dwarf spheroidal galaxy population in this group. We use archival HST/ACS data to study their resolved stellar content. The resulting metallicity distribution functions reveal metal‐poor populations and wide metallicity spreads for each galaxy. We find no clear trend of the derived physical properties as a function of galaxy position in the group. Finally, we compare our results to the dwarf population of the Local Group, and find no outstanding differences (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

On the origin of the helium-rich population in ω Centauri

To study the possible origin of the huge helium enrichment attributed to the stars on the blue main sequence of ω Centauri, we make use of a chemical evolution model that has proven able to reproduce other major observed properties of the cluster, namely its stellar metallicity distribution function, age–metallicity relation and trends of several abundance ratios with metallicity. In this framework, the key condition to satisfy all the available observational constraints is that a galactic-scale outflow develops in a much more massive parent system, as a consequence of multiple supernova explosions in a shallow potential well. This galactic wind must carry out preferentially the metals produced by explosive nucleosynthesis in supernovae, whereas elements restored to the interstellar medium through low-energy stellar winds by both asymptotic giant branch (AGB) stars and massive stars must be mostly retained. Assuming that helium is ejected through slow winds by both AGB stars and fast-rotating massive stars, the interstellar medium of ω Centauri's parent galaxy gets naturally enriched in helium in the course of its evolution.

The stellar content of the Hamburg/ESO survey

We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely "best and brightest stars first". [...] We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for [Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Z_cr = 10^{-3.4} * Z_Sun reproduces the sharp drop at [Fe/H] ~-3.6 present in the HES MDF. [...] A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant. [ABSTRACT ABRIDGED]

On the initial cluster mass distribution inferred from synthesis models

In this contribution we examine the problem of inferring ages and initial cluster masses from synthesis models at the limit of low-mass clusters (M≤ a few ×104 M⊙). We show that it is not possible to apply directly synthesis models using standard methods to such clusters, since the basic hypothesis implicit in the models (a fixed proportionality between the number of stars in different evolutionary phases) is not fulfilled due to an insufficient number of stars for a reliable sampling of the stellar initial mass function. The consequence of this incomplete sampling is a non-Gaussian distribution of the mass–luminosity relation for clusters that share the same evolutionary conditions (age, metallicity and stellar initial mass distribution function). We review some tests, that can be performed before the start of the analysis, to estimate if the observed cluster can be analyzed with synthesis models following traditional procedures (like χ 2 minimization) or if it is necessary make use of synthesis models in a probabilistic framework. Finally, we show the implications of these results for estimating the low-mass tail in the initial cluster mass distribution function.

Influence of Population III stars on cosmic chemical evolution

New observations from the Hubble ultra deep field suggest that the star formation rate at z>7 drops off faster than previously thought. Using a newly determined star formation rate for the normal mode of Population II/I stars (PopII/I), including this new constraint, we compute the Thomson scattering optical depth and find a result that is marginally consistent with WMAP5 results. We also reconsider the role of Population III stars (PopIII) in light of cosmological and stellar evolution constraints. While this input may be needed for reionization, we show that it is essential in order to account for cosmic chemical evolution in the early Universe. We investigate the consequences of PopIII stars on the local metallicity distribution function of the Galactic halo (from the recent Hamburg/ESO survey of metal-poor stars) and on the evolution of abundances with metallicity (based on the ESO large program on very metal-poor stars), with special emphasis on carbon-enhanced metal-poor stars. Our most important results show that the nucleosynthetic yields of PopIII stars lead to abundance patterns in agreement with those observed in extremely metal-poor stars. In this chemical approach to cosmic evolution, PopIII stars prove to be a compulsory ingredient, and extremely metal-poor stars are inevitably born at high redshift. (Abridged)