Stellar Metallicity Distribution Research Articles

PROBING BARYONIC PROCESSES AND GASTROPHYSICS IN THE FORMATION OF THE MILKY WAY DWARF SATELLITES. I. METALLICITY DISTRIBUTION PROPERTIES

In this paper, we study the chemical properties of the stars in the dwarf satellites around the MW-like host galaxies, and explore the possible effects of several baryonic processes, including supernova (SN) feedback, the reionization of the universe and H$_2$ cooling, on them and how current and future observations may put some constraints on these processes. We use a semi-analytical model to generate MW-like galaxies, for which a fiducial model can reproduce the luminosity function and the stellar metallicity--stellar mass correlation of the MW dwarfs. Using the simulated MW-like galaxies, we focus on investigating three metallicity properties of their dwarfs: the stellar metallicity--stellar mass correlation of the dwarf population, and the metal-poor and metal-rich tails of the stellar metallicity distribution in individual dwarfs. We find that (1) the slope of the stellar metallicity--stellar mass correlation is sensitive to the SN feedback strength and the reionization epoch; (2) the extension of the metal-rich tails is mainly sensitive to the SN feedback strength; (3) the extension of the metal-poor tails is mainly sensitive to the reionization epoch; (4) none of the three chemical properties are sensitive to the H$_2$ cooling process; and (5) comparison of our model results with the current observational slope of the stellar metallicity--stellar mass relation suggests that the local universe is reionized earlier than the cosmic average and local sources may have a significant contribution to the reionization in the local region, and an intermediate to strong SN feedback strength is preferred. Future observations of metal-rich and metal-poor tails of stellar metallicity distributions will put further constraints on the SN feedback and the reionization processes.

The colour–magnitude relation of globular clusters in Centaurus and Hydra

We investigate the colour-magnitude relation of metal-poor globular clusters, the 'blue tilt', in the Hydra and Centaurus galaxy clusters and constrain the primordial conditions for star cluster self-enrichment. We analyse U,I photometry for about 2500 globular clusters in the central regions of Hydra and Centaurus, based on FORS1@VLT data. We convert the measured colour-magnitude relations into mass-metallicity space and obtain a scaling of Z \propto M^{0.27 \pm 0.05} for Centaurus GCs and Z \propto M^{0.40 \pm 0.06} for Hydra GCs, consistent with results in other environments. We find that the GC mass-metallicity relation already sets in at present-day masses of a few 10^5 solar masses and is well established in the luminosity range of massive MW clusters like omega Centauri. We compare the mass-metallicity relation with predictions from the star cluster self-enrichment model by Bailin & Harris (2009). For this we include effects of dynamical and stellar evolution and a physically well motivated primordial mass-radius scaling. The self-enrichment model reproduces the observed relations well for average primordial half-light radii r_h ~ 1-1.5 pc, star formation efficiencies f_* ~ 0.3-0.4, and pre-enrichment levels of [Fe/H] ~ -1.7 dex. Within the self-enrichment scenario, the observed blue tilt implies a correlation between GC mass and width of the stellar metallicity distribution. We find that this implied correlation matches the trend of width with GC mass measured in Galactic GCs, including extreme cases like omega Cen and M54. We conclude that 1. A primordial star cluster mass-radius relation provides a significant improvement to the self-enrichment model fits. 2. Broadenend metallicity distributions as found in some massive MW globular clusters may have arisen naturally from self-enrichment processes, without the need of a dwarf galaxy progenitor.

Chemical evolution of classical and ultra-faint dwarf spheroidal galaxies

We present updated chemical evolution models of two dwarf spheroidal galaxies (Sculptor and Carina) and the first detailed chemical evolution models of two ultra-faint dwarfs (Hercules and Boötes I). Our results suggest that the dwarf spheroidals evolve with a low efficiency of star formation, confirming previous results, and the ultra-faint dwarfs with an even lower one. Under these assumptions, we can reproduce the stellar metallicity distribution function, the [α/Fe] versus [Fe/H] abundance patterns and the total stellar and gas masses observed at the present time in these objects. In particular, for the ultra-faint dwarfs we assume a strong initial burst of star formation, with the mass of the system being already in place at early times. On the other hand, for the classical dwarf spheroidals the agreement with the data is found by assuming the star formation histories suggested by the colour–magnitude diagrams and a longer time-scale of formation via gas infall. We find that all these galaxies should experience galactic winds, starting in all cases before 1 Gyr from the beginning of their evolution. From comparison with Galaxy data, we conclude that it is unlikely that the ultra-faint dwarfs have been the building blocks of the whole Galactic halo, although more data are necessary before drawing firm conclusions.

Studying the metallicity gradient in Virgo ellipticals with European-Extremely Large Telescope photometry of resolved stars

The next generation of large aperture ground based telescopes will offer the opportunity to perform accurate stellar photometry in very crowded fields. This future capability will allow one to study in detail the stellar population in distant galaxies. In this paper we explore the effect of photometric errors on the stellar metallicity distribution derived from the color distribution of the Red Giant Branch stars in the central regions of galaxies at the distance of the Virgo cluster. We focus on the analysis of the Color-Magnitude Diagrams at different radii in a typical giant Elliptical galaxy obtained from synthetic data constructed to exemplify observations of the European Extremely Large Telescope. The simulations adopt the specifications of the first light high resolution imager MICADO and the expected performance of the Multi-Conjugate Adaptive Optics Module MAORY. We find that the foreseen photometric accuracy allows us to recover the shape of the metallicity distribution with a resolution $\lesssim 0.4$ dex in the inner regions ($\mu_{\rm B}$ = 20.5 mag arcsec$^{-2}$) and $\simeq 0.2$ dex in regions with $\mu_{\rm B}$ = 21.6 mag arcsec$^{-2}$, that corresponds to approximately half of the effective radius for a typical giant elliptical in Virgo. At the effective radius ($\mu_{\rm B} \simeq 23$ mag arcsec$^{-2}$), the metallicity distribution is recovered with a resolution of $\simeq 0.1$ dex. It will thus be possible to study in detail the metallicity gradient of the stellar population over (almost) the whole extension of galaxies in Virgo. We also evaluate the impact of moderate degradations of the Point Spread Function from the assumed optimal conditions and find similar results, showing that this science case is robust.

Theoretical predictions for the effect of nebular emission on the broad-band photometry of high-redshift galaxies

By combining optical and near-IR observations from the Hubble Space Telescope with near-IR photometry from the Spitzer Space Telescope, it is possible to measure the rest-frame UV–optical colours of galaxies at z = 4–8. The UV–optical spectral energy distribution of star formation dominated galaxies is the result of several different factors. These include the joint distribution of stellar masses, ages and metallicities (solely responsible for the pure stellar spectral energy distribution), and the subsequent reprocessing by dust and gas in the interstellar medium. Using a large cosmological hydrodynamical simulation (MassiveBlack-II), we investigate the predicted spectral energy distributions of galaxies at high redshift with a particular emphasis on assessing the potential contribution of nebular emission. We find that the average (median) pure stellar UV–optical colour correlates with both luminosity and redshift such that galaxies at lower redshift and higher luminosity are typically redder. Assuming that the escape fraction of ionizing photons is close to zero, the effect of nebular emission is to redden the UV–optical 1500 − Vw colour by, on average, 0.4 mag at z = 8 declining to 0.25 mag at z = 4. Young and low-metallicity stellar populations, which typically have bluer pure stellar UV–optical colours, produce larger ionizing luminosities and are thus more strongly affected by the reddening effects of nebular emission. This causes the distribution of 1500 − Vw colours to narrow and the trends with luminosity and redshift to weaken. The strong effect of nebular emission leaves observed-frame colours critically sensitive to the redshift of the source. For example, increasing the redshift by 0.1 can result in observed-frame colours changing by up to ∼0.6. These predictions reinforce the need to include nebular emission when modelling the spectral energy distributions of galaxies at high redshift and also highlight the difficultly in interpreting the observed colours of individual galaxies without precise redshift information.

The stellar metallicity distribution of the Milky Way from the Beijing–Arizona–Taiwan–Connecticut survey

Using the stellar atmospheric parameters such as effective temperature and metallicity derived from SDSS spectra for 2200 main sequence (MS) stars which were also observed by Beijing - Arizona - Taiwan - Connecticut (BATC) photometric system, we develop the polynomial photometric calibration method to evaluate the stellar effective temperature and metallicity for BATC multi-color photometric data. This calibration method has been applied to about 160 000 MS stars from 67 BATC observed fields. Those stars have colors and magnitudes in the ranges 0.1 < d - h < 1.4 and 14.0 < d < 21.0. We find that there is a peak of metallicity distribution at [Fe/H] ~ -1.5 in the distance from the Galactic plane |Z| > 5 kpc which corresponds to the halo component and a peak at [Fe/H] ~ -0.7 in the region 2 < |Z| < 5 kpc where is dominated by the thick disk stars. The mean stellar metallicity smoothly decreases from -0.65 to -0.78 in the interval 0.5 < |Z|< 2 kpc. Metallicity distributions in the halo and the thick disk seem invariant with the distance from the Galactic plane.

Modeling the chemical evolution of the Galaxy halo

We study the chemical evolution and formation of the Galactic halo through the analysis of its stellar metallicity distribution function and some key elemental abundance patterns. Starting from the two-infall model for the Galaxy, which predicts too few low-metallicity stars, we add a gas outflow during the halo phase with a rate proportional to the star formation rate through a free parameter, lambda. In addition, we consider a first generation of massive zero-metal stars in this two-infall + outflow model adopting two different top-heavy initial mass functions and specific population III yields. The metallicity distribution function of halo stars, as predicted by the two-infall + outflow model shows a good agreement with observations, when the parameter lambda=14 and the time scale for the first infall, out of which the halo formed, is not longer than 0.2 Gyr, a lower value than suggested previously. Moreover, the abundance patterns [X/Fe] vs. [Fe/H] for C, N and alpha-elements O, Mg, Si, S, Ca show a good agreement with the observational data. If population III stars are included, under the assumption of different initial mass functions, the overall agreement of the predicted stellar metallicity distribution function with observational data is poorer than in the case without population III. We conclude that it is fundamental to include both a gas infall and outflow during the halo formation to explain the observed halo metallicity distribution function, in the framework of a model assuming that the stars in the inner halo formed mostly in situ. Moreover, we find that it does not exist a satisfactory initial mass function for population III stars which reproduces the observed halo metallicity distribution function. As a consequence, there is no need for a first generation of only massive stars to explain the evolution of the Galactic halo.

The stellar metallicity distribution of the Galaxy from the BATC survey

AbstractUsing ~2200 BATC main sequence (MS) stars which include SDSS stellar atmospheric parameters, we develop the polynomial photometric calibration method to evaluate the stellar effective temperature and metallicity for BATC data. This calibration method has been applied to about 160 000 MS stars. The mean stellar metallicity smoothly decreases from −0.65 to −0.78 dex in the interval 0.5 &lt; |Z| ≤ 2 kpc. Metallicity distributions in the halo and the thick disk seem invariant with the distance from the Galactic plane.

THE STELLAR METALLICITY DISTRIBUTION FUNCTION OF THE GALACTIC HALO FROM SDSS PHOTOMETRY

We explore the stellar metallicity distribution function of the Galactic halo based on SDSS ugriz photometry. A set of stellar isochrones is calibrated using observations of several star clusters and validated by comparisons with medium-resolution spectroscopic values over a wide range of metal abundance. We estimate distances and metallicities for individual main-sequence stars in the multiply scanned SDSS Stripe 82, at heliocentric distances in the range 5 - 8 kpc and |b| > 35 deg, and find that the in situ photometric metallicity distribution has a shape that matches that of the kinematically-selected local halo stars from Ryan & Norris. We also examine independent kinematic information from proper-motion measurements for high Galactic latitude stars in our sample. We find that stars with retrograde rotation in the rest frame of the Galaxy are generally more metal poor than those exhibiting prograde rotation, which is consistent with earlier arguments by Carollo et al. that the halo system comprises at least two spatially overlapping components with differing metallicity, kinematics, and spatial distributions. The observed photometric metallicity distribution and that of Ryan & Norris can be described by a simple chemical evolution model by Hartwick (or by a single Gaussian distribution); however, the suggestive metallicity-kinematic correlation contradicts the basic assumption in this model that the Milky Way halo consists primarily of a single stellar population. When the observed metallicity distribution is deconvolved using two Gaussian components with peaks at [Fe/H] ~ -1.7 and -2.3, the metal-poor component accounts for ~20% - 35% of the entire halo population in this distance range.

ARGOS – II. The Galactic bulge survey

Abstract We describe the motivation, field locations and stellar selection for the Abundances and Radial velocity Galactic Origins Survey (ARGOS) spectroscopic survey of 28 000 stars in the bulge and inner disc of the Milky Way galaxy across latitudes of b = −5° to −10°. The primary goal of this survey is to constrain the formation processes of the bulge and establish whether it is predominantly a merger or instability remnant. From the spectra (R = 11 000), we have measured radial velocities and determined stellar parameters, including metallicities and [α/Fe] ratios. Distances were estimated from the derived stellar parameters and about 14 000 stars are red giants within 3.5 kpc of the Galactic Centre. In this paper, we present the observations and analysis methods. Subsequent papers (III and IV) will discuss the stellar metallicity distribution and kinematics of the Galactic bulge and inner disc, and the implications for the formation of the bulge.

The stellar metallicity distribution of disc galaxies and bulges in cosmological simulations

By means of high-resolution cosmological hydrodynamical simulations of Milky Way-like disc galaxies, we conduct an analysis of the associated stellar metallicity distribution functions (MDFs). After undertaking a kinematic decomposition of each simulation into spheroid and disc sub-components, we compare the predicted MDFs to those observed in the solar neighbourhood and the Galactic bulge. The effects of the star formation density threshold are visible in the star formation histories, which show a modulation in their behaviour driven by the threshold. The derived MDFs show median metallicities lower by 0.2-0.3 dex than the MDF observed locally in the disc and in the Galactic bulge. Possible reasons for this apparent discrepancy include the use of low stellar yields and/or centrally-concentrated star formation. The dispersions are larger than the one of the observed MDF; this could be due to simulated discs being kinematically hotter relative to the Milky Way. The fraction of low metallicity stars is largely overestimated, visible from the more negatively skewed MDF with respect to the observational sample. For our fiducial Milky Way analog, we study the metallicity distribution of the stars born "in situ" relative to those formed via accretion (from disrupted satellites), and demonstrate that this low-metallicity tail to the MDF is populated primarily by accreted stars. Enhanced supernova and stellar radiation energy feedback to the surrounding interstellar media of these pre-disrupted satellites is suggested as an important regulator of the MDF skewness.

An empirical prediction for stellar metallicity distributions in nearby galaxies

We combine star-formation histories derived from observations of high redshift galaxies with measurements of the z ∼ 0 relation between gas-phase metallicity, stellar mass and star-formation rate to make an explicit and completely empirical connection between near-field and distant galaxy observations. Our approach relies on two basic assumptions: (1) galaxies’ average paths through time in stellar mass versus star-formation rate space are represented by a family of smooth functions that are determined by the galaxies’ final stellar mass and (2) galaxies grow and become enriched with heavy elements such that they always evolve along the mass–metallicity–star-formation rate relation. By integrating over these paths, we can track the chemical evolution of stars in galaxies in a model-independent way, without the need for explicit assumptions about gas inflow, outflow or star-formation efficiency. Using this approach, we present predictions of stellar metallicity (i.e. O/H) distribution functions for present day star-forming galaxies of different stellar masses and the evolution of the α-element stellar metallicity–mass relation since z ∼ 1. The metallicity distribution functions are fairly well described as Gaussians, truncated at high metallicity, with power-law tails to low metallicity. We find that the stellar metallicity distribution for Milky Way mass galaxies is in reasonable agreement with observations for our Galaxy, and that the predicted stellar mass versus mean stellar metallicity relation at z = 0 agrees quite well with results derived from galaxy surveys. This validates the assumptions that are implicit in our simple approach. Upcoming observations will further test these assumptions and their range of validity, by measuring the mean stellar mass–metallicity relation up to z ∼ 1, and by measuring the stellar metallicity distributions over a range of galaxy masses.

The distribution of metals in cosmological hydrodynamical simulations of dwarf disc galaxies

We examine the chemical properties of 5 cosmological hydrodynamical simulations of an M33-like disc galaxy which have been shown to be consistent with the morphological characteristics and bulk scaling relations expected of late-type spirals. These simulations are part of the Making Galaxies In a Cosmological Context (MaGICC) Project, in which stellar feedback is tuned to match the stellar mass -- halo mass relationship. Each realisation employed identical initial conditions and assembly histories, but differed from one another in their underlying baryonic physics prescriptions, including (a) the efficiency with which each supernova energy couples to the ISM, (b) the impact of feedback associated with massive star radiation pressure, (c) the role of the minimum shut-off time for radiative cooling of Type II SNe remnants, (d) the treatment of metal diffusion, and (e) varying the IMF. Our analysis focusses on the resulting stellar metallicity distribution functions (MDFs) in each simulated (analogous) `solar neighbourhood' and central `bulge' region. We compare the simulated MDFs' skewness, kurtosis, and dispersion (inter-quartile, inter-decile, inter-centile, and inter-tenth-percentile regions) with that of the empirical solar neighbourhood MDF and Local Group dwarfs. We find that the MDFs of the simulated discs are more negatively skewed, with higher kurtosis, than those observed locally. We can trace this difference to the simulations' tight and correlated age-metallicity relations (compared with that of the Milky Way), suggesting that these relations within `dwarf' discs might be steeper than in L* discs and/or the degree of stellar orbital re-distribution and migration inferred locally has not been captured in their entirety, at the resolution of our simulations. The important role of metal diffusion in ameliorating the over-production of extremely metal-poor stars is highlighted.

Ionised gas abundances in barred spiral galaxies

This is the third paper of a series devoted to study the properties of bars from long slit spectroscopy to understand their formation, evolution and their influence on the evolution of disk galaxies. In this work we aim to determine the gas metallicity distribution of a sample of 20 barred early-type galaxies. We compare the nebular and stellar metallicity distributions to conclude about the origin of the warm gas. We compare the results of nebular emission metallicities using different semi-empirical methods. We carry out AGN diagnostic diagrams along the radius to determine the radius of influence of the AGN and the nuclei nature of the studied galaxies. We then derive the gas metallicities along the bars and compare the results to the distribution of stellar metallicities in the same regions. Most of the gas emission is centrally concentrated, although 15 galaxies also show emission along the bar. In the central regions, gas oxygen abundances are in the range 12+$\log$(O/H)= 8.4-9.1. The nebular metallicity gradients are very shallow in the bulge and bar regions. For three galaxies (one of them a LINER), the gas metallicities lie well below the stellar ones in the bulge region. These results do not depend on the choice of the semi-empirical calibration used to calculate the abundances. We see that the galaxies with the lowest abundances are those with the largest rotational velocities. The presence of gas of significantly lower metallicity than the stellar abundances in three of our galaxies, points to an external origin as the source of the gas that fuels the present star formation in the centre of some early-type barred galaxies. The fact that the bar/disk nebular metallicities are higher than the central ones might be indicating that the gas could be accreted via cooling flows instead of radial accretion from gas sitting in the outer parts of the disk.

A spectroscopic survey of Andromeda’s Western Shelf

The Andromeda galaxy (M31) shows many tidal features in its halo, including the Giant Southern Stream (GSS) and a sharp ledge in surface density on its western side (the W Shelf). Using DEIMOS on the Keck telescope, we obtain radial velocities of M31's giant stars along its NW minor axis, in a radial range covering the W Shelf and extending beyond its edge. In the space of velocity versus radius, the sample shows the wedge pattern expected from a radial shell, which is detected clearly here for the first time. This confirms predictions from an earlier model of formation of the GSS, which proposed that the W Shelf is a shell from the third orbital wrap of the same tidal debris stream that produces the GSS, with the main body of the progenitor lying in the second wrap. We calculate the distortions in the shelf wedge pattern expected from its outward expansion and angular momentum, and show that these effects are echoed in the data. In addition, a hot, relatively smooth spheroid population is clearly present. We construct a bulge-disk-halo N-body model that agrees with surface brightness and kinematic constraints, and combine it with a simulation of the GSS. From the contrasting kinematic signatures of the hot spheroid and shelf components, we decompose the observed stellar metallicity distribution into contributions from each component using a non-parametric mixture model. The shelf component's metallicity distribution matches previous observations of the GSS superbly, further strengthening the evidence they are connected and bolstering the case for a massive progenitor of this stream.

The stellar metallicity distribution in intermediate-latitude fields with BATC and SDSS data

Based on the Beijing-Arizona-Taiwan-Connecticut (BATC) and Sloan Digital Sky Survey (SDSS) photometric data, we adopt SEDs fitting Method to evaluate the metallicity distribution for \sim40,000 main-sequence stars in the Galaxy. According to the derived photometric metallicities of these sample stars, we find that the metallicity distribution shift from metal-rich to metal-poor with the increase of distance from the Galactic center. The mean metallicity is about of 1.5 \pm 0.2dex in the outer halo and 1.3 \pm 0.1 dex in the inner halo. The mean metallicity smoothly decreases from -0.4 to -0.8 in interval 0 < r \leq 5 kpc. The fluctuation in the mean metallicity with Galactic longitude can be found in interval 4 < r \leq 8 kpc. There is a vertical abundance gradients d[Fe/H]/dz\sim -0.21 \pm 0.05 dex kpc-1 for the thin disk (z \leq 2 kpc). At distance 2 < z \leq 5 kpc, where the thick disk stars are dominated, the gradients are about of -0.16 \pm0.06 dex kpc-1, it can be interpreted as a mixture of stellar population with different mean metallicities at all z levels. The vertical metallicity gradient is - 0.05 \pm0.04 dex kpc-1 for the halo (z > 5 kpc). So there is little or no metallicity gradient in the halo.

TWO-COMPONENT GALACTIC BULGE PROBED WITH RENEWED GALACTIC CHEMICAL EVOLUTION MODEL

Results of recent observations of the Galactic bulge demand that we discard a simple picture of its formation, suggesting the presence of two stellar populations represented by two peaks of stellar metallicity distribution (MDF) in the bulge. To assess this issue, we construct Galactic chemical evolution models that have been updated in two respects: First, the delay time distribution (DTD) of type Ia supernovae (SNe Ia) recently revealed by extensive SN Ia surveys is incorporated into the models. Second, the nucleosynthesis clock, the s-processing in asymptotic giant branch (AGB) stars, is carefully considered in this study. This novel model first shows that the Galaxy feature tagged by the key elements, Mg, Fe, Ba for the bulge as well as thin and thick disks is compatible with a short-delay SN Ia. We present a successful modeling of a two-component bulge including the MDF and the evolutions of [Mg/Fe] and [Ba/Mg], and reveal its origin as follows. A metal-poor component (<[Fe/H]>~-0.5) is formed with a relatively short timescale of ~1 Gyr. These properties are identical to the thick disk's characteristics in the solar vicinity. Subsequently from its remaining gas mixed with a gas flow from the disk outside the bulge, a metal-rich component (<[Fe/H]>~+0.3) is formed with a longer timescale (~4 Gyr) together with a top-heavy initial mass function that might be identified with the thin disk component within the bulge.

Spectral fitting of SDSS passive galaxies with α-enhanced single stellar populations

AbstractThe power of population synthesis as a mean to estimate the star-formation and chemical histories of galaxies has been well established in the last decade. The major developments were due to a huge avalanche of methods, codes and high-quality galaxy data sets, such as the 2dF, 6dF and SDSS surveys. Semi-empirical spectral synthesis allows for the decomposition of a galaxy spectrum in terms of linear combinations of base elements, i.e. Single Stellar Populations (SSPs) of different ages and metallicities, which are computed from evolutionary synthesis codes (BPASS, GALEV, GALAXEV, MILES, PÉGASE, etc. . .), containing distinct ingredients like: stellar library, evolutionary tracks, metallicities and Initial Mass Function. In general, they have solar-scaled relative abundances, but this is about to change with the unfolding of new α-enhanced SSP models (Coelho et al. 2007). However, passive galaxies have some spectral features corresponding to “enhanced-ratios” ([E/Fe]), like O, Ne, Si, S, Mg, Na, C and N over Fe that are not well modeled using solar-scaled SSPs (Trager et al. 2000), leading to residuals between observed and modeled spectra, which also correlate with the velocity dispersion (σ*) and stellar mass (M*): Massive galaxies exhibit a larger [E/Fe] discrepancy than less massive ones. This result can be interpreted as a signature of distinct previous star-formation efficiencies in passive galaxies, leading to distinctive ratios of type Ia and II SNe.We have applied the starlight spectral synthesis code (Cid Fernandes et al. 2005) to a sample of ~ 1000 passive galaxies from the SDSS DR7 with a S/N at the continuum ≥ 20 to investigate possible enhancements in the derived [E/Fe] ratios. Three sets of SSPs based on Coelho et al. (2007) theoretical models and Walcher et al. (2009) prescriptions were computed for [α/Fe]=0.0, [α/Fe]=0.2 and [α/Fe]=0.4. Our aim is to determine: (1) the quality of the fits, (2) the mean stellar age and metallicity distributions, and (3) the star-formation history of passive galaxies.Using [α/Fe]=0.0 SSPs, we have identified the strongest residuals in the CN (4142.125-4177.125 Å), Na D (5876.875-5909.375 Å) and Mg (5069.125-5196.625 Å) bands. On the other hand, [α/Fe]=0.2 and [α/Fe]=0.4 SSP models tend to reproduce better the Mg band, as compared to solar-scaled SSPs ([α/Fe]=0.0). The residuals are decreased by 1.77 Å ([α/Fe]=0.2) and 2.92 Å ([α/Fe]=0.4). However, as expected, these α--enhanced models lead to worse fits for the CN and Na D bands. These residuals may even reach up to 2.08 Å (CN) and 4.20 Å (Na D), using [α/Fe]=0.2 SSPs and 2.28 Å (CN) and 7.94 Å (Na D), using [α/Fe]=0.4 SSPs.In terms of mean stellar ages and metallicities, we obtain non-negligible biases in both quantities when we compare the solar-scaled SSPs with α-enhanced ones, which tend to have mean stellar ages by 0.12 dex ([α/Fe]=0.2) and 0.14 dex ([α/Fe]=0.4) higher and mean stellar metallicities by 0.1 dex ([α/Fe]=0.2) and 0.2 dex ([α/Fe]=0.4) lower.

Resolved Stellar Population of Distant Galaxies in the ELT Era

ABSTRACTThe expected imaging capabilities of future Extremely Large Telescopes (ELTs) will offer the unique possibility to investigate the stellar populations of distant galaxies from the photometry of the stars in very crowded fields. Using simulated images and photometric analysis we explore here two representative science cases aimed at recovering the characteristics of the stellar populations in the inner regions of distant galaxies. Specifically, case (1) at the center of the disk of a giant spiral in the Centaurus Group (μB ∼ 21, distance of 4.6 Mpc) and case (2) at half of the effective radius of a giant elliptical in the Virgo Cluster (μI ∼ 19.5, distance of 18 Mpc). We generate synthetic frames by distributing model stellar populations and adopting a representative instrumental setup, i.e., a 42 m telescope operating close to the diffraction limit. The effect of crowding is discussed in detail, showing how stars are measured preferentially brighter than they really are as the confusion limit is approached. We find that (1) accurate photometry (σ ∼ 0.1, completeness ≳90%) can be obtained for case 2 down to I ∼ 28.5, J ∼ 27.5, allowing us to recover the stellar metallicity distribution in the inner regions of ellipticals in Virgo to within ∼0.1 dex; (2) the same photometric accuracy holds for the science case 1 down to J ∼ 28.0, K ∼ 27.0, enabling reconstruction of the star formation history up to the Hubble time via simple star counts in diagnostic boxes. For the latter case, we discuss the possibility of deriving more detailed information on the star formation history from the analysis of their horizontal-branch stars. We show that the combined features of high sensitivity and angular resolution of ELTs may open a new era for our knowledge of the stellar content of galaxies of different morphological types up to the distance of the Virgo cluster.

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.