Ages and Metallicities in Elliptical Galaxies from the H_beta, , and Mg2 Diagnostics (original) (raw)
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On the metallicity distribution in the nuclei of elliptical galaxies
Monthly Notices of the Royal Astronomical Society, 1997
Using current models of spectrophotometric properties of single age, single metallicity and stellar populations (SSPs) I have computed the Mg 2 , HP, Fe52 and Fe53 line strengths for stellar populations with a metallicity spread. The comparison of these models with the nuclear indices of early-type galaxies yields the following major conclusions. The metallicity distribution of the closed box, simple model for the chemical evolution of galaxies is not able to account for Mg 2 and Fe52, Fe53 values in excess of '" 0.27,3 and 2.7, respectively, which are observed in the nuclei of a large fraction of ellipticals. To reproduce the line strengths in these galaxies high average metallicities, small metallicity dispersion and high ages are required. In particular, Mg 2 values of '" 0.3 are reproduced only with a metallicity distribution ranging from "" 0.5 to "" 3 Zo' and "" 15 Gyr old stellar populations. I interpret the data as indicating that the gas out of which the nuclei of ellipticals formed was preenriched, to larger metallicities for increasing Mg 2 • The presence of a metallicity dispersion does not alter the relation between Mg 2 and iron indices with respect to the SSP models. Thus, the need for a Mg/Fe overabundance in the strongest lined galaxies is confirmed, and I present a simple way to estimate the [Mg/Fe] ratio on the basis of existing models with solar abundance ratios.
The magnesium Mg2 index as an indicator of metallicity in elliptical galaxies
The Astronomical Journal, 1992
A quantitative calibration of the Mg 2 index [Faber et al, AJ, 82, 941 (1977)] is attempted deriving a metallicity scale for elliptical galaxies. The dependence of the index on stellar temperature, gravity, and metallicity has been studied through spectroscopic observations of 87 standards applying the derived calibration to models for stellar population synthesis. Buzzoni's [ApJS, 71,817 (1989) ] computational code has been used to explore the behavior of the index versus age, IMF, and metallicity of simple stellar populations (SSPs) inferring galactic metallicity for the Davies et al. [ApJS, 69, 581 (1987)] extensive observational database. It appears .that ellipticals are old metal-rich systems, with age about 15 Gyr and [Fe/H] = + 0.15. A large spread of nearly one order of magnitude is however derived for metallicity among single galaxies confirming that [Fe/H] is the driving parameter inducing the color spread in the galaxy population. Evolutionary behavior of Mg 2 is briefly discussed giving its expected variations at early epochs for comparison with high-redshift galaxies.
The ages and metallicities of galaxies in the local universe
Monthly Notices of …, 2005
We derive stellar metallicities, light-weighted ages and stellar masses for a magnitudelimited sample of 175,128 galaxies drawn from the Sloan Digital Sky Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of these parameters using a large library of model spectra at medium-high resolution, covering a comprehensive range of star formation histories. The constraints we derive are set by the simultaneous fit of five spectral absorption features, which are well reproduced by our population synthesis models. By design, these constraints depend only weakly on the α/Fe element abundance ratio. Our sample includes galaxies of all types spanning the full range in star formation activity, from dormant early-type to actively starforming galaxies. By analysing a subsample of 44,254 high-quality spectra, we show that, in the mean, galaxies follow a sequence of increasing stellar metallicity, age and stellar mass at increasing 4000Å-break strength. For galaxies of intermediate mass, stronger Balmer absorption at fixed 4000Å-break strength is associated with higher metallicity and younger age. We investigate how stellar metallicity and age depend on total galaxy stellar mass. Low-mass galaxies are typically young and metal-poor, massive galaxies old and metal-rich, with a rapid transition between these regimes over the stellar mass range 3 × 10 9 M * 3 × 10 10 M ⊙ . Both high-and low-concentration galaxies follow these relations, but there is a large dispersion in stellar metallicity at fixed stellar mass, especially for low-concentration galaxies of intermediate mass.
1997
The line strength indices Hb, Mg2, and , together with their gradients and dependence on the galaxy luminosity (mass), observed in elliptical galaxies are customarily considered as reliable indicators of systematic differences in age and abundances of Mg and Fe. In this paper, we address the question whether or not the indices Mg2 and (and their gradients) are real indicators of chemical abundances and enhancement of these or other effects must be taken into account. We show that this is not the case because the above indices are severely affected by the unknown relative percentage of stars as function of the metallicity. In order to cast the problem, first we provide basic calibrations for the variations dHb, dMg2, and d as a function of the age Dlog(t) (in Gyr), metallicity Dlog(Z/Z_o), and D[Mg/Fe]. Second, we analyze the implications of the gradients in Mg2 and observed across these systems. Finally, the above calibration is used to explore the variations from galaxy to galaxy o...
Arxiv preprint astro-ph/ …, 2006
We exploit recent constraints on the ages and metallicities of early-type galaxies in the Sloan Digital Sky Survey (SDSS) to gain new insight into the physical origin of two fundamental relations obeyed by these galaxies: the colour-magnitude and the Mg 2 -σ V relations. Our sample consists of 26,003 galaxies selected from the SDSS Data Release Two (DR2) on the basis of their concentrated light profiles, for which we have previously derived median-likelihood estimates of stellar metallicity, light-weighted age and stellar mass. Our analysis provides the most unambiguous demonstration to date of the fact that both the colour-magnitude and the Mg 2 -σ V relations are primarily sequences in stellar mass and that total stellar metallicity, α-elements-to-iron abundance ratio and light-weighted age all increase with mass along the two relations. For high-mass ellipticals, the dispersion in age is small and consistent with the error. At the low-mass end, there is a tail towards younger ages, which dominates the scatter in colour and index strength at fixed mass. A small, but detectable, intrinsic scatter in the mass-metallicity relation also contributes to the scatter in the two observational scaling relations, even at high masses. Our results suggest that the chemical composition of an early-type galaxy is more tightly related to its dynamical mass (including stars and dark matter) than to its stellar mass. The ratio between stellar mass and dynamical mass appears to decrease from the least massive to the most massive galaxies in our sample.
The evolution of the mass-metallicity relation in galaxies of different morphological types
Astronomy and Astrophysics, 2009
Aims. By means of chemical evolution models for ellipticals, spirals and irregular galaxies, we aim at investigating the physical meaning and the redshift evolution of the mass-metallicity relation as well as how this relation is connected with galaxy morphology. Methods. Our models distinguish among different morphological types through the use of different infall, outflow, and star formation prescriptions. We assume that galaxy morphologies do not change with cosmic time. We present a method to account for a spread in the epochs of galaxy formation and to refine the galactic mass grid. To do that, we extract the formation times randomly and assume an age dispersion ∆ t . We compare our predictions to observational results obtained for galaxies between redshifts 0.07 and 3.5. Results. We reproduce the mass-metallicity (MZ) relation mainly by means of an increasing efficiency of star formation with mass in galaxies of all morphological types, without any need to invokegalactic outflows favoring the loss of metals in the less massive galaxies. Our predictions can help constraining the slope and the zero point of the observed local MZ relation, both affected by uncertainties related to the use of different metallicity calibrations. We show how, by considering the MZ, the O/H vs star formation rate (SFR), and the SFR vs galactic mass diagrams at various redshifts, it is possible to constrain the morphology of the galaxies producing these relations. Our results indicate that the galaxies observed at z = 3.5 should be mainly proto-ellipticals, whereas at z = 2.2 the observed galaxies consist of a morphological mix of proto-spirals and proto-ellipticals. At lower redshifts, the observed MZ relation is well reproduced by considering both spirals and irregulars. Galaxies with different star formation histories may overlap in the MZ diagram, but measures of abundance ratios such as [O/Fe] can help to break this degeneracy. Predictions for the MZ relations for other elements (C, N, Mg, Si, Fe) are also presented, with largest dispersions predicted for elements produced in considerable amounts by Type Ia SNe, owing to the long lifetimes of their progenitors.
On Extending the Mass‐Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
The Astrophysical Journal, 2006
We report 4.5 µm luminosities for 27 nearby (D 5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 µm luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 µm L-Z relation is 12+log(O/H) = (5.78 ± 0.21) + (−0.122 ± 0.012) M [4.5] , where M [4.5] is the absolute magnitude at 4.5 µm. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity (M * -Z) relation is 12+log(O/H) = (5.65 ± 0.23) + (0.298 ± 0.030) log M * , and extends the SDSS M * -Z relation to lower mass by about 2.5 dex. We find that the dispersion in the M * -Z relation is similar over five orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L-Z and M * -Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed.
Astronomy & Astrophysics, 2014
We study the relationship between age, metallicity, and α-enhancement of FGK stars in the Galactic disk. The results are based upon the analysis of high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore the limitations of the observed dataset, i.e. the accuracy of stellar parameters and the selection effects that are caused by the photometric target preselection. We find that the colour and magnitude cuts in the survey suppress old metal-rich stars and young metal-poor stars. This suppression may be as high as 97% in some regions of the age-metallicity relationship. The dataset consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr, Galactocentric distances from 6 kpc to 9.5 kpc, and vertical distances from the plane 0 < |Z| < 1.5 kpc. On this basis, we find that i) the observed age-metallicity relation is nearly flat in the range of ages between 0 Gyr and 8 Gyr; ii) at ages older than 9 Gyr, we see a decrease in [Fe/H] and a clear absence of metal-rich stars; this cannot be explained by the survey selection functions; iii) there is a significant scatter of [Fe/H] at any age; and iv) [Mg/Fe] increases with age, but the dispersion of [Mg/Fe] at ages >9 Gyr is not as small as advocated by some other studies. In agreement with earlier work, we find that radial abundance gradients change as a function of vertical distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In addition, we show that the inner disk is not only more α-rich compared to the outer disk, but also older, as traced independently by the ages and Mg abundances of stars.