Showing papers by "Anna Gallazzi published in 2006"

Ages and metallicities of early-type galaxies in the sloan digital sky survey: new insight into the physical origin of the colour–magnitude and the Mg2–σV relations

Abstract: 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 2 (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.

Ages and metallicities of early-type galaxies in the SDSS: new insight into the physical origin of the colour-magnitude and the Mg2-sigmaV relations

Abstract: 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 Mg2-sigmaV relations. Our sample consists of 26,003 galaxies selected from the SDSS 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 relations are primarily sequences in stellar mass and that total stellar metallicity, alpha-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.

Hα surface photometry of galaxies in nearby clusters. V. The survey completion

Abstract: We present the Halpha imaging observations of 273 late-type galaxies in the nearby rich galaxy clusters Virgo, A1367, Coma, Cancer, Hercules and in the Great Wall, carried out primarily with the 2.1m telescope of the San Pedro Martir Observatory (SPM) and with the ESO/3.6m telescope. We derived the Halpha+[NII] fluxes and equivalent widths. The Halpha survey reached completion for an optically selected sample of nearby galaxies in and outside rich clusters. Taking advantage of the completeness of the data set, the dependence of Halpha properties on the Hubble type was determined for late-type galaxies in the Virgo cluster. Differences in the gaseous content partly account for the large scatter of the Halpha E.W. within each Hubble-type class. We studied the radial distributions of the Halpha E.W. around Coma+A1367 and the Virgo clusters in two luminosity bins. Luminous galaxies show a decrease in their average Halpha E.W. in the inner 1 virial radius, while low-luminosity galaxies do not show this trend.

The colours of elliptical galaxies

Abstract: We have compiled a sample of 2728 nearby (z < 0.08) elliptical galaxies with photometry in the g, r, i, z bands from the Sloan Digital Sky Survey (SDSS) and J, H, K photometry from the Two-Micron All-Sky Survey (2MASS). Stellar masses, stellar velocity dispersions and structural parameters such as sizes and surface mass densities are also available for these objects. In order to correct the aperture mismatch between SDSS and 2MASS, we correct the SDSS magnitudes to the isophotal circular radius where the 2MASS magnitudes are measured. We compare the correlations between optical, optical-infrared and infrared colours and galaxy luminosity, stellar mass, velocity dispersion and surface mass density. We find that all galaxy colours correlate more strongly with stellar mass and velocity dispersion than with any other structural parameter. The dispersion about these two relations is also smaller. We also study the correlations between a variety of stellar absorption-line indices and the same set of galaxy parameters and we reach very similar conclusions. Finally, we analyse correlations between absorption-line indices and colour. Our results suggest that the optical colours of elliptical galaxies are sensitive to a combination of age, metallicity and α-enhancement, while the optical-infrared colours are sensitive to metallicity and to α-enhancement, but are somewhat less sensitive to age.

A census of the physical parameters of nearby galaxies

Abstract: We combine together physical parameters estimates for a large sample of galaxies drawn from the Sloan Digital Sky Survey Data Release Two (SDSS DR2), spanning wide ranges in galaxy properties, in order to provide a census of the physical parameters of galaxies in the local Universe. Stellar metallicities, masses and ages for this sample have been estimated comparing the strengths of selected absorption features in each observed spectrum to a comprehensive Monte Carlo library of star formation histories, following a Bayesian statistical approach. This allows us also to derive the total amount of metals locked up in stars today, and hence the average stellar metallicity of galaxies in the local Universe, finding a value of 1.03 −0.15Z . We also quantify the relative contribution to the total densities of mass and metals in stars from galaxies with different properties.