Spectral Line Indices Research Articles

Colour-magnitude relations and spectral line strengths in the Coma cluster

We use the C24668, Fe4383, HγA and HδA spectral absorption line indices, together with U- and V-band photometry of 101 galaxies in the Coma cluster, to investigate how mean age and metal abundance correlate with galaxy luminosity. In particular, we use the line index measurements to address the origin of the colour–magnitude relation (CMR). We find that the CMR in Coma is driven primarily by a luminosity–metallicity correlation. We additionally show evidence for a relation between age and luminosity, in the direction predicted by the semi-analytic hierarchical clustering models of Kauffmann & Charlot, but this is only present in the C24668 index models, and could be an effect of the lack of non-solar abundance ratios in the Worthey models used. By comparing deviations from the CMR with deviations in absorption index from analogous ‘index–magnitude’ relations, we find that colour deviations bluewards of the mean relation are strongly correlated with the hydrogen Balmer line series absorption. We show that the properties of these blue galaxies are consistent with the presence of a young stellar population in the galaxies, rather than with a reduced metallicity.

Stellar populations of cluster E and S0 galaxies

Spectral line indices for a sample of 290 nearby E and S0 galaxies are used to investigate the stellar populations of these galaxies. Relations are established between the line indices (Mg2, <Fe>, Hbeta) and the velocity dispersions, the masses of the galaxies, the M/L ratios and the cluster environment. The difference between the slope of the Mg2-sigma relation and the slope of the <Fe>-sigma relation indicates that the abundance ratio [Mg/Fe] increases with the velocity dispersion. The <Fe> index is stronger correlated with the projected cluster surface density, than with the galaxy mass or the velocity dispersion. Also the Mg2 index depends on the cluster environment. The dependence on the environment implies that [Mg/Fe] decreases with increasing density. The M/L ratios are strongly correlated with the Mg2 and Hbeta, while <Fe> is only weakly correlated with the M/L ratio. Based on current stellar population models it is not yet possible to derive unique physical parameters (mean age, mean abundances, mean IMF, and fraction of dark matter) from the observables (line indices, velocity dispersion, mass, M/L ratio).

THE CHEMICAL AND DYNAMICAL EVOLUTION OF THE GALAXY

ABSTRACT Theoretical models of the chemical and dynamical evolution of galaxies are developed and a Galactic structure survey is presented -- both are invaluable tools for uncovering the history of the formation of our Galaxy. Important advances in my theoretical models are non-instantaneous recycling of newly synthesized elements such as iron from Type Ia supernovae (mergers of white dwarf binaries). For the first time, Type Ia supernovae are incorporated in models of stellar remnants as baryonic dark matter. If dark halos consist mainly of white dwarfs, as has been proposed, then an excessive Type Ia supernovae rate is predicted unless the binary fraction of precursor stars is virtually zero. The observed constancy of chemical abundance ratios in halo stars e.g., [O/Fe] ~/= +0.5, signifies enrichment from only Type II supernovae. I explore the timescale for the onset of Type Ia supernovae as an upper limit to the timescale of halo formation. The apparent inconsistency between age differences of 2-3 Gyr in halo stars and the timescale inferred from the onset of Type Ia supernovae (~<0.1 Gyr for a range of models) suggests galaxies formed hierarchically from substructures in which star formation was disrupted before the onset of Type Ia supernovae. Chemical and dynamical evolution models of viscous disks with non-instantaneous recycling are also explored. The patterns of chemical element ratios and scatter in the age-metallicity relationship are calculated. Characterized by radial flow of gas, these models are ideal for exploring new ideas on the formation of galactic bulges from inflow of disk gas. The observed high metallicity of bulge stars may not require an ad hoc variation of the stellar initial mass function, if the bulge is an extension of the disk metallicity gradient. In addition, I present data from an unbiased spectroscopic survey of F-G stars in Selected Areas 51, 57 and 68. Radial velocities accurate to ~<10 km/s are derived. Comparison of the velocity distributions with predictions of Galactic structure models suggests Vrot ~/= 170, sigmaV ~/= 50, sigmaU ~/= 80 km/s for the thick disk. Preliminary distances are derived by photometric parallax, and 3-dimensional velocities calculated using published proper motions. Stellar metallicities can be derived by analysis of spectral line indices composed of weak iron features. Calibration against metallicity standard stars is presented. Evaluation of metallicities of program stars are forthcoming. These data are useful for constraining the existence of a metallicity gradient in the Galactic halo, and determining the correlation of kinematics and chemistry in the disk/halo transition.

Spectral synthesis in the ultraviolet. I - Far-ultraviolet stellar library

A library of mean stellar energy distributions for use in population synthesis of the ultraviolet spectra of active star-forming galaxies is derived from the ultraviolet spectrophotometry in the IUE Spectral Atlas. The spectra extend from 1230 to 1930 A with a resolution of 6 A. The library contains eight main-sequence groups from O3 to A7 V, four giant groups from O5 to B9 III, and three supergiant groups from O9 to A0 I. Several continuum and spectral line indices are computed, and their usefulness as temperature and luminosity discriminants is discussed. The advantages of analyzing composite stellar populations in the far-ultraviolet, where cool starlight is strongly suppressed, are examined.