Recovering galaxy star formation and metallicity histories from spectra using VESPA

We introduce versatile spectral analysis (VESPA): a new method which aims to recover robust star formation and metallicity histories from galactic spectra. VESPA uses the full spectral range to construct a galaxy history from synthetic models. We investigate the use of an adaptative parametrization grid to recover reliable star formation histories on a galaxy-by-galaxy basis. Our goal is robustness as opposed to high-resolution histories, and the method is designed to return high time resolution only where the data demand it. In this paper we detail the method and we present our findings when we apply VESPA to synthetic and real Sloan Digital Sky Survey (SDSS) spectroscopic data. We show that the number of parameters that can be recovered from a spectrum depends strongly on the signal-to-noise ratio, wavelength coverage and presence or absence of a young population. For a typical SDSS sample of galaxies, we can normally recover between two and five stellar populations. We find very good agreement between VESPA and our previous analysis of the SDSS sample with MOPED.     

Instabilities of galactic discs in the presence of star formation

We discuss the stability of galactic discs in which the energy of interstellar clouds is gained in encounters with expanding supernova (SN) remnants and lost in inelastic collisions. Energy gain and loss processes introduce a phase difference between the pressure and density perturbations, making discs unstable on small scales for several recipes of star formation. This is in contrast to the standard stability analysis in which small-scale perturbations are stabilized by pressure. In the limit of small scales, the dispersion relation for the growth rate reduces to that of thermal instabilities in a fluid without gravity. If instabilities lead to star formation, then our results imply a secondary mode of star formation that operates on small scales and feeds on the existence of a primary mode on intermediate scales. This may be interpreted as positive feedback. Further, the standard stability criterion on intermediate scales is significantly modified.     

The metallicity distribution of stars in the galactic bulge and disc

The chemical evolution of the 3-component system of halo-bulge-disc is calculated. If the bulge accretes primordial halo matter quickly and forms stars rapidly before the gas is ejected by a galactic wind after 10⁹ yr, the metallicity distribution of the bulge K-giants (Rich, 1988) is reproduced. The metal-enriched matter in a wind from the bulge is mixed with the halo gas which is accreted into the disc. The metallicity distribution of the G-dwarfs and Twarog's age-metallicity relatin in the solar neighbourhood can be well reproduced by assuming reasonable bulge-to-disc mass ratio.     

Induced star formation and non-linear multipopulation models for galactic evolution

I discuss the physical ingredients necessary for a coherent treatment of the star formation processes on galactic scales.     

The recent star formation history of the Hipparcos solar neighbourhood

We use data from the Hipparcos catalogue to construct colour–magnitude diagrams for the solar neighbourhood, which are then treated using advanced Bayesian analysis techniques to derive the star formation rate history, SFR ( t ), of this region over the last 3 Gyr. The method we use allows the recovery of the underlying SFR ( t ) without the need of assuming any a priori structure or condition on SFR ( t ), and hence yields a highly objective result. The remarkable accuracy of the data permits the reconstruction of the local SFR ( t ) with an unprecedented time resolution of ≈50 Myr. An SFR ( t ) that has an oscillatory component of period ≈0.5 Gyr is found, superimposed on a small level of constant star formation activity. Problems arising from the non-uniform selection function of the Hipparcos satellite are discussed and treated. Detailed statistical tests are then performed on the results, which confirm the inferred SFR ( t ) to be compatible with the observed distribution of stars.     

Probes of cosmic star formation history

I summarize X-ray diagnostic studies of cosmic star formation history in terms of evolutionary schemes for X-ray binary evolution in normal galaxies with evolving star formation. Deep X-ray imaging studies byChandra andXMM-Newton are now beginning to constrain both the X-ray luminosity evolution of galaxies and the logN- logS diagnostics of the X-ray background. I discuss these in the above context, summarizing current understanding and future prospects.     

Abundance analysis of selected cepheids and the galactic distribution of metallicity

We have determined the atmospheric abundances of selected Cepheids in order to study the large-scale chemical inhomogeneities across the galactic disk. The classical Cepheids were selected as probes to study the variation of metallicity in the galactic disk, because of their high intrinsic luminosity, small age and the existence of period-luminosity and period-age relationships. High dispersion spectra of programme stars WZ Sgr, X Sgr, ? Gem, T Mon and S V Mon were obtained using the 102-cm reflector of Kavalur Observatory. The atmospheric abundances were determined by theoretically synthesizing the selected portions of the stellar spectrum and comparing with the observed spectra. In order to compute the theoretical spectrum, the formal solution of the equation of radiative transfer was numerically evaluated with the simplifying assumptions of local thermodynamical equilibrium, plane-parallel geometry and hydrostatic equilibrium. These assumptions are reasonably good for the metallic lines of F-G supergiants and hence the observations were confined to the phases where Cepheids behave like nonvariable F-G supergiants. The atmospheric abundances of iron-peak elements, Fe, Cr, Ti, Ca and heavier s-process elements Y, Ba, La, Ce, Sm were obtained by synthesizing a selected spectral region in the range 4330 Å — 4650 Å. We derive a radial abundance gradient for iron \(\frac{{d(Fe/H)}}{{dr_{gc} }} = - 0.056 \pm 0.08\) for the region of galactic disk between 6.7 and 10.9 kpc from the galactic centre (assuming r_gc = 8.5 kpc for the Sun). This value agrees with the one obtained from the general sample of Cepheids for which spectroscopic abundances are available, and also with the existing photometric determinations, but is shallower than the one derived by Luck (1982). Abundances of the elements derived in the present investigation do not show any significant correlation with atomic number. Also the abundance ratio of s-process elements does not show any correlation with Fe. This lack of correlation for disk population stars shows the inadequacy of simple models of galactic chemical evolution and favours the infall models. Alternately, the evolution of [s/Fe] may be determined by the ratio of intermediate-mass stars (which contribute s-process nuclei) to high-mass stars (which contribute Fe peak nuclei). Thus the different behaviour of halo and disk population may indicate a difference in the mass spectrum of star formation.     

Observational evidence for the evolution of nuclear metallicity and star formation rate with the merger stage

We investigate the evolution of nuclear gas-phase oxygen abundance and star formation rate(SFR) of local far-infrared selected star-forming galaxies along the merger sequence, as traced by their optical morphologies. The sample was drawn from a cross-correlation analysis of the IRAS Point Source Catalog Redshift Survey and 1 Jy ultraluminous infrared galaxy sample with the Sloan Digital Sky Survey Data Release 7 database. The investigation is done by comparing our sample to a control sample matched in the normalized redshift distribution in two diagnostics, which are the nuclear gas-phase metallicity vs.stellar mass and the nuclear SFR vs. stellar mass diagrams. Galaxies with different morphological types show different mass-metallicity relations(MZRs). Compared to the MZR defined by the control sample,isolated spirals have comparable metallicities with the control sample at a given stellar mass. Spirals in pairs and interacting galaxies with projected separations of rp 20 kpc show a mild metallicity dilution of0.02–0.03 dex. Interacting galaxies with rp 20 kpc, pre-mergers and advanced mergers are underabundant by～0.06,～0.05 and～0.04 dex, respectively. This shows an evolutionary trend that the metallicity is increasingly depressed as the merging proceeds and it is diluted most dramatically when two galaxies are closely interacting. Afterwards, the interstellar medium(ISM) is enriched when the galaxies coalesce.This is the first time that such ISM enrichment at the final coalescence stage has been observed, which demonstrates the importance of supernova explosions in affecting the nuclear metallicity. Moreover, the central SFR enhancement relative to the control sample evolves simultaneously with the nuclear gas-phase oxygen abundance. Our results support the predictions from numerical simulations.     

On a simple model for self-regulating star formation in the galactic disk

Sternentstehung in Galaxien ist ein Prozeß mit Rückkopplung zum interstellaren Medium (ISM) und möglicherweise ein Teil eines selbstregu-lierenden Zyklus. DOPITA (1985) hat ein Modell vorgeschlagen, in dem Sternentstehung in Spiral- und irregulären Galaxien über den Druck im ISM selbstregulierend wirkt. In der vorliegenden Arbeit wird gezeigt, daß die verfügbaren Daten für die radialen Verteilungen von Gas, Gesamtmasse und Lymankontinuumphotonenfluß in der Scheibe unserer Galaxis dieses einfache Modell nicht stützen. Verschiedene mögliche Ursachen werden diskutiert.     

Influence of galactic nuclear explosions on the efficiency of the process of star formation

Periodic explosions in the nucleus of a galaxy generate strong shock waves. The shock waves, in moving outwards, produce highly compressed thin layers of gas at distances much larger than the thickness of the layer. When the gas in this layer undergoes fragmentation, the Jeans mass is found to be much less than that if the fragmentation proceeded under normal gravitational pull. It is, therefore, concluded that the explosive events in the galactic centres make the process of star formation highly efficient in the central region of galaxies.