Spectrophotometry of galaxies in the Virgo Cluster. I. The star formation history

As a result of an extensive observational campaign targeting the Virgo Cluster, we obtained integrated (drift-scan mode) optical spectra and multiwavelength (UV, U, B, V, H) photometry for 124 and 330 galaxies, respectively, spanning the whole Hubble sequence, and with m(p) less than or equal to 16 (M-p less than or equal to -15). These data were combined to obtain galaxy Spectral Energy Distributions (SEDs) extending from 2000 to 22000 Angstrom. By fitting these SEDs with synthetic ones derived using Bruzual & Charlot population synthesis models we try to constrain observationally the star formation history (SFH) of galaxies in the rich cluster of galaxies nearest to us. Assuming a Salpeter IMF and an analytical form for the SFH, the fit free parameters are the age ( T) of the star formation event, its characteristic timescale (tau), and the initial metallicity (Z). In this work we test the ( simplistic) case in which all galaxies have a common age T = 13 Gyr, exploring a SFH with delayed exponential form ( which we call a la Sandage), thus allowing for an increasing SFR with time. This SFH is consistent with the full range of observed SEDs, provided that the characteristic timescale tau is let free to vary between 0.1 (quasi-instantaneous burst) and 25 Gyr ( increasing SFR) and Z between 1/50 and 2.5 Z(circle dot). Elliptical galaxies (including dEs) are best fitted with short timescales ( 3 Gyr) and metallicity varying between 1/5 and Z(circle dot). The model metallicity is found to increase as a function of H-band luminosity. Spiral galaxies require that both and metallicity correlate with H-band luminosity: low-mass Im+BCD have subsolar Z and tau greater than or equal to 10 Gyr, whereas giant spirals have solar metallicities and tau similar to3 Gyr, consistent with elliptical galaxies. Moreover, we find that the SFH of spiral galaxies in the Virgo Cluster depends upon the presence at their interior of fresh gas capable of sustaining the star formation. In fact, the residuals of the tau vs. L-H relation depend significantly on the H I content. H I deficient galaxies have shorter (up to a factor of 4) tau (truncated SFH) than spirals with normal H I content.