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 mp ≤ 16 (Mp ≤ −15). These data were combined to obtain galaxy Spectral Energy Distributions (SEDs) extending from 2000 to 22000 ˚A. 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 time-scale (τ) and the initial metallicity (Z). In this work we test the (simplistic) case where 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 time-scale τ 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⊙. Elliptical galaxies (including dEs) are best