SPITZER 24 mu m DETECTIONS OF STARBURST GALAXIES IN ABELL 851

Spitzer-MIPS 24 mu m observations and ground-based optical imaging and spectroscopy of the rich galaxy cluster Abell 851 at z = 0.41 are used to derive and compare star formation rates from the mid-IR 24 mu m and from [O II] lambda lambda 3727 emission. Many cluster galaxies have star formation rates SFR(24 mu m)/SFR([O II]) >> 1, indicative of star formation in regions highly obscured by dust. We focus on the substantial minority of A851 cluster members where strong Balmer absorption points to a starburst on a 10(8)-10(9) year timescale. As is typical, two types of galaxies with strong Balmer absorption are found in A851: with optical emission (starforming), and without optical emission (post-starburst). Our principal result is that the starforming variety, so-called e(a) galaxies, are mostly detected (9 out of 12) at 24 mu m-for these we find typically SFR(24 mu m)/SFR([O II]) similar to 4. Strong Balmer absorption and high values of SFR(24 mu m)/SFR([O II]) indicate moderately active starbursts (SB); both observations support the picture that e(a) galaxies are the active starbursts that feed the post-starburst population. While 24 mu m detections are frequent with Balmer-strong objects (even 6 out of 18 of the supposedly "post-starburst" galaxies are detected), only two out of seven of the continuously starforming 'e(c)' galaxies (with weak Balmer absorption) are detected-for them, SFR(24 mu m)/SFR([O II]) similar to 1. Their optical spectra resemble present-epoch spirals that dominate today's universe; we strengthen this association by showing that SFR(24 mu m)/SFR([O II]) similar to 1 is the norm today. That is, not just the amount of star formation but also its mode has evolved strongly from z similar to 0.4 to the present. We fit spectrophotometric models in order to measure the strength and duration of the bursts and to quantify the evolutionary sequence from active to post-starburst. Our results harden the evidence that moderately active starbursts are the defining feature of starforming cluster galaxies at z similar to 0.4.