Spitzer 24 μm detections of starburst galaxies in abell 851

Spitzer-MIPS 24 μm 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 μm and from [O II] λλ3727 emission. Many cluster galaxies have star formation rates SFR(24μm)/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 10⁸-10 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 μm - for these we find typically SFR(24 μm)/SFR([O II]) 4. Strong Balmer absorption and high values of SFR(24 μm)/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 μm 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 μm)/SFR([O II]) 1. Their optical spectra resemble present-epoch spirals that dominate today's universe; we strengthen this association by showing that SFR(24 μm)/SFR([O II]) 1 is the norm today. That is, not just the amount of star formation but also its mode has evolved strongly from z 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 0.4.