Spitzer 24 μ m 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 SFR(24 μm) /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 year timescale. As is typical, A851 galaxies with strong Balmer absorption occur in two types: with optical emission (starforming), and without (post-starburst). Our principal result is that the starforming variety, socalled 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] ) both indicate moderately active starbursts and support the picture that e(a) galaxies are the active starbursts that feed the