THE NATURE OF DUSTY STARBURST GALAXIES IN A RICH CLUSTER AT z= 0.4: THE PROGENITORS OF LENTICULARS?

We present the results of a Spitzer Infrared Spectrograph survey of 24 μm selected luminous infrared galaxies (LIRGs, LIR > 1011L ) in the rich cluster Cl 0024+16 at z = 0.4. Optically, these LIRGs resemble unremarkable spiral galaxies with e(a)/e(c) spectral classifications and [O ii]-derived star formation rates (SFRs) of 2 M yr−1, generally indistinguishable from the “quiescent” star-forming population in the cluster. Our IRS spectra show that the majority of the 24 μm detected galaxies exhibit polycyclic aromatic hydrocarbon emission with implied SFRs∼ 30–60 M yr−1, with only one (< 10%) in the sample displaying unambiguous evidence of an active galactic nucleus in the mid-infrared. This confirms the presence of a large population of obscured starburst galaxies in distant clusters, which comprise the bulk of the star formation occurring in these environments at z ∼ 0.5. We suggest that, although several mechanisms could be at play, these dusty starbursts could be the signature of an important evolutionary transition converting gas-rich spiral galaxies in distant clusters into the passive, bulge-dominated lenticular galaxies that become increasingly abundant in the cores of rich clusters in the ∼ 4 Gyr to the present day.