2009ApJ...691..783G

We present the results of a Spitzer Infrared Spectrograph survey of 24 µm selected luminous infrared galaxies (LIRGs, L_IR> 10¹¹ L_☉) 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, 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, 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.