Low Star Formation Rates for z = 1 Early‐Type Galaxies in the Very Deep GOODS MIPS Imaging: Implications for Their Optical/Near‐Infrared Spectral Energy Distributions

Abstract

We measure the obscured star formation in z~1 early-type galaxies. Thisconstrains the influence of star formation on their optical/near-IR colors,which, we found, are redder than predicted by the model by Bruzual & Charlot(2003). From deep ACS imaging we construct a sample of 95 morphologicallyselected early-type galaxies in the HDF-N and CDF-S with spectroscopicredshifts in the range 0.85 2 sigma detections (~25 muJy} is17(-4,+9)%. Of the 15 galaxies with significant detections at least six have anAGN. Stacking the MIPS images of the galaxies without significant detectionsand adding the detected galaxies without AGN we find an upper limit on the meanstar formation rate (SFR) of 5.2+/-3.0 Msol yr^-1, and on the mean specific SFRof 4.6+/-2.2 * 10^-11 yr^-1. Under the assumption that the average SFR willdecline at the same rate as the cosmic average, the in situ growth in stellarmass of the early-type galaxy population is less than 14+/-7% between z=1 andthe present. We show that the typically low IR luminosity and SFR imply thatthe effect of obscured star formation (or AGN) on their rest-frameoptical/near-IR SEDs is negligible for ~90% of the galaxies in our sample.Hence, their optical/near-IR colors are most likely dominated by evolvedstellar populations. This implies that the colors predicted by the Bruzual &Charlot (2003) model for stellar populations with ages similar to those of z~1early-type galaxies (~1-3 Gyr) are most likely too blue, and that stellarmasses of evolved, high-redshift galaxies can be overestimated by up to afactor of ~2.