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. This constrains the influence of star formation on their optical/near-IR colors, which, we found, are redder than predicted by the model by Bruzual & Charlot. From deep ACS imaging we construct a sample of 95 morphologically selected early-type galaxies in the HDF-N and CDF-S with spectroscopic redshifts in the range 0.85<z<1.15. We measure their 24 μm fluxes from the deep GOODS MIPS imaging and derive the IR luminosities and star formation rates (SFRs). The fraction of galaxies with >2 σ detections (~25 μJy) is 17⁺⁹_-4%. Of the 15 galaxies with significant detections, at least six have an AGN. Stacking the MIPS images of the galaxies without significant detections and adding the detected galaxies without AGNs we find an upper limit on the mean SFR of 5.2+/-3.0 M_solar yr^-1, and on the mean specific SFR of 4.6+/-2.2×10^-11 yr^-1. Under the assumption that the average SFR will decline at the same rate as the cosmic average, the in situ growth in stellar mass of the early-type galaxy population is less than 14%+/-7% between z=1 and the present. We show that the typically low IR luminosity and SFR imply that the effect of obscured star formation (or AGNs) on their rest-frame optical/near-IR SEDs is negligible for ~90% of the galaxies in our sample. Hence, their optical/near-IR colors are most likely dominated by evolved stellar populations. This implies that the colors predicted by the Bruzual & Charlot model for stellar populations with ages similar to those of z~1 early-type galaxies (~1-3 Gyr) are most likely too blue, and that stellar masses of evolved, high-redshift galaxies can be overestimated by up to a factor of ~2.

Based on observations with the Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555, and observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407.