The Stellar Mass Density and Specific Star Formation Rates of the Universe at Z ∼ 7

We use a robust sample of 11 z ∼ 7 galaxies (z850-dropouts) to estimate the stellar mass density of the universe when it was only ∼750 Myr old. We combine the very deep optical to near-Infrared photometry from the HST ACS and NICMOS cameras with mid-Infrared Spitzer IRAC imaging available through the GOODS program. After carefully removing the flux from contaminating foreground sources we have obtained reliable photometry in the 3.6 μm and 4.5 μm IRAC channels. The spectral shapes of these sources, including their rest frame optical colors, strongly support their being at z∼7 with a mean photometric redshift of 〈z〉 =7.2±0.5. We use Bruzual & Charlot (2003) synthetic stellar population models to constrain their stellar masses and star formation histories. We find stellar masses that range over 0.1 13×109 M⊙ and ages from 20 Myr to up to 400 Myr with a mean of ∼ 200 Myr, suggesting that in some of these galaxies most of the stars were formed at z & 8. The best fits to the observed SEDs are consistent with little or no dust extinction, in agreement with recent results at z ∼ 4− 6. The star formation rates (SFR) are typically 10 M⊙ yr. From this sample we measure a stellar mass density of 7.4 −3.8 × 10 M⊙ Mpc to a limit of MUV,AB < −20 (or 0.4L∗z=3). Combined with a fiducial lower limit for their ages (80 Myr) this implies a maximum SFR density of 0.0013 M⊙ yr Mpc. This is well below the critical level needed to reionize the universe at z ∼ 8 using standard assumptions. We expect that there is probably a substantial amount of SF that is not seen since we are only observing the bright end of the LF (> L). Strikingly, we find that the specific SFR is constant from z ∼ 7 to z ∼ 2 but drops substantially at more recent times. Subject headings: galaxies: evolution — galaxies: high-redshift