The Masses and Ages of Galaxies in the Era of Reionization

Details The Masses and Ages of Galaxies in the Era of Reionization The Masses and Ages of Galaxies in the Era of Reionization

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007sptz.prop40311l&link_type=abstract

Spitzer Proposal ID #40311

Computer Science

Scientific paper

Our understanding of the z~6 galaxy population is mostly based on observations in the relatively small GOODS fields, and is thus vulnerable to the effects of cosmic variance. We propose to use IRAC to followup 6 deep fields observed with the now defunct HST/ACS to sample a substantially larger volume at these redshifts. We previously identified a population of star-forming galaxies at z~6 through the Lyman-break technique (I-drops) and confirmed many with spectroscopy on Keck/Gemini/VLT. We used IRAC observations in the GOODS-S field to discover significant Balmer/4000A breaks in many of these I-drops, indicating dominant stellar populations with ages of 200-700Myr and formation redshifts of z~7-18. We suggested that these massive galaxies could have contributed signficantly to the reionization of the universe at z>7 through earlier vigorous episodes of star formation. However, our sample is small, as it comprises only a handful of bright and isolated z~6 galaxies. The GOODS dataset covers two pointings, and our quantitative conclusions are limited by cosmic variance. To properly constrain the stellar mass density and contribution of earlier star formation to reionization, requires a larger sample over many different sightlines. We have deep ACS I- and z-band images in 6 fields (deeper than GOODS), from which we generated extensive catalogs of I-drop galaxies. With IRAC imaging, we can measure their ages and stellar masses through stellar population fitting. By observing with all 4 IRAC channels we can also constrain the metallicities and dust reddenings of galaxies at z~6; our analysis of GOODS-South favors low extinction and low metallicity, but a larger sample observed out to 8um is crucial to confirm this important finding. By quadrupling the number of sightlines, and the number of z~6 galaxies with Spitzer detections, we can address whether the presence of established systems at z=6 suggests long-lived sources at earlier epochs (z>7) played a key role in reionizing the Universe.

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