A New Measurement of the Stellar Mass Density at z~5: Implications for the Sources of Cosmic Reionization
Abstract
We present a new measurement of the integrated stellar mass per comoving volume at redshift 5 determined via spectral energy fitting drawn from a sample of 214 photometrically-selected galaxies with z'<26.5 in the southern GOODS field. We estimate stellar masses for various sub-samples for which reliable and unconfused Spitzer IRAC detections are available. A spectroscopic sample of 14 of the most luminous sources with =4.92 provides a firm lower limit of 2e6 Msun/Mpc^3. We then consider a larger sample whose photometric redshifts in the publicly-available GOODS-MUSIC catalog lie in the range 4.4<z<5.6. After excising probable stellar contaminants and using the z'-J color to exclude any remaining foreground red galaxies, we conclude that 153 sources are likely to be at z~5. The implied mass density from the unconfused IRAC fraction of this sample, scaled to the total available, is 7e6 Msun/Mpc^3. We discuss the likelihood that we have underestimated the true mass density. Including fainter and quiescent sources the total integrated density could be as high as 2e7 Msun/Mpc^3. Such a high mass density only 1.2 Gyr after the Big Bang has interesting consequences for the implied past average star formation during the period when cosmic reionization is now thought to have taken place. Using the currently available (but highly uncertain) rate of decline in the star formation history over 5<z<10, it is only possible to account for the assembled mass at z~5 by appealing to significant dust extinction at early times or extending the luminosity function to very faint limits. As mass density estimates improve at z~5-6, our method is likely to provide one of the tightest constraints on the question of whether star forming sources were responsible for reionizing the Universe. [abridged]Keywords
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