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Register a new userDiscovery of z~8 Galaxies in the HUDF from ultra-deep WFC3/IR Observations
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R.J. Bouwens
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We utilize the newly-acquired, ultra-deep WFC3/IR observations over the HUDF to search for star-forming galaxies at z~8-8.5, only 600 million years from recombination, using a Y_{105}-dropout selection. The new 4.7 arcmin**2 WFC3/IR observations reach to ~28.8 AB mag (5 sigma) in the Y_{105}J_{125}H_{160} bands. These remarkable data reach ~1.5 AB mag deeper than the previous data over the HUDF, and now are an excellent match to the HUDF optical ACS data. For our search criteria, we use a two-color Lyman-Break selection technique to identify z~8-8.5 Y_{105}-dropouts. We find 5 likely z~8-8.5 candidates. The sources have H_{160}-band magnitudes of ~28.3 AB mag and very blue UV-continuum slopes, with a median estimated beta of <~-2.5 (where f_{lambda}propto lambda^{beta}). This suggests that z~8 galaxies are not only essentially dust free but also may have very young ages or low metallicities. The observed number of Y_{105}-dropout candidates is smaller than the 20+/-6 sources expected assuming no evolution from z~6, but is consistent with the 5 expected extrapolating the Bouwens et al. 2008 LF results to z~8. These results provide evidence that the evolution in the LF seen from z~7 to z~3 continues to z~8. The remarkable improvement in the sensitivity of WFC3/IR has enabled HST to cross a threshold, revealing star-forming galaxies at z~8-9.
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We present the results of a systematic search for galaxies in the redshift range z = 6 - 9, within the new, deep, near-infrared imaging of the Hubble Ultra Deep Field provided by the Wide Field Camera 3 (WFC3) on HST. We have performed full SED fitting to the optical+infrared photometry of all high-redshift galaxy candidates detected at greater than 5-sigma in at least one of the WFC3/IR broad-band filters. After rejection of contaminants, the result is a sample of 49 galaxies with primary redshift solutions z > 5.9. Our sample, selected without recourse to specific colour cuts, re-selects all but the faintest one of the 16 z-drops selected by Oesch et al. (2009), recovers all 5 of the Y-drops reported by Bouwens et al. (2009), and adds a further 29 galaxy candidates, of which 12 lie beyond z = 6.3, and 4 lie beyond z = 7. We also present confidence intervals on our photometric redshift estimates, and caution that acceptable low-redshift (z < 2) solutions exist for 28 out of the 37 galaxies at z > 6.3, and for all 8 galaxy candidates at z > 7.5. Nevertheless, the very highest redshift candidates appear to be strongly clustered in the field. We derive new estimates of the ultraviolet galaxy luminosity function at z = 7 and z = 8. Where our results are most robust, at a characteristic luminosity M(1500) ~ -19.5 (AB), we find that the comoving number density of galaxies declines by a factor of ~ 2.5 between z = 6 and z = 7, and by a further factor of ~ 2 by z = 8. These results suggest that it is difficult for the observed population of high-redshift star-forming galaxies to achieve reionisation by z ~ 6 without a significant contribution from galaxies well below the detection limits, plus alterations in the escape fraction of ionising photons and/or continued vigorous star formation at z > 15.
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We present the rest-frame optical morphologies of active galactic nucleus (AGN) host galaxies at 1.5<z<3, using near-infrared imaging from the Hubble Space Telescope Wide Field Camera 3, the first such study of AGN host galaxies at these redshifts. The AGN are X-ray selected from the Chandra Deep Field South and have typical luminosities of 1E42 < L_X < 1E44 erg/s. Accreting black holes in this luminosity and redshift range account for a substantial fraction of the total space density and black hole mass growth over cosmic time; they thus represent an important mode of black hole growth in the universe. We find that the majority (~80%) of the host galaxies of these AGN have low Sersic indices indicative of disk-dominated light profiles, suggesting that secular processes govern a significant fraction of the cosmic growth of black holes. That is, many black holes in the present-day universe grew much of their mass in disk-dominated galaxies and not in early-type galaxies or major mergers. The properties of the AGN host galaxies are furthermore indistinguishable from their parent galaxy population and we find no strong evolution in either effective radii or morphological mix between z~2 and z~0.05.
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