Galaxy Formation In The Reionization Epoch As Hinted By Wide Field Camera 3 Observations Of The Hubble Ultra Deep Field

TL;DR

This study uses HST/WFC3 observations of the Hubble Ultra Deep Field to identify high-redshift galaxy candidates, revealing unexpected surface densities at z~8-10 and proposing evolving luminosity functions that suggest a rapid emergence of low-luminosity galaxies during reionization.

Contribution

It presents a large sample of z~7--10 galaxy candidates and proposes new evolving luminosity function parameters consistent with the data, indicating a rapid increase in low-luminosity galaxies at high redshift.

Findings

Surface densities of Y- and J-dropouts increase sharply at faint magnitudes.

Proposed Schechter function parameters suggest a decrease in L* and increase in Phi* at z~8 and 10.

Early low-luminosity galaxies could account for reionization photon budget.

Abstract

We present a large sample of candidate galaxies at z~7--10, selected in the HUDF using the new observations made by the HST/WFC3. Our sample is composed of 20 z-dropouts, 15 Y-dropouts, and 20 J-dropouts. The surface densities of the z-dropouts are close to what predicted by earlier studies, however, those of the Y- and J-dropouts are quite unexpected. While no Y- or J-dropouts have been found at AB < 28.0 mag, their surface densities seem to increase sharply at fainter levels. While some of these candidates seem to be close to foreground galaxies and thus could possibly be gravitationally lensed, the overall surface densities after excluding such cases are still much higher than what would be expected if the luminosity function does not evolve from z~7 to 10. Motivated by such steep increases, we tentatively propose a set of Schechter function parameters to describe the LFs at z~8 and 10. As compared to their counterpart at z~7, here L* decreases by ~ 6.5x and Phi* increases by 17--90x. Although such parameters are not yet demanded by the existing observations, they are allowed and seem to agree with the data better than other alternatives. If these LFs are still valid beyond our current detection limit, this would imply a sudden emergence of a large number of low-luminosity galaxies when looking back in time to z~10, which, while seemingly exotic, would naturally fit in the picture of the cosmic hydrogen reionization. These early galaxies could easily account for the ionizing photon budget required by the reionization, and they would imply that the global star formation rate density might start from a very high value at z~10, rapidly reach the minimum at z~7, and start to rise again towards z~6. In this scenario, the majority of the stellar mass that the universe assembled through the reionization epoch seems still undetected by current observations at z~6. [Abridged]