Probing Very Bright End of Galaxy Luminosity Function at z >~ 7 Using Hubble Space Telescope Pure Parallel Observations

TL;DR

This study uses Hubble Space Telescope pure parallel imaging to identify extremely bright galaxy candidates at z > 7, challenging existing luminosity function models and suggesting the presence of very luminous early galaxies.

Contribution

First application of HST pure parallel imaging to find bright z > 7 galaxies, revealing objects brighter than predicted by current models.

Findings

Discovered two very luminous galaxy candidates at z > 7.

Brightest candidate's existence challenges current luminosity function estimates.

Sample benefits from reduced cosmic variance due to multiple sightlines.

Abstract

We report the first results from the Hubble Infrared Pure Parallel Imaging Extragalactic Survey, which utilizes the pure parallel orbits of the Hubble Space Telescope to do deep imaging along a large number of random sightlines. To date, our analysis includes 26 widely separated fields observed by the Wide Field Camera 3, which amounts to 122.8 sq.arcmin in total area. We have found three bright Y098-dropouts, which are candidate galaxies at z >~ 7.4. One of these objects shows an indication of peculiar variability and its nature is uncertain. The other two objects are among the brightest candidate galaxies at these redshifts known to date L>2L*. Such very luminous objects could be the progenitors of the high-mass Lyman break galaxis (LBGs) observed at lower redshifts (up to z~5). While our sample is still limited in size, it is much less subject to the uncertainty caused by "cosmic variance" than other samples because it is derived using fields along many random sightlines. We find that the existence of the brightest candidate at z~7.4 is not well explained by the current luminosity function (LF) estimates at z~8. However, its inferred surface density could be explained by the prediction from the LFs at z~7 if it belongs to the high-redshift tail of the galaxy population at z~7.