Discovery of Lyman Break Galaxies at z ~ 7 from the zFourGE Survey
Kelson, D. D.; Ferguson, H. C.; Finkelstein, S. L.; Papovich, C.; Koekemoer, A. M.; Fazio, G. G.; Faber, S. M.; van Dokkum, P.; Glazebrook, K.; Kacprzak, G. G.; Ashby, M. L. N.; Labbé, I.; Tilvi, V.; Straatman, C. M. S.; Monson, A.; Persson, S. E.; Salmon, B.; Willner, S. P.; Grogin, N. A.; McCarthy, P. J.; Spitler, L. R.; Newman, J. A.; Murphy, D.; Tran, K. -V. H.; Quadri, R. F.
United States, Netherlands, Australia, France
Abstract
Star-forming galaxies at redshifts z > 6 are likely responsible for the reionization of the universe, and it is important to study the nature of these galaxies. We present three candidates for z ~ 7 Lyman break galaxies (LBGs) from a 155 arcmin2 area in the CANDELS/COSMOS field imaged by the deep FourStar Galaxy Evolution (zFourGE) survey. The FourStar medium-band filters provide the equivalent of R ~ 10 spectroscopy, which cleanly distinguishes between z ~ 7 LBGs and brown dwarf stars. The distinction between stars and galaxies based on an object's angular size can become unreliable even when using Hubble Space Telescope imaging; there exists at least one very compact z ~ 7 candidate (FWHM ~ 0.5-1 kpc) that is indistinguishable from a point source. The medium-band filters provide narrower redshift distributions compared with broadband-derived redshifts. The UV luminosity function derived using the three z ~ 7 candidates is consistent with previous studies, suggesting an evolution at the bright end (M UV ~ -21.6 mag) from z ~ 7 to z ~ 5. Fitting the galaxies' spectral energy distributions, we predict Lyα equivalent widths for the two brightest LBGs, and find that the presence of a Lyα line affects the medium-band flux thereby changing the constraints on stellar masses and UV spectral slopes. This illustrates the limitations of deriving LBG properties using only broadband photometry. The derived specific star-formation rates for the bright LBGs are ~13 Gyr-1, slightly higher than the lower-luminosity LBGs, implying that the star-formation rate increases with stellar mass for these galaxies.