INTRINSIC SHAPE OF STAR-FORMING BzK GALAXIES AT z ~ 2 IN GOODS-N

Abstract

We study the structure of star-forming galaxies at z ~ 2 in a Great Observatories Origins Deep Survey North field selected as star-forming BzK (sBzK) galaxies down to K_{AB} < 24.0 mag. Among 1029 sBzK galaxies, 551 galaxies (54%) show a single component in the Advanced Camera for Survey (ACS)/F850LP image obtained with the Hubble Space Telescope; the rest show multiple components. We fit the single-component sBzK galaxies with the single Sérsic profile using the ACS/F850LP image and find that a majority of them (64%) show a Sérsic index of n = 0.5-2.5, indicating that they have a disk-like structure. The resulting effective radii typically range from 1.0 to 3.0 kpc in the rest-frame UV wavelength. After correcting the effective radii to those in the rest-frame optical wavelength, we find that the single-component sBzK galaxies are located in the region where the local and z ~ 1 disk galaxies are distributed in the stellar-mass-size diagram, suggesting comparable surface stellar-mass density between the sBzK and z ~ 0-1 disk galaxies. All these properties suggest that the single-component sBzK galaxies are progenitors of the present-day disk galaxies. However, by studying their intrinsic shape through comparison between the observed distribution of apparent axial ratios and the distribution for triaxial models with axes A > B > C, we find that the mean B/A ratio is 0.61[+0.05]_{ –0.08} and disk thickness C/A is 0.28[+0.03]_{ –0.04}. This indicates that the single-component sBzK galaxies at z ~ 2 have a bar-like or oval shape rather than a round disk shape. The shape seems to resemble a bar/oval structure that forms through bar instability; if this is the case, the intrinsic shape may give us a clue to understand dynamical evolution of baryonic matter in a dark matter halo.