Kinematics of Galaxies in the Hubble Deep Field-South: Discovery of a Very Massive Spiral Galaxy at z = 0.6

Abstract

We report the first results from a study of the internal kinematics, based on spatially resolved Hα velocity profiles, of three galaxies at redshift z~0.6 and one at redshift z~0.8, detected by ISOCAM in the Hubble Deep Field-South. The kinematics are derived from high-resolution near-IR Very Large Telescope spectroscopy. One of the galaxies is a massive spiral that possesses a very large rotational velocity of 460 km s^-1 and contains a mass of 10¹² M_solar (within 20 kpc), significantly higher than the dynamical masses measured in most other local and high-redshift spiral galaxies. Two of the galaxies comprise a counterrotating interacting system, while the fourth is also a large spiral. The observed galaxies are representative examples of the morphologies encountered among ISOCAM galaxies. The mass-to-light (M/L_bol) ratios of ISOCAM galaxies lie between those of local luminous IR galaxies and massive spiral galaxies. We measure an offset of 1.6+/-0.3 mag in the rest-frame B band and of 0.7+/-0.3 mag in the rest-frame I band when we compare the four ISOCAM galaxies with the local Tully-Fisher B- and I-band relations. We conclude that the large IR luminosity of the ISOCAM population results from a combination of large mass and efficient triggering of star formation. Since ISOCAM galaxies contribute significantly to the cosmic infrared background, our results imply that a relatively small number of very massive and IR-luminous objects contribute significantly to the IR background and star-formation activity near z~0.7. Based on observations collected at the European Southern Observatory, Chile: ESO observations 65.O-0612 and 67.A-0518.