Kinematics of Galaxies in the Hubble Deep Field–South: Discovery of a Very Massive Spiral Galaxy atz= 0.6

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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☉ (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/Lbol) 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.
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