Low-redshift quasars as the active nuclei of cosmologically distant interacting galaxies - A spectroscopic investigation

Mathematics – Logic

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Astronomical Spectroscopy, Cosmology, Galactic Nuclei, Interacting Galaxies, Quasars, Red Shift, Radial Velocity, Stellar Evolution

Scientific paper

We present CCD spectra obtained with the 4-m telescope at Kitt Peak of 21 apparent companion galaxies to 15 low-redshift quasars. With one exception, each galaxy is located at an apparent projected distance of < 50 kpc from its quasar. We have determined redshifts of 19 of these galaxies (15 redshifts relying at least partially on stellar absorption lines). In 18 of the 19 total cases (95%), the galaxy and quasar radial velocities agree to < l0 km s-1 in the quasar rest frame. For absorption-line redshifts, the results are similar (14 of 15 at the quasar redshifts). We argue that our very high "success rate" relative to previous similar investigations stems primarily from our selection of close (in projection) apparent companions, which reduces contamination by foreground/background galaxies. Combining our data with similar data in the literature yields the following: 86% of the apparent close companions (<50 kpc projected quasar/galaxy separation) share the quasar redshift, compared to only ˜40% of the more distant (in projection) apparent companions. By virtue of its direct and general nature, we believe the present data represent the most convincing evidence yet for a cosmological origin of quasar redshifts. Our data also imply that imaging data alone are sufficient to test whether low-redshift quasars are preferentially associated with close companions (since the correction for unrelated line-of-sight galaxies will be small). This lends support to the hypothesis that galaxy interactions are an important means of triggering quasar activity. The low relative velocities of the quasars and companions (<1000 km s-1) are consistent with other evidence which indicates that low-z quasars are rarely found in rich relaxed clusters of galaxies. Finally, we point out that the close-companion galaxies in our sample do not themselves exhibit unusually strong nuclear activity and/or unusually high rates of star formation. This is consistent with data on nearby galaxy pairs, the great majority of which are apparently experiencing only modest nuclear stimulation.

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