EDP Sciences
Free access
Volume 394, Number 2, November I 2002
Page(s) 405 - 414
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20021152

A&A 394, 405-414 (2002)
DOI: 10.1051/0004-6361:20021152


A. Omar1, K. S. Dwarakanath1, 2, M. Rupen2 and K. R. Anantharamaiah1

1  Raman Research Institute, C.V. Raman Avenue, Bangalore, 560 080, India
2  National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA

(Received 19 June 2002 / Accepted 2 August 2002 )

We present Giant Meterwave Radio Telescope (GMRT) observations of the $\ion{H}{i}$ 21 cm line and Very Large Array (VLA) observations of the OH 18 cm line from the Seyfert 2 galaxy Mrk 1. $\ion{H}{i}$ emission is detected from both Mrk 1 and its companion NGC 451. The $\ion{H}{i}$ emission morphology and the velocity field of Mrk 1 are disturbed. We speculate that the nuclear activities of Mrk 1 are triggered by tidal interactions. We estimate the $\ion{H}{i}$ masses of Mrk 1 and NGC 451 to be $8.0(\pm0.6)\,\times\,10^{8}~M_\odot$ and $1.3(\pm0.1)\times10^{9}~M_\odot$ respectively. We have also detected the $\ion{H}{i}$ 21 cm line and the OH 18 cm line in absorption toward the nucleus of Mrk 1 at a blueshifted velocity with respect to its systemic velocity indicating an outflow of atomic and molecular gas. Two OH lines, at 1665 and 1667 MHz, are detected. Each of the profiles of the $\ion{H}{i}$ and OH absorption consists of two components that are separated by ~125 km s -1. Gaussian fitting gave dispersions of ~ 44 km s -1 for both the components of the $\ion{H}{i}$ absorption. The profile of the OH absorption is qualitatively similar to that of the $\ion{H}{i}$ absorption. Both components of the OH absorption are thermally excited. The peak optical depths of the two components of the $\ion{H}{i}$ absorption are $(7.3\pm0.4)\times10^{-2}$ and $(3.2\pm0.4)\times10^{-2}$. The corresponding peak optical depths of the 1667 MHz OH absorption are $(2.3\pm0.3)\times10^{-2}$ and $(1.1\pm0.3)\times10^{-2}$. The higher velocity components of the $\ion{H}{i}$ and OH (1667 MHz) absorption lines are blueshifted from the [ $\ion{O}{iii}$] $\lambda5007$, [ $\ion{O}{i}$] $\lambda6300$, and the systemic velocity by ~100 km s -1, but are consistent with the [ $\ion{O}{ii}$] $\lambda3727$ velocity. We explain these velocity discrepancies as due to shock ionization of a region which is pushed forward due to shocks in front of the radio nucleus thereby giving apparent blueshift to $\ion{H}{i}$, OH, and [ $\ion{O}{ii}$] velocities. The optical depth ratios $\tau_{\rm\ion{H}{i}}/\tau_{\rm OH}^{1667}$ of both the components of the $\ion{H}{i}$ and OH absorption are ~3, indicating their origin in dense molecular clouds. Using OH/ $A_{\rm v}$ values for the Galactic molecular clouds, we obtain $9 < A_{\rm v}< 90$ toward the line of sight of Mrk 1.

Key words: galaxies: active -- galaxies: interactions -- galaxies: individual: Mrk 1, NGC 451 -- galaxies: ISM

Offprint request: A. Omar, aomar@rri.res.in

© ESO 2002