Optical and NIR spectroscopy of Mrk 1210: constraints and physical conditions of the active nucleus ^*,**

¹ IATE, Observatorio Astronómico Córdoba, Laprida 854, X5000BGR, Córdoba, Argentina e-mail: ximena@oac.uncor.edu
² Laboratório Nacional de Astrofísica, Rua dos Estados Unidos 154, Itajubá, MG, Brazil

Received: 13 September 2005
Accepted: 22 August 2006

Abstract

Aims.Mrk 1210 is an outstanding Seyfert 2 galaxy because it displays signatures of recent circumnuclear star formation and a high level of X-ray activity, in addition to the classical spectral characteristics typical of an AGN. Here we investigate the extinction affecting the nuclear and extended emitting gas, the kinematics of the narrow-line region, and the physical properties and conditions of that gas.

Methods.Near-infrared and optical spectra of the nuclear and extended emission region of Mrk 1210 are presented, covering the interval 0.4-2.4 μm. Emission and absorption lines were used to infer, respectively, the geometrical extension of the ionized gas and the contribution of the underlying stellar population to the observed integrated continuum. The emission line profiles were employed to study the kinematics in the NLR. The reddening and physical condition of the gas were investigated by means of flux ratios among permitted and forbidden lines.

Results.The NIR nuclear spectrum is dominated by and recombination lines, as well as [], [], and [] forbidden lines. Coronal lines of [], [], [], [], and [], in addition to molecular H₂ lines, were also detected. The  1.618 μm overtone bandhead helped to estimate the contribution of the stellar population to the continuum. It was found that of the H-band continuum has a stellar origin. It improves previous estimates, which claimed that at least 50% of the observed continuum was attributed to the AGN. Analysis of the emission line profiles, both allowed and forbidden, shows a narrower ( km s^-1) line on top of a broad ( km s^-1) blue-shifted component. This seems to be associated to a nuclear outflow. This hypothesis is supported by 6 cm VLBI observations, which show a radio ejecta extending up to ~30 pc from the nucleus. This result does not require the presence of the hidden BLR claimed to be present in previous NIR observations of this object. Internal extinction, calculated by means of several indicators including [] flux ratios not previously used before in AGNs, reveals a dusty AGN, while the extended regions are barely affected by dust, if at all. The density and temperature are calculated for the NLR using optical and NIR lines as diagnostic ratios. The results show electronic temperatures from 10 000 K up to 40 000 K and densities between  cm^-3. The higher temperatures show that shocks, most probably related to the radio outflow, must contribute to the line emission.