Rotation and outflow in the central kiloparsec of the water-megamaser galaxies IC 2560, NGC 1386, NGC 1052, and Mrk 1210

Abstract

Optical emission-line profiles were evaluated in order to explore the structure of galactic nuclei containing H2O megamaser sources. Long-slit spectra of IC 2560, NGC 1386, NGC 1052 and Mrk 1210 were obtained at ∼100 km s-1 spectral and ∼2″ × 2″ spatial resolution. The following individual properties of the objects were found: The active nucleus of IC 2560 (innermost ±2″) emits lines typical for a high-ionization Seyfert-2 spectrum albeit with comparatively narrow profiles (FWHM ∼ 200 km s-1). Line wings are stronger on the blue side than on the red side, suggesting outflow. The central velocity gradient fits into the general velocity curve of the galaxy. Attributing it to a rotating disk coplanar with the galaxy leads to a Keplerian mass of ∼107 M⊙ inside a radius of 100 pc. - The central 6″ sized structure seen on HST Hα and [OIII] images of NGC 1386 appears to be the inner part of a near-edge-on warped rotating spiral disk that is traced in Hα within a diameter of 17″ along PA 23°. This interpretation is based on observed kinematic continuity and a typical S-shaped dust lane crossing the kinematical center. The central velocity gradient yields a Keplerian mass estimate of ∼108 M⊙ inside R = 100 pc and 5 × 109 M⊙ inside 0.8 kpc. The total mass of the ionized gas of 105-6 M⊙ is small compared to the dynamical mass of the spiral disk. - The kinematical gradient of the rotating gas disk in the center of the elliptical galaxy NGC 1052 yields a mass of ∼6 × 108 M⊙ inside 166 pc. Two components are found: component A arises in a rotating disk, component B is blueshifted by ∼400 km s-1 relative to the disk. B likely originates from outflowing gas distributed within a wide cone. There is no need for a broad Hα component in unpolarized flux. The peculiar polarization seen in [OI]λ6300 by Barth et al. (1999) may be related to the outflow component. - In Mrk 1210, a redshifted component R contributes to the exceptional width of [OI]λ6300. An additional blueshifted component B strong in [OIII] allows to fit the Hα + [NII] blend without a broad component of Hα. B and R are likely to be outflow components. The location of the brightness maximum BM is slightly (∼ 1″) shifted relative to the kinematical center KC. Locating the true obscured nucleus in KC makes it understandable that BM displays faint broad scattered Hα in polarized light. - Galactic rotation and outflow of narrow-line gas are common features of this sample of water-megamaser galaxies. All decomposed line-systems exhibit AGN typical line ratios. Recent detections of H2O megamasers in starburst galaxies and the apparent association of one megamaser with a Seyfert 1 AGN suggest that megamasers can possibly be triggered by optically detectable outflows. The frequently encountered edge-on geometry favoring large molecular column densities appears to be verified for NGC 1386 and IC 2560. For NGC 1052 and Mrk 1210, maser emission triggered by the optically detected outflow components cannot be ruled out.