Asymmetry in the Spectrum of High-Velocity H2O Maser Emission Features in Active Galactic Nuclei

Asymmetry in the Spectrum of High-Velocity Н2O Maser Emission Features in Active Galactic Nuclei

A. V. Nesterenok* and D. A. Varshalovich** Ioffe Physical - Technical Institute, ul. Politekhnicheskaya 26, St. Petersburg, 194021 Russia Received July 16, 2009

Abstract - We suggest a mechanism for the amplification of high-velocity water-vapor maser emission features from the central regions of active galactic nuclei. The model of an emitting accretion disk is considered. The high-velocity emission features originate in the right and left wings of the Keplerian disk. The hyperfine splitting of the signal levels leads to an asymmetry in the spectral profile of the water vapor maser line at a frequency of 22.235 GHz. We show that the gain profile asymmetry must lead to an enhanced brightness of the blueshifted high-velocity emission features compared to the redshifted ones. Such a situation is observed in the source UGC 3789. DOI: 10.1134/S1063773710010019 Key words: active galactic nuclei, maser emission, spectral profile, hyperfine structure.

INTRODUCTION

Investigation of the maser emission that originates in accretion disks in the central regions of galaxies opens a unique opportunity to directly measure the Hubble constant. The high intensity of the maser emission allows the spatial structure of the system of sources to be investigated by radio-interferometry methods. The narrowness of the spectral features makes it possible to measure the frequencies of emission lines with a high accuracy. This allows one to model the geometry and kinematics of the accretion disk and to determine the “geometric” distance to the system. The accuracy of determining cosmological parameters, such as the Hubble constant, depends significantly on the calibration of the intergalactic distance scale. At present, this scale is based on observations of Cepheids in the Large Magellanic Cloud and in the nearest galaxies. Since the distance to the Large Magellanic Cloud is uncertain (its independent estimates can differ by as much as 25%) and since the effect of metallicity on the period – luminosity relation for Cepheids is unclear, the accuracy of distance determination by this method is limited (Macri et al. 2006). One way of refining the intergalactic distance scale is to

E-mail: alex-n10@yandex.ru ** E-mail: varsh@astro.ioffe.ru

2 use the systems whose distances can be found by “geometric” methods as “reference points”. One of such systems is the galaxy NGC 4258, whose geometric distance was obtained on the basis of detailed interferometric measurements and by modeling the geometry and kinematics of the accretion disk around a central black hole.

The H2O maser emission from the central region of NGC 4258 was detected by Claussen et al. (1984); the emission frequency shift corresponds to the galaxy’s recession velocity of about 500 km s-1, and the “isotropic luminosity” in the 1.35-cm maser line (22.235 GHz) is 120 Lʘ. Apart from the brightest central source, Nakai et al. (1993) detected less intense, high-velocity emission features with velocities relative to the central source of about ±1000 km s-1. Miyoshi et al. (1995) were the first to map the spatial distribution of maser sources by means of VLBI. As a result, it was established that the maser emission from the central region of NGC 4258 originates in a thin, differentially rotating accretion disk seen edge-on. The emission whose frequency corresponds to the systemic velocity of the galaxy comes to us from the disk foreground, where the rotation velocity of the emitting gas is perpendicular to the line of sight. The high-velocity maser emission features originate in the right and left disk sectors, where the orbital velocity of the emitting gas is parallel and antiparallel to the line of sight. The orbital radii of the high- velocity features lie within the range from 0.11 to 0.3 pc, and their velocities fall on a Keplerian rotation curve with an error of <1% (Argon et al. 2007). The central mass of the system calculated from the rotation curve is about 3.8 × 107 M๏ (Herrnstein et al. 2005).

The relative velocities of the spectral emission features in the central source undergo an acceleration of about 8 km s-1 yr-1, which corresponds to the centripetal acceleration of emitting gas clouds. Based on the interferogram data and the accelerations of the emission features in the central source, Herrnstein et al. (1999) determined the geometric distance to the system, 7.2 ± 0.3(rand.) ± 0.4(syst.) Mpc. A further refinement of the geometry of the spatial distribution of maser sources and their accelerations wi

…(Full text truncated)…