The saturation states of compact and diffuse components of OH megamaser galaxies

Abstract

Abstract.A sample of 9 OH megamaser galaxies detected in the soft X-ray domain was compiled. Using available OH and X-ray data a striking correlation was found between the X-ray luminosity and the width of the OH line. This correlation may indicate that the X-ray heating of a molecular gas may increase the collisional excitation of the maser emission. However, this result should be considered as a tentative one because of the insufficient number of galaxies. An analysis of the saturation states of compact and diffuse components of OH emission was performed. The results of the analysis support the assumption that both the compact and diffuse OH maser emissions in the megamaser galaxies are saturated. The diffuse component might show unsaturated masing under certain conditions, such as the appropriate relation between the intensities of compact and diffuse components and a relevant number of the IR photons to pump the maser emission.

Properties of OH Megamaser Galaxies in the Radio Continuum. II. Data Analysis

“…If roughly 10% of this radiation is thermal, then the expected monochromatic luminosity at 1 keV would be 10 28 erg·s -1 Hz -1 (and 10 26 erg·s -1 Hz -1 at 6 keV). However, the observed x-ray luminosity at 1 keV for megamasers is several (more than 3) orders of magnitude lower than 10 28 erg·s -1 Hz -1 [27]. Thus, in OH megamasers it is unlikely that roughly 10% of the radio emission is thermal in nature.…”

“…In addition, the radio emission from the nucleus either is uncorrelated or is only weakly correlated with such characteristics as the infrared luminosity, spectral index, mass of molecular gas, etc. The results of this section and the features of megamasers listed above, as well as the linear relationship between the OH and FIR luminosities [3,5,11,26], a theoretical analysis [27,28], and the variability of maser OH emission from the source IRAS 21272+2514 [29], suggest that an unsaturated gain mechanism and infrared pumping are not the main emission mechanism for OH megamasers, although such a mechanism may occur under certain conditions. In OH megamasers saturated gain and collisional pumping are probably the preferred emission mechanism.…”

“…This circumstance reduces the probability that a certain part of the radio emission in these galaxies might consist of thermal free-free radiation. Otherwise, the x-ray emission from the megamasers might be very high, which is not, in fact, observed [27]. Condon et al [1] state that in galaxies with strong infrared emission, thermal free-free radiation may form up to 12% of the radio emission at 1.49 GHz.…”

Class I methanol megamasers: a potential probe of starburst activity and feedback in active galaxies

“…The target sample for our survey includes eight water megamaser galaxies (among them two galaxies which also host OH megamasers) which also exhibit soft X-ray emission detected in Chandra and/or XMM-Newton observations (Zhang et al 2006;Brassington, Ponman & Read 2007). In addition, we also include eight OH megamaser galaxies with extended X-ray emission detected in ROSAT observations (Kandalyan 2003). The two different sub-samples of water and OH megamaser galaxies considered in our observations also allow us to investigate whether there are differences between them in the characteristics of any detected methanol megamaser emission.…”

Some Features of the X-Ray and Radio Emission from OH Megamaser Galaxies

“…The gradient of the velocity field of the emission suggests that the compact OH maser emission from these galaxies is associated with a massive central object. The observed relationship between the x-ray emission of megamasers and the width of the OH emission line [10] indicates that the molecular disk in these galaxies may also be responsible for…”