Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007pabei..25...36z&link_type=abstract
Progress in Astronomy (ISSN 1000-8349), Vol. 25, No. 1, p. 36 - 44 (2007)
Astronomy and Astrophysics
Astrophysics
Astrophysics, Water Maser, Agn, Seyfert 2 Galaxies, Liners Galaxies, Astrophysics, Review, Water Masers, Seyfert 2 Galaxies, Liner Galaxies
Scientific paper
Extragalactic H2O masers have been observed for nearly 30 years and to date there are about 70 sources detected, including 64 being reported (as of May 2006). The masers are associated with either star formation or active galactic nuclei (AGNs). It can be classified as related to star formation, nuclear accretion disks (disk-masers), interactions of nuclear jets with ambient molecular clouds or amplification of the jet's seed photons by suitably located foreground clouds (jet-masers) and nuclear outflows. The statistical analysis of extragalactic H2O masers shows that almost all the detected H2O megamasers (LH2O > 10 Lsolar) are Seyfert 2 or LINERs. Most of them arise from heavily obscured (NH > 1023 cm-2) AGN, even half from Compton-thick circumstance (NH > 1024 cm-2). And kilomasers (LH2O < 10 Lsolar), mainly associated with star formation, are always Compton-thin, i.e. their absorbing column densities are NH < 1024 cm-2. The positional deviation between maser spots and nuclear X-ray source as well as a high degree of clumpiness in the circumnuclear interstellar medium may be responsible for the weakness of the correlation between isotropic maser luminosity and X-ray absorbing column density. The masers associated with star formation have been used to pinpoint sites of massive star-forming regions and to estimate the geometric distance of host galaxies by comparing radial velocity and proper motion in groups of maser spots, and to determine the dynamical model of Local Group. VLBI observation of AGN-related masers has become a very powerful tool to investigate the innermost region of AGN. In addition, the H2O masers are detectable at high redshift, potentially providing a new cosmological observable for testing the reality and properties of dark energy and eventually the cosmological model.
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