Andrew Hopkins, Ron Ekers, Carole Jackson, Lawrence Cram, Anne Green, Dick Manchester, Lister Staveley-Smith and Ray Norris, PASA, 16 (2), in press.
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Masers and spectral lines - Ray Norris
The interest in masers stems primarily from their use as tools to investigate the kinematics of regions containing them. The most spectacular such use was the discovery that the H2O masers in NGC4258 are confined to a thin molecular disk, only 0.5pc in diameter, surrounding the central engine (Figure 5). This discovery has provided the best evidence to date for the existence of massive black holes (MBH) in active galactic nuclei (AGN). As a result, H2O megamasers are now becoming one of the most powerful tools available to us for probing the inner parsecs of active galaxies, for example providing a mass estimate, accurate to a few percent, of the MBH, and exploring the turn-on of the radio jet a fraction of a parsec from the MBH. The Square Kilometre Array will allow us to detect and study such megamasers up to a redshift of 0.5, and thus allow us to explore the properties of the MBH as a function of galaxy type and evolution.
As a result of enormously increasing the number of objects known to contain a MBH, many other questions will be able to be addressed. Do megamasers occur only in Sy2 and Liners, or in other galaxy types too? Does the mass of the MBH vary with redshift? Are the large black holes a result of many mergers of small black holes, or do small gas-rich galaxies already contain large black holes? Can we see accretion disks around black holes in merging galaxies? Can we trace the kinematics as the black holes merge? With the Square Kilometre Array we will also be able to study the kinematics of the accretion disc, and thus mechanism for fuelling the MBH.
To detect water megamasers the proposed instrument needs to have a frequency range extending up to 22GHz. Methanol masers, at 6.7GHz, could be used as a tracer of circumnuclear star formation in galaxies. The most luminous Galactic methanol masers (G340.78-0.10) would have a flux density of 2.6mJy in NGC 253. The Square Kilometre Array could detect these with a signal-to-noise ratio of about 5 in 3 seconds.
Other maser work has concentrated on the life and death-processes of stars, using masers in the interstellar and circumstellar environment to map the kinematics and properties of the accreting or outflowing gas. The Square Kilometre Array will enable us to extend this work to nearby galaxies, so we can compare star formation processes in our own galaxy with those in starburst galaxies such as NGC253.
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