At radio wavelengths, the most common molecules found to produce maser
emission in extragalactic environments are hydroxyl and water. In
particular, water mega-maser-emission at 22 GHz has proven to be a
powerful tool for astrophysical studies of Active Galactic Nuclei (AGN),
allowing a deep investigation of the structure and kinematics of the gas
close to and around their nuclear engines. Indeed, one of the best
determination of the central black-hole mass and of the accretion-disc
geometry and dynamics in AGN is provided by water maser studies (see the
sketch in the image).
Despite deep searches of water-maser emission in large surveys of
galaxies, the detection rate has usually been below 10% so that only
about 200 of known water-maser emitting galaxies are known so far.
A novel approach to this field has been provided by the search of
water-maser emission in a hard X-ray selected sample of AGN, from the
INTEGRAL/IBIS survey above 20 keV. Of 380 sources inspected, half of
them were observed at 22 GHz in search for maser emission and the
resulting detection rate is 15+/-3%. From this total sample we derived a
volume-limited sub-sample of 87 sources, for which we obtained new
observations with the Green Bank and Effelsberg telescopes (for 35
sources). In this way, new masers can be detected (see, e.g., POM
December 2011) and the radio coverage of the complete sub-sample have
increased to 75%. The detection fraction in the complete sub-sample
increases to 19+/-5%.
We confirm that a high fraction of water masers in AGN is related to the
presence of high obscuration (see image, top panel). In optically
classified sources as type 2, the fraction of water maser detection is
22+/-5% and 31+/-10% for the total and complete samples, respectively.
In Compton-thick AGN (sources with an X-ray column density higher than
about 10e24 /cm^2), these fractions raise to 56+/-18% and 50+/-35% for
the total and complete samples, respectively. Considering a model where
the maser medium is located in the very edge-on portion of the obscuring
medium, the large fraction of Compton-thick AGN with water-maser
emission is then explained in terms of geometrical effects.
The correlation between the water-maser luminosity and the X-ray (2-10
keV) and hard X-ray (20-100 keV) luminosity is not significant, however
a possible decline in detection fraction with increasing luminosity
might suggest that an extremely luminous nuclear environment does not
favour maser emission.
Very promisingly, the hard X–ray selection has proven to be very
efficient in the detection of water masers, compared to large optical or
infrared surveys. The possibility of increasing the number of observable
sources for future water-maser detections will be certainly provided by
the discovery of new heavily absorbed sources thanks to the increased
sensitivity of the ongoing INTEGRAL/IBIS and Swift/BAT surveys.
