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2006A&A...450..933Z - Astronomy and Astrophysics, volume 450, 933-944 (2006/5-2)

Extragalactic H2O masers and X-ray absorbing column densities.

ZHANG J.S., HENKEL C., KADLER M., GREENHILL L.J., NAGAR N., WILSON A.S. and BRAATZ J.A.

Abstract (from CDS):

Having conducted a search for the λ∼1.3cm (22GHz) water vapor line towards galaxies with nuclear activity, large nuclear column densities or high infrared luminosities, we present H2O spectra for NGC2273, UGC5101, and NGC3393 with isotropic luminosities of 7, 1500, and 400L. The H2O maser in UGC5101 is by far the most luminous yet found in an ultraluminous infrared galaxy. NGC3393 reveals the classic spectrum of a disk maser'', represented by three distinct groups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below 1000 km/s. Based on the literature and archive data, X-ray absorbing column densities are compiled for the 64 galaxies with reported maser sources beyond the Magellanic Clouds. For NGC2782 and NGC5728, we present Chandra archive data that indicate the presence of an active galactic nucleus in both galaxies. Modeling the hard nuclear X-ray emission, NGC2782 is best fit by a high energy reflection spectrum with NH>1024cm–2. For NGC5728, partial absorption with a power law spectrum indicates NH∼8x1023cm–2. The correlation between absorbing column and H2O emission is analyzed. There is a striking difference between kilo- and megamasers with megamasers being associated with higher column densities. All kilomasers (LH2O<10L) except NGC2273 and NGC5194 are Compton-thin, i.e. their absorbing columns are <1024cm–2. Among the H2O megamasers, 50% arise from Compton-thick and 85% from heavily obscured (>1023cm–2) active galactic nuclei. These values are not larger but consistent with those from samples of Seyfert 2 galaxies not selected on the basis of maser emission. The similarity in column densities can be explained by small deviations in position between maser spots and nuclear X-ray source and a high degree of clumpiness in the circumnuclear interstellar medium.

Journal keyword(s): masers - galaxies: active - galaxies: nuclei - galaxies: statistics - radio lines: galaxies - X-rays: galaxies

CDS comments: Table 5 AM2158-380NED02 is MCG-06-48-013 in Simbad.