Extragalactic H₂O masers and X-ray absorbing column densities

Abstract

Having conducted a search for the λ ∼ 1.3 cm (22 GHz) water vapor line towards galaxies with nuclear activity, large nuclear column densities or high infrared luminosities, we present H 2 O spectra for NGC 2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and 400 L . The H 2 O maser in UGC 5101 is by far the most luminous yet found in an ultraluminous infrared galaxy. NGC 3393 reveals the classic spectrum of a "disk maser", represented by three distinct groups of Doppler components. As in all other known cases except NGC 4258, the rotation velocity of the putative masing disk is well below 1000 km s −1 . 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 NGC 2782 and NGC 5728, we present Chandra archive data that indicate the presence of an active galactic nucleus in both galaxies. Modeling the hard nuclear X-ray emission, NGC 2782 is best fit by a high energy reflection spectrum with N H > ∼ 10 24 cm −2 . For NGC 5728, partial absorption with a power law spectrum indicates N H ∼ 8 × 10 23 cm −2 . The correlation between absorbing column and H 2 O emission is analyzed. There is a striking difference between kilo-and megamasers with megamasers being associated with higher column densities. All kilomasers (L H 2 O < 10 L ) except NGC 2273 and NGC 5194 are Compton-thin, i.e. their absorbing columns are <10 24 cm −2 . Among the H 2 O megamasers, 50% arise from Compton-thick and 85% from heavily obscured (>10 23 cm −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.

New H2O masers in Seyfert and FIR bright galaxies

“…The value for the nuclear column densities is a continuous function that spans a range of values between 10 21.08 cm −2 (M 82) and 10 24.75 cm −2 (M 51; although the values for NGC 6300 and NGC 2273 are only lower limits). Indeed, as qualitatively suggested by Zhang et al (2006), the kilomasers associated, or most likely associated, with AGN activity are found in galaxies with higher nuclear column densities (10 23 cm −2 < N H < 10 25 cm −2 ), different from those with a star-forming origin, which are found in galaxies with nuclear column densities <10 23 cm −2 . The only exception is represented by the maser in NGC 253 that, despite being quite confidently associated with star formation (Hofner et al 2006;Brunthaler et al 2009), Histogram showing the number of H 2 O kilomaser galaxies as a function of their nuclear column density derived from X-ray spectroscopy.…”

“…N H > 10 24 cm −2 . In particular, Zhang et al (2006) found that, for kilomasers, two groups were apparently present, one with nuclear column densities of 10 21−22 cm −2 and another with nuclear column densities of >10 23.3 cm −2 . Furthermore, it was put forward that the few nuclear kilomasers have an average column density that is indistinguishable from those of the entire megamaser sample.…”

“…Are there intrinsic differences in the host galaxy that, in the absence of studies at high (<1 ) spatial resolution, can allow us to distinguish between kilomasers associated with star formation and those associated with AGN activity? A possible suggestion is briefly introduced by Zhang et al (2006) and, more recently, by Greenhill et al (2008) in the broader framework of the association between maser phenomena and nuclear X-ray column density. Their general conclusion was that the distributions of kilomasers and megamasers are different, with the latter associated with galaxies with higher nuclear column densities.…”

“…10, we show an updated plot from Zhang et al (2006) (their Fig. 6a), with the number of H 2 O kilomasers as a function of nuclear column density derived from X-ray spectroscopy for all 19 kilomaser galaxies (Zhang et al 2006: 12 galaxies) for which such information was available. From Fig.…”

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

“…The observing sample is considered towards the extended X-ray galaxies which show megamaser emission from water or OH, since the detection rate of class I methanol masers should tend to be higher in sources with megamasers. 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).…”