ON THE NUCLEAR OBSCURATION OF H₂O MASER GALAXIES

Abstract

To shed light onto the circumnuclear environment of 22 GHz (λ ∼ 1.3 cm) H 2 O maser galaxies, we have analyzed some of their multi-wavelength properties, including the far infrared luminosity (FIR), the luminosity of the [O III]λ5007 emission line, the nuclear X-ray luminosity, and the equivalent width of the neutral iron Kα emission line (EW (K α )). Our statistical analysis includes a total of 85 sources, most of them harboring an active galactic nucleus (AGN). There are strong anti-correlations between EW (K α ) and two "optical thickness parameters", i.e. the ratios of the X-ray luminosity versus the presumably more isotropically radiated [O III] and far infrared (FIR) luminosities. Based on these anticorrelations, a set of quantitative criteria, EW (K α )>300 eV, L 2−10 keV <2 L [O III] and L FIR >600 L 2−10 keV can be established for Compton-thick nuclear regions. 18 H 2 O maser galaxies belong to this category. There are no obvious correlations between the EW (K α ), the [O III] luminosity and the isotropic H 2 O maser luminosity. When comparing samples of Seyfert 2s with and without detected H 2 O maser lines, there seem to exist differences in EW (K α ) and the fraction of Compton-thick nuclei. This should be studied further. For AGN masers alone, there is no obvious correlation between FIR and H 2 O maser luminosities. However, including masers associated with star forming regions, a linear correlation is revealed. Overall, the extragalactic FIR-H 2 O data agree with the corresponding relation for Galactic maser sources, extrapolated by several orders of magnitude to higher luminosities.

Radio properties of H2O maser host galaxies

“…For the entire sample, the distribution of log L 6 cm peaks around 29 (here and throughout the article, L 6 cm Notes. ( * ) Radio properties for the published extragalactic H 2 O maser host galaxies with luminosity distance D > 0.5 Mpc (Zhang et al 2010).…”

“…Column 1: source; For Arp 299, the emission from NGC 3690 was only taken (Neff et al 2004;Tarchi et al 2011). Column 2: types of nuclear activity from Zhang et al (2010). SBG: starburst galaxy; Sy1.5, Sy1.9, Sy2: Seyfert type; LINER: low-ionization nuclear emission line region; ULIRG: ultra luminous infrared galaxy; FR II: Fanaroff-Riley type II radio galaxy; H II: H II galaxies, i.e., dwarf galaxies undergoing a burst of star formation (i.e., Bordalo et al and L 20 cm are given in units of erg s −1 Hz −1 ) and the mean value is 28.8 ± 0.2 (Throughout the paper, given errors are the standard deviation of the mean).…”

“…The absorbing column densities along the lines of sight also differ. Among 31 AGN-masers with available data, about 60% are Compton-thick (Zhang et al 2010). Moreover, 6 cm or 20 cm measurements as indicators of intrinsic nuclear power may introduce some uncertainties.…”

Narrow-line Seyfert 1 galaxies: an amasing class of AGN

“…For example, the most prolific class of AGN, the Sy 2 galaxies, have detection rates of up to ∼20%, while in "pure" Sy 1 galaxies the percentage of water maser detections never exceeds 1% (e.g. Braatz et al 2004;Bennert et al 2009;Zhang et al 2010;Ramolla et al 2011). …”

“…However, the presence of nuclear optically thick gas and/or the partial covering of the nucleus, possibly related to the aforementioned disks/tori inclination, has been invoked to describe the X-ray characteristics of a number of NLS1s, including our target galaxies (Guainazzi et al 2010, for NGC 5506;Turner et al 2006, for Mrk 766;and Terashima et al 2009, for NGC 4051). Given the relation between nuclear obscuration and maser emission reported for relatively large samples of maser galaxies (hence, mostly type 2 Seyfert) by, e.g., Greenhill et al (2008), Zhang et al (2010), this could partly justify the relatively high maser detection rate in NLS1s, especially w.r.t. that in Sy 1.…”