22 GHz Water Maser Search in 37 Nearby Galaxies - Four New Water
  Megamasers in Seyfert 2 and OH Maser/Absorber Galaxies
  We report four new 22 GHz water masers found in a Green Bank Telescope search
toward 37 nearby objects. Our goal was to find new maser galaxies, AGN
disk-masers, and objects where both hydroxyl and water maser species coexist.
We observed 37 sources within 250 Mpc that were selected by high X-ray
luminosity (L_X > 10^40 W) and high absorbing column density (N_H ~ 10^22
cm^-2). Sources also included dual or triple AGN and interacting systems. We
further included objects detected in hydroxyl (OH). The selection consisted of
16 new sources, 13 previous non-detections to follow up with a factor ten
higher sensitivity, 10 OH masers and one deep OH absorber, of which 37 were
observed. Water megamasers were detected towards the Sy 2 galaxy 2MFGC 13581 (a
disk-maser), towards the 6 GHz OH absorber NGC 4261 (3C 270 with a twin-jet and
dusty torus; broad water maser emission), and towards the two 1.6 GHz OH maser
sources IRAS 17526+3253 and IRAS 20550+1656 (possible star formation water
masers). We set upper limits on 33 non-detections. The search increased the
number of known "dual-species" objects containing both OH and H2O masers to
eight. The detection rate was 25 % in OH galaxies and 11 % overall. Combined
with other searches, a total of 95 objects have now been searched for both OH
and H2O masers. We found the overall dual-species detection rate (8 in 95) to
be of the order of the joint probability of both species independently
occurring in the same object (1 % lower bound). However, this needs to be
verified by a more detailed analysis of selection criteria. Lastly, we see a
lack of H2O kilomasers in OH megamaser objects already noted by Tarchi et al.
(2011). This may be due to sensitivity bias rather than astrophysical reasons.
