A search for H2O masers in 100 active dwarf galaxies

We present the results of the first dedicated survey for 22 GHz H2O maser emission in dwarf galaxies outside of the Local Group, with the aim of discovering disc masers. Studies of disc masers yield accurate and precise measurements of black hole (BH) mass, and such measurements in dwarf galaxies would be key to understanding the low-mass end of BH-galaxy coevolution. We used the Green Bank Telescope to survey 100 nearby (z less than or similar to 0.055) dwarf galaxies (M-* less than or similar to 10(9.5) M-circle dot) with optical emission line ratios indicative of accretion on to a massive black hole. We detected no new masers down to a limit of similar to 12 mJy (5 sigma). We compared the properties of our sample with those of similar to 1850 known detections and non-detections in massive galaxies. We find, in agreement with previous studies, that masers are preferentially hosted by Seyferts and highly obscured, [O III]-bright active galactic nuclei (AGNs). Our sample has fewer Seyferts, is less obscured, and is [O III]-faint. Though the overall maser detection rate is similar to 3 per cent in massive galaxies, the predicted rate for our sample, weighted by its optical properties, is similar to 0.6-1.7 per cent, corresponding to a probability of making no detections of similar to 20-50 per cent. We also found a slight increase in the detection rate with increased stellar mass in previously surveyed galaxies. However, further observations are required to discern whether there is an intrinsic difference between the maser fraction in active dwarf galaxies and in their massive counterparts for the same AGN properties.