A search for H$_2$O masers in 100 active dwarf galaxies

Abstract

We present the results of the first dedicated survey for 22 GHz H$_2$O 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 mass, and such measurements in dwarf galaxies would be key to understanding the low-mass end of BH-galaxy co-evolution. We used the Green Bank Telescope to survey 100 nearby ($z \lesssim 0.055$) dwarf galaxies ($M_* \lesssim 10^{9.5}~M_{\odot}$) with optical emission line ratios indicative of accretion onto a massive black hole. We detected no new masers down to a limit of $\sim$12 mJy (5$\sigma$). We compared the properties of our sample with those of $\sim$1,850 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 AGNs. Our sample has fewer Seyferts, is less obscured, and is [O III]-faint. Though the overall maser detection rate is $\sim$3% in massive galaxies, the predicted rate for our sample, weighted by its optical properties, is $\sim$0.6-1.7%, corresponding to a probability of making no detections of $\sim$20-50%. 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.