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