The Discovery of Water Maser Emission from Eight Nearby Galaxies.pdf
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The Discovery of Water Maser Emission from Eight Nearby
Galaxies
J.A. Braatz
National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903
jbraatz@nrao.edu
and
N.E. Gugliucci
Department of Astronomy, University of Virginia, Charlottesville, VA 22904
neg9j@virginia.edu
ABSTRACT
Using the Green Bank Telescope, we conducted a “snapshot” survey for water
maser emission toward the nuclei of 611 galaxies and detected eight new sources.
The sample consisted of nearby (v 5000 km s?1) and luminous (MB -19.5)
galaxies, some with known nuclear activity but most not previously known to
host AGNs. Our detections include both megamasers associated with AGNs and
relatively low luminosity masers probably associated with star formation. The
detection in UGC 3789 is particularly intriguing because the spectrum shows both
systemic and high-velocity lines indicative of emission from an AGN accretion
disk seen edge-on. Based on six months of monitoring, we detected accelerations
among the systemic features ranging from 2 to 8 km s?1 yr?1, the larger values
belonging to the most redshifted systemic components. High-velocity maser lines
in UGC 3789 show no detectable drift over the same period. Although UGC 3789
was not known to be an AGN prior to this survey, the presence of a disk maser
is strong evidence for nuclear activity, and an optical spectrum obtained later
has confirmed it. With follow up observations, it may be possible to measure a
geometric distance to UGC 3789.
Subject headings: galaxies: active — galaxies: nuclei — galaxies: Seyfert — ISM:
molecules — masers — radio lines: galaxies
– 2 –
1. Introduction
Water maser emission at 22 GHz is commonly detected toward Galactic H II regions,
pinpointing the locations of massive star formation. The median isotropic line luminosity
in these sources is 10?4 L⊙ (Genzel Downes, 1977), although in exceptional cases such as
W49 the lumin
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