400 research outputs found
The Megamaser Cosmology Project: I. VLBI observations of UGC 3789
The Megamaser Cosmology Project (MCP) seeks to measure the Hubble Constant
(Ho) in order to improve the extragalactic distance scale and constrain the
nature of dark energy. We are searching for sources of water maser emission
from AGN with sub-pc accretion disks, as in NGC 4258, and following up these
discoveries with Very Long Baseline Interferometric (VLBI) imaging and spectral
monitoring. Here we present a VLBI map of the water masers toward UGC 3789, a
galaxy well into the Hubble Flow. We have observed masers moving at rotational
speeds up to 800 km/s at radii as small as 0.08 pc. Our map reveals masers in a
nearly edge-on disk in Keplerian rotation about a 10^7 Msun supermassive black
hole. When combined with centripetal accelerations, obtained by observing
spectral drifts of maser features (to be presented in Paper II), the UGC 3789
masers may provide an accurate determination of Ho, independent of luminosities
and metallicity and extinction corrections.Comment: 18 pages, 3 figures, 4 table
The Discovery of Water Maser Emission from Eight Nearby Galaxies
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) and luminous (M_B <
-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/yr, 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.Comment: to appear in Astrophysical Journal, 1 May 200
The Megamaser Cosmology Project. V. An Angular Diameter Distance to NGC 6264 at 140 Mpc
We present the direct measurement of the Hubble constant, yielding the direct
measurement of the angular-diameter distance to NGC 6264 using the HO
megamaser technique. Our measurement is based on sensitive observations of the
circumnuclear megamaser disk from four observations with the Very Long Baseline
Array, the Green Bank Telescope and the Effelsberg Telescope. We also monitored
the maser spectral profile for 2.3 years using the Green Bank Telescope to
measure accelerations of maser lines by tracking their line-of-sight velocities
as they change with time. The measured accelerations suggest that the systemic
maser spots have a significantly wider radial distribution than in the
archetypal megamaser in NGC 4258. We model the maser emission as arising from a
circumnuclear disk with orbits dominated by the central black hole. The best
fit of the data gives a Hubble constant of 689 km s
Mpc, which corresponds to an angular-diameter distance of 14419
Mpc. In addition, the fit also gives a mass of the central black hole of
(3.090.42) . The result demonstrates the
feasibility of measuring distances to galaxies located well into the Hubble
flow by using circumnuclear megamaser disks.Comment: 29 pages, 8 figures, Accepted by Ap
The innermost region of the water megamaser radio galaxy 3C403
The standard unified scheme of active galactic nuclei requires the presence
of high column densities of gas and dust potentially obscuring the central
engine. So far, few direct subarcsecond resolution studies of this material
have been performed toward radio galaxies. The goal of this paper is to
elucidate the nuclear environment of the prototypical X-shaped Fanaroff-Riley
type II radio galaxy 3C403, the only powerful radio galaxy known to host a
water megamaser. Very Large Array A-array and single-dish Green Bank and
Effelsberg 1.3 cm measurements were performed to locate and monitor the water
maser emission. Very Long Baseline Interferometry 6 cm continuum observations
were taken to analyze the spatial structure of the nuclear environment at even
smaller scales, while the CO J=1-0 and 2-1 transitions were observed with the
IRAM 30-m telescope to search for thermal emission from a spatially extended,
moderately dense gas component.[abridged]Comment: 11 pages, 12 figures, accepted by A&A. For a version with high
quality figures, see http://erg.ca.astro.it/~atarchi/3C403
The Megamaser Cosmology Project: IV. A Direct Measurement of the Hubble Constant from UGC 3789
In Papers I and II from the Megamaser Cosmology Project (MCP), we reported
initial observations of water masers in an accretion disk of a supermassive
black hole at the center of the galaxy UGC 3789, which gave an angular-diameter
distance to the galaxy and an estimate of Ho with 16% uncertainty. We have
since conducted more VLBI observations of the spatial-velocity structure of
these water masers, as well as continued monitoring of its spectrum to better
measure maser accelerations. These more extensive observations, combined with
improved modeling of the masers in the accretion disk of the central
supermassive black hole, confirm our previous results, but with signifcantly
improved accuracy. We find Ho = 68.9 +/- 7.1 km/s/Mpc; this estimate of Ho is
independent of other methods and is accurate to +/-10%, including sources of
systematic error. This places UGC 3789 at a distance of 49.6 +/- 5.1 Mpc, with
a central supermassive black hole of (1.16 +/- 0.12) x 10^7 Msun.Comment: to appear in Ap
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