311 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 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 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
Discovery of water vapor megamaser emission from Mrk1419 (NGC 2690): An analogue of NGC 4258?
Water vapor emission at 22 GHz is reported from the nucleus of the LINER
galaxy Mrk 1419 (NGC 2960). Single-dish spectra of the maser source show
properties that are similar to those seen in NGC 4258, namely (1) a cluster of
systemic H2O features, (2) two additional H2O clusters, one red- and one
blue-shifted by about 475 km/s, (3) a likely acceleration of the systemic
features, and (4) no detectable velocity drifts in the red- and blue-shifted
features. Interpreting the data in terms of the paradigm established for NGC
4258, i.e. assuming the presence of an edge-on Keplerian circumnuclear annulus
with the systemic emission arising from the near side of its inner edge, the
following parameters are derived: Rotational velocity: 330-600 km/s; radius:
0.13-0.43 pc; binding mass: about 10 million solar masses. With the galaxy
being approximately ten times farther away than NGC 4258, a comparison of
linear and angular scales (the latter via Very Long Baseline Interferometry)
may provide an accurate geometric distance to Mrk 1419 that could be used to
calibrate the cosmic distance scale.Comment: 4 pages, 5 Postscript figures, A&A Letter
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