1,344 research outputs found
HI Epoch of Reionization Arrays
There are few data available with which to constrain the thermal history of
the intergalactic medium (IGM) following global recombination. Thus far, most
constraints flow from analyses of the Cosmic Microwave Background and optical
spectroscopy along a few lines of sight. However, direct study of the IGM in
emission or absorption against the CMB via the 1S hyperfine transition of
Hydrogen would enable broad characterization thermal history and source
populations. New generations of radio arrays are in development to measure this
line signature. Bright foreground emission and the complexity of instrument
calibration models are significant hurdles. How to optimize these is uncertain,
resulting in a diversity in approaches. We discuss recent limits on line
brightness, array efforts including the new Large Aperture Experiment to Detect
the Dark Ages (LEDA), and the next generation Hydrogen Reionization Array
(HERA) concept.Comment: 8 pages, 4 figures, 1 table. Invited review to the 11th Asian-Pacific
Regional IAU Meeting 2011, NARIT Conference Series, Vol. 1 eds. S.
Komonjinda, Y. Kovalev, and D. Ruffolo (2012
A 43GHz VLBI mapping of SiO maser emission associated with Orion-KL IRC-2
A milliarcsecond resolution spot map of the SiO maser emission associated with IRC-2 in Orion-KL is presented. The two dominant groups of spectral features, near V(LRS) = -6 and 16 km/s, were observed in the 43 GHz, v = 1 to 0 transition of SiO, using a Mark III VLBI system. The 74 km baseline ran from Haystack Observatory in Westford, Massachusetts to Five College Radio Astronomy Observatory (FCRAO) in New Salem, Massachusetts. Five distinct maser features were observed: -8.5 to -6.5 km/s; -5 to -1.5 km/s; 12 to 13.5 km/s; 16.5 to 19 km/s; and 20 to 21 km/s (stellar velocity = 5 km/s). The relative positions were established, from an analysis of fringe phases, to an accuracy of about 5 milliarcseconds. All the features lay within an area of radius 0.08 arcseconds or 6x10(14) cm, at a distance of 500 pc. Previous interferometric studies were only able to measure the gross separation between the red and the blue shifted groups. Our measurement of the separation between these two gropus is consistent with those of the previous studies, indicating the persistence of these two centers of activity. The absolute positions of the masers with respect to IRC-2 are only known to an accuracy of about 1 arcsecond. It is assumed that IRC-2 is centered between the red shifted and the blue shifted maser features. The relative placement of these two groups of maser features agrees with observations of thermal emission from SO, which traces the outflow on a much larger scale. The SiO masers trace the neutral outflow from IRC-2 on the smallest scale yet observed
Toward a New Distance to the Active Galaxy NGC 4258: II. Centripetal Accelerations and Investigation of Spiral Structure
We report measurements of centripetal accelerations of maser spectral
components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second
paper of a series, in which the goal is determination of a new geometric maser
distance to NGC 4258 accurate to possibly ~3%. We measure accelerations using a
formal analysis method that involves simultaneous decomposition of maser
spectra for all epochs into multiple, Gaussian components. Components are
coupled between epochs by linear drifts (accelerations) from their centroid
velocities at a reference epoch. For high-velocity emission, accelerations lie
in the range -0.7 to +0.7 km/s/yr indicating an origin within 13 degrees of the
disk midline (the perpendicular to the line-of-sight to the black hole).
Comparison of high-velocity emission projected positions in VLBI images, with
those derived from acceleration data, provides evidence that masers trace real
gas dynamics. High-velocity emission accelerations do not support a model of
trailing shocks associated with spiral arms in the disk. However, we find
strengthened evidence for spatial periodicity in high-velocity emission, of
wavelength 0.75 mas. This supports suggestions of spiral structure due to
density waves in the nuclear accretion disk of an active galaxy. Accelerations
of low-velocity (systemic) emission lie in the range 7.7 to 8.9 km/s/yr,
consistent with emission originating from a concavity where the thin, warped
disk is tangent to the line-of-sight. A trend in accelerations of low-velocity
emission as a function of Doppler velocity may be associated with disk geometry
and orientation, or with the presence of spiral structure.Comment: Accepted to ApJ, 48 pages and 20 figure
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
VLBI Imaging of Water Maser Emission from the Nuclear Torus of NGC 1068
We have made the first VLBI synthesis images of the H2O maser emission
associated with the central engine of the Seyfert galaxy NGC 1068. Emission
extends about +/-300 km/s from the systemic velocity. Images with
submilliarcsecond angular resolution show that the red-shifted emission lies
along an arc to the northwest of the systemic emission. (The blue-shifted
emission has not yet been imaged with VLBI.) Based on the maser velocities and
the relative orientation of the known radio jet, we propose that the maser
emission arises on the surface of a nearly edge-on torus, where physical
conditions are conducive to maser action. The visible part of the torus is
axially thick, with comparable height and radius. The velocity field indicates
sub-Keplerian differential rotation around a central mass of about 1e7 Msun
that lies within a cylindrical radius of about 0.65 pc. The estimated
luminosity of the central engine is about 0.5 of the Eddington limit. There is
no detectable compact radio continuum emission near the proposed center of the
torus (T_B< 5e6 K on size scales of about 0.1 pc), so that the observed
flat-spectrum core cannot be direct self-absorbed synchrotron radiation.Comment: 12 pages, 4 figures. To appear in ApJ Part 2. Also available at
http://www.physics.ucsb.edu/~vlbiweb
21cm Absorption by Compact Hydrogen Disks Around Black Holes in Radio-Loud Nuclei of Galaxies
The clumpy maser disks observed in some galactic nuclei mark the outskirts of
the accretion disk that fuels the central black hole and provide a potential
site of nuclear star formation. Unfortunately, most of the gas in maser disks
is currently not being probed; large maser gains favor paths that are
characterized by a small velocity gradient and require rare edge-on
orientations of the disk. Here we propose a method for mapping the atomic
hydrogen distribution in nuclear disks through its 21cm absorption against the
radio continuum glow around the central black hole. In NGC 4258, the 21cm
optical depth may approach unity for high angular-resolution (VLBI) imaging of
coherent clumps which are dominated by thermal broadening and have the column
density inferred from X-ray absorption data, ~10^{23}/cm^2. Spreading the 21cm
absorption over the full rotation velocity width of the material in front of
the narrow radio jets gives a mean optical depth of ~0.1. Spectroscopic
searches for the 21cm absorption feature in other galaxies can be used to
identify the large population of inclined gaseous disks which are not masing in
our direction. Follow-up imaging of 21cm silhouettes of accelerating clumps
within these disks can in turn be used to measure cosmological distances.Comment: 4 page
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