34 research outputs found
On Measuring Accurate 21-cm Line Profiles with the Robert C. Byrd Green Bank Telescope
We use observational data to show that 21 cm line profiles measured with the
Green Bank Telescope (GBT) are subject to significant inaccuracy. These include
~10% errors in the calibrated gain and significant contribution from distant
sidelobes. In addition, there are ~60% variations between the GBT and
Leiden/Argentine/Bonn 21 cm line profile intensities, which probably occur
because of the high main-beam efficiency of the GBT. Stokes V profiles from the
GBT contain inaccuracies that are related to the distant sidelobes.
We illustrate these problems, define physically motivated components for the
sidelobes, and provide numerical results showing the inaccuracies. We provide a
correction scheme for Stokes I 21 cm line profiles that is fairly successful
and provide some rule-of-thumb comments concerning the accuracy of Stokes V
profiles.Comment: 39 pages, 20 figures, accepted for publication in PAS
HI Imaging of LGS 3 and an Apparently Interacting High-Velocity Cloud
We present a 93' by 93' map of the area near the Local Group dwarf galaxy LGS
3, centered on an HI cloud 30' away from the galaxy. Previous authors
associated this cloud with LGS 3 but relied on observations made with a 36'
beam. Our high-resolution (3.4'), wide-field Arecibo observations of the region
reveal that the HI cloud is distinct from the galaxy and suggest an interaction
between the two. We point out faint emission features in the map that may be
gas that has been tidally removed from the HI cloud by LGS 3. We also derive
the rotation curve of the cloud and find that it is in solid-body rotation out
to a radius of 10', beyond which the rotation velocity begins to decline.
Assuming a spherical geometry for the cloud, the implied mass is 2.8 x 10^7
(d/Mpc) M_{Sun}, where d is the distance in Mpc. The observed HI mass is 5.5 x
10^6 (d/Mpc)^2 M_{Sun}, implying that the cloud is dark-matter dominated unless
its distance is at least 1.9 Mpc. We propose that the cloud is a high-velocity
cloud that is undergoing a tidal interaction with LGS 3 and therefore is
located roughly 700 kpc away from the Milky Way. The cloud then contains a
total mass of ~2.0 x 10^7 M_{Sun}, 82% of which consists of dark matter.Comment: 5 pages, 2 color figures. Accepted for publication in ApJ Letter
Parsec-scale magnetic fields in Arp 220
We present the first very-long-baseline interferometry (VLBI) detections of
Zeeman splitting in another galaxy. We used Arecibo Observatory, the Green Bank
Telescope, and the Very Long Baseline Array to perform dual-polarization
observations of OH maser lines in the merging galaxy Arp 220. We measured
magnetic fields of 1-5 mG associated with three roughly parsec-sized
clouds in the nuclear regions of Arp 220. Our measured magnetic fields have
comparable strengths and the same direction as features at the same velocity
identified in previous Zeeman observations with Arecibo alone. The agreement
between single dish and VLBI results provides critical validation of previous
Zeeman splitting observations of OH megamasers that used a single large dish.
The measured magnetic field strengths indicate that magnetic energy densities
are comparable to gravitational energy in OH maser clouds. We also compare our
total intensity results to previously published VLBI observations of OH
megamasers in Arp 220. We find evidence for changes in both structure and
amplitude of the OH maser lines that are most easily explained by variability
intrinsic to the masing region, rather than variability produced by
interstellar scintillation. Our results demonstrate the potential for using
high-sensitivity VLBI to study magnetic fields on small spatial scales in
extragalactic systems.Comment: 9 pages, accepted to MNRA
Extragalactic Zeeman Detections in OH Megamasers
We have measured the Zeeman splitting of OH megamaser emission at 1667 MHz
from five (ultra)luminous infrared galaxies ([U]LIRGs) using the 305 m Arecibo
telescope and the 100 m Green Bank Telescope. Five of eight targeted galaxies
show significant Zeeman-splitting detections, with 14 individual masing
components detected and line-of-sight magnetic field strengths ranging from
~0.5-18 mG. The detected field strengths are similar to those measured in
Galactic OH masers, suggesting that the local process of massive star formation
occurs under similar conditions in (U)LIRGs and the Galaxy, in spite of the
vastly different large-scale environments. Our measured field strengths are
also similar to magnetic field strengths in (U)LIRGs inferred from synchrotron
observations, implying that milligauss magnetic fields likely pervade most
phases of the interstellar medium in (U)LIRGs. These results provide a
promising new tool for probing the astrophysics of distant galaxies.Comment: 32 pages, 14 figures, 8 tables. Accepted for publication in The
Astrophysical Journal v680n2, June 20, 2008; corrected 2 typo
Gas Rich Dwarf Spheroidals
We present evidence that nearly half of the dwarf spheroidal galaxies (dSph
and dSph/dIrr) in the Local Group are associated with large reservoirs of
atomic gas, in some cases larger than the stellar mass. The gas is sometimes
found at large distance (~10 kpc) from the center of a galaxy and is not
necessarily centered on it. Similarly large quantities of ionized gas could be
hidden in these systems as well. The properties of some of the gas reservoirs
are similar to the median properties of the High-Velocity Clouds (HVCs); two of
the HI reservoirs are catalogued HVCs. The association of the HI with the dwarf
spheroidals might thus provide a link between the HVCs and stars. We show that
the HI content of the Local Group dSphs and dIrrs exhibits a sharp decline if
the galaxy is within 250 kpc of either the Milky Way or M31. This can be
explained if both galaxies have a sufficiently massive x-ray emitting halo that
produces ram-pressure stripping if a dwarf ventures too close to either giant
spiral. We also investigate tidal stripping of the dwarf galaxies and find that
although it may play a role, it cannot explain the apparent total absence of
neutral gas in most dSph galaxies at distances less than 250 kpc. For the
derived mean density of the hot gas, n_0 = 2.5e-5 cm^-2, ram-pressure stripping
is found to be more than an order of magnitude more effective in removing the
gas from the dSph galaxies. The hot halo, with an inferred mass of 1e10 solar
masses, may represent a reservoir of ~1000 destroyed dwarf systems, either HVCs
or true dwarf galaxies similar to those we observe now.Comment: AASTex preprint style, 27 pages including 12 figures. Submitted to
ApJ. See also http://astro.berkeley.edu/~robisha
Spectral Polarization of the Redshifted 21 cm Absorption Line Toward 3C 286
A re-analysis of the Stokes-parameter spectra obtained of the z=0.692 21 cm
absorption line toward 3C 286 shows that our original claimed detection of
Zeeman splitting by a line-of-sight magnetic field, B_los = 87 microgauss is
incorrect. Because of an insidious software error, what we reported as Stokes V
is actually Stokes U: the revised Stokes V spectrum indicates a 3-sigma upper
limit of B_los < 17 microgauss. The correct analysis reveals an absorption
feature in fractional polarization that is offset in velocity from the Stokes I
spectrum by -1.9 km/s. The polarization position-angle spectrum shows a dip
that is also significantly offset from the Stokes I feature, but at a velocity
that differs slightly from the absorption feature in fractional polarization.
We model the absorption feature with 3 velocity components against the core-jet
structure of 3C 286. Our chisquare minimization fitting results in components
with differing (1) ratios of H I column density to spin temperature, (2)
velocity centroids, and (3) velocity dispersions. The change in polarization
position angle with frequency implies incomplete coverage of the background jet
source by the absorber. It also implies a spatial variation of the polarization
position angle across the jet source, which is observed at frequencies higher
than the 839.4 MHz absorption frequency. The multi-component structure of the
gas is best understood in terms of components with spatial scales of ~100 pc
comprised of hundreds of low-temperature (T < 200 K) clouds with linear
dimensions of about 1 pc.Comment: Accepted for Publication by the Astrophysical Journa