69 research outputs found
The Spiral Structure of the Outer Milky Way in Hydrogen
We produce a detailed map of the perturbed surface density of neutral
hydrogen in the outer Milky Way disk demonstrating that the Galaxy is a
non-axisymmetric multi-armed spiral. Spiral structure in the southern half of
the Galaxy can be traced out to at least 25 kpc, implying a minimum radius for
the gas disk. Overdensities in the surface density are coincident with regions
of reduced gas thickness. The ratio of the surface density to the local median
surface density is relatively constant along an arm. Logarithmic spirals can be
fit to the arms with pitch angles of 20-25 degrees.Comment: 10 pages with 4 color figures. Accepted for publication in Science.
Embargoed for discussion in the popular press until publication in
ScienceXpress. Higher resolution versions of figures 3 and 4 are available at
http://astron.berkeley.edu/~elevin
Velocity Spectrum for HI at High Latitudes
In this paper we present the results of the statistical analysis of
high-latitude HI turbulence in the Milky Way. We have observed HI in the 21 cm
line, obtained with the Arecibo L-Band Feed Array (ALFA) receiver at the
Arecibo radio telescope. For recovering of velocity statistics we have used the
Velocity Coordinate Spectrum (VCS) technique. In our analysis we have used
direct fitting of the VCS model, as its asymptotic regimes are questionable for
Arecibo's resolution and given the restrictions from thermal smoothing of the
turbulent line. We have obtained a velocity spectral index , an
injection scale of pc, and an HI cold phase temperature of K. The spectral index is steeper than the Kolmogorov index and can be
interpreted as being due to shock-dominated turbulence.Comment: Accepted to Ap
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
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