675 research outputs found
Small-Scale Structure of O VI Interstellar Gas in the Direction of the Globular Cluster NGC 6752
In order to study the small-scale structure of the hot interstellar gas, we
obtained Far Ultraviolet Spectroscopic Explorer interstellar O VI interstellar
absorption spectra of 4 four post-extreme horizontal branch stars in the
globular cluster NGC 6752 [(l,b) = (336.50,-25.63), d = 3.9 kpc, z= -1.7 kpc].
The good quality spectra of these stars allow us to measure both lines of the O
VI doublet at 1031.926 \AA and 1037.617 \AA. The close proximity of these stars
permits us to probe the hot interstellar gas over angular scale of only
2\farcm2 - 8\farcm9, corresponding to spatial scales \la 2.5-10.1 pc. On
these scales we detect no variations in the O VI column density and velocity
distribution. The average column density is log = 14.34 \pm 0.02 (log
= 13.98). The measured velocity dispersions of the O VI
absorption are also indistinguishable. Including the earlier results of Howk et
al., this study suggests that interstellar O VI is smooth on scales \Delta
\theta \la 12\arcmin, corresponding to a spatial scale of less than 10 pc, and
quite patchy at larger scales. Although such small scales are only probed in a
few directions, this suggests a characteristic size scale for the regions
producing collisionally-ionized O VI in the Galaxy.Comment: Accepted for publication in the PASP (to appear in the October 2004
issue
Extraplanar Dust in the Edge-On Spiral NGC 891
We present high-resolution (<0.65") optical broad-band images of the edge-on
Sb galaxy NGC 891 obtained with the WIYN 3.5-m telescope. These BVR images
reveal a complex network of hundreds of dust absorbing structures far from the
mid-plane of the galaxy. The dust structures have a wide range of morphologies
and are clearly visible to |z|<1.5 kpc from the mid-plane. In this paper we
discuss the general characteristics of the population of absorbing structures,
as well as physical properties of 12 individual features. These 12 structures
are characterised by N_H >10^21 cm^-2, with masses estimated to be more than
2x10^5 - 5x10^6 solar masses, assuming Galactic gas-to-dust relationships. The
gravitational potential energies of the individual dust structures, given their
observed heights and derived masses, lie in the range of 20-200x10^51 ergs.
Rough number counts of extraplanar dust features suggest the mass of high-z gas
associated with extraplanar dust in NGC 891 likely exceeds 2x10^8 solar masses,
or ~2% of the total neutral ISM mass of the galaxy.
We discuss several mechanisms which may produce high-z dust structures such
as those seen in the images presented here. It is not yet known which of these
mechanisms are primarily responsible for the extensive extraplanar dust
structures seen in our images. The data presented are part of a larger program
to search for and characterize off-plane dust structures in edge-on systems.
(Abstract Abridged)Comment: To appear in the Astronomical Journal: 37 pages, Latex; 9 separate
figures; the paper and high-resolution figures are also available at
http://www.astro.wisc.edu/~howk/Papers/papers.htm
Near-infrared reddening of extra-galactic GMCs in a face-on geometry
[Abridged] We describe the near-infrared reddening signature of giant
molecular clouds (GMCs) in external galaxies. In particular, we examine the
E(J-H) and E(H-K) color-excesses, and the effective extinction law observed in
discrete GMC regions. We also study the effect of the relative scale height of
the GMC distribution to the color-excesses, and to the observed mass function
of GMCs. We perform Monte Carlo radiative transfer simulations with 3D models
of stellar radiation and clumpy dust distributions, resembling a face-on
geometry. The scattered light is included in the models, and near-infrared
color maps are calculated from the simulated data. The effective near-infrared
reddening law, i.e. the ratio E(J-H)/E(H-K), has a value close to unity in GMC
regions. The ratio depends on the relative scale height of GMCs, xi, and for xi
values 0.1...0.75 we find the typical ratios of 0.6...1.1. The effective
extinction law turns out to be very flat in GMC regions. We find the ratios of
apparent extinctions of A(H)/A(K)=1.35...1.55 and A(J)/A(H)=1.15. The effect of
the scattered flux on the effective reddening law, as well as on the effective
extinction law, is significant. Regarding the GMC mass function, we find no
correlation between the input and observed slopes of the mass functions.
Rather, the observed slope reflects the parameter and the dynamical range
of the mass function. We estimate that only a fraction of 10...20 % of the
total mass of GMCs is recovered, if the observed color-excess values are
transformed to masses using the Galactic reddening law. In the case of
individual clouds the fraction can vary between ~0...50 %.Comment: 8 pages, 10 figures, accepted for publication in A&A. Added missing
histograms in Fig.
A Lion in the Path of Oman's Nationalization: Insurgency in Oman from the 1950s through the 1970s Examined through Social Movement Theory; Strategic Insights, v. 7 issue 2 (April 2008)
This article appeared in Strategic Insights, v.7 issue 2 (April 2008)Approved for public release; distribution is unlimited
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