81 research outputs found
The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
We use the Gaussian-fit results of Paper I to investigate the properties of
interstellar HI in the Solar neighborhood. The Warm and Cold Neutral Media (WNM
and CNM) are physically distinct components. The CNM spin temperature histogram
peaks at about 40 K. About 60% of all HI is WNM. At z=0, we derive a volume
filling fraction of about 0.50 for the WNM; this value is very rough. The
upper-limit WNM temperatures determined from line width range upward from about
500 K; a minimum of about 48% of the WNM lies in the thermally unstable region
500 to 5000 K. The WNM is a prominent constituent of the interstellar medium
and its properties depend on many factors, requiring global models that include
all relevant energy sources, of which there are many. We use Principal
Components Analysis, together with a form of least squares fitting that
accounts for errors in both the independent and dependent parameters, to
discuss the relationships among the four CNM Gaussian parameters. The spin
temperature T_s and column density N(HI) are, approximately, the two most
important eigenvectors; as such, they are sufficient, convenient, and
physically meaningful primary parameters for describing CNM clouds. The Mach
number of internal macroscopic motions for CNM clouds is typically 2.5, but
there are wide variations. We discuss the historical tau-T_s relationship in
some detail and show that it has little physical meaning. We discuss CNM
morphology using the CNM pressure known from UV stellar absorption lines.
Knowing the pressure allows us to show that CNM structures cannot be isotropic
but instead are sheetlike, with length-to-thickness aspect ratios ranging up to
about 280. We present large-scale maps of two regions where CNM lies in very
large ``blobby sheets''.Comment: Revised submission to Ap.J. Changes include: (1) correction of
turbulent Mach number in equation 16 and figure 12; the new typical value is
1.3 versus the old, incorrect value 2.5. (2) smaller typeface for the
astro-ph version to conserve paper. 60 pages, 16 figure
A Fractal Analysis of the HI Emission from the Large Magellanic Cloud
A composite map of HI in the LMC using the ATCA interferometer and the Parkes
multibeam telescope was analyzed in several ways in an attempt to characterize
the structure of the neutral gas and to find an origin for it. Fourier
transform power spectra in 1D, 2D, and in the azimuthal direction were found to
be approximate power laws over 2 decades in length. Delta-variance methods also
showed the same power-law structure. Detailed models of these data were made
using line-of-sight integrals over fractals that are analogous to those
generated by simulations of turbulence with and without phase transitions. The
results suggested a way to measure directly for the first time the
line-of-sight thickness of the cool component of the HI disk of a nearly
face-on galaxy. The signature of this thickness was found to be present in all
of the measured power spectra.
The character of the HI structure in the LMC was also viewed by comparing
positive and negative images of the integrated emission. The geometric
structure of the high-emission regions was found to be filamentary, whereas the
geometric structure of the low-emission (intercloud) regions was found to be
patchy and round. This result suggests that compressive events formed the
high-emission regions, and expansion events, whether from explosions or
turbulence, formed the low-emission regions. The character of the structure was
also investigated as a function of scale using unsharp masks.
All of these results suggest that most of the ISM in the LMC is fractal,
presumably the result of pervasive turbulence, self-gravity, and self-similar
stirring.Comment: 30 pages, 21 figures, scheduled for ApJ Vol 548n1, Feb 10, 200
Magnetic Field Structure of the Large Magellanic Cloud from Faraday Rotation Measures of Diffuse Polarized Emission
We present a study of the magnetic field of the Large Magellanic Cloud (LMC),
carried out using diffuse polarized synchrotron emission data at 1.4 GHz
acquired at the Parkes Radio Telescope and the Australia Telescope Compact
Array. The observed diffuse polarized emission is likely to originate above the
LMC disk on the near side of the galaxy. Consistent negative rotation measures
(RMs) derived from the diffuse emission indicate that the line-of-sight
magnetic field in the LMC's near-side halo is directed coherently away from us.
In combination with RMs of extragalactic sources that lie behind the galaxy, we
show that the LMC's large scale magnetic field is likely to be of quadrupolar
geometry, consistent with the prediction of dynamo theory. On smaller scales,
we identify two brightly polarized filaments southeast of the LMC, associated
with neutral hydrogen arms. The filaments' magnetic field potentially aligns
with the direction towards the Small Magellanic Cloud. We suggest that tidal
interactions between the Small and the Large Magellanic Clouds in the past 10^9
years is likely to have shaped the magnetic field in these filaments.Comment: 42 pages, 22 figures, 2 tables. Accepted for publication in ApJ.
Electronic version of Table 2 is available via email from the first autho
Star Formation from Galaxies to Globules
The empirical laws of star formation suggest that galactic-scale gravity is
involved, but they do not identify the actual triggering mechanisms for
clusters in the final stages. Many other triggering processes satisfy the
empirical laws too, including turbulence compression and expanding shell
collapse. The self-similar nature of the gas and associated young stars
suggests that turbulence is more directly involved, but the small scale
morphology of gas around most embedded clusters does not look like a random
turbulent flow. Most clusters look triggered by other nearby stars. Such a
prominent local influence makes it difficult to understand the universality of
the Kennicutt and Schmidt laws on galactic scales. A unified view of
multi-scale star formation avoids most of these problems. Ambient self-gravity
produces spiral arms and drives much of the turbulence that leads to
self-similar structures, while localized energy input from existing clusters
and field supernovae triggers new clusters in pre-existing clouds. The
hierarchical structure in the gas made by turbulence ensures that the
triggering time scales with size, giving the Schmidt law over a wide range of
scales and the size-duration correlation for young star fields. The efficiency
of star formation is determined by the fraction of the gas above a critical
density of around 10^5 m(H2)/cc. Star formation is saturated to its largest
possible value given the fractal nature of the interstellar medium.Comment: accepted for ApJ, 42 pages, Dannie Heineman prize lecture, January
200
Primordialists and Constructionists: a typology of theories of religion
This article adopts categories from nationalism theory to classify theories of religion. Primordialist explanations are grounded in evolutionary psychology and emphasize the innate human demand for religion. Primordialists predict that religion does not decline in the modern era but will endure in perpetuity. Constructionist theories argue that religious demand is a human construct. Modernity initially energizes religion, but subsequently undermines it. Unpacking these ideal types is necessary in order to describe actual theorists of religion. Three distinctions within primordialism and constructionism are relevant. Namely those distinguishing: a) materialist from symbolist forms of constructionism; b) theories of origins from those pertaining to the reproduction of religion; and c) within reproduction, between theories of religious persistence and secularization. This typology helps to make sense of theories of religion by classifying them on the basis of their causal mechanisms, chronology and effects. In so doing, it opens up new sightlines for theory and research
Physical Properties of Giant Molecular Clouds in the Large Magellanic Cloud
The Magellanic Mopra Assessment (MAGMA) is a high angular resolution CO
mapping survey of giant molecular clouds (GMCs) in the Large and Small
Magellanic Clouds using the Mopra Telescope. Here we report on the basic
physical properties of 125 GMCs in the LMC that have been surveyed to date. The
observed clouds exhibit scaling relations that are similar to those determined
for Galactic GMCs, although LMC clouds have narrower linewidths and lower CO
luminosities than Galactic clouds of a similar size. The average mass surface
density of the LMC clouds is 50 Msol/pc2, approximately half that of GMCs in
the inner Milky Way. We compare the properties of GMCs with and without signs
of massive star formation, finding that non-star-forming GMCs have lower peak
CO brightness than star-forming GMCs. We compare the properties of GMCs with
estimates for local interstellar conditions: specifically, we investigate the
HI column density, radiation field, stellar mass surface density and the
external pressure. Very few cloud properties demonstrate a clear dependence on
the environment; the exceptions are significant positive correlations between
i) the HI column density and the GMC velocity dispersion, ii) the stellar mass
surface density and the average peak CO brightness, and iii) the stellar mass
surface density and the CO surface brightness. The molecular mass surface
density of GMCs without signs of massive star formation shows no dependence on
the local radiation field, which is inconsistent with the
photoionization-regulated star formation theory proposed by McKee (1989). We
find some evidence that the mass surface density of the MAGMA clouds increases
with the interstellar pressure, as proposed by Elmegreen (1989), but the
detailed predictions of this model are not fulfilled once estimates for the
local radiation field, metallicity and GMC envelope mass are taken into
account.Comment: 28 pages, 10 figures, accepted by MNRA
Supplement: "Localization and broadband follow-up of the gravitational-wave transient GW150914" (2016, ApJL, 826, L13)
This Supplement provides supporting material for Abbott et al. (2016a). We briefly summarize past electromagnetic (EM) follow-up efforts as well as the organization and policy of the current EM follow-up program. We compare the four probability sky maps produced for the gravitational-wave transient GW150914, and provide additional details of the EM follow-up observations that were performed in the different bands
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