171 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
ROSAT X-ray sources in the field of the LMC I.Total LMC gas from the background AGN spectral fits
We analyzed a sample of 26 background X-ray sources in a ~60 square degree
field of the Large Magellanic Cloud observed with the ROSAT PSPC. The sample
has been selected from previously classified and optically identified X-ray
sources. In addition pointlike and spectrally hard sources with at least 100 to
200 observed counts have been used for the analysis. We performed X-ray
spectral fitting and derived total hydrogen absorbing column densities due to
LMC gas in the range 10^20 - 2. 10^21 cm^-2. We compared these columns with the
HI columns derived from a 21-cm Parkes survey of the LMC. For 7 optically
identified sources we find, within the uncertainties derived from the X-ray
spectral fit, agreement for both columns. For further 19 sources we constrain
the LMC columns from the X-ray spectral fit assuming that the powerlaw photon
index is that of AGN type spectra. We derive for 20 sources gas columns which
are within the uncertainties in agreement with the HI columns. We derive for
two background sources (RX J0536.9-6913 and RX J0547.0-7040) hydrogen absorbing
column densities due to LMC gas, which are in excess to the HI columns. These
sources - located in regions of large (~3. 10^21 cm^-2) LMC HI column densities
- could be seen through additional gas which may be warm and diffuse, cold or
molecular. For 10 sources we derive upper limits for the gas columns additional
to HI and constrain the molecular mass fraction to <(30-140)%.Comment: Accepted by A&
Cool Gas in the Magellanic Stream
We present the first direct detection of cold atomic gas in the Magellanic
Stream, through 21 cm line absorption toward a background radio source, J0119 -
6809, using the Australia Telescope Compact Array. Two absorption components
were identified at heliocentric velocities 218.6 km/s and 227.0 km/s, with
optical depths of tau ~ 0.02. The corresponding H I emission region has a
column density in excess of 2 x 10^20 cm^{-2}. The inferred spin temperature of
the emitting gas is ~70 K. We failed to find cool gas in observations of three
other radio continuum sources. Although we have definitively detected cool gas
in the Stream, its spin temperature is higher than similar components in the
LMC, SMC and Bridge, and its contribution to the total H I density is probably
lower. No corresponding 12CO(J = 1 -> 0) or dust appears to be associated with
the cool gas, suggesting that the cloud is not forming stars
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
Wavefront sensing and control in space-based coronagraph instruments using Zernike’s phase-contrast method
Future space telescopes with coronagraph instruments will use a wavefront sensor (WFS) to measure and correct for phase errors and stabilize the stellar intensity in high-contrast images. The HabEx and LUVOIR mission concepts baseline a Zernike wavefront sensor (ZWFS), which uses Zernike’s phase contrast method to convert phase in the pupil into intensity at the WFS detector. In preparation for these potential future missions, we experimentally demonstrate a ZWFS in a coronagraph instrument on the Decadal Survey Testbed in the High Contrast Imaging Testbed facility at NASA’s Jet Propulsion Laboratory. We validate that the ZWFS can measure low- and mid-spatial frequency aberrations up to the control limit of the deformable mirror (DM), with surface height sensitivity as small as 1 pm, using a configuration similar to the HabEx and LUVOIR concepts. Furthermore, we demonstrate closed-loop control, resolving an individual DM actuator, with residuals consistent with theoretical models. In addition, we predict the expected performance of a ZWFS on future space telescopes using natural starlight from a variety of spectral types. The most challenging scenarios require ∼1 h of integration time to achieve picometer sensitivity. This timescale may be drastically reduced by using internal or external laser sources for sensing purposes. The experimental results and theoretical predictions presented here advance the WFS technology in the context of the next generation of space telescopes with coronagraph instruments
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
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