1,652 research outputs found
In ISM Modeling, The Devil is in the Details: You Show Me Your OVI and I'll Show You Mine
The three sections of this paper illustrate the importance of rationalizing
ISM theory and modeling with the observational information.Comment: To be published in "From Observations to Self-Consistent Modelling of
the ISM in Galaxies", eds. M. Avillez & D. Breitschwerdt, Kluwer Astrophys. &
Space Sci. Serie
Supernova Remnant in a Stratified Medium: Explicit, Analytical Approximations for Adiabatic Expansion and Radiative Cooling
We propose simple, explicit, analytical approximations for the kinematics of
an adiabatic blast wave propagating in an exponentially stratified ambient
medium, and for the onset of radiative cooling, which ends the adiabatic era.
Our method, based on the Kompaneets implicit solution and the Kahn
approximation for the radiative cooling coefficient, gives straightforward
estimates for the size, expansion velocity, and progression of cooling times
over the surface, when applied to supernova remnants (SNRs). The remnant shape
is remarkably close to spherical for moderate density gradients, but even a
small gradient in ambient density causes the cooling time to vary substantially
over the remnant's surface, so that for a considerable period there will be a
cold dense expanding shell covering only a part of the remnant. Our
approximation provides an effective tool for identifying the approximate
parameters when planning 2-dimensional numerical models of SNRs, the example of
W44 being given in a subsequent paper.Comment: ApJ accepted, 11 pages, 2 figures embedded, aas style with
ecmatex.sty and lscape.sty package
3D MHD Modeling of the Gaseous Structure of the Galaxy: Synthetic Observations
We generated synthetic observations from the four-arm model presented in
Gomez & Cox (2004) for the Galactic ISM in the presence of a spiral
gravitational perturbation. We found that velocity crowding and diffusion have
a strong effect in the l-v diagram. The v-b diagram presents structures at the
expected spiral arm velocities, that can be explained by the off-the-plane
structure of the arms presented in previous papers of this series. Such
structures are observed in the Leiden/Dwingeloo HI survey. The rotation curve,
as measured from the inside of the modeled galaxy, shows similarities with the
observed one for the Milky Way Galaxy, although it has large deviations from
the smooth circular rotation corresponding to the background potential. The
magnetic field inferred from a synthetic synchrotron map shows a largely
circular structure, but with interesting deviations in the midplane due to
distortion of the field from circularity in the interarm regions.Comment: Accepted for publication in ApJ. Better quality figures in
http://www.astro.umd.edu/~gomez/publica/3d_galaxy-3.pd
Models of Veritcal Disturbances in the Interstellar Medium
This paper describes some interesting properties of waves in, and
oscillations of, the interstellar medium in the direction normal to the plane
of the Galaxy. Our purpose is to examine possible reasons for four observed
phenomena: the falling sky in the northern hemisphere; the apparent presence of
clouds in absorption spectra when a sightline is occupied primarily only by
warm intercloud gas; the peculiar structuring of spiral arms involving clumps,
spurs, and feathering; and the existence of an abundance of high stage ions far
off the plane of the Galaxy.
We explored the reaction of the interstellar medium - in the vertical
direction only - to large imposed disturbances (initial displacements,
expansive velocities, and compressions), and to the introduction of small
amplitude waves via oscillation of the midplane. Our findings included: 1) the
anticipated growth in amplitude of high frequency waves with height; 2) the
four lowest normal modes for the oscillation of the atmosphere as a whole, as
functions of the height of the outer boundary; 3) the time for material to
`bounce' from one unusually dense state to the next as a function of height;
and 4) the tendency for the disk to develop a hot outer halo, either after the
passage of a single shock from a large event, or in response to a continuous
stream of small amplitude waves.Comment: This paper contains 31 pages of text and 23 figures. This paper was
submitted to The Astrophysical Journal on 6/6/2000. It was revised and
resubmitted on 9/29/200
Comparison of NOAA-9 ERBE measurements with Cirrus IFO satellite and aircraft measurements
Earth Radiation Budget Experiment (ERBE) measurements onboard the NOAA-9 are compared for consistency with satellite and aircraft measurements made during the Cirrus Intensive Field Observation (IFO) of October 1986. ERBE scene identification is compared with NOAA-9 TIROS Operational Vertical Sounder (TOVS) cloud retrievals; results from the ERBE spectral inversion algorithms are compared with High resolution Interferometer Sounder (HIS) measurements; and ERBE radiant existance measurements are compared with aircraft radiative flux measurements
Sensitive observations at 1.4 and 250 GHz of z > 5 QSOs
We present 1.4 and 5 GHz observations taken with the Very Large Array (VLA),
and observations at 250 GHz obtained with the Max-Planck millimeter bolometer
(MAMBO) at the IRAM 30~m telescope, of ten optically selected Quasi-stellar
Objects (QSOs) at 5.0 < z < 6.28. Four sources are detected at 1.4 GHz two of
which are radio loud and are also detected at 5 GHz. These results are roughly
consistent with there being no evolution of the radio-loud QSO fraction out to
z~6.
Three sources have been detected at 250 GHz or 350 GHz at much higher levels
than their 1.4 GHz flux densities suggesting that the observed mm emission is
likely thermal emission from warm dust, although more exotic possibilities
cannot be precluded.
The highest redshift source in our sample (J1030+0524 at z=6.28) is not
detected at 1.4 or 250 GHz, but four fairly bright radio sources (flux density
at 1.4GHz > 0.2 mJy) are detected in a 2' field centered on the QSO, including
an edge-brightened ('FRII') double radio source with an extent of about 1'.
A similar over-density of radio sources is seen in the field of the highest
redshift QSO J1148+5251. We speculate that these over-densities of radio
sources may indicate clusters along the lines-of-sight, in which case
gravitational lensing by the cluster could magnify the QSO emission by a factor
2 or so without giving rise to arcsecond-scale distortions in the optical
images of the QSOs.Comment: 25 pages, 12 figures. accepted by A
The Evolution of Adiabatic Supernova Remnants in a Turbulent, Magnetized Medium
(Abridged) We present the results of three dimensional calculations for the
MHD evolution of an adiabatic supernova remnant in both a uniform and turbulent
interstellar medium using the RIEMANN framework of Balsara. In the uniform
case, which contains an initially uniform magnetic field, the density structure
of the shell remains largely spherical, while the magnetic pressure and
synchrotron emissivity are enhanced along the plane perpendicular to the field
direction. This produces a bilateral or barrel-type morphology in synchrotron
emission for certain viewing angles. We then consider a case with a turbulent
external medium as in Balsara & Pouquet, characterized by .
Several important changes are found. First, despite the presence of a uniform
field, the overall synchrotron emissivity becomes approximately spherically
symmetric, on the whole, but is extremely patchy and time-variable, with
flickering on the order of a few computational time steps. We suggest that the
time and spatial variability of emission in early phase SNR evolution provides
information on the turbulent medium surrounding the remnant. The
shock-turbulence interaction is also shown to be a strong source of
helicity-generation and, therefore, has important consequences for magnetic
field generation. We compare our calculations to the Sedov-phase evolution, and
discuss how the emission characteristics of SNR may provide a diagnostic on the
nature of turbulence in the pre-supernova environment.Comment: ApJ, in press, 5 color figure
Thermal Emission from Warm Dust in the Most Distant Quasars
We report new continuum observations of fourteen z~6 quasars at 250 GHz and
fourteen quasars at 1.4 GHz. We summarize all recent millimeter and radio
observations of the sample of the thirty-three quasars known with
5.71<=z<=6.43, and present a study of the rest frame far-infrared (FIR)
properties of this sample. These quasars were observed with the Max Plank
Millimeter Bolometer Array (MAMBO) at 250 GHz with mJy sensitivity, and 30% of
them were detected. We also recover the average 250 GHz flux density of the
MAMBO undetected sources at 4 sigma, by stacking the on-source measurements.
The derived mean radio-to-UV spectral energy distributions (SEDs) of the full
sample and the 250 GHz non-detections show no significant difference from that
of lower-redshift optical quasars. Obvious FIR excesses are seen in the
individual SEDs of the strong 250 GHz detections, with FIR-to-radio emission
ratios consistent with that of typical star forming galaxies. Most 250
GHz-detected sources follow the L_{FIR}--L_{bol} relationship derived from a
sample of local IR luminous quasars (L_{IR}>10^{12}L_{\odot}), while the
average L_{FIR}/L_{bol} ratio of the non-detections is consistent with that of
the optically-selected PG quasars. The MAMBO detections also tend to have
weaker Ly\alpha emission than the non-detected sources. We discuss possible FIR
dust heating sources, and critically assess the possibility of active star
formation in the host galaxies of the z~6 quasars. The average star formation
rate of the MAMBO non-detections is likely to be less than a few hundred
M_{\odot} yr^{-1}, but in the strong detections, the host galaxy star formation
is probably at a rate of \gtrsim10^{3} M_{\odot} yr^{-1}, which dominates the
FIR dust heating.Comment: 32 pages with 6 figures; ApJ, in press; Added references; Corrected
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