498 research outputs found
First mid-infrared spectrum of a faint high-z galaxy: Observations of CFRS 14.1157 with the Infrared Spectrograph on the Spitzer Space Telescope
The unprecedented sensitivity of the Infrared Spectrograph on the Spitzer
Space Telescope allows for the first time the measurement of mid-infrared
spectra from 14 to 38 microns of faint high-z galaxies. This unique capability
is demonstrated with observations of sources having 16 micron fluxes of 3.6 mJy
(CFRS 14.1157) and 0.35 mJy (CFRS 14.9025). A spectral-fitting technique is
illustrated which determines the redshift by fitting emission and absorption
features characteristic of nearby galaxies to the spectrum of an unknown
source. For CFRS 14.1157, the measured redshift is z = 1.00+/-0.20 in agreement
with the published result of z = 1.15. The spectrum is dominated by emission
from an AGN, similar to the nucleus of NGC 1068, rather than a typical
starburst with strong PAH emission like M82. Such spectra will be crucial in
characterizing the nature of newly discovered distant galaxies, which are too
faint for optical follow-up.Comment: Accepted in ApJ Sup. Spitzer Special Issue, 4 pages, 5 figure
A Spitzer Infrared Spectrograph Survey of Warm Molecular Hydrogen in Ultra-luminous Infrared Galaxies
We have conducted a survey of Ultra-luminous Infrared Galaxies (ULIRGs) with
the Infrared Spectrograph on the Spitzer Space Telescope, obtaining spectra
from 5.0-38.5um for 77 sources with 0.02<z <0.93. Observations of the pure
rotational H2 lines S(3) 9.67um, S(2) 12.28um, and S(1) 17.04um are used to
derive the temperature and mass of the warm molecular gas. We detect H2 in 77%
of the sample, and all ULIRGs with F(60um)>2Jy. The average warm molecular gas
mass is ~2x10^8solar-masses. High extinction, inferred from the 9.7um silicate
absorption depth, is not observed along the line of site to the molecular gas.
The derived H2 mass does not depend on F(25um)/F(60um), which has been used to
infer either starburst or AGN dominance. Similarly, the molecular mass does not
scale with the 25 or 60um luminosities. In general, the H2 emission is
consistent with an origin in photo-dissociation regions associated with star
formation. We detect the S(0) 28.22um emission line in a few ULIRGs. Including
this line in the model fits tends to lower the temperature by ~50-100K,
resulting in a significant increase in the gas mass. The presence of a cooler
component cannot be ruled out in the remainder of our sample, for which we do
not detect the S(0) line. The measured S(7) 5.51um line fluxes in six ULIRGs
implies ~3x10^6 solar-masses of hot (~1400K) H2. The warm gas mass is typically
less than 1% of the cold gas mass derived from CO observations.Comment: Accepted ApJ 01 September 2006, v648n1 issue. 14 pages 12 figures
IRAS 06361-6217 the f25/f60 ratio is 0.10 not 1.0
Spectroscopic Redshifts to z > 2 for Optically Obscured Sources Discovered with the Spitzer Space Telescope
We have surveyed a field covering 9.0 degrees^2 within the NOAO Deep
Wide-Field Survey region in Bootes with the Multiband Imaging Photometer on the
Spitzer Space Telescope (SST) to a limiting 24 um flux density of 0.3 mJy.
Thirty one sources from this survey with F(24um) > 0.75 mJy which are optically
very faint (R > 24.5 mag) have been observed with the low-resolution modules of
the Infrared Spectrograph on SST. Redshifts derived primarily from strong
silicate absorption features are reported here for 17 of these sources; 10 of
these are optically invisible (R > 26 mag), with no counterpart in B_W, R, or
I. The observed redshifts for 16 sources are 1.7 < z < 2.8. These represent a
newly discovered population of highly obscured sources at high redshift with
extreme infrared to optical ratios. Using IRS spectra of local galaxies as
templates, we find that a majority of the sources have mid-infrared spectral
shapes most similar to ultraluminous infrared galaxies powered primarily by
AGN. Assuming the same templates also apply at longer wavelengths, bolometric
luminosities exceed 10^13 L(solar).Comment: Accepted for publication on 7 Feb 2005 in ApJL. 7 pages 2 figure
The role of body rotation in bacterial flagellar bundling
In bacterial chemotaxis, E. coli cells drift up chemical gradients by a
series of runs and tumbles. Runs are periods of directed swimming, and tumbles
are abrupt changes in swimming direction. Near the beginning of each run, the
rotating helical flagellar filaments which propel the cell form a bundle. Using
resistive-force theory, we show that the counter-rotation of the cell body
necessary for torque balance is sufficient to wrap the filaments into a bundle,
even in the absence of the swirling flows produced by each individual filament
Spitzer IRS Spectra of Optically Faint Infrared Sources with Weak Spectral Features
Spectra have been obtained with the low-resolution modules of the Infrared
Spectrograph (IRS) on the Spitzer Space Telescope (Spitzer) for 58 sources
having f(24 micron) > 0.75 mJy. Sources were chosen from a survey of
8.2 deg within the NOAO Deep Wide-Field Survey region in Bootes (NDWFS)
using the Multiband Imaging Photometer (MIPS) on the Spitzer Space Telescope.
Most sources are optically very faint (I > 24mag). Redshifts have previously
been determined for 34 sources, based primarily on the presence of a deep 9.7
micron silicate absorption feature, with a median z of 2.2. Spectra are
presented for the remaining 24 sources for which we were previously unable to
determine a confident redshift because the IRS spectra show no strong features.
Optical photometry from the NDWFS and infrared photometry with MIPS and the
Infrared Array Camera on the Spitzer Space Telescope (IRAC) are given, with K
photometry from the Keck I telescope for some objects. The sources without
strong spectral features have overall spectral energy distributions (SEDs) and
distributions among optical and infrared fluxes which are similar to those for
the sources with strong absorption features. Nine of the 24 sources are found
to have feasible redshift determinations based on fits of a weak silicate
absorption feature. Results confirm that the "1 mJy" population of 24 micron
Spitzer sources which are optically faint is dominated by dusty sources with
spectroscopic indicators of an obscured AGN rather than a starburst. There
remain 14 of the 58 sources observed in Bootes for which no redshift could be
estimated, and 5 of these sources are invisible at all optical wavelengths.Comment: Accepted by Ap
Accelerated convergence to steady state by gradual far-field damping
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76495/1/AIAA-11054-168.pd
The Eastern Arm of M83 Revisited: High-Resolution Mapping of 12CO 1-0 Emission
We have used the Owens Valley Millimeter Array to map 12CO (J=1-0) along a
3.5 kpc segment of M83's eastern spiral arm at resolutions of 6.5"x3.5", 10",
and 16". The CO emission in most of this segment lies along the sharp dust lane
demarking the inner edge of the spiral arm, but beyond a certain point along
the arm the emission shifts downstream from the dust lane to become better
aligned with the young stars seen in blue and H-beta images. This morphology
resembles that of the western arm of M100. Three possibilities, none of which
is wholly satisfactory, are considered to explain the deviation of the CO arm
from the dust lane: heating of the CO by UV radiation from young stars, heating
by low-energy cosmic rays, and a molecular medium consisting of two (diffuse
and dense) components which react differently to the density wave. Regardless,
the question of what CO emission traces along this spiral arm is a complicated
one. Strong tangential streaming is observed where the arm crosses the
kinematic major axis of the galaxy, implying that the shear becomes locally
prograde in the arms. Inferred from the streaming is a very high gas surface
density of about 230 solar masses/pc**2 and an arm-interarm contrast greater
than 2.3 in the part of the arm near the major axis. Using two different
criteria, we find that the gas at this location is well above the threshold for
gravitational instability -- much more clearly so than in either M51 or M100.Comment: Accepted for publication in ApJ. 25 pages, 5 figures. Manuscript in
LaTeX, figures in pdf. Fig 3 in colo
Spitzer-IRS Spectroscopy of the Prototypical Starburst Galaxy NGC7714
We present observations of the starburst galaxy NGC 7714 with the Infrared
Spectrograph IRS on board the Spitzer Space Telescope. The spectra yield a
wealth of ionic and molecular features that allow a detailed characterization
of its properties. NGC 7714 has an HII region-like spectrum with strong PAH
emission features. We find no evidence for an obscured active galactic nucleus,
and with [NeIII]/[NeII]~0.73, NGC7714 lies near the upper end of
normal-metallicity starburst galaxies. With very little slicate absorption and
a temperature of the hottest dust component of 340K, NGC 7714 is the perfect
template for a young, unobscured starburstComment: To appear in the special ApJSS issue on early results from Spitze
Conformal Mapping on Rough Boundaries II: Applications to bi-harmonic problems
We use a conformal mapping method introduced in a companion paper to study
the properties of bi-harmonic fields in the vicinity of rough boundaries. We
focus our analysis on two different situations where such bi-harmonic problems
are encountered: a Stokes flow near a rough wall and the stress distribution on
the rough interface of a material in uni-axial tension. We perform a complete
numerical solution of these two-dimensional problems for any univalued rough
surfaces. We present results for sinusoidal and self-affine surface whose slope
can locally reach 2.5. Beyond the numerical solution we present perturbative
solutions of these problems. We show in particular that at first order in
roughness amplitude, the surface stress of a material in uni-axial tension can
be directly obtained from the Hilbert transform of the local slope. In case of
self-affine surfaces, we show that the stress distribution presents, for large
stresses, a power law tail whose exponent continuously depends on the roughness
amplitude
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