70 research outputs found
Infrared Properties of Radio-Selected Submillimeter Galaxies in the Spitzer First Look Survey Verification Field
We report on submillimeter and infrared observations of 28 radio-selected
galaxies in the Spitzer First Look Survey Verification field (FLSV). All of the
radio-selected galaxies that show evidence for emission at 850um with SCUBA
have Spitzer counterparts at 24um, while only half of the radio-selected
galaxies without 850um emission have detectable counterparts at 24um. The data
show a wide range of infrared colors (S70/S24 < 5--30, S8/S3.6 < 0.3--4),
indicative of a mixture of infrared-warm AGN and cooler starburst dominated
sources. The galaxies showing 850um emission have Spitzer flux densities and
flux density ratios consistent with the range of values expected for
high-redshift (z=1--4) ultraluminous infrared galaxies.Comment: Accepted for Spitzer ApJS Special Edition, 12 pages including 4
figures and 1 tabl
Characterization of Extragalactic 24micron Sources in the Spitzer First Look Survey
In this Letter, we present the initial characterization of extragalactic 24um
sources in the Spitzer First Look Survey (FLS) by examining their counterparts
at 8um and R-band. The color-color diagram of 24-to-8 vs. 24-to-0.7um is
populated with 18,734 sources brighter than the 3sigma flux limit of 110uJy,
over an area of 3.7sq.degrees. The 24-to-0.7um colors of these sources span
almost 4 orders of magnitudes, while the 24-to-8um colors distribute at least
over 2 orders of magnitudes. In addition to identifying ~30% of the total
sample with infrared quiescent, mostly low redshift galaxies, we also found
that: (1) 23% of the 24um sources (~1200/sq.degrees) have very red 24-to-8 and
24-to-0.7 colors and are probably infrared luminous starbursts with
L(IR)>3x10^(11)Lsun at z>1. In particular, 13% of the sample (660/sq.degrees)
are 24um detected only, with no detectable emission in either 8um or R-band.
These sources are the candidates for being ULIRGs at z>2. (2) 2% of the sample
(85/sq.degrees) have colors similar to dust reddened AGNs, like Mrk231 at
z~0.6-3. (3) We anticipate that some of these sources with extremely red colors
may be new types of sources, since they can not be modelled with any familiar
type of spectral energy distribution. We find that 17% of the 24um sources have
no detectable optical counterparts brighter than R limit of 25.5mag. Optical
spectroscopy of these optical extremely faint 24um sources would be very
difficult, and mid-infrared spectroscopy from the Spitzer would be critical for
understanding their physical nature (Abridged).Comment: Accepted for publication in ApJ (Spitzer Special Issue
Obscured and unobscured active galactic nuclei in the Spitzer Space Telescope First Look Survey
Selection of active galactic nuclei (AGN) in the infrared allows the
discovery of AGN whose optical emission is extinguished by dust. In this paper,
we use the Spitzer Space Telescope First Look Survey (FLS) to assess what
fraction of AGN with mid-infrared luminosities comparable to quasars are missed
in optical quasar surveys due to dust obscuration. We begin by using the Sloan
Digital Sky Survey (SDSS) database to identify 54 quasars within the 4 deg^2
extragalactic FLS. These quasars occupy a distinct region in mid-infrared color
space by virtue of their strong, red, continua. This has allowed us to define a
mid-infrared color criterion for selecting AGN candidates. About 2000 FLS
objects have colors consistent with them being AGN, but most are much fainter
in the mid-infrared than the SDSS quasars, which typically have 8 micron flux
densities, S(8.0), ~1 mJy. We have investigated the properties of the 43
objects with S(8.0) >= 1 mJy satisfying our AGN color selection. This sample
should contain both unobscured quasars, and AGN which are absent from the SDSS
survey due to extinction in the optical. After removing 16 known quasars, three
probable normal quasars, and eight spurious or confused objects from the
initial sample of 43, we are left with 16 objects which are likely to be
obscured quasars or luminous Seyfert-2 galaxies. This suggests the numbers of
obscured and unobscured AGN are similar in samples selected in the mid-infrared
at S(8.0)~1 mJy.Comment: To appear in the ApJS Spitzer Special Issu
Damage spreading transition in glasses: a probe for the ruggedness of the configurational landscape
We consider damage spreading transitions in the framework of mode-coupling
theory. This theory describes relaxation processes in glasses in the mean-field
approximation which are known to be characterized by the presence of an
exponentially large number of meta-stable states. For systems evolving under
identical but arbitrarily correlated noises we demonstrate that there exists a
critical temperature which separates two different dynamical regimes
depending on whether damage spreads or not in the asymptotic long-time limit.
This transition exists for generic noise correlations such that the zero damage
solution is stable at high-temperatures being minimal for maximal noise
correlations. Although this dynamical transition depends on the type of noise
correlations we show that the asymptotic damage has the good properties of an
dynamical order parameter such as: 1) Independence on the initial damage; 2)
Independence on the class of initial condition and 3) Stability of the
transition in the presence of asymmetric interactions which violate detailed
balance. For maximally correlated noises we suggest that damage spreading
occurs due to the presence of a divergent number of saddle points (as well as
meta-stable states) in the thermodynamic limit consequence of the ruggedness of
the free energy landscape which characterizes the glassy state. These results
are then compared to extensive numerical simulations of a mean-field glass
model (the Bernasconi model) with Monte Carlo heat-bath dynamics. The freedom
of choosing arbitrary noise correlations for Langevin dynamics makes damage
spreading a interesting tool to probe the ruggedness of the configurational
landscape.Comment: 25 pages, 13 postscript figures. Paper extended to include
cross-correlation
Circumstellar material in the Vega inner system revealed by CHARA/FLUOR
Only a handful of debris disks have been imaged up to now. Due to the need
for high dynamic range and high angular resolution, very little is known about
the inner planetary region, where small amounts of warm dust are expected to be
found. We investigate the close neighbourhood of Vega with the help of infrared
stellar interferometry and estimate the integrated K-band flux originating from
the central 8 AU of the debris disk. We performed precise visibility
measurements at both short (~30 m) and long (~150 m) baselines with the FLUOR
beam-combiner installed at the CHARA Array (Mt Wilson, California) in order to
separately resolve the emissions from the extended debris disk (short
baselines) and from the stellar photosphere (long baselines). After revising
Vega's K-band angular diameter (3.202+/-0.005 mas), we show that a significant
deficit in squared visibility (1.88+/-0.34%) is detected at short baselines
with respect to the best-fit uniform disk stellar model. This deficit can be
either attributed to the presence of a low-mass stellar companion around Vega,
or as the signature of the thermal and scattered emissions from the debris
disk. We show that the presence of a close companion is highly unlikely, as
well as other possible perturbations (stellar morphology, calibration), and
deduce that we have most probably detected the presence of dust in the close
neighbourhood of Vega. The resulting flux ratio between the stellar photosphere
and the debris disk amounts to 1.29+/-0.19% within the FLUOR field-of-view
(~7.8 AU). Finally, we complement our K-band study with archival photometric
and interferometric data in order to evaluate the main physical properties of
the inner dust disk. The inferred properties suggest that the Vega system could
be currently undergoing major dynamical perturbations.Comment: A&A, accepted -- Press release available at
http://www2.cnrs.fr/presse/communique/848.ht
ISOPHOT - Photometric Calibration of Point Sources
All observations by the aperture photometer (PHT-P) and the far-infrared (FIR) camera section (PHT-C) of ISOPHOT included reference measurements against stable internal fine calibration sources (FCS) to correct for temporal drifts in detector responsivities. The FCSs were absolutely calibrated in-orbit against stars, asteroids and planets, covering wavelengths from 3.2 to 240 micron. We present the calibration concept for point sources within a flux-range from 60 mJy up to 4500 Jy for staring and raster observations in standard configurations and discuss the requisite measurements and the uncertainties involved. In this process we correct for instrumental effects like nonlinearities, signal transients, time variable dark current, misalignments and diffraction effects. A set of formulae is developed that describes the calibration from signal-level to flux densities. The scatter of 10 to 20 % of the individual data points around the derived calibration relations is a measure of the consistency and typical accuracy of the calibration. The reproducibility over longer periods of time is better than 10 %. The calibration tables and algorithms have been implemented in the final versions of the software for offline processing and interactive analysis
Do dusty A stars exhibit accretion signatures in their photospheres?
We determined abundances of O, Ca, Fe, Ba and Y for a sample of dusty and
dust-free A stars, taken from the list of Cheng et al. (1992). Five of the
stars have an infrared-excess due to circumstellar dust. Ongoing accretion from
their circumstellar surroundings might have modified the abundances in the
photospheres of these stars, but our results clearly show, that there is no
difference in the photospheric composition of the dusty and dust-free stars.
Instead all of them show the typical diffusion pattern which diminishes towards
larger rotational velocities.Comment: 8 pages, 3 figures, accepted for publication in A&
- …