967 research outputs found
Limits on Dust and Metallicity Evolution of Lya Forest Clouds from COBE
We consider possible observational consequences of dust and metals in \lya
forest clouds. We relate the dust content, , to the metal
evolution of the absorbers and assume that dust is heated by the ultraviolet
background radiation and by the CMB. We find that the dust temperature deviates
from by at most 10% at redshift . The \lya cloud dust opacity to
redshift sources around the observed wavelength m is , and could affect observations of the distant universe in
that band. The expected CMB spectral distortions due to high- dust in \lya
clouds is smaller than the current COBE upper limit, depending
on the metallicity evolution of the clouds. If \lya clouds are clustered, the
corresponding CMB anisotropy due to dust is on angular scales
\theta \simlt 10'' at frequencies probed by various future/ongoing FIR
missions, which makes these fluctuations potentially detectable in the near
future. Emission from CII fine-structure transitions could considerably
contribute to submm range of the FIR background radiation. Depending on the
ionization of carbon and on the density of metal enriched regions, this
contribution can be comparable with the observed residual flux at
mm, after CMB subtraction. We argue that constraints on
metal evolution versus redshift can be obtained from the observed flux in that
range.Comment: 8 pages, 3 figures, MNRAS, in pres
Dissecting the Cosmic Infrared Background with 3D Instruments
The cosmic infrared background (CIB) consists of emission from distant,
dusty, star-forming galaxies. Energetically, the CIB is very important as it
contains as much energy as the extragalactic optical background. The nature and
evolutionary status of the objects making up the background are, however,
unclear. The CIB peaks at ~150 microns, and as such is most effectively studied
from space. The limited apertures of space-borne telescopes set the angular
resolution that can be attained, and so even Herschel, with its 3.5m diameter,
will be confusion-limited at this wavelengths at ~5mJy. The bulk of the
galaxies contributing to the CIB are fainter than this, so it is difficult to
study them without interferometry. Here we present the results of a preliminary
study of an alternative way of probing fainter than the continuum confusion
limit using far-IR imaging spectroscopy. An instrument capable of such
observations is being planned for SPICA - a proposed Japanese mission with an
aperture equivalent to that of Herschel and more than 2 orders of magnitude
more sensitive. We investigate the potential of imaging spectrometers to break
the continuum confusion limit. We have simulated the capabilities of a
spectrometer with modest field of view (2'x2'), moderate spectral resolution
(R~1-2000) and high sensitivity. We find that such an instrument is capable of
not only detecting line emission from sources with continuum fluxes
substantially below the confusion limit, but also of determining their
redshifts and, where multiple lines are detected, some emission line
diagnostics. We conclude that 3-D imaging spectrometers on cooled far-IR space
telescopes will be powerful new tools for extragalactic far-IR astronomy.Comment: Accepted for publication in Astronomy & Astrophysic
A Semi-Empirical Model of the Infra-Red Universe
We present a simple model of the infra-red universe, based as much as
possible on local observations. We model the luminosity and number evolution of
disk and starburst galaxies, including the effects of dust, gas and spectral
evolution. Although simple, our approach is able to reproduce observations of
galaxy number counts and the infra-red and sub-millimeter extra-galactic
backgrounds. It provides a useful probe of galaxy formation and evolution out
to high redshift. The model demonstrates the significant role of the starburst
population and predicts high star formation rates at redshifts 3 to 4,
consistent with recent extinction-corrected observations of Lyman break
galaxies. Starbursting galaxies are predicted to dominate the current SCUBA
surveys. Their star formation is driven predominantly by strong tidal
interactions and mergers of galaxies. This leads to the creation of spheroidal
stellar systems, which may act as the seeds for disk formation as gas infalls.
We predict the present-day baryonic mass in bulges and halos is comparable to
that in disks. From observations of the extra-galactic background, the model
predicts that the vast majority of star formation in the Universe occurs at
z<5.Comment: 23 pages including 9 figures. To appear in ApJ. Model results
available electronically at http://astro.berkeley.edu/~jt/irmodel.htm
350 Micron Dust Emission from High Redshift Objects
We report observations of a sample of high redshift sources (1.8<z<4.7),
mainly radio-quiet quasars, at 350 microns using the SHARC bolometer camera at
the Caltech Submillimeter Observatory. Nine sources were detected (>4-sigma)
and upper limits were obtained for 11 with 350 micron flux density limits
(3-sigma) in the range 30-125mJy. Combining published results at other
far-infrared and millimeter wavelengths with the present data, we are able to
estimate the temperature of the dust, finding relatively low values, averaging
50K. From the spectral energy distribution, we derive dust masses of a few 10^8
M_sun and luminosities of 4-33x10^{12} L_sun (uncorrected for any
magnification) implying substantial star formation activity. Thus both the
temperature and dust masses are not very different from those of local
ultraluminous infrared galaxies. For this redshift range, the 350 micron
observations trace the 60-100 micron rest frame emission and are thus directly
comparable with IRAS studies of low redshift galaxies.Comment: 5 pages, 2 PS figures. Accepted for publication in Astrophysical
Journal Letter
A Spitzer Unbiased Ultradeep Spectroscopic Survey
We carried out an unbiased, spectroscopic survey using the low-resolution
module of the infrared spectrograph (IRS) on board Spitzer targeting two 2.6
square arcminute regions in the GOODS-North field. IRS was used in spectral
mapping mode with 5 hours of effective integration time per pixel. One region
was covered between 14 and 21 microns and the other between 20 and 35 microns.
We extracted spectra for 45 sources. About 84% of the sources have reported
detections by GOODS at 24 microns, with a median F_nu(24um) ~ 100 uJy. All but
one source are detected in all four IRAC bands, 3.6 to 8 microns. We use a new
cross-correlation technique to measure redshifts and estimate IRS spectral
types; this was successful for ~60% of the spectra. Fourteen sources show
significant PAH emission, four mostly SiO absorption, eight present mixed
spectral signatures (low PAH and/or SiO) and two show a single line in
emission. For the remaining 17, no spectral features were detected. Redshifts
range from z ~ 0.2 to z ~ 2.2, with a median of 1. IR Luminosities are roughly
estimated from 24 microns flux densities, and have median values of 2.2 x
10^{11} L_{\odot} and 7.5 x 10^{11} L_{\odot} at z ~ 1 and z ~ 2 respectively.
This sample has fewer AGN than previous faint samples observed with IRS, which
we attribute to the fainter luminosities reached here.Comment: Published in Ap
Linking stellar mass and star formation in Spitzer/MIPS 24 micron galaxies
We present deep Ks<21.5 (Vega) identifications, redshifts and stellar masses
for most of the sources composing the bulk of the 24 micron background in the
GOODS/CDFS. Our identified sample consists of 747 Spitzer/MIPS 24 micron
objects, and includes ~94% of all the 24 micron sources in the GOODS-South
field which have fluxes Snu(24)>83 microJy (the 80% completeness limit of the
Spitzer/GTO 24 micron catalog). 36% of our galaxies have spectroscopic
redshifts (mostly at z<1.5) and the remaining ones have photometric redshifts
of very good quality, with a median of |dz|=|zspec-zphot|/(1+zspec)=0.02. We
find that MIPS 24 micron galaxies span the redshift range z~0-4, and that a
substantial fraction (28%) lie at high redshifts z>1.5. We determine the
existence of a bump in the redshift distribution at z~1.9, indicating the
presence of a significant population of galaxies with PAH emission at these
redshifts. Massive (M>10^11 Msun) star-forming galaxies at redshifts 2<z<3 are
characterized by very high star-formation rates (SFR>500 Msun/yr), and some of
them are able to construct a mass of 10^10-10^11 Msun in a single burst
lifetime (~0.01-0.1 Gyr). At lower redshifts z<2, massive star-forming galaxies
are also present, but appear to be building their stars on long timescales,
either quiescently or in multiple modest burst-like episodes. At redshifts
z~1-2, the ability of the burst-like mode to produce entire galaxies in a
single event is limited to some lower (M<7x10^10 Msun) mass systems, and it is
basically negligible at z<1. Our results support a scenario where
star-formation activity is differential with assembled stellar mass and
redshift, and where the relative importance of the burst-like mode proceeds in
a down-sizing way from high to low redshifts. (abridged)Comment: Accepted for publication in the ApJ. 19 pages, 10 figures. Uses
emulateap
Implications of Cosmological Gamma-Ray Absorption - I.Evolution of the Metagalactic Radiation Field
Gamma-ray absorption due to gamma-gamma-pair creation on cosmological scales
depends on the line-of-sight integral of the evolving density of low-energy
photons in the Universe, i.e. on the history of the diffuse, isotropic
radiation field. Here we present and discuss a semi-empirical model for this
metagalactic radiation field based on stellar light produced and reprocessed in
evolving galaxies. With a minimum of parameters and assumptions, the
present-day background intensity is obtained from the far-IR to the ultraviolet
band. Predicted model intensities are independent of cosmological parameters,
since we require that the comoving emissivity, as a function of redshift,
agrees with observed values obtained from deep galaxy surveys. The far-infrared
background at present day prediced from optical galaxy surveys falls short in
explaining the observed one, and we show that this deficit can be removed by
taking into account (ultra)luminous infrared galaxies (ULIGs/LIGs) with a
seperate star formation rate. The accuracy and reliability of the model, out to
redshifts of 5, allow a realistic estimate of the attenuation length of
GeV-to-TeV gamma-rays and its uncertainty, which is the focus of a subsequent
paper.Comment: 12 pages, 6 figures; accepted for publication in Astronomy and
Astrophysic
The Canada-UK Deep Submillimetre Survey: The Survey of the 14-hour field
We have used SCUBA to survey an area of 50 square arcmin, detecting 19
sources down to a 3sigma sensitivity limit of 3.5 mJy at 850 microns. We have
used Monte-Carlo simulations to assess the effect of source confusion and noise
on the SCUBA fluxes and positions, finding that the fluxes of sources in the
SCUBA surveys are significantly biased upwards and that the fraction of the 850
micron background that has been resolved by SCUBA has been overestimated. The
radio/submillmetre flux ratios imply that the dust in these galaxies is being
heated by young stars rather than AGN. We have used simple evolution models
based on our parallel SCUBA survey of the local universe to address the major
questions about the SCUBA sources: (1) what fraction of the star formation at
high redshift is hidden by dust? (2) Does the submillimetre luminosity density
reach a maximum at some redshift? (3) If the SCUBA sources are
proto-ellipticals, when exactly did ellipticals form? However, we show that the
observations are not yet good enough for definitive answers to these questions.
There are, for example, acceptable models in which 10 times as much
high-redshift star formation is hidden by dust as is seen at optical
wavelengths, but also acceptable ones in which the amount of hidden star
formation is less than that seen optically. There are acceptable models in
which very little star formation occurred before a redshift of three (as might
be expected in models of hierarchical galaxy formation), but also ones in which
30% of the stars have formed by this redshift. The key to answering these
questions are measurements of the dust temperatures and redshifts of the SCUBA
sources.Comment: 41 pages (latex), 17 postscript figures, to appear in the November
issue of the Astronomical Journa
Automatically Discovering Hidden Transformation Chaining Constraints
Model transformations operate on models conforming to precisely defined
metamodels. Consequently, it often seems relatively easy to chain them: the
output of a transformation may be given as input to a second one if metamodels
match. However, this simple rule has some obvious limitations. For instance, a
transformation may only use a subset of a metamodel. Therefore, chaining
transformations appropriately requires more information. We present here an
approach that automatically discovers more detailed information about actual
chaining constraints by statically analyzing transformations. The objective is
to provide developers who decide to chain transformations with more data on
which to base their choices. This approach has been successfully applied to the
case of a library of endogenous transformations. They all have the same source
and target metamodel but have some hidden chaining constraints. In such a case,
the simple metamodel matching rule given above does not provide any useful
information
Effects of a burst of formation of first-generation stars on the evolution of galaxies
First-generation (Population III) stars in the universe play an important
role inearly enrichment of heavy elements in galaxies and intergalactic medium
and thus affect the history of galaxies. The physical and chemical properties
of primordial gas clouds are significantly different from those of present-day
gas clouds observed in the nearby universe because the primordial gas clouds do
not contain any heavy elements which are important coolants in the gas.
Previous theoretical considerations have suggested that typical masses of the
first-generation stars are between several and
although it has been argued that the formation of very massive stars (e.g., ) is also likely. If stars with several are most popular
ones at the epoch of galaxy formation, most stars will evolve to hot (e.g.,
K), luminous () stars with gaseous and dusty
envelope prior to going to die as white dwarf stars. Although the duration of
this phase is short (e.g., yr), such evolved stars could contribute
both to the ionization of gas in galaxies and to the production of a lot of
dust grains if the formation of intermediate-mass stars is highly enhanced. We
compare gaseous emission-line properties of such nebulae with some interesting
high-redshift galaxies such asIRAS F10214+4724 and powerful radio galaxies.Comment: 25 pages, 7 figures, ApJ, in pres
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