163 research outputs found
The 21cm angular-power spectrum from the dark ages
At redshifts z >~ 30 neutral hydrogen gas absorbs CMB radiation at the 21cm
spin-flip frequency. In principle this is observable and a high-precision probe
of cosmology. We calculate the linear-theory angular power spectrum of this
signal and cross-correlation between redshifts on scales much larger than the
line width. In addition to the well known redshift-distortion and density
perturbation sources a full linear analysis gives additional contributions to
the power spectrum. On small scales there is a percent-level linear effect due
to perturbations in the 21cm optical depth, and perturbed recombination
modifies the gas temperature perturbation evolution (and hence spin temperature
and 21cm power spectrum). On large scales there are several post-Newtonian and
velocity effects; although negligible on small scales, these additional terms
can be significant at l <~ 100 and can be non-zero even when there is no
background signal. We also discuss the linear effect of reionization
re-scattering, which damps the entire spectrum and gives a very small
polarization signal on large scales. On small scales we also model the
significant non-linear effects of evolution and gravitational lensing. We
include full results for numerical calculation and also various approximate
analytic results for the power spectrum and evolution of small scale
perturbations.Comment: 29 pages; significant extensions including: self-absorption terms
(i.e. change to background radiation due to 21cm absorption); ionization
fraction perturbations; estimates of non-linear effects; approximate analytic
results; results for sharp redshift window functions. Code available at
http://camb.info/sources
A Cosmic Relation between Extinction and Star Formation
We study the relation between the star formation intensity of galaxies and
the extinction by dust of their emitted light. We employ a detailed statistical
analysis of Hubble Deep Field North data to show a clear positive correlation
between the extinction and star formation intensity at all epochs from redshift
0.4 to 6.5. The extinction evidently increases with time for a given star
formation intensity, consistent with the expected increase in the metallicity
with time. Our observational results are well fitted at all epochs by a double
power-law model with a fixed shape that simply shifts with redshift. The
correlation between the extinction and the star formation intensity can be
interpreted by combining two other trends: the correlation between the star
formation rate and the gas content of galaxies, and the evolution of the
dust-to-gas ratio in galaxies. If we assume that Kennicutt's observed relation
for the former is valid at each redshift, then our findings imply an
interesting variation in the dust-to-gas ratio in galaxies within each epoch
and with time, and suggest new ways to investigate the cosmic evolution of this
quantity.Comment: 5 pages, 5 figures, MNRAS Letters, revised versio
Fast calculation of a family of elliptical mass gravitational lens models
Because of their simplicity, axisymmetric mass distributions are often used
to model gravitational lenses. Since galaxies are usually observed to have
elliptical light distributions, mass distributions with elliptical density
contours offer more general and realistic lens models. They are difficult to
use, however, since previous studies have shown that the deflection angle (and
magnification) in this case can only be obtained by rather expensive numerical
integrations. We present a family of lens models for which the deflection can
be calculated to high relative accuracy (one part in 100000) with a greatly
reduced numerical effort, for small and large ellipticity alike. This makes it
easier to use these distributions for modelling individual lenses as well as
for applications requiring larger computing times, such as statistical lensing
studies. A program implementing this method can be obtained from the author (or
at http://www.sns.ias.edu/~barkana/ellip.html).Comment: 13 pages, 3 figures, submitted to ApJ, also available at
http://www.sns.ias.edu/~barkana/ellip.htm
High resolution observations and mass modelling of the CLASS gravitational lens B1152+199
We present a series of high resolution radio and optical observations of the
CLASS gravitational lens system B1152+199 obtained with the Multi-Element
Radio-Linked Interferometer Network (MERLIN), Very Long Baseline Array (VLBA)
and Hubble Space Telescope (HST). Based on the milliarcsecond-scale
substructure of the lensed radio components and precise optical astrometry for
the lensing galaxy, we construct models for the system and place constraints on
the galaxy mass profile. For a single galaxy model with surface mass density
Sigma(r) propto r^-beta, we find that 0.95 < beta < 1.21 at 2-sigma confidence.
Including a second deflector to represent a possible satellite galaxy of the
primary lens leads to slightly steeper mass profiles.Comment: 7 pages, post-referee revision for MNRA
A novel approach for extracting time-delays from lightcurves of lensed quasar images
We present a new method to estimate time delays from light curves of lensed
quasars. The method is based on chi^2 minimization between the data and a
numerical model light curve. A linear variation can be included in order to
correct for slow long-term microlensing effects in one of the lensed images. An
iterative version of the method can be applied in order to correct for higher
order microlensing effects. The method is tested on simulated light curves.
When higher order microlensing effects are present the time delay is best
constrained with the iterative method. Analysis of a published data set for the
lensed double Q0957+561 yields results in agreement with other published
estimates.Comment: 6 pages, accepted for publication in A&
Dark Energy, scalar-curvature couplings and a critical acceleration scale
We study the effects of coupling a cosmologically rolling scalar field to
higher order curvature terms. We show that when the strong coupling scale of
the theory is on the 10^{-3}-10^{-1}eV range, the model passes all experimental
bounds on the existence of fifth forces even if the field has a mass of the
order of the Hubble scale in vacuum and non-suppressed couplings to SM fields.
The reason is that the coupling to certain curvature invariant acts as an
effective mass that grows in regions of large curvature. This prevents the
field from rolling down its potential near sources and makes its effects on
fifth-force search experiments performed in the laboratory to be observable
only at the sub-mm scale. We obtain the static spherically symmetric solutions
of the theory and show that a long-range force appears but it is turned on only
below a fixed Newtonian acceleration scale of the order of the Hubble constant.
We comment on the possibility of using this feature of the model to alleviate
the CDM small scale crisis and on its possible relation to MOND.Comment: 12 pages, 2 figure
Hot Cores : Probes of High-Redshift Galaxies
The very high rates of second generation star formation detected and inferred
in high redshift objects should be accompanied by intense millimetre-wave
emission from hot core molecules. We calculate the molecular abundances likely
to arise in hot cores associated with massive star formation at high redshift,
using several independent models of metallicity in the early Universe. If the
number of hot cores exceeds that in the Milky Way Galaxy by a factor of at
least one thousand, then a wide range of molecules in high redshift hot cores
should have detectable emission. It should be possible to distinguish between
independent models for the production of metals and hence hot core molecules
should be useful probes of star formation at high redshift.Comment: Updated to correspond to version accepted by MNRA
21-cm cosmology
Imaging the Universe during the first hundreds of millions of years remains
one of the exciting challenges facing modern cosmology. Observations of the
redshifted 21 cm line of atomic hydrogen offer the potential of opening a new
window into this epoch. This would transform our understanding of the formation
of the first stars and galaxies and of the thermal history of the Universe. A
new generation of radio telescopes is being constructed for this purpose with
the first results starting to trickle in. In this review, we detail the physics
that governs the 21 cm signal and describe what might be learnt from upcoming
observations. We also generalize our discussion to intensity mapping of other
atomic and molecular lines.Comment: 64 pages, 20 figures, submitted to Reports on Progress in Physics,
comments welcom
Is a Classical Language Adequate in Assessing the Detectability of the Redshifted 21cm Signal from the Early Universe?
The classical radiometer equation is commonly used to calculate the
detectability of the 21cm emission by diffuse cosmic hydrogen at high
redshifts. However, the classical description is only valid in the regime where
the occupation number of the photons in phase space is much larger than unity
and they collectively behave as a classical electromagnetic field. At redshifts
z<20, the spin temperature of the intergalactic gas is dictated by the
radiation from galaxies and the brightness temperature of the emitting gas is
in the range of mK, independently from the existence of the cosmic microwave
background. In regions where the observed brightness temperature of the 21cm
signal is smaller than the observed photon energy, of 68/(1+z) mK, the
occupation number of the signal photons is smaller than unity. Neverethless,
the radiometer equation can still be used in this regime because the weak
signal is accompanied by a flood of foreground photons with a high occupation
number (involving the synchrotron Galactic emission and the cosmic microwave
background). As the signal photons are not individually distinguishable, the
combined signal+foreground population of photons has a high occupation number,
thus justifying the use of the radiometer equation.Comment: 4 pages, Accepted for publication in JCA
Lyman-alpha emission galaxies at a redshift of z = 5.7 in the FORS Deep Field
We present the results of a search for Lyman-alpha emission galaxies at z~
5.7 in the FORS Deep Field. The objective of this study is to improve the faint
end of the luminosity function of high-redshift Lyman-alpha emitting galaxies
and to derive properties of intrinsically faint Lyman-alpha emission galaxies
in the young universe. Using FORS2 at the ESO VLT and a set of special
interference filters, we identified candidates for high-redshift Lyman-alpha
galaxies. We then used FORS2 in spectroscopic mode to verify the
identifications and to study their spectral properties. The narrow-band
photometry resulted in the detection of 15 likely Lyman-alpha emission
galaxies. Spectra with an adequate exposure time could be obtained for eight
galaxies. In all these cases the presence of Lyman-alpha emission at z = 5.7
was confirmed spectroscopically. The line fluxes of the 15 candidates range
between 3 and 16 * 10^-21 Wm^-2, which corresponds to star-formation rates not
corrected for dust between 1 and 5 Msun/yr. The luminosity function derived for
our photometrically identified objects extends the published luminosity
functions of intrinsically brighter Lyman-alpha galaxies. With this technique
the study of high-redshift Lyman-alpha emission galaxies can be extended to low
intrinsic luminosities.Comment: 9 pages, 17 figures. Accepted by A&A. PDF version with higher
resolution figures here:
http://www.lsw.uni-heidelberg.de/users/jheidt/fdf/pubs/fdflae5_7_110406.pd
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