6,588 research outputs found
A new model for the infrared emission of IRAS F10214+4724
We present a new model for the infrared emission of the high redshift
hyperluminous infrared galaxy IRAS F10214+4724 which takes into account recent
photometric data from Spitzer and Herschel that sample the peak of its spectral
energy distribution. We first demonstrate that the combination of the AGN
tapered disc and starburst models of Efstathiou and coworkers, while able to
give an excellent fit to the average spectrum of type 2 AGN measured by
Spitzer, fails to match the spectral energy distribution of IRAS F10214+4724.
This is mainly due to the fact that the nuSnu distribution of the galaxy falls
very steeply with increasing frequency (a characteristic of heavy absorption by
dust) but shows a silicate feature in emission. We propose a model that assumes
two components of emission: clouds that are associated with the narrow-line
region and a highly obscured starburst. The emission from the clouds must
suffer significantly stronger gravitational lensing compared to the emission
from the torus to explain the observed spectral energy distribution.Comment: 4 pages, 1 figure, to be published in 'The Spectral Energy
Distribution of Galaxies' J. Tuffs \& C.C.Popescu, ed
H0 Revisited
I reanalyse the Riess et al. (2011, hereafter R11) Cepheid data using the
revised geometric maser distance to NGC 4258 of Humphreys et al. (2013). I
explore different outlier rejection criteria designed to give a reduced
chi-squared of unity and compare the results with the R11 rejection algorithm,
which produces a reduced chi-squared that is substantially less than unity and,
in some cases, to underestimates of the errors on parameters. I show that there
are sub-luminous low metallicity Cepheids in the R11 sample that skew the
global fits of the period-luminosity relation. This has a small but
non-negligible impact on the global fits using NGC 4258 as a distance scale
anchor, but adds a poorly constrained source of systematic error when using the
Large Magellanic Cloud (LMC) as an anchor. I also show that the small Milky Way
(MW) Cepheid sample with accurate parallax measurements leads to a distance to
NGC 4258 that is in tension with the maser distance. I conclude that H0 based
on the NGC 4258 maser distance is H0 = 70.6 +/- 3.3 km/s/Mpc compatible within
1 sigma with the recent determination from Planck for the base six-parameter
LCDM cosmology. If the H-band period-luminosity relation is assumed to be
independent of metallicity and the three distance anchors are combined, I find
H0 = 72.5 +/- 2.5 km/s/Mpc, which differs by 1.9 sigma from the Planck value.
The differences between the Planck results and these estimates of H0 are not
large enough to provide compelling evidence for new physics at this stage.Comment: 14 page
Constraining the equation of state of the Universe from Distant Type Ia Supernovae and Cosmic Microwave Background Anisotropies
We analyse the constraints that can be placed on a cosmological constant or
quintessence-like component by combining observations of Type Ia supernovae
with measurements of anisotropies in the cosmic microwave background. We use
the recent supernovae sample of Perlmutter et al and observations of the CMB
anisotropies to constraint the equation of state (w_Q = p/rho) in
quintessence-like models via a likelihood analysis. The 2 sigma upper limits
are w_Q < -0.6 if the Universe is assumed to be spatially flat, and w_Q < -0.4
for universes of arbitrary spatial curvature. The upper limit derived for a
spatially flat Universe is close to the lower limit (w_Q approx -0.7) allowed
for simple potentials, implying that additional fine tuning may be required to
construct a viable quintessence model.Comment: 9 pages, 8 Postscript figures, uses mn.sty. submitted to MNRA
How Stochastic is the Relative Bias Between Galaxy Types?
Examining the nature of the relative clustering of different galaxy types can
help tell us how galaxies formed. To measure this relative clustering, I
perform a joint counts-in-cells analysis of galaxies of different spectral
types in the Las Campanas Redshift Survey (LCRS). I develop a
maximum-likelihood technique to fit for the relationship between the density
fields of early- and late-type galaxies. This technique can directly measure
nonlinearity and stochasticity in the biasing relation. At high significance, a
small amount of stochasticity is measured, corresponding to a correlation
coefficient of about 0.87 on scales corresponding to 15 Mpc/h spheres. A large
proportion of this signal appears to derive from errors in the selection
function, and a more realistic estimate finds a correlation coefficient of
about 0.95. These selection function errors probably account for the large
stochasticity measured by Tegmark & Bromley (1999), and may have affected
measurements of very large-scale structure in the LCRS. Analysis of the data
and of mock catalogs shows that the peculiar geometry, variable flux limits,
and central surface-brightness selection effects of the LCRS do not seem to
cause the effect.Comment: 38 pages, 14 figures. Submitted to Apj. Modified from a chapter of my
Ph.D. Thesis at Princeton University, available at
http://www-astro-theory.fnal.gov/Personal/blanton/thesis/index.htm
A Maximum Likelihood Analysis of the Low CMB Multipoles from WMAP
The amplitudes of the quadrupole and octopole measured from the Wilkinson
Microwave Anisotropy Probe (WMAP) appear to be lower than expected according to
the concordance Lambda CDM cosmology. However, the pseudo-Cl estimator used by
the WMAP team is non-optimal. In this paper, we discuss the effects of Galactic
cuts on pseudo-Cl and quadratic maximum likelihood estimators. An application
of a quadratic maximum likelihood estimator to Galaxy subtracted maps produced
by the WMAP team and Tegmark, de Oliveira-Costa and Hamilton (2003) shows that
the amplitudes of the low multipoles are stable to different Galactic cuts. In
particular, the quadrupole and octopole amplitudes are found to lie in the
ranges 176 - 250 (micro K)**2 794 - 1183 (micro K)**2 (and more likely to be at
the upper ends of these ranges) rather than the values of 123 (micro K)**2 and
611 (micro K)**2 found by the WMAP team. These results indicate that the
discrepancy with the concordance Lambda CDM model at low multipoles is not
particularly significant and is in the region of a few percent. This conclusion
is consistent with an analysis of the low amplitude of the angular correlation
function computed from quadratic maximum likelihood power spectrum estimates.Comment: MNRAS (2004) 348 885. Resubmission matches published versio
Reconstruction of cosmological initial conditions from galaxy redshift catalogues
We present and test a new method for the reconstruction of cosmological
initial conditions from a full-sky galaxy catalogue. This method, called
ZTRACE, is based on a self-consistent solution of the growing mode of
gravitational instabilities according to the Zel'dovich approximation and
higher order in Lagrangian perturbation theory. Given the evolved
redshift-space density field, smoothed on some scale, ZTRACE finds via an
iterative procedure, an approximation to the initial density field for any
given set of cosmological parameters; real-space densities and peculiar
velocities are also reconstructed. The method is tested by applying it to
N-body simulations of an Einstein-de Sitter and an open cold dark matter
universe. It is shown that errors in the estimate of the density contrast
dominate the noise of the reconstruction. As a consequence, the reconstruction
of real space density and peculiar velocity fields using non-linear algorithms
is little improved over those based on linear theory. The use of a
mass-preserving adaptive smoothing, equivalent to a smoothing in Lagrangian
space, allows an unbiased (although noisy) reconstruction of initial
conditions, as long as the (linearly extrapolated) density contrast does not
exceed unity. The probability distribution function of the initial conditions
is recovered to high precision, even for Gaussian smoothing scales of ~ 5
Mpc/h, except for the tail at delta >~ 1. This result is insensitive to the
assumptions of the background cosmology.Comment: 19 pages, MN style, 12 figures included, revised version. MNRAS, in
pres
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