2,971 research outputs found
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
The APM Galaxy Survey III: An Analysis of Systematic Errors in the Angular Correlation Function and Cosmological Implications
We present measurements of the angular two-point galaxy correlation function,
, from the APM Galaxy Survey. The performance of various estimators
of is assessed using simulated galaxy catalogues and analytic arguments.
Several error analyses show that residual plate-to-plate errors do not bias our
estimates of by more than . Direct comparison between our
photometry and external CCD photometry of over 13,000 galaxies from the Las
Campanas Deep Redshift Survey shows that the rms error in the APM plate zero
points lies in the range 0.04-0.05 magnitudes, in agreement with our previous
estimates. We estimate the effects on of atmospheric extinction and
obscuration by dust in our Galaxy and conclude that these are negligible. We
use our best estimates of the systematic errors in the survey to calculate
corrected estimates of . Deep redshift surveys are used to determine the
selection function of the APM Galaxy Survey, and this is applied in Limber's
equation to compute how scales as a function of limiting magnitude. Our
estimates of are in excellent agreement with the scaling relation,
providing further evidence that systematic errors in the APM survey are small.
We explicitly remove large-scale structure by applying filters to the APM
galaxy maps and conclude that there is still strong evidence for more
clustering at large scales than predicted by the standard scale-invariant cold
dark matter (CDM) model. We compare the APM and the three dimensional power
spectrum derived by inverting , with the predictions of scale-invariant CDM
models. We show that the observations require in the range
0.2-0.3 and are incompatible with the value of the standard CDM
model.Comment: 102 pages, plain TeX plus 41 postscript figures. Submitted to MNRA
First Structure Formation: A Simulation of Small Scale Structure at High Redshift
We describe the results of a simulation of collisionless cold dark matter in
a LambdaCDM universe to examine the properties of objects collapsing at high
redshift (z=10). We analyze the halos that form at these early times in this
simulation and find that the results are similar to those of simulations of
large scale structure formation at low redshift. In particular, we consider
halo properties such as the mass function, density profile, halo shape, spin
parameter, and angular momentum alignment with the minor axis. By understanding
the properties of small scale structure formation at high redshift, we can
better understand the nature of the first structures in the universe, such as
Population III stars.Comment: 31 pages, 14 figures; accepted for publication in ApJ. Figure 1 can
also be viewed at http://cfa-www.harvard.edu/~hjang/research
Forecast B-modes detection at large scales in presence of noise and foregrounds
We investigate the detectability of the primordial CMB polarization B-mode
power spectrum on large scales in the presence of instrumental noise and
realistic foreground contamination. We have worked out a method to estimate the
errors on component separation and to propagate them up to the power spectrum
estimation. The performances of our method are illustrated by applying it to
the instrumental specifications of the Planck satellite and to the proposed
configuration for the next generation CMB polarization experiment COrE. We
demonstrate that a proper component separation step is required in order
achieve the detection of B-modes on large scales and that the final sensitivity
to B-modes of a given experiment is determined by a delicate balance between
noise level and residual foregrounds, which depend on the set of frequencies
exploited in the CMB reconstruction, on the signal-to-noise of each frequency
map, and on our ability to correctly model the spectral behavior of the
foreground components. We have produced a flexible software tool that allows
the comparison of performances on B-mode detection of different instrumental
specifications (choice of frequencies, noise level at each frequency, etc.) as
well as of different proposed approaches to component separation.Comment: 7 pages, 2 tables, 1 figure, accepted by MNRA
An Analytical Approach to Inhomogeneous Structure Formation
We develop an analytical formalism that is suitable for studying
inhomogeneous structure formation, by studying the joint statistics of dark
matter halos forming at two points. Extending the Bond et al. (1991) derivation
of the mass function of virialized halos, based on excursion sets, we derive an
approximate analytical expression for the ``bivariate'' mass function of halos
forming at two redshifts and separated by a fixed comoving Lagrangian distance.
Our approach also leads to a self-consistent expression for the nonlinear
biasing and correlation function of halos, generalizing a number of previous
results including those by Kaiser (1984) and Mo & White (1996). We compare our
approximate solutions to exact numerical results within the excursion-set
framework and find them to be consistent to within 2% over a wide range of
parameters. Our formalism can be used to study various feedback effects during
galaxy formation analytically, as well as to simply construct observable
quantities dependent on the spatial distribution of objects. A code that
implements our method is publicly available at
http://www.arcetri.astro.it/~evan/GeminiComment: 41 Pages, 11 figures, published in ApJ, 571, 585. Reference added,
Figure 2 axis relabele
Formation of early-type galaxies from cosmological initial conditions
We describe high resolution Smoothed Particle Hydrodynamics (SPH) simulations
of three approximately field galaxies starting from \LCDM initial
conditions. The simulations are made intentionally simple, and include
photoionization, cooling of the intergalactic medium, and star formation but
not feedback from AGN or supernovae. All of the galaxies undergo an initial
burst of star formation at , accompanied by the formation of a
bubble of heated gas. Two out of three galaxies show early-type properties at
present whereas only one of them experienced a major merger. Heating from
shocks and -PdV work dominates over cooling so that for most of the gas the
temperature is an increasing function of time. By a significant
fraction of the final stellar mass is in place and the spectral energy
distribution resembles those of observed massive red galaxies. The galaxies
have grown from on average by 25% in mass and in size by gas poor
(dry) stellar mergers. By the present day, the simulated galaxies are old
(), kinematically hot stellar systems surrounded by hot
gaseous haloes. Stars dominate the mass of the galaxies up to
effective radii ( kpc). Kinematic and most photometric properties
are in good agreement with those of observed elliptical galaxies. The galaxy
with a major merger develops a counter-rotating core. Our simulations show that
realistic intermediate mass giant elliptical galaxies with plausible formation
histories can be formed from \LCDM initial conditions even without requiring
recent major mergers or feedback from supernovae or AGN.Comment: accepted for publication in Ap
The mass function
We present the mass functions for different mass estimators for a range of
cosmological models. We pay particular attention to how universal the mass
function is, and how it depends on the cosmology, halo identification and mass
estimator chosen. We investigate quantitatively how well we can relate observed
masses to theoretical mass functions.Comment: 14 pages, 12 figures, to appear in ApJ
Sub-millimetre observations of hyperluminous infrared galaxies
We present sub-mm photometry for 11 Hyperluminous Infrared Galaxies (HLIRGs)
and use radiative transfer models for starbursts and AGN to investigate the IR
emission. In all sources both a starburst and AGN are required to explain the
IR emission. The mean starburst fraction is 35%, with a range spanning 80%
starburst dominated to 80% AGN dominated. In all cases the starburst dominates
at rest-frame wavelengths >50 microns, with star formation rates >500 solar
masses per year. The trend of increasing AGN fraction with increasing IR
luminosity seen in IRAS galaxies peaks in HLIRGs, and is not higher than the
fraction seen in bright ULIRGs. The AGN and starburst luminosities correlate,
suggesting that a common physical factor, plausibly the dust masses, governs
their luminosities. Our results suggest that the HLIRG population is comprised
both of ULIRG-like galaxy mergers, and of young galaxies going through their
maximal star formation periods whilst harbouring an AGN. The coeval AGN and
starburst activity in our sources implies that starburst and AGN activity, and
the peak starburst and AGN luminosities, can be coeval in active galaxies
generally. When extrapolated to high-z our sources have comparable sub-mm
fluxes to sub-mm survey sources. At least some sub-mm survey sources are
therefore likely to be comprised of similar galaxy populations to those found
in the HLIRG population. It is also plausible from these results that high-z
sub-mm sources harbour heavily obscured AGN. The differences in X-ray and
sub-mm properties between HLIRGs at z~1 and sub-mm sources at z~3 implies
evolution between the two epochs. Either the mean AGN obscuration level is
greater at z~3 than at z~1, or the fraction of IR-luminous sources at z~3 that
contain AGN is smaller than that at z~1.Comment: 15 pages. Accepted for publication in MNRA
New and Old Tests of Cosmological Models and Evolution of Galaxies
We describe the classical cosmological tests, such as the Log-Log,
redshift-magnitude and angular diameter tests, and propose some new tests of
the evolution of galaxies and the universe. Most analyses of these tests treat
the problem in terms of a luminosity function and its evolution which can lead
to incorrect conclusions when dealing with high redshift sources. We develop a
proper treatment in three parts. In the first part we describe these tests
based on the isophotal values of the quantities such as flux, size or surface
brightness. We show the shortcomings of the simple point source approximation
based solely on the luminosity function and consideration of the flux limit. We
emphasize the multivariate nature of the problem and quantify the effects of
other selection biases due to the surface brightness and angular size
limitations. In these considerations the surface brightness profile plays a
critical role. In the second part we show that considerable simplification over
the complicated isophotal scheme is achieved if these test are carried out in
some sort of metric scheme, for example that suggested by Petrosian (1976).
This scheme, however, is limited to well resolved sources. Finally, we describe
the new tests, which use the data to a fuller extent than the isophotal or
metric based tests, and amount to simply counting the pixels or adding their
intensities as a function of the pixel surface brightness, instead of dealing
with surface brightness, sizes and fluxes of individual galaxies. We show that
the data analysis and its comparison with the theoretical models of the
distributions and evolution of galaxies has the simplicity of the metric test
and utilizes the data more fully than the isophotal test.Comment: 29 pages including 8 figures.
http://www-bigbang.stanford.edu/~vahe/papers/finals/newtest.ps. To appear in
ApJ, Oct. 199
The Apm Galaxy Survey IV: Redshifts of Rich Clusters of Galaxies
We present redshifts for a sample of 229 clusters selected from the APM
Galaxy Survey, 189 of which are new redshift determinations. Non-cluster galaxy
redshifts have been rejected from this sample using a likelihood ratio test
based on the projected and apparent magnitude distributions of the cluster
fields. We test this technique using cluster fields in which redshifts have
been measured for more than 10 galaxies. Our redshift sample is nearly complete
and has been used in previous papers to study the three dimensional
distribution of rich clusters of galaxies. 157 of the clusters in our sample
are listed in the Abell catalogue or supplement, and the remainder are new
cluster identifications.Comment: 15 pages UUencoded compressed postscript. Submitted to Monthly
Notices of the R.A.
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