2,360 research outputs found
The local space density of dwarf galaxies
We estimate the luminosity function of field galaxies over a range of ten
magnitudes (-22 < M_{B_J} < -12 for H_0 = 100 km/s/Mpc) by counting the number
of faint APM galaxies around Stromlo-APM redshift survey galaxies at known
distance. The faint end of the luminosity function rises steeply at M_{B_J}
\approx -15, implying that the space density of dwarf galaxies is at least two
times larger than predicted by a Schechter function with flat faint-end slope.
Such a high abundance of dwarf galaxies at low redshift can help explain the
observed number counts and redshift distributions of faint galaxies without
invoking exotic models for galaxy evolution.Comment: 20 pages, 5 included postscript figures, uses AAS LaTex macros.
Accepted for publication in the Astrophysical Journal. Two figures and
associated discussion added; results and conclusions unchange
The Galaxy Angular Correlation Functions and Power Spectrum from the Two Micron All Sky Survey
We calculate the angular correlation function of galaxies in the Two Micron
All Sky Survey. We minimize the possible contamination by stars, dust, seeing
and sky brightness by studying their cross correlation with galaxy density, and
limiting the galaxy sample accordingly. We measure the correlation function at
scales between 1-18 arcdegs using a half million galaxies. We find a best fit
power law to the correlation function has a slope of 0.76 and an amplitude of
0.11. However, there are statistically significant oscillations around this
power law. The largest oscillation occurs at about 0.8 degrees, corresponding
to 2.8 h^{-1} Mpc at the median redshift of our survey, as expected in halo
occupation distribution descriptions of galaxy clustering.
We invert the angular correlation function using Singular Value Decomposition
to measure the three-dimensional power spectrum and find that it too is in good
agreement with previous measurements. A dip seen in the power spectrum at small
wavenumber k is statistically consistent with CDM-type power spectra. A fit of
CDM-type power spectra to k < 0.2 h Mpc^{-1} give constraints of
\Gamma_{eff}=0.116 and \sigma_8=0.96. This suggest a K_s-band linear bias of
1.1+/-0.2. This \Gamma_{eff} is different from the WMAP CMB derived value. On
small scales the power-law shape of our power spectrum is shallower than that
derived for the SDSS. These facts together imply a biasing of these different
galaxies that might be nonlinear, that might be either waveband or luminosity
dependent, and that might have a nonlocal origin.Comment: 14 pages, 20 figures, to be published in ApJ January 20th, revision
included two new figures, version with high resolution figures can be found
here http::ww
Faint blue objects on the Hubble Deep Field North & South as possible nearby old halo white dwarfs
Using data derived from the deepest and finest angular resolution images of
the universe yet acquired by astronomers at optical wavelengths using the
Hubble Space Telescope (HST) in two postage-stamp sections of the sky (Williams
et al. 1996a,b), plus simple geometrical and scaling arguments, we demonstrate
that the faint blue population of point-source objects detected on those two
fields (M\'endez et al. 1996) could actually be ancient halo white dwarfs at
distances closer than about 2 kpc from the Sun. This finding has profound
implications, as the mass density of the detected objects would account for
about half of the missing dark matter in the Milky-Way (Bahcall and Soneira
1980), thus solving one of the most controversial issues of modern astrophysics
(Trimble 1987, Ashman 1992). The existence of these faint blue objects points
to a very large mass locked into ancient halo white dwarfs. Our estimate
indicates that they could account for as much as half of the dark matter in our
Galaxy, confirming the suggestions of the MACHO microlensing experiment (Alcock
et al. 1997). Because of the importance of this discovery, deep follow-up
observations with HST within the next two years would be needed to determine
more accurately the kinematics (tangential motions) for these faint blue old
white dwarfs.Comment: Accepted for publication on The Astrophysical Journal, Part 1. 8
pages (AAS Latex macros V4.0), 1 B&W postscript figure, 2 color postscript
figure
Redshifts in the Southern Abell Redshift Survey Clusters. I. The Data
The Southern Abell Redshift Survey contains 39 clusters of galaxies with
redshifts in the range 0.0 < z < 0.31 and a median redshift depth of z =
0.0845. SARS covers the region 0 21h (while
avoiding the LMC and SMC) with b > 40. Cluster locations were chosen from the
Abell and Abell-Corwin-Olowin catalogs while galaxy positions were selected
from the Automatic Plate Measuring Facility galaxy catalog with
extinction-corrected magnitudes in the range 15 <= b_j < 19. SARS utilized the
Las Campanas 2.5 m duPont telescope, observing either 65 or 128 objects
concurrently over a 1.5 sq deg field. New redshifts for 3440 galaxies are
reported in the fields of these 39 clusters of galaxies.Comment: 20 pages, 5 figures, accepted for publication in the Astronomical
Journal, Table 2 can be downloaded in its entirety from
http://trotsky.arc.nasa.gov/~mway/SARS1/sars1-table2.cs
Large-scale structure and matter in the universe
This paper summarizes the physical mechanisms that encode the type and
quantity of cosmological matter in the properties of large-scale structure, and
reviews the application of such tests to current datasets. The key lengths of
the horizon size at matter-radiation equality and at last scattering determine
the total matter density and its ratio to the relativistic density; acoustic
oscillations can diagnose whether the matter is collisionless, and small-scale
structure or its absence can limit the mass of any dark-matter relic particle.
The most stringent constraints come from combining data on present-day galaxy
clustering with data on CMB anisotropies. Such an analysis breaks the
degeneracies inherent in either dataset alone, and proves that the universe is
very close to flat. The matter content is accurately consistent with pure Cold
Dark Matter, with about 25% of the critical density, and fluctuations that are
scalar-only, adiabatic and scale-invariant. It is demonstrated that these
conclusions cannot be evaded by adjusting either the equation of state of the
vacuum, or the total relativistic density.Comment: 17 Pages. Review paper from the January 2003 Royal Society Discussion
Meeting, "The search for dark matter and dark energy in the universe
Radio Sources in the 2dF Galaxy Redshift Survey. I. Radio Source Populations
We present the first results from a study of the radio continuum properties
of galaxies in the 2dF Galaxy Redshift Survey, based on thirty 2dF fields
covering a total area of about 100 square degrees. About 1.5% of galaxies with
b(J) < 19.4 mag are detected as radio continuum sources in the NRAO VLA Sky
Survey (NVSS). Of these, roughly 40% are star-forming galaxies and 60% are
active galaxies (mostly low-power radio galaxies and a few Seyferts). The
combination of 2dFGRS and NVSS will eventually yield a homogeneous set of
around 4000 radio-galaxy spectra, which will be a powerful tool for studying
the distriibution and evolution of both AGN and starburst galaxies out to
redshift z=0.3.Comment: 14 pages, 7 figures, accepted for publication in PAS
The Apparent and Intrinsic Shape of the APM Galaxy Clusters
We estimate the distribution of intrinsic shapes of APM galaxy clusters from
the distribution of their apparent shapes. We measure the projected cluster
ellipticities using two alternative methods. The first method is based on
moments of the discrete galaxy distribution while the second is based on
moments of the smoothed galaxy distribution. We study the performance of both
methods using Monte Carlo cluster simulations covering the range of APM cluster
distances and including a random distribution of background galaxies. We find
that the first method suffers from severe systematic biases, whereas the second
is more reliable. After excluding clusters dominated by substructure and
quantifying the systematic biases in our estimated shape parameters, we recover
a corrected distribution of projected ellipticities. We use the non-parametric
kernel method to estimate the smooth apparent ellipticity distribution, and
numerically invert a set of integral equations to recover the corresponding
distribution of intrinsic ellipticities under the assumption that the clusters
are either oblate or prolate spheroids. The prolate spheroidal model fits the
APM cluster data best.Comment: 8 pages, including 7 figures, accepted for publication in MNRA
Probing dark energy with steerable wavelets through correlation of WMAP and NVSS local morphological measures
Using local morphological measures on the sphere defined through a steerable
wavelet analysis, we examine the three-year WMAP and the NVSS data for
correlation induced by the integrated Sachs-Wolfe (ISW) effect. The steerable
wavelet constructed from the second derivative of a Gaussian allows one to
define three local morphological measures, namely the signed-intensity,
orientation and elongation of local features. Detections of correlation between
the WMAP and NVSS data are made with each of these morphological measures. The
most significant detection is obtained in the correlation of the
signed-intensity of local features at a significance of 99.9%. By inspecting
signed-intensity sky maps, it is possible for the first time to see the
correlation between the WMAP and NVSS data by eye. Foreground contamination and
instrumental systematics in the WMAP data are ruled out as the source of all
significant detections of correlation. Our results provide new insight on the
ISW effect by probing the morphological nature of the correlation induced
between the cosmic microwave background and large scale structure of the
Universe. Given the current constraints on the flatness of the Universe, our
detection of the ISW effect again provides direct and independent evidence for
dark energy. Moreover, this new morphological analysis may be used in future to
help us to better understand the nature of dark energy.Comment: 12 pages, 10 figures, replaced to match version accepted by MNRA
Power Spectrum of Velocity Fluctuations in the Universe
We investigate the power spectrum of velocity fluctuations in the universe,
, starting from four different measures of velocity: (1) the power
spectrum of velocity fluctuations from peculiar velocities of galaxies; (2) the
rms peculiar velocity of galaxy clusters; (3) the power spectrum of velocity
fluctuations from the power spectrum of density fluctuations in the galaxy
distribution; (4) and the bulk velocity from peculiar velocities of galaxies.
We show that measures (1) and (2) are not consistent with each other and either
the power spectrum from peculiar velocities of galaxies is overestimated or the
rms cluster peculiar velocity is underestimated. The amplitude of velocity
fluctuations derived from the galaxy distribution (measure 3) depends on the
parameter . We estimate the parameter on the basis of measures
(2) and (4). The power spectrum of velocity fluctuations from the galaxy
distribution in the Stromlo-APM redshift survey is consistent with the observed
rms cluster velocity and with the observed large-scale bulk flow when the
parameter is in the range 0.4-0.5. In this case the value of the
function at wavelength Mpc is km s
and the rms amplitude of the bulk flow at the radius Mpc is km s. The velocity dispersion of galaxy systems originates mostly
from the large-scale velocity fluctuations with wavelengths Mpc.Comment: Astrophysical Journal, Vol. 493, in press: 23 pages, uses AAS Latex,
and 14 separate postscript figure
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