2,385 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
Correlation Function of Superclusters of Galaxies
We present a study of the two-point correlation function of superclusters of
galaxies. The largest catalogs are used. The results show negligible
correlation less than 0.1-0.2 for separations up to 500-600 h^{-1} Mpc. Small
correlations are obtained using various estimates and samples. Seemingly there
are no structures of superclusters of galaxies.Comment: 19 pages, 3 figures, 4 tables. To appear in 1998 ApJ, 506, No. 2 (Oct
20
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
Two-Dimensional Topology of the 2dF Galaxy Redshift Survey
We study the topology of the publicly available data released by the 2dFGRS.
The 2dFGRS data contains over 100,000 galaxy redshifts with a magnitude limit
of b_J=19.45 and is the largest such survey to date. The data lie over a wide
range of right ascension (75 degree strips) but only within a narrow range of
declination (10 degree and 15 degree strips). This allows measurements of the
two-dimensional genus to be made.
The NGP displays a slight meatball shift topology, whereas the SGP displays a
bubble like topology. The current SGP data also have a slightly higher genus
amplitude. In both cases, a slight excess of overdense regions are found over
underdense regions. We assess the significance of these features using mock
catalogs drawn from the Virgo Consortium's Hubble Volume LCDM z=0 simulation.
We find that differences between the NGP and SGP genus curves are only
significant at the 1 sigma level. The average genus curve of the 2dFGRS agrees
well with that extracted from the LCDM mock catalogs.
We compare the amplitude of the 2dFGRS genus curve to the amplitude of a
Gaussian random field with the same power spectrum as the 2dFGRS and find,
contradictory to results for the 3D genus of other samples, that the amplitude
of the GRF genus curve is slightly lower than that of the 2dFGRS. This could be
due to a a feature in the current data set or the 2D genus may not be as
sensitive as the 3D genus to non-linear clustering due to the averaging over
the thickness of the slice in 2D. (Abridged)Comment: Submitted to ApJ A version with Figure 1 in higher resolution can be
obtained from http://www.physics.drexel.edu/~hoyle
Cumulant Correlators from the APM
This work presents a set of new statistics, the cumulant correlators, aimed
at high precision analysis of the galaxy distribution. They form a symmetric
matrix, , related to moment correlators the same way as cumulants are
related to the moments of the distribution. They encode more information than
the usual cumulants, 's, and their extraction from data is similar to the
calculation of the two-point correlation function. Perturbation theory (PT),
its generalization, the extended perturbation theory (EPT), and the
hierarchical assumption (HA) have simple predictions for these statistics. As
an example, the factorial moment correlators measured by Szapudi, Dalton,
Efstathiou & Szalay (1996) in the APM catalog are reanalyzed using this
technique. While the previous analysis assumed hierarchical structure
constants, this method can directly investigate the validity of HA, along with
PT, and EPT. The results in agreement with previous findings indicate that, at
the small scales used for this analysis, the APM data supports HA. When all
non-linear corrections are taken into account it is a good approximation at the
20 percent level. It appears that PT, and a natural generalization of EPT for
cumulant correlators does not provide such a good fit for the APM at small
scales. Once the validity the HA is approximately established, cumulant
correlators can separate the amplitudes of different tree-types in the
hierarchy up to fifth order. As an example, the weights for the fourth order
tree topologies are calculated including all non-linear corrections.Comment: 9 pages+.ps figure, final version accepted for publication in Apj
Letter
The Ha Luminosity Function and Star Formation Rate at z\sim 0.2
We have measured the Ha+[N II] fluxes of the I-selected Canada-France
Redshift Survey (CFRS) galaxies lying at a redshift z below 0.3, and hence
derived the Ha luminosity function. The magnitude limits of the CFRS mean that
only the galaxies with M(B) > -21 mag were observed at these redshifts. We
obtained a total Ha luminosity density of at least 10^{39.44\pm 0.04}
erg/s/Mpc^{3} at a mean z=0.2 for galaxies with rest-fame EW(Ha+[N II]) > 10
Angs. This is twice the value found in the local universe by Gallego et al.
1995. Our Ha star formation rate, derived from Madau (1997) is higher than the
UV observations at same z, implying a UV dust extinction of about 1 mag. We
found a strong correlation between the Ha luminosity and the absolute magnitude
in the B-band: M(B(AB)) = 46.7 - 1.6 log L(Ha). This work will serve as a basis
of future studies of Ha luminosity distributions measured from
optically-selected spectroscopic surveys of the distant universe, and it will
provide a better understanding of the physical processes responsible for the
observed galaxy evolution.Comment: Accepted for publication in ApJ, 14 pages, LaTeX (macro aas2pp4.sty),
6 figure
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
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