1,215 research outputs found
Angular correlations of galaxy distribution
We study the angular correlations of various galaxy catalogs (CfA1, SSRS1,
Perseus-Pisces, APM Bright Galaxies and Zwicky). We find that the angular
correlation exponent is rather than as
usually found by the standard correlation function . We
identify the problem in the artificial decay of . Moreover we
find that no characteristic angular scale is present in any of the analyzed
catalogs. Finally we show that all the available data are consistent with each
other and the angular distribution of galaxies is quite naturally compatible
with a fractal structure with .Comment: 16 pages, latex, 3 postscript figures. Accepted for publication in
Astrophysical Journal Letters. This paper is also available at
http://www.phys.uniroma1.it/DOCS/PIL/pil.htm
Spectral Analysis of the Stromlo-APM Survey II. Galaxy luminosity function and clustering by spectral type
We study the luminosity function and clustering properties of subsamples of
local galaxies selected from the Stromlo-APM survey by the rest-frame
equivalent widths of their Halpha and Oii emission lines. The b_J luminosity
function of star-forming galaxies has a significantly steeper faint-end slope
than that for quiescent galaxies: the majority of sub-L* galaxies are currently
undergoing significant star formation. Emission line galaxies are less strongly
clustered, both amongst themselves, and with the general galaxy population,
than quiescent galaxies. Thus as well as being less luminous, star-forming
galaxies also inhabit lower-density regions of the Universe than quiescent
galaxies.Comment: 8 pages, 7 figures, MNRAS, in pres
Spectral Analysis of the Stromlo-APM Survey I. Spectral Properties of Galaxies
We analyze spectral properties of 1671 galaxies from the Stromlo-APM survey,
selected to have 15 < b_J < 17.15 and having a mean redshift z = 0.05. This is
a representative local sample of field galaxies, so the global properties of
the galaxy population provide a comparative point for analysis of more distant
surveys. We measure Halpha, Oii 3727, Sii 6716, 6731, Nii 6583 and Oi 6300
equivalent widths and the D_4000 break index. The 5A resolution spectra use an
8 arcsec slit, which typically covers 40-50% of the galaxy area. We find no
evidence for systematic trends depending on the fraction of galaxy covered by
the slit, and further analysis suggests that our spectra are representative of
integrated galaxy spectra.
We classify spectra according to their Halpha emission, which is closely
related to massive star formation. Overall we find 61% of galaxies are Halpha
emitters with rest-frame equivalent widths EW(Halpha) >= 2A. The emission-line
galaxy (ELG) fraction is smaller than seen in the CFRS at z = 0.2 and is
consistent with a rapid evolution of Halpha luminosity density. The ELG
fraction, and EW(Halpha), increase at fainter absolute magnitudes, smaller
projected area and smaller D_4000. In the local Universe, faint, small galaxies
are dominated by star formation activity, while bright, large galaxies are more
quiescent. This picture of the local Universe is quite different from the
distant one, where bright galaxies appear to show rapidly-increasing activity
back in time.
(Abridged)Comment: 40 pages, 25 figures, MNRAS, in pres
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
Galaxy types in the Sloan Digital Sky Survey using supervised artificial neural networks
Supervised artificial neural networks are used to predict useful properties of galaxies in the Sloan Digital Sky Survey, in this instance morphological classifications, spectral types and redshifts. By giving the trained networks unseen data, it is found that correlations between predicted and actual properties are around 0.9 with rms errors of order ten per cent. Thus, given a representative training set, these properties may be reliably estimated for galaxies in the survey for which there are no spectra and without human intervention
Structures of solid hydrogen at 300K
We present results predicting experimentally measurable structural quantities
from molecular dynamics studies of hydrogen. In doing this, we propose a
paradigm shift for experimentalists -- that the predictions from such
calculations should be seen as the most likely hypotheses. Specifically, the
experimental results should be aiming to distinguish between the candidate
low-energy structures, rather than aiming to solve the simplest structure
consistent with the data. We show that the room temperature X-ray diffraction
patterns for hydrogen phases I, III, IV and V are very similar, with only small
peaks denoting symmetry-breaking from the hcp Phase I. Because they incorporate
atomic displacements the XRD patterns implied by molecular dynamics
calculations are very different from those arising from the static minimum
enthalpy structures found by structure searching. Simulations also show that
within Phase I the molecular becomes increasingly confined to the basal plane
and suggest the possibility of an unusual critical point terminating the Phase
I-III boundary line
Intrinsic Axis Ratio Distribution of Early-type Galaxies From Sloan Digital Sky Survey
Using Sloan Digital Sky Survey Data Release 5, we have investigated the
intrinsic axis ratio distribution (ARD) for early-type galaxies. We have
constructed a volume-limited sample of 3,922 visually-inspected early-type
galaxies at carefully considering sampling biases
caused by the galaxy isophotal size and luminosity. We attempt to de-project
the observed ARD into three-dimensional types (oblate, prolate, and triaxial),
which are classified in terms of triaxiality. We confirm that no linear
combination of -distributed axis ratios of the three types can
reproduce the observed ARD. However, using Gaussian intrinsic distributions, we
have found reasonable fits to the data with preferred mean axis ratios for
oblate, prolate, and triaxial (triaxials in two axis ratios), where the fractions of
oblate, prolate and triaxial types are
\textrm{O:P:T}=0.29^{\pm0.09}:0.26^{\pm0.11}:0.45^{\pm0.13}-23.3 < M_r \leq -21.2-21.2 < M_r <-19.3$) sample does. Oblate is
relatively more abundant among the less luminous galaxies. Interestingly, the
preferences of axis ratios for triaxial types in the two luminosity classes are
remarkably similar. We have not found any significant influence of the local
galaxy number density on ARD. We show that the results can be seriously
affected by the details in the data selection and type classification scheme.
Caveats and implications on galaxy formation are discussed.Comment: 9 pages, 11 figures, Accepted for publication in Ap
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