358 research outputs found
The Seyfert Population in the Local Universe
The magnitude-limited catalog of the Southern Sky Redshift Survey (SSRS2), is
used to characterize the properties of galaxies hosting Active Galactic Nuclei.
Using emission-line ratios, we identify a total of 162 (3%) Seyfert galaxies
out of the parent sample with 5399 galaxies. The sample contains 121 Seyfert 2
galaxies and 41 Seyfert 1. The SSRS2 Seyfert galaxies are predominantly in
spirals of types Sb and earlier, or in galaxies with perturbed appearance as
the result of strong interactions or mergers. Seyfert galaxies in this sample
are twice as common in barred hosts than the non-Seyferts. By assigning
galaxies to groups using a percolation algorithm we find that the Seyfert
galaxies in the SSRS2 are more likely to be found in binary systems, when
compared to galaxies in the SSRS2 parent sample. However, there is no
statistically significant difference between the Seyfert and SSRS2 parent
sample when systems with more than 2 galaxies are considered. The analysis of
the present sample suggests that there is a stronger correlation between the
presence of the AGN phenomenon with internal properties of galaxies
(morphology, presence of bar, luminosity) than with environmental effects
(local galaxy density, group velocity dispersion, nearest neighbor distance).Comment: 35 pages, 13 figures, Accepted to be publised in Astronomical Journa
The Age-Redshift Relation for Standard Cosmology
We present compact, analytic expressions for the age-redshift relation
for standard Friedmann-Lema\^ \itre-Robertson-Walker (FLRW)
cosmology. The new expressions are given in terms of incomplete Legendre
elliptic integrals and evaluate much faster than by direct numerical
integration.Comment: 13 pages, 3 figure
A New Observational Upper Limit to the Low Redshift Ionizing Background Radiation
We report a new Fabry-Perot search for Halpha emission from the intergalactic
cloud HI 1225+01 in an attempt to measure the low redshift ionizing background
radiation. We set a new 2 sigma upper limit on Halpha emission of 8 mR (5 x
10^{-20} ergs cm^{-2} s^{-1} arcsec^{-2}). Conversion of this limit to limits
on the strength of the ionizing background requires knowledge of the ratio of
the projected to total surface area of this cloud, which is uncertain. We
discuss the plausible range of this ratio, and within this range find that the
strength of the ionizing backround is in the lower range of, but consistent
with, previous observational and theoretical estimates.Comment: 46 pages including 9 figures (7 ps, 2 gif
Superconductivity and Antiferromagnetism: Hybridization Impurities in a Two-Band Spin-Gapped Electron System
We present the exact solution of a one-dimensional model of a spin-gapped
correlated electron system with hybridization impurities exhibiting both
magnetic and mixed-valence properties. The host supports superconducting
fluctuations, with a spin gap. The localized electrons create a band of
antiferromagnetic spin excitations inside the gap for concentrations x of the
impurities below some critical value x_c. When x = x_c the spin gap closes and
a ferrimagnetic phase appears. This is the first example of an exactly solvable
model with coexisting superconducting and antiferromagnetic fluctuations which
in addition supports a quantum phase transition to a (compensated)
ferrimagnetic phase. We discuss the possible relevance of our results for
experimental systems, in particular the U-based heavy-fermion materials.Comment: 4 page
On the Hadronic Beam Model for Gamma-ray Production in Blazars
We consider, herein, a model for gamma-ray production in blazars in which a
relativistic, highly-collimated electron-proton beam interacts with a dense,
compact cloud as the jet propagates through the broad and perhaps narrow line
regions (BLR and NLR) of active galactic nuclei (AGN). During the propagation
of the beam through the cloud, the process of excitation of plasma waves
becomes an important energy loss mechanism, especially for mildly relativistic
proton beams. We compute the expected spectra of gamma-rays from the decay of
neutral pions produced in hadronic collisions of the beam with the cloud,
taking into account collisionless losses of the electron-proton beam. This
model may explain the X-ray and TeV gamma-ray (both low and high emission
states) of Mrk 421 as a result of synchrotron emission of secondary pairs from
the decay of charged pions and gamma-ray emission from the decay of neutral
pions for the plausible cloud parameters. However clouds can not be too hot and
too dense. Otherwise the TeV gamma-rays can be attenuated by the bremsstrahlung
radiation in the cloud and the secondary pairs are not able to efficiently
produce synchrotron flares because of the dominant role of inverse Compton
scattering.
The non-variable -ray emission observed from Mrk 421 in the EGRET
energy range cannot be described by the -rays from decay of neutral
pions provided that the spectrum of protons in the beam is well described by a
simple power law. These -rays might only be produced by secondary pairs
scattering the soft non-variable X-rays which might originate in the inner part
of the accretion disk.Comment: 14 pages,3 figures, latex, submitted to Ap
Evolution of the Scale Factor with a Variable Cosmological Term
Evolution of the scale factor a(t) in Friedmann models (those with zero
pressure and a constant cosmological term Lambda) is well understood, and
elegantly summarized in the review of Felten and Isaacman [Rev. Mod. Phys. 58,
689 (1986)]. Developments in particle physics and inflationary theory, however,
increasingly indicate that Lambda ought to be treated as a dynamical quantity.
We revisit the evolution of the scale factor with a variable Lambda-term, and
also generalize the treatment to include nonzero pressure. New solutions are
obtained and evaluated using a variety of observational criteria. Existing
arguments for the inevitability of a big bang (ie., an initial state with a=0)
are substantially weakened, and can be evaded in some cases with Lambda_0 (the
present value of Lambda) well below current experimental limits.Comment: 29 pages, 12 figures (not included), LaTeX, uses Phys Rev D style
files (revtex.cls, revtex.sty, aps.sty, aps10.sty, prabib.sty). To appear in
Phys Rev
DA white dwarfs from the LSS-GAC survey DR1: the preliminary luminosity and mass functions and formation rate
Modern large-scale surveys have allowed the identification of large numbers
of white dwarfs. However, these surveys are subject to complicated target
selection algorithms, which make it almost impossible to quantify to what
extent the observational biases affect the observed populations. The LAMOST
(Large Sky Area Multi-Object Fiber Spectroscopic Telescope) Spectroscopic
Survey of the Galactic anti-center (LSS-GAC) follows a well-defined set of
criteria for selecting targets for observations. This advantage over previous
surveys has been fully exploited here to identify a small yet
well-characterised magnitude-limited sample of hydrogen-rich (DA) white dwarfs.
We derive preliminary LSS-GAC DA white dwarf luminosity and mass functions. The
space density and average formation rate of DA white dwarfs we derive are
0.83+/-0.16 x 10^{-3} pc^{-3} and 5.42 +/- 0.08 x 10^{-13} pc^{-3} yr^{-1},
respectively. Additionally, using an existing Monte Carlo population synthesis
code we simulate the population of single DA white dwarfs in the Galactic
anti-center, under various assumptions. The synthetic populations are passed
through the LSS-GAC selection criteria, taking into account all possible
observational biases. This allows us to perform a meaningful comparison of the
observed and simulated distributions. We find that the LSS-GAC set of criteria
is highly efficient in selecting white dwarfs for spectroscopic observations
(80-85 per cent) and that, overall, our simulations reproduce well the observed
luminosity function. However, they fail at reproducing an excess of massive
white dwarfs present in the observed mass function. A plausible explanation for
this is that a sizable fraction of massive white dwarfs in the Galaxy are the
product of white dwarf-white dwarf mergers.Comment: 23 pages, 14 figures and 5 tables. Accepted for publication by MNRA
V838 Monocerotis: A Geometric Distance from Hubble Space Telescope Polarimetric Imaging of its Light Echo
Following the outburst of the unusual variable star V838 Monocerotis in 2002,
a spectacular light echo appeared. A light echo provides the possibility of
direct geometric distance determination, because it should contain a ring of
highly linearly polarized light at a linear radius of ct, where t is the time
since the outburst. We present imaging polarimetry of the V838 Mon light echo,
obtained in 2002 and 2005 with the Advanced Camera for Surveys onboard the
Hubble Space Telescope, which confirms the presence of the highly polarized
ring. Based on detailed modeling that takes into account the outburst light
curve, the paraboloidal echo geometry, and the physics of dust scattering and
polarization, we find a distance of 6.1+-0.6 kpc. The error is dominated by the
systematic uncertainty in the scattering angle of maximum linear polarization,
taken to be theta_{max}=90^o +- 5^o. The polarimetric distance agrees
remarkably well with a distance of 6.2+-1.5 kpc obtained from the entirely
independent method of main-sequence fitting to a sparse star cluster associated
with V838 Mon. At this distance, V838 Mon at maximum light had M_V\simeq-9.8,
making it temporarily one of the most luminous stars in the Local Group. Our
validation of the polarimetric method offers promise for measurement of
extragalactic distances using supernova light echoes.Comment: 43 pages, 17 figures, 3 tables; accepted for publication in the
Astronomical Journal. Version with high-quality figures available at
http://www.stsci.edu/~bond/v838monpolariz.pd
The Southern Sky Redshift Survey
We report redshifts, magnitudes and morphological classifications for 5369
galaxies with and 57 galaxies fainter than this limit, in two
regions covering a total of 1.70 steradians in the southern celestial
hemisphere. The galaxy catalog is drawn primarily from the list of non-stellar
objects identified in the Guide Star Catalog (Lasker et al. 1990, AJ 99, 2019;
hereafter GSC). The galaxies have positions accurate to about 1 arc sec and
magnitudes with an rms scatter of about 0.3 mag. We compute magnitudes
() from the relation between instrumental GSC magnitudes and the
photometry by Lauberts & Valentijn (1989). From a comparison with CCD
photometry, we find that our system is homogeneous across the sky and
corresponds to magnitudes measured at the isophotal level 26 mag
arcsec. The precision of the radial velocities is of \sim 40 km/s and
the redshift survey is more than 99% complete to the = 15.5
magnitude limit. This sample is in the direction opposite to the CfA2; in
combination the two surveys provide an important database for studies of the
properties of galaxies and their large-scale distribution in the nearby
Universe.Comment: 20 pages, uses AASTeX 4.0 macros. Includes 3 tables and 4 Postscripts
figures. Scheduled for publication in the AJ 1998 July issu
The Cosmological Constant
This is a review of the physics and cosmology of the cosmological constant.
Focusing on recent developments, I present a pedagogical overview of cosmology
in the presence of a cosmological constant, observational constraints on its
magnitude, and the physics of a small (and potentially nonzero) vacuum energy.Comment: 50 pages. Submitted to Living Reviews in Relativity
(http://www.livingreviews.org/), December 199
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