4,383 research outputs found
The UV variability of the Seyfert 1: Galaxies 3 Zw 2 and Markarian 509
The two galaxies differ markedly in their radio properties. III Zw 2 is a strong source with a highly variable compact component while MK 509 is a very weak source. Both galaxies show significant variations in X-rays and MK 509 has shown variations at optical wavelengths as well. Simultaneous observations were made in the ultraviolet, optical and infrared in order to examine three fundamental aspects of the origin of the continuum emission: are these thermal and nonthermal components; how large is the emitting region; and does the UV flux originate in the same region responsible for the optical, IR, radio and/or X-ray continuum emission
Current-Carrying Ground States in Mesoscopic and Macroscopic Systems
We extend a theorem of Bloch, which concerns the net orbital current carried
by an interacting electron system in equilibrium, to include mesoscopic
effects. We obtain a rigorous upper bound to the allowed ground-state current
in a ring or disc, for an interacting electron system in the presence of static
but otherwise arbitrary electric and magnetic fields. We also investigate the
effects of spin-orbit and current-current interactions on the upper bound.
Current-current interactions, caused by the magnetic field produced at a point
r by a moving electron at r, are found to reduce the upper bound by an amount
that is determined by the self-inductance of the system. A solvable model of an
electron system that includes current-current interactions is shown to realize
our upper bound, and the upper bound is compared with measurements of the
persistent current in a single ring.Comment: 7 pager, Revtex, 1 figure available from [email protected]
Clustering of Galaxies in a Hierarchical Universe: I. Methods and Results at z=0
We introduce a new technique for following the formation and evolution of
galaxies in cosmological N-body simulations. Dissipationless simulations are
used to track the formation and merging of dark matter halos as a function of
redshift. Simple prescriptions, taken directly from semi-analytic models of
galaxy formation, are adopted for cooling, star formation, supernova feedback
and the merging of galaxies within the halos. This scheme enables us to study
the clustering properties of galaxies and to investigate how selection by type,
colour or luminosity influences the results. In this paper, we study properties
of the galaxy distribution at z=0. These include luminosity functions, colours,
correlation functions, pairwise peculiar velocities, cluster M/L ratios and
star formation rates. We focus on two variants of a CDM cosmology: a high-
density model with Gamma=0.21 (TCDM) and a low-density model with Omega=0.3 and
Lambda=0.7 (LCDM). Both are normalized to reproduce the I-band Tully-Fisher
relation near a circular velocity of 220 km/s. Our results depend strongly both
on this normalization and on the prescriptions for star formation and feedback.
Very different assumptions are required to obtain an acceptable model in the
two cases. For TCDM, efficient feedback is required to suppress the growth of
galaxies low-mass field halos. Without it, there are too many galaxies and the
correlation function turns over below 1 Mpc. For LCDM, feedback must be weak,
otherwise too few L* galaxies are produced and the correlation function is too
steep. Given the uncertainties in modelling some of the key physical processes,
we conclude that it is not yet possible to draw conclusions about the values of
cosmological parameters from studies of this kind. Further work on global star
formation and feedback effects is required to narrow the range of possibilitiesComment: 43 pages, Latex, 16 figures included, 2 additional GIF format
figures, submitted to MNRA
An excess of damped Lyman alpha galaxies near QSOs
We present a sample of 33 damped Lyman alpha systems (DLAs) discovered in the
Sloan Digital Sky Survey (SDSS) whose absorption redshifts (z_abs) are within
6000 km/s of the QSO's systemic redshift (z_sys). Our sample is based on 731
2.5 < z_sys < 4.5 non-broad-absorption-line (non-BAL) QSOs from Data Release 3
(DR3) of the SDSS. We estimate that our search is ~100 % complete for absorbers
with N(HI) >= 2e20 cm^-2. The derived number density of DLAs per unit redshift,
n(z), within v < 6000 km/s is higher (3.5 sigma significance) by almost a
factor of 2 than that of intervening absorbers observed in the SDSS DR3, i.e.
there is evidence for an overdensity of galaxies near the QSOs. This provides a
physical motivation for excluding DLAs at small velocity separations in surveys
of intervening 'field' DLAs. In addition, we find that the overdensity of
proximate DLAs is independent of the radio-loudness of the QSO, consistent with
the environments of radio-loud and radio-quiet QSOs being similar.Comment: Accepted for publication in MNRAS (13 pages, 6 figures
Adaptive Density Estimation on the Circle by Nearly-Tight Frames
This work is concerned with the study of asymptotic properties of
nonparametric density estimates in the framework of circular data. The
estimation procedure here applied is based on wavelet thresholding methods: the
wavelets used are the so-called Mexican needlets, which describe a nearly-tight
frame on the circle. We study the asymptotic behaviour of the -risk
function for these estimates, in particular its adaptivity, proving that its
rate of convergence is nearly optimal.Comment: 30 pages, 3 figure
Using x ray images to detect substructure in a sample of 40 Abell clusters
Using a method for constraining the dynamical state of a galaxy cluster by examining the moments of its x-ray surface brightness distribution, we determine the statistics of cluster substructure for a sample of 40 Abell clusters. Using x-ray observations from the Einstein Observatory Imaging Proportional Counter (IPC), we measure the first moment M1(r), the ellipsoidal orientation angle theta2(r), and the axial ratio eta(r) at several different radii in the cluster. We determine the effects of systematics such as x-ray point source emission, telescope vignetting, Poisson noise, and characteristics of the IPC by measuring the same parameters on an ensemble of simulated cluster images. Due to the small band-pass of the IPC, the ICM emissivity is nearly independent of temperature so the intensity at each point in the IPC images is simply proportional to the emission measure calculated along the line of sight through the cluster (e.g. Fabricant et al. 1980). Therefore, barring a change superposition of two x-ray emitting clusters, a significant variation in the image centroid M1(r) as a function of radius indicates that the center of mass of the intra-cluster medium (ICM) varies with radius. We argue that such a configuration (essentially an m = 1 component in the ICM density distribution) is a non-equilibrium component; it results from an off-center subclump or a recent merger in the ICM
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