106 research outputs found
Precision Photometry for Q0957+561 Images A and B
Since the persuasive determination of the time-delay in Q0957+561, much
interest has centered around shifting and subtracting the A and B light-curves
to look for residuals due to microlensing. Solar mass objects in the lens
galaxy produce variations on timescales of decades, with amplitudes of a few
tenths of a magnitude, but MACHO's (with masses of order to
) produce variations at only the 5% level. To detect such small
variations, highly precise photometry is required.
To that end, we have used 200 observations over three nights to examine the
effects of seeing on the light-curves. We have determined that seeing itself
can be responsible for correlated 5% variations in the light-curves of A and B.
We have found, however, that these effects can be accurately removed, by
subtracting the light from the lens galaxy, and by correcting for cross
contamination of light between the closely juxtaposed A and B images. We find
that these corrections improve the variations due to seeing from 5% to a level
only marginally detectable over photon shot noise (0.5%).Comment: 21 Pages with 9 PostScript figures, AASTeX 4 (preprint style
Brightest Cluster Galaxies and Core Gas Density in REXCESS Clusters
We investigate the relationship between brightest cluster galaxies (BCGs) and
their host clusters using a sample of nearby galaxy clusters from the
Representative XMM Cluster Structure Survey (REXCESS). The sample was imaged
with the Southern Observatory for Astrophysical Research (SOAR) in R band to
investigate the mass of the old stellar population. Using a metric radius of
12h^-1 kpc, we found that the BCG luminosity depends weakly on overall cluster
mass as L_BCG \propto M_cl^0.18+-0.07, consistent with previous work. We found
that 90% of the BCGs are located within 0.035 r_500 of the peak of the X-ray
emission, including all of the cool core (CC) clusters. We also found an
unexpected correlation between the BCG metric luminosity and the core gas
density for non-cool core (non-CC) clusters, following a power law of n_e
\propto L_BCG^2.7+-0.4 (where n_e is measured at 0.008 r_500). The correlation
is not easily explained by star formation (which is weak in non-CC clusters) or
overall cluster mass (which is not correlated with core gas density). The trend
persists even when the BCG is not located near the peak of the X-ray emission,
so proximity is not necessary. We suggest that, for non-CC clusters, this
correlation implies that the same process that sets the central entropy of the
cluster gas also determines the central stellar density of the BCG, and that
this underlying physical process is likely to be mergers.Comment: 16 pages, 8 figures, accepted Astrophysical Journa
Radio Wavelength Constraints on the Sources of the Far Infrared Background
The cosmic far infrared background detected recently by the COBE-DIRBE team
is presumably due, in large part, to the far infrared (FIR) emission from all
galaxies. We take the well-established correlation between FIR and radio
luminosity for individual galaxies and apply it to the FIR background. We find
that these sources make up about half of the extragalactic radio background,
the other half being due to AGN. This is in agreement with other radio
observations, which leads us to conclude that the FIR-radio correlation holds
well for the very faint sources making up the FIR background, and that the FIR
background is indeed due to star-formation activity (not AGN or other possible
sources). If these star-forming galaxies have a radio spectral index between
0.4 and 0.8, and make up 40 to 60% of the extragalactic radio background, we
find that they have redshifts between roughly 1 and 2, in agreement with recent
estimates by Madau et al. of the redshift of peak star-formation activity. We
compare the observed extragalactic radio background to the integral over the
logN-logS curve for star-forming radio sources, and find that the slope of the
curve must change significantly below about 1 microjansky. At 1 microjansky,
the faint radio source counts predict about 25 sources per square arcminute,
and these will cause SIRTF to be confusion limited at 160micron.Comment: 10 pages including 1 figure, AASTeX, accepted by Ap
Faint Radio Sources and Star Formation History
Faint extragalactic radio sources provide important information about the
global history of star formation. Sensitive radio observations of the Hubble
Deep Field and other fields have found that sub-mJy radio sources are
predominantly associated with star formation activity rather than AGN. Radio
observations of star forming galaxies have the advantage of being independent
of extinction by dust. We use the FIR-radio correlation to compare the radio
and FIR backgrounds, and make several conclusions about the star forming
galaxies producing the FIR background. We then use the redshift distribution of
faint radio sources to determine the evolution of the radio luminosity
function, and thus estimate the star formation density as a function of
redshift.Comment: 12 pages, 9 figures, latex using texas.sty, to appear in the CD-ROM
Proceedings of the 19th Texas Symposium on Relativistic Astrophysics and
Cosmology, held in Paris, France, Dec. 14-18, 1998. Eds.: J. Paul, T.
Montmerle, and E. Aubourg (CEA Saclay). No changes to paper, just updated
publication info in this commen
The Central Component of Gravitational Lens Q0957+561
In 1981, a faint radio source (G') was detected near the center of the
lensing galaxy of the famous "twin quasar" Q0957+561. It is still unknown
whether this central radio source is a third quasar image or an active nucleus
of the lensing galaxy, or a combination of both. In an attempt to resolve this
ambiguity, we observed Q0957+561 at radio wavelengths of 13cm, 18cm, and 21cm,
using the Very Long Baseline Array in combination with the phased Very Large
Array and the Green Bank Telescope. We measured the spectrum of G' for the
first time and found it to be significantly different from the spectra of the
two bright quasar images. This finding suggests that the central component is
primarily or entirely emission from the foreground lens galaxy, but the
spectrum is also consistent with the hypothesis of a central quasar image
suffering free-free absorption. In addition, we confirm the previously-reported
VLBI position of G' just north of the optical center of the lens galaxy. The
position slightly favors the hypothesis that G' originates in the lens, but is
not conclusive. We discuss the prospects for further clarification of this
issue.Comment: 18 pages, accepted for publication in A
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