294 research outputs found
Constraints on the Abundance of Highly Ionized Proto-Cluster Regions from the Absence of Large Voids in the Lyman Alpha Forest
Energetic feedback processes during the formation of galaxy clusters may have
heated and ionized a large fraction of the intergalactic gas in proto-cluster
regions. When such a highly ionized hot ``super-bubble'' falls along the
sightline to a background quasar, it would be seen as a large void, with little
or no absorption, in the Lyman alpha forest. We examine the spectra of 137
quasars in the Sloan Digital Sky Survey, to search for such voids, and find no
clear evidence of their existence. The size distribution of voids in the range
5-70 Angstrom (corresponding to physical sizes of approximately 3-35 comoving
Mpc/h) is consistent with the standard model for the Lyman alpha forest without
additional hot bubbles. We adapt a physical model for HII bubble growth during
cosmological reionization (Furlanetto, Zaldarriaga and Hernquist 2004), to
describe the expected size-distribution of hot super-bubbles at redshift around
z = 3. This model incorporates the conjoining of bubbles around individual
neighboring galaxies. Using the non-detection of voids, we find that models in
which the volume filling factor of hot bubbles exceeds approximately 20 percent
at z=3 can be ruled out, primarily because they overproduce the number of large
(40-50 Angstrom) voids. We conclude that any pre-heating mechanism that
explains galaxy cluster observations must avoid heating the low-density gas in
the proto-cluster regions, either by operating relatively recently (z<3) or by
depositing entropy in the high-density regions.Comment: submitted to ApJ, 9 emulateapj pages with 3 figure
A method for optimal image subtraction
We present a new method designed for optimal subtraction of two images with
different seeing. Using image subtraction appears to be essential for the full
analysis of the microlensing survey images, however a perfect subtraction of
two images is not easy as it requires the derivation of an extremely accurate
convolution kernel. Some empirical attempts to find the kernel have used the
Fourier transform of bright stars, but solving the statistical problem of
finding the best kernel solution has never really been tackled. We demonstrate
that it is possible to derive an optimal kernel solution from a simple least
square analysis using all the pixels of both images, and also show that it is
possible to fit the differential background variation at the same time. We also
show that PSF variations can also be easily handled by the method. To
demonstrate the practical efficiency of the method, we analyzed some images
from a Galactic Bulge field monitored by the OGLE II project.
We find that the residuals in the subtracted images are very close to the
photon noise expectations. We also present some light curves of variable stars,
and show that, despite high crowding levels, we get an error distribution close
to that expected from photon noise alone. We thus demonstrate that nearly
optimal differential photometry can be achieved even in very crowded fields. We
suggest that this algorithm might be particularly important for microlensing
surveys, where the photometric accuracy and completeness levels could be very
significantly improved by using this method.Comment: 8,pages, 4 Postscript figures, emulateapj.sty include
Survey for Transiting Extrasolar Planets in Stellar Systems: III. A Limit on the Fraction of Stars with Planets in the Open Cluster NGC 1245
We analyze a 19-night photometric search for transiting extrasolar planets in
the open cluster NGC 1245. An automated transit search algorithm with
quantitative selection criteria finds six transit candidates; none are bona
fide planetary transits. We characterize the survey detection probability via
Monte Carlo injection and recovery of realistic limb-darkened transits. We use
this to derive upper limits on the fraction of cluster members with close-in
Jupiter-radii, RJ, companions. We carefully analyze the random and systematic
errors of the calculation. For similar photometric noise and weather properties
as this survey, observing NGC 1245 twice as long results in a tighter
constraint on "Hot Jupiter", HJ, companions than observing an additional
cluster of similar richness as NGC 1245 for the same length of time as this
survey. This survey observed ~870 cluster members. If 1% of stars have 1.5 RJ
HJ companions, we expect to detect one planet for every 5000 dwarf stars
observed for a month. To reach a ~2% upper limit on the fraction of stars with
1.5 RJ HJ companions, we conclude a total sample size of ~7400 dwarf stars
observed for at least a month will be needed. Results for 1.0 RJ companions,
without substantial improvement in the photometric precision, will require a
small factor larger sample size.Comment: 24 pages, 15 figures, submitted A
Observational Evidence for the Effect of Amplification Bias in Gravitational Microlensing Experiments
Recently Alard\markcite{alard1996} proposed to detect the shift of a star's
image centroid, , as a method to identify the lensed source among
blended stars. Goldberg & Wo\'zniak\markcite{goldberg1997} actually applied
this method to the OGLE-1 database and found that 7 out of 15 events showed
significant centroid shifts of arcsec. The amount of
centroid shift has been estimated theoretically by
Goldberg.\markcite{goldberg1997} However, he treated the problem in general and
did not apply it to a particular survey or field, and thus based his estimates
on simple toy model luminosity functions (i.e., power laws). In this paper, we
construct the expected distribution of for Galactic bulge events by
using the precise stellar LF observed by Holtzman et al.\markcite{holtzman1998}
using HST. Their LF is complete up to (),
corresponding to faint M-type stars. In our analysis we find that regular
blending cannot produce a large fraction of events with measurable centroid
shifts. By contrast, a significant fraction of events would have measurable
centroid shifts if they are affected by amplification-bias blending. Therefore,
Goldberg & Wo\'zniak's measurements of large centroid shifts for a large
fraction of microlensing events confirms the prediction of Han and Alard that a
large fraction of Galactic bulge events are affected by amplification-bias
blending.Comment: total 15 pages, including 6 figures, and no Table, submitted to ApJ
on Apr 26 1998, email [email protected]
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