879 research outputs found
Formation of the First Stars and Quasars
We review observable signatures of the first generation of stars and
low-luminosity quasars, including the metal enrichment, radiation background,
and dust opacity/emission that they produce. We compute the formation history
of collapsed baryonic halos based on an extension of the Press-Schechter
formalism, incorporating the effects of pressure and H2-dissociation, and
calibrate this history to observational data at redshifts z<5. To be consistent
with the lack of faint point-sources in the Hubble Deep Field, we introduce a
lower limit of 75 km/s for the circular velocities of halos harboring central
black holes. Based on our models, we predict that future satellite instruments
such as MAP, Planck, and NGST will be able to detect the signatures small star
clusters and low-luminosity quasars at redshifts exceeding z=10.Comment: 10 pages, 5 figures included. Invited talk at the 32nd COSPAR
Scientific Assemby, 15-17 July, 1998, Nagoya, Japan; to appear in Adv. of
Space Researc
Testing the binary hypothesis: pulsar timing constraints on supermassive black hole binary candidates
The advent of time domain astronomy is revolutionizing our understanding of
the Universe. Programs such as the Catalina Real-time Transient Survey (CRTS)
or the Palomar Transient Factory (PTF) surveyed millions of objects for several
years, allowing variability studies on large statistical samples. The
inspection of 250k quasars in CRTS resulted in a catalogue of 111
potentially periodic sources, put forward as supermassive black hole binary
(SMBHB) candidates. A similar investigation on PTF data yielded 33 candidates
from a sample of 35k quasars. Working under the SMBHB hypothesis, we
compute the implied SMBHB merger rate and we use it to construct the expected
gravitational wave background (GWB) at nano-Hz frequencies, probed by pulsar
timing arrays (PTAs). After correcting for incompleteness and assuming virial
mass estimates, we find that the GWB implied by the CRTS sample exceeds the
current most stringent PTA upper limits by almost an order of magnitude. After
further correcting for the implicit bias in virial mass measurements, the
implied GWB drops significantly but is still in tension with the most stringent
PTA upper limits. Similar results hold for the PTF sample. Bayesian model
selection shows that the null hypothesis (whereby the candidates are false
positives) is preferred over the binary hypothesis at about and
for the CRTS and PTF samples respectively. Although not decisive,
our analysis highlights the potential of PTAs as astrophysical probes of
individual SMBHB candidates and indicates that the CRTS and PTF samples are
likely contaminated by several false positives.Comment: 14 pages, 11 figures, 3 tables. Resubmitted to the Astrophysical
Journal after some major revision of the results including a proper estimate
of the intrinsic mass of the binary candidate
Effects of the complex mass distribution of dark matter halos on weak lensing cluster surveys
Gravitational lensing effects arise from the light ray deflection by all of
the mass distribution along the line of sight. It is then expected that weak
lensing cluster surveys can provide us true mass-selected cluster samples. With
numerical simulations, we analyze the correspondence between peaks in the
lensing convergence -map and dark matter halos. Particularly we
emphasize the difference between the peak value expected from a dark
matter halo modeled as an isolated and spherical one, which exhibits a
one-to-one correspondence with the halo mass at a given redshift, and that of
the associated -peak from simulations. For halos with the same expected
, their corresponding peak signals in the -map present a wide
dispersion. At an angular smoothing scale of , our
study shows that for relatively large clusters, the complex mass distribution
of individual clusters is the main reason for the dispersion. The projection
effect of uncorrelated structures does not play significant roles. The
triaxiality of dark matter halos accounts for a large part of the dispersion,
especially for the tail at high side. Thus lensing-selected clusters
are not really mass-selected. (abridged)Comment: ApJ accepte
A smoother end to the dark ages
Independent lines of evidence suggest that the first stars, which ended the
cosmic dark ages, came in pairs, rather than singly. This could change the
prevailing view that the early Universe had a Swiss-cheese-like appearance.Comment: Nature News and Views, April 7, 201
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