1,940 research outputs found
Can Virialization Shocks be Detected Around Galaxy Clusters Through the Sunyaev-Zel'dovich Effect?
In cosmological structure formation models, massive non-linear objects in the
process of formation, such as galaxy clusters, are surrounded by large-scale
shocks at or around the expected virial radius. Direct observational evidence
for such virial shocks is currently lacking, but we show here that their
presence can be inferred from future, high resolution, high-sensitivity
observations of the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters. We study
the detectability of virial shocks in mock SZ maps, using simple models of
cluster structure (gas density and temperature distributions) and noise
(background and foreground galaxy clusters projected along the line of sight,
as well as the cosmic microwave background anisotropies). We find that at an
angular resolution of 2'' and sensitivity of 10 micro K, expected to be reached
at ~ 100 GHz frequencies in a ~ 20 hr integration with the forthcoming ALMA
instrument, virial shocks associated with massive M ~ 10^15 M_Sun clusters will
stand out from the noise, and can be detected at high significance. More
generally, our results imply that the projected SZ surface brightness profile
in future, high-resolution experiments will provide sensitive constraints on
the density profile of cluster gas.Comment: 15 pages, submitted to Ap
A Population of Short-Period Variable Quasars from PTF as Supermassive Black Hole Binary Candidates
Supermassive black hole binaries (SMBHBs) at sub-parsec separations should be
common in galactic nuclei, as a result of frequent galaxy mergers.
Hydrodynamical simulations of circumbinary discs predict strong periodic
modulation of the mass accretion rate on time-scales comparable to the orbital
period of the binary. As a result, SMBHBs may be recognized by the periodic
modulation of their brightness. We conducted a statistical search for periodic
variability in a sample of 35,383 spectroscopically confirmed quasars in the
photometric database of the Palomar Transient Factory (PTF). We analysed
Lomb-Scargle periodograms and assessed the significance of our findings by
modeling each individual quasar's variability as a damped random walk (DRW). We
identified 50 quasars with significant periodicity beyond the DRW model,
typically with short periods of a few hundred days. We find 33 of these to
remain significant after a re-analysis of their periodograms including
additional optical data from the intermediate-PTF and the Catalina Real-Time
Transient Survey (CRTS). Assuming that the observed periods correspond to the
redshifted orbital periods of SMBHBs, we conclude that our findings are
consistent with a population of unequal-mass SMBHBs, with a typical mass ratio
as low as q = M2/M1 ~ 0.01.Comment: MNRAS (accepted), new section 4.
Constraints from Gravitational Recoil on the Growth of Supermassive Black Holes at High Redshift
Recent studies have shown that during their coalescence, binary supermassive
black holes (SMBHs) experience a gravitational recoil with velocities of 100
km/s < v(kick) < 600 km/s. These velocities exceed the escape velocity v(esc)
from typical dark matter (DM) halos at high-redshift (z>6), and therefore put
constraints on scenarios in which early SMBHs grow at the centers of DM halos.
Here we quantify these constraints for the most distant known SMBHs, with
inferred masses in excess of 10^9 M(sun), powering the bright quasars
discovered in the Sloan Digital Sky Survey at z>6. We assume that these SMBHs
grew via a combination of accretion and mergers between pre-existing seed BHs
in individual progenitor halos, and that mergers between progenitors with
v(esc) < v(kick) disrupt the BH growth process. Our results suggest that under
these assumptions, the z=6 SMBHs had a phase during which gained mass
significantly more rapidly than under an Eddington-limited exponential growth
rate.Comment: submitted to ApJ Letters, 5 emulateapj pages with 1 figur
On the Cosmological Evolution of the Luminosity Function and the Accretion Rate of Quasars
We consider a class of models for the redshift evolution (between 0\lsim z
\lsim 4) of the observed optical and X-ray quasar luminosity functions (LFs),
with the following assumptions: (i) the mass-function of dark matter halos
follows the Press-Schechter theory, (ii) the black hole (BH) mass scales
linearly with the halo mass, (iii) quasars have a constant universal lifetime,
and (iv) a thin accretion disk provides the optical luminosity of quasars,
while the X-ray/optical flux ratio is calibrated from a sample of observed
quasars. The mass accretion rate onto quasar BHs is a free parameter
of the models, that we constrain using the observed LFs. The accretion rate
inferred from either the optical or X-ray data under these assumptions
generally decreases as a function of cosmic time from to . We find that a comparable accretion rate is inferred from the X-ray and
optical LF only if the X-ray/optical flux ratio decreases with BH mass. Near
, drops to substantially sub-Eddington values at which
advection-dominated accretion flows (ADAFs) exist. Such a decline of ,
possibly followed by a transition to radiatively inefficient ADAFs, could
explain both the absence of bright quasars in the local universe and the
faintness of accreting BHs at the centers of nearby galaxies. We argue that a
decline of the accretion rate of the quasar population is indeed expected in
cosmological structure formation models.Comment: Latex, 23 pages, 9 figures, accepted for publication in Ap
Constraining the Lifetime of Quasars from their Spatial Clustering
The lifetime t_Q of the luminous phase of quasars is constrained by current
observations to be between 10^6 and 10^8 years, but is otherwise unkown. We
model the quasar luminosity function in detail in the optical and X-ray bands
using the Press-Schechter formalism, and show that the expected clustering of
quasars depends strongly on their assumed lifetime. We quantify this
dependence, and find that existing measurements of the correlation length of
quasars are consistent with the range 10^6 < t_Q < 10^8 years. We then show
that future measurements of the power spectrum of quasars out to z=3, from the
2dF or Sloan Digital Sky Survey, can significantly improve this constraint, and
in principle allow a precise determination of t_Q. We estimate the systematic
errors introduced by uncertainties in the modeling of the quasar-halo
relationship, as well as by the possible existence of obscured quasars.Comment: ApJ, in press (emulateapj
Witnessing the formation of a brightest cluster galaxy at z>2
We present deep observations taken with the HST Advanced Camera for Surveys
of the central massive galaxy in a forming cluster at z=2.2. The galaxy hosting
the powerful radio source MRC 1138-262 is associated with one of the most
extensive merger systems known in the early universe. Our HST/ACS image shows
many star-forming galaxies merging within a ~200 kpc region that emits both
diffuse line emission and continuum in the rest-frame UV. Because this galaxy
lives in an overdense environment, it represents a rare view of a brightest
cluster galaxy in formation at z>2 which may serve as a testbed for predictions
of massive cluster galaxy formation.Comment: Contribution to the proceedings of "The Fate of Gas in Galaxies",
Dwingeloo, July 2006, with 2 colour figures. To appear in New Astronomy
Reviews, Vol. 51 (2007), eds. Morganti, Oosterloo, Villar-Martin & van Gorko
Gravitational-wave memory revisited: memory from the merger and recoil of binary black holes
Gravitational-wave memory refers to the permanent displacement of the test
masses in an idealized (freely-falling) gravitational-wave interferometer.
Inspiraling binaries produce a particularly interesting form of memory--the
Christodoulou memory. Although it originates from nonlinear interactions at 2.5
post-Newtonian order, the Christodoulou memory affects the gravitational-wave
amplitude at leading (Newtonian) order. Previous calculations have computed
this non-oscillatory amplitude correction during the inspiral phase of binary
coalescence. Using an "effective-one-body" description calibrated with the
results of numerical relativity simulations, the evolution of the memory during
the inspiral, merger, and ringdown phases, as well as the memory's final
saturation value, are calculated. Using this model for the memory, the
prospects for its detection are examined, particularly for supermassive black
hole binary coalescences that LISA will detect with high signal-to-noise
ratios. Coalescing binary black holes also experience center-of-mass recoil due
to the anisotropic emission of gravitational radiation. These recoils can
manifest themselves in the gravitational-wave signal in the form of a "linear"
memory and a Doppler shift of the quasi-normal-mode frequencies. The prospects
for observing these effects are also discussed.Comment: 6 pages, 2 figures; accepted to the proceedings of the 7th
International LISA Symposium; v2: updated figures and signal-to-noise ratios,
several minor changes to the tex
The Stellar Population of the M31 Spiral Arm Around OB Association A24
A study of the stellar population of the M31 spiral arm around OB association
A24 was carried out based on the photometric data obtained from deep V and JHK
imaging. The luminosity function was obtained for -7 <~ Mbol <~ -3.5 by
applying the extinction correction corresponding to Av=1 and the bolometric
correction BC(K) as an empirical function of (J-K)o. In comparing the observed
color-luminosity diagrams with semitheoretical isochrones modified for the
dust-shell effects, we found the young population of t <~ 30 Myr with
supergiants of Mbol <~ -5, the bulk of the intermediate-age population of t ~
0.2 - 2.5 Gyr with bright asymptotic giant branch (AGB) stars of -5 <~ Mbol <~
-4, and old populations of t ~> 3 Gyr with AGB and red giant branch (RGB) stars
of Mbol ~> -4. The average star formation rate was estimated to be ~1.8x10^4
M_o/Myr and ~0.7x10^4 M_o/Myr per deprojected disk area of 1 kpc^2 from the
number density of B0 V stars around Mv=-4.0 (age ~10 Myr) and the number
density of bright AGB stars around Mbol = -4.3 (age ~1 Gyr), respectively. A
study of the local variation in the V and the J and H luminosity functions
revealed a kind of anticorrelation between the population of the young
component and that of the intermediate-age component when subdomains of ~100 pc
scales were concerned. This finding suggests that the disk domain around the
A24 area experienced a series of star formation episodes alternatively among
different subdomains with a timescale of a few spiral passage periods. Brief
discussions are given about the interstellar extinction and about the lifetimes
of bright AGB stars and the highly red objects (HROs) in the same area.Comment: 27 pages, 11 figures, accepted: ApJ, July 1, 199
The Refined Topological Vertex
We define a refined topological vertex which depends in addition on a
parameter, which physically corresponds to extending the self-dual graviphoton
field strength to a more general configuration. Using this refined topological
vertex we compute, using geometric engineering, a two-parameter (equivariant)
instanton expansion of gauge theories which reproduce the results of Nekrasov.
The refined vertex is also expected to be related to Khovanov knot invariants.Comment: 70 Pages, 23 Figure
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