491 research outputs found
Hydrodynamic Simulation of the Cosmological X-ray Background
(Abridged) We use a hydrodynamic simulation of a LambdaCDM model to predict
the extragalactic X-ray background (XRB), focussing on emission from the
intergalactic medium (IGM). We also include X-rays from point sources
associated with galaxies in the simulation, and make maps of the angular
distribution of the emission. We find that filaments in the maps are not
evident, being diluted by projection. In the soft (0.5-2 keV) band, the mean
intensity of radiation from intergalactic and cluster gas is 2.3*10^-12
ergdeg^-2cm^-2s^-1, 35% of the total soft band emission. This is compatible at
the ~1 sigma level with estimates of the unresolved soft background from ROSAT
and {\it Chandra}. Only 4% of the hard (2-10 keV) emission is associated with
the IGM. Relative to AGN flux, the IGM component peaks at a lower redshift
(median z~0.45) so its clustering makes an important contribution to that of
the total XRB. The angular correlations on 0.1-10 arcmin scales are
significant, with an amplitude roughly consistent with an extrapolation of
recent ROSAT results to small scales. A cross-correlation of the XRB against
nearby galaxies taken from a simulated redshift survey also yields a strong
signal from the IGM. Although some recent papers have argued that the expected
soft band intensity from gas in galaxy, group, and cluster halos would exceed
XRB limits unless much of the gas is expelled by supernova feedback, we obtain
reasonable compatibility with current observations in a simulation that
incorporates cooling, star formation, and only modest feedback. A prediction of
our model is that the unresolved portion of the soft XRB will remain mostly
unresolved.Comment: Improved referencing of related papers. Submitted to ApJ, 19 pages,
17 postscript figures, most reduced in resolution, emulateapj.sty, for full
resolution version, see http://cfa-www.harvard.edu/~rcroft/xray.ps.g
Changes in left atrial deformation in hypertrophic cardiomyopathy: Evaluation by vector velocity imaging.
OBJECTIVES: Hypertrophic cardiomyopathy (HCM) represents a generalized myopathic process affecting both ventricular and atrial myocardium. We assessed the global and regional left atrial (LA) function and its relation to left ventricular (LV) mechanics and clinical status in patients with HCM using Vector Velocity Imaging (VVI). METHODS: VVI of the LA and LV was acquired from apical four- and two-chamber views of 108 HCM patients (age 40 ± 19years, 56.5% men) and 33 healthy subjects, all had normal LV systolic function. The LA subendocardium was traced to obtain atrial volumes, ejection fraction, velocities, and strain (ϔ)/strain rate (SR) measurements. RESULTS: Left atrial reservoir (ϔsys,SRsys) and conduit (early diastolic SRe) function were significantly reduced in HCM compared to controls (P  - 1.8s(- 1) was 81% sensitive and 30% specific, SRa> - 1.5s(- 1) was 73% sensitive and 40% specific. By multivariate analysis global LVϔsys and LV septal thickness are independent predictors for LAϔsys, while end systolic diameter is the only independent predictor for SRsys, P < .001. CONCLUSION: Left atrial reservoir and conduit function as measured by VVI were significantly impaired while contractile function was preserved among HCM patients. Left atrial deformation was greatly influenced by LV mechanics and correlated to severity of phenotype
Constraints on Warm Dark Matter from Cosmological Reionization
We study the constraints that high-redshift structure formation in the
universe places on warm dark matter (WDM) dominated cosmological models. We
modify the extended Press-Schechter formalism to derive the halo mass function
in WDM models. We show that our predictions agree with recent numerical
simulations at low redshift over the halo masses of interest. Applying our
model to galaxy formation at high redshift, we find that the loss of power on
small scales, together with the delayed collapse of low-mass objects, results
in strong limits on the root-mean-square velocity dispersion v_rms of the WDM
particles at z=0. For fermions decoupling while relativistic, these limits are
equivalent to constraints on the mass m_X of the particles. The presence of a 4
billion solar mass black hole at z=5.8, believed to power the quasar SDSS
1044-1215, implies m_X > 0.5 keV (or v_rms < 0.10 km/s), assuming that the
quasar is unlensed and radiating at or below the Eddington limit. Reionization
by z=5.8 also implies a limit on m_X. If high-redshift galaxies produce
ionizing photons with an efficiency similar to their z=3 counterparts, we find
m_X > 1.2 keV (or v_rms < 0.03 km/s). However, given the uncertainties in
current measurements from the proximity effect of the ionizing background at
redshift 3, values of m_X as low as 0.75 keV (v_rms = 0.06 km/s) are not ruled
out. The limit weakens further if, instead, the ionizing-photon production
efficiency is greater at high z, but this limit will tighten considerably if
reionization is shown in the future to have occurred at higher redshifts. WDM
models with m_X 0.04 km/s) produce a low-luminosity cutoff in
the high-redshift galaxy luminosity function which is directly detectable with
the Next Generation Space Telescope (abridged).Comment: 38 pages, 10 figures, to appear in ApJ. One figure added, some
discussion revise
The redshift-dependence of gamma-ray absorption in the environments of strong-line AGN
The case of gamma-ray absorption due to photon-photon pair production of jet
photons in the external photon environment like accretion disk and broad-line
region radiation field of gamma-ray loud active galactic nuclei (AGN) that
exhibit strong emission lines is considered. I demonstrate that this ''local
opacity'', if detected, will almost unavoidably be redshift-dependent in the
sub-TeV range. This introduces non-negligible biases, and complicates
approaches for studying the evolution of the extragalactic background light
with contemporary GeV instruments like e.g. the Gamma-ray Large Area Space
Telescope (GLAST), etc., where the gamma-ray horizon is probed by means of
statistical analysis of absorption features (e.g. Fazio-Stecker relation, etc.)
in AGN spectra at various redshifts. It particularly applies to strong-line
quasars where external photon fields are potentially involved in gamma-ray
production.Comment: 19 pages, 5 figures; accepted for publication in Ap
Hierarchical build-up of galactic bulges and the merging rate of supermassive binary black holes
The hierarchical build-up of galactic bulges should lead to the build-up of
present-day supermassive black holes by a mixture of gas accretion and merging
of supermassive black holes. The tight relation between black hole mass and
stellar velocity dispersion is thereby a strong argument that the supermassive
black holes in merging galactic bulges do indeed merge. Otherwise the ejection
of supermassive black holes by gravitational slingshot would lead to excessive
scatter in this relation. At high redshift the coalescence of massive black
hole binaries is likely to be driven by the accretion of gas in the major
mergers signposted by optically bright QSO activity. If massive black holes
only form efficiently by direct collapse of gas in deep galactic potential
wells with v_c > 100 km/s as postulated in the model of Kauffmann & Haehnelt
(2000) LISA expects to see event rates from the merging of massive binary black
holes of about 0.1-1 yr^{-1} spread over the redshift range 0 < z < 5. If,
however, the hierarchical build-up of supermassive black holes extends to
pre-galactic structures with significantly shallower potential wells event
rates may be as high as 10-100 yr^{-1} and will be dominated by events from
redshift z > 5.Comment: 8 pages, 4 postscript figures. Proceedings of the 4th International
LISA Symposium, Penn State University, 19-24 July 2002, ed. L S Fin
Quasars and Ultraluminous Infrared Galaxies: At the Limit?
We have detected the host galaxies of 16 nearby, radio-quiet quasars using
images obtained with the Near-Infrared Camera and Multi-Object Spectrometer
(NICMOS). We confirm that these luminous quasars tend to live in luminous,
early-type host galaxies, and we use the host-galaxy magnitudes to refine the
luminosity/host-mass limit inferred from ground-based studies. If quasars obey
the relation found for massive dark
objects in nonactive galaxies, then our analysis implies that they radiate at
up to of the Eddington rate. An analogous analysis for ultraluminous
infrared galaxies shows them to accrete at up to similar Eddington fractions,
consistent with the hypothesis that some of them are powered by embedded
quasars.Comment: 9 pages, includes 2 eps figs, accepted to ApJLet
Measuring Feedback Using the Intergalactic Medium State and Evolution Inferred from the Soft X-ray Background
We explore the intergalactic medium (IGM) as a potential source of the
unresolved soft X-ray background (XRB) and the feasibility to extract the IGM
state and evolution from XRB observations. We build two analytical models, the
continuum field model and the halo model, to calculate the IGM XRB mean flux,
angular auto correlation and cross correlation with galaxies. Our results
suggest that the IGM may contribute a significant fraction to the unresolved
soft XRB flux and correlations. We calibrated non-Gaussian errors estimated
against our moving mesh hydro simulation and estimate that the ROSAT
all sky survey plus Sloan galaxy photometric redshift survey would allow a
accuracy in the IGM XRB-galaxy cross correlation power spectrum
measurement for and a accuracy in the redshift resolved
X-ray emissivity-galaxy cross correlation power spectrum measurement for
. At small scales, non-gravitational heating, e.g. feedback,
dominates over gravity and leaves unique signatures in the IGM XRB, which
allows a comparable accuracy in the measurement of the amount of
non-gravitational heating and the length scales where non-gravitational energy
balances gravity.Comment: 17 pages, 5 figures. Will appear on ApJ May issu
The X-ray surface brightness distribution from diffuse gas
We use simulations to predict the X-ray surface brightness distribution
arising from hot, cosmologically distributed diffuse gas. The distribution is
computed for two bands: 0.5-2 keV and 0.1-0.4 keV, using a
cosmological-constant dominated cosmology that fits many other observations. We
examine a number of numerical issues such as resolution, simulation volume and
pixel size and show that the predicted mean background is sensitive to
resolution such that higher resolution systematically increases the mean
predicted background. Although this means that we can compute only lower bounds
to the predicted level, these bounds are already quite restrictive. Since the
observed extra-galactic X-ray background is mostly accounted for by compact
sources, the amount of the observed background attributable to diffuse gas is
tightly constrained. We show that without physical processes in addition to
those included in the simulations (such as radiative cooling or
non-gravitational heating), both bands exceed observational limits. In order to
examine the effect of non-gravitational heating we explore a simple modeling of
energy injection and show that substantial amounts of heating are required
(i.e. 5 keV per particle when averaged over all baryons). Finally, we also
compute the distribution of surface brightness on the sky and show that it has
a well-resolved characteristic shape. This shape is substantially modified by
non-gravitational heating and can be used as a probe of such energy injection.Comment: 11 pages, 11 figures, submitted to Ap
- âŠ