103 research outputs found
Investigating the origins of the CMB-XRB cross correlation
Recently, we presented evidence for a cross-correlation of the WMAP satellite
map of the cosmic microwave background (CMB) and the HEAO1 satellite map of the
hard X-ray background (XRB) with a dimensionless amplitude of 0.14 +- 0.05
normalized to the product of the rms fluctuations of the CMB and XRB (Boughn &
Crittenden, 2004). Such a correlation is expected in a universe dominated by a
cosmological constant via the integrated Sachs-Wolfe (ISW) effect and the level
of the correlation observed is consistent with that predicted by the currently
favored Lambda cold dark matter model of the universe. Since this offers
independent confirmation of the cosmological model, it is important to verify
the origin of the correlation. Here we explore in detail some possible
foreground sources of the correlation. The present evidence all supports an ISW
origin.Comment: 8 pages, 7 eps figure
On Radio and X-ray Emission Mechanisms in Nearby, X-ray Bright Galactic Nuclei
It has been suggested that advection-dominated accretion flows (ADAFs) are responsible for the X-ray activity in nearby galactic nuclei. These X-ray bright galactic nuclei (XBGN) are a heterogeneous group which includes LINERs, low to moderate luminosity Seyferts, and narrow-line X-ray galaxies with 2-10 keV X-ray luminosities in the range ~10**39 to ~10**43 erg/s. In the absence of a radio jet, the core 15 GHz radio luminosity of an ADAF is relatively low and roughly proportional to the mass of the central black hole. The predicted radio luminosity depends primarily on the black hole mass and for XBGN typically falls in the range 10**35-10**39 erg/s. We designate these as ``radio quiet'' XBGN. However, some level of jet activity seems to be present in most sources and the radio emission can be considerably larger than that from the ADAF core. We discuss connections between radio-bright XBGN and radio-loud, powerful active galactic nuclei (AGN) and suggest that the radio activities are directly correlated with black hole spins in both cases. Even in the presence of a radio jet, high resolution, high frequency radio imaging of nearby XBGN could identify compact, inverted spectrum ADAF radio sources. The unique radio/X-ray luminosity relation is confirmed in a few cases where black hole masses are known and could be used as a tool to estimate unknown black hole masses. For radio-dim (L_10**43 erg/s) sources, which are primarily Seyferts, the X-ray emission mechanism is not accounted for by pure ADAFs and radio activities are likely to be similar to those of the radio-quiet AGN
The Large-Scale Structure of the X-ray Background and its Cosmological Implications
A careful analysis of the HEAO1 A2 2-10 keV full-sky map of the X-ray
background (XRB) reveals clustering on the scale of several degrees. After
removing the contribution due to beam smearing, the intrinsic clustering of the
background is found to be consistent with an auto-correlation function of the
form (3.6 +- 0.9) x 10^{-4} theta^{-1} where theta is measured in degrees. If
current AGN models of the hard XRB are reasonable and the cosmological
constant-cold dark matter cosmology is correct, this clustering implies an
X-ray bias factor of b_X ~ 2. Combined with the absence of a correlation
between the XRB and the cosmic microwave background, this clustering can be
used to limit the presence of an integrated Sachs-Wolfe (ISW) effect and
thereby to constrain the value of the cosmological constant, Omega_Lambda <
0.60 (95 % C.L.). This constraint is inconsistent with much of the parameter
space currently favored by other observations. Finally, we marginally detect
the dipole moment of the diffuse XRB and find it to be consistent with the
dipole due to our motion with respect to the mean rest frame of the XRB. The
limit on the amplitude of any intrinsic dipole is delta I / I < 5 x 10^{-3} at
the 95 % C.L. When compared to the local bulk velocity, this limit implies a
constraint on the matter density of the universe of Omega_m^{0.6}/b_X(0) >
0.24.Comment: 15 pages, 8 postscript figures, to appear in the Astrophysical
Journal. The postscript version appears not to print, so use the PDF versio
RMS Radio Source Contributions to the Microwave Sky
Cross-correlations of the WMAP full sky K, Ka, Q, V, and W band maps with the
1.4 GHz NVSS source count map and the HEAO I A2 2-10 keV full sky X-ray flux
map are used to constrain rms fluctuations due to unresolved microwave sources
in the WMAP frequency range. In the Q band (40.7 GHz), a lower limit, taking
account of only those fluctuations correlated with the 1.4 GHz radio source
counts and X-ray flux, corresponds to an rms Rayleigh-Jeans temperature of ~ 2
microKelvin for a solid angle of one square degree. The correlated fluctuations
at the other bands are consistent with a beta = -2.1 +- 0.4 frequency spectrum.
Using the rms fluctuations of the X-ray flux and radio source counts, and the
cross-correlation of these two quantities as a guide, the above lower limit
leads to a plausible estimate of ~ 5 microKelvin for Q-band rms fluctuations in
one square degree. This value is similar to that implied by the excess, small
angular scale fluctuations observed in the Q band by WMAP, and is consistent
with estimates made by extrapolating low-frquency source counts.Comment: 17 pages, 8 figures, submitted to Ap
The large-scale bias of the hard X-ray background
Recent deep X-ray surveys combined with spectroscopic identification of the
sources have allowed the determination of the rest-frame 2-8 keV luminosity as
a function of redshift. In addition, an analysis of the HEAO1 A2 2-10 keV
full-sky map of the X-ray background (XRB) reveals clustering on the scale of
several degrees. Combining these two results in the context of the currently
favored Lambda-CDM cosmological model implies an average X-ray bias factor,
b_x, of b_x^2 = 1.12 +- 0.33, i.e., b_x = 1.06 +- 0.16. These error estimates
include only statistical error; the systematic error sources, while comparable,
appear to be sub-dominant. This result is in contrast to the large biases of
some previous estimates and is more in line with current estimates of the
optical bias of L* galaxies.Comment: 6 pages, 3 eps figures, accepted for ApJ, vol. 612, 10 September 200
Signatures of Galaxy-Cluster Interactions: Tully-Fisher Observations at z~0.1
We have obtained new optical imaging and spectroscopic observations of 78
galaxies in the fields of the rich clusters Abell 1413 (z = 0.14), Abell 2218
(z = 0.18) and Abell 2670 (z = 0.08). We have detected line emission from 25
cluster galaxies plus an additional six galaxies in the foreground and
background, a much lower success rate than what was found (65%) for a sample of
52 lower-richness Abell clusters in the range 0.02 < z < 0.08. We have combined
these data with our previous observations of Abell 2029 and Abell 2295 (both at
z = 0.08), which yields a sample of 156 galaxies. We evaluate several
parameters as a function of cluster environment: Tully-Fisher residuals,
H-alpha equivalent width, and rotation curve asymmetry, shape and extent.
Although H-alpha is more easily detectable in galaxies that are located further
from the cluster cores, we fail to detect a correlation between H-alpha extent
and galaxy location in those where it is detected, again in contrast with what
is found in the clusters of lesser richness. We fail to detect any
statistically significant trends for the other parameters in this study. The
zero-point in the z~0.1 Tully-Fisher relation is marginally fainter (by 1.5
sigma) than that found in nearby clusters, but the scatter is essentially
unchanged.Comment: 27 pages including 5 figures; accepted for publication in the
Astronomical Journa
Cross-Correlation of the Cosmic Microwave Background with Radio Sources: Constraints on an Accelerating Universe
We present a new limit on the cosmological constant based on the absence of
correlations between the cosmic microwave background (CMB) and the distribution
of distant radio sources. In the cosmological constant-cold dark matter models
currently favored, such correlations should have been produced via the
integrated Sachs-Wolfe effect, assuming that radio sources trace the local
(z=1) matter density. We find no evidence of correlations between the COBE 53Hz
microwave map and the NVSS 1.4 GHz radio survey. The implied 95% CL limit on
the cosmological constant is Lambda < 0.74, in marginal agreement with the
values suggested by recent measurements of the CMB anisotropy and type-IA
supernovae observations, 0.6 < Lambda < 0.7. If the cosmological model does lie
in this range, then the integrated Sachs-Wolfe effect should be detectable with
upcoming CMB maps and radio surveys.Comment: 5 pages; 3 figures; submitted to PR
Evidence for Inverted Spectrum 20 GHz Emission in the Galactic Plane
A comparison of a 19 GHz full-sky map with the WMAP satellite K band (23 GHz)
map indicates that the bulk of the 20 GHz emission within 7 degrees of the
Galactic plane has an inverted (rising) spectrum with an average spectral index
alpha = 0.21 +/- 0.05. While such a spectrum is inconsistent with steep
spectrum synchrotron (alpha ~ -0.7) and flat spectrum free-free (alpha ~ -0.1)
emission, it is consistent with various models of electric dipole emission from
thermally excited spinning dust grains as well as models of magnetic dipole
emission from ferromagnetic dust grains. Several regions in the plane, e.g.,
near the Cygnus arm, have spectra with even larger alpha. While low signal to
noise of the 19 GHz data precludes a detailed map of spectral index, especially
off the Galactic plane, it appears that the bulk of the emission in the plane
is correlated with the morphology of dust. Regions with higher 23 GHz flux tend
to have harder spectra. Off the plane, at Galactic latitudes between 7 and 20
degree the spectrum steepens to alpha = -0.16 +/- 0.15.Comment: 11 page, 3 figure
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