300 research outputs found
On Weyl group equivariant maps
We prove an equivariant analogue of Chevalley's isomorphism theorem for polynomial, C(infinity) or C(omega) maps
A Fatou- type theorem for harmonic functions on symmetric spaces
First published in the Bulletin of the American Mathematical Society in Vol.74, 1968, published by the American Mathematical Societ
Chandra and XMM-Newton X-ray observations of AWM 7 - I: Investigating X-ray surface brightness fluctuations
We investigate the levels of small scale structure in surface brightness
images of the core of the X-ray bright cool-core galaxy cluster AWM 7. After
subtraction of a model of the smooth cluster emission, we find a number of
approximately radial surface brightness depressions which are not present in
simulated images and are seen in both the Chandra and XMM-Newton data. The
depressions are most strongly seen in the south of the cluster and have a
magnitude of around 4 per cent in surface brightness. We see these features in
both an energy band sensitive to the density (0.6 to 5 keV) and a band more
sensitive to the pressure (3.5 to 7.5 keV). Histograms of surface brightness in
the data, when compared to realisations of a smooth model, reveal stronger
surface brightness variations. We use the Delta-variance technique to
characterise the magnitude of the fluctuations as a function of length scale.
We find that the spectrum in the 0.6 to 5 keV band is flatter than expected for
Kolmogorov index fluctuations. If characterised by a power spectrum, on large
scales it would have an index around -1.7, rather than -3.7. The implied 3D
density fluctuations have a standard deviation of around 4 per cent. The
implied 3D pressure variations are at most 4 per cent. Most of the longer-scale
power in the density spectrum is contributed by the southern half of the
cluster, where the depressions are seen. The density variations implied by the
spectrum of the northern sector have a standard deviation of about 2 per cent.Comment: 17 pages, accepted by MNRAS, high resolution version available at
http://www-xray.ast.cam.ac.uk/papers/awm7.pd
Testing the Hubble Law with the IRAS 1.2 Jy Redshift Survey
We test and reject the claim of Segal et al. (1993) that the correlation of
redshifts and flux densities in a complete sample of IRAS galaxies favors a
quadratic redshift-distance relation over the linear Hubble law. This is done,
in effect, by treating the entire galaxy luminosity function as derived from
the 60 micron 1.2 Jy IRAS redshift survey of Fisher et al. (1995) as a distance
indicator; equivalently, we compare the flux density distribution of galaxies
as a function of redshift with predictions under different redshift-distance
cosmologies, under the assumption of a universal luminosity function. This
method does not assume a uniform distribution of galaxies in space. We find
that this test has rather weak discriminatory power, as argued by Petrosian
(1993), and the differences between models are not as stark as one might expect
a priori. Even so, we find that the Hubble law is indeed more strongly
supported by the analysis than is the quadratic redshift-distance relation. We
identify a bias in the the Segal et al. determination of the luminosity
function, which could lead one to mistakenly favor the quadratic
redshift-distance law. We also present several complementary analyses of the
density field of the sample; the galaxy density field is found to be close to
homogeneous on large scales if the Hubble law is assumed, while this is not the
case with the quadratic redshift-distance relation.Comment: 27 pages Latex (w/figures), ApJ, in press. Uses AAS macros,
postscript also available at
http://www.astro.princeton.edu/~library/preprints/pop682.ps.g
A Photometric and Kinematic Study of AWM 7
We have measured redshifts and Kron-Cousins R-band magnitudes for a sample of
galaxies in the poor cluster AWM 7. We have measured redshifts for 172
galaxies; 106 of these are cluster members.
We determine the luminosity function from a photometric survey of the central
1.2 h^{-1} x 1.2 h^{-1} Mpc. The LF has a bump at the bright end and a
faint-end slope of \alpha = -1.37+-0.16, populated almost exclusively by
absorption-line galaxies.
The cluster velocity dispersion is lower in the core (\sim 530 km/s) than at
the outskirts (\sim 680 km/s), consistent with the cooling flow seen in the
X-ray. The cold core extends \sim 150 h^{-1} kpc from the cluster center. The
Kron-Cousins R-band mass-to-light ratio of the system is 650+-170 h
M_\odot/L_\odot, substantially lower than previous optical determinations, but
consistent with most previous X-ray determinations.
We adopt H_0 = 100 h km/s/Mpc throughout this paper; at the mean cluster
redshift, (5247+-76 km/s), 1 h^{-1} Mpc subtends 65\farcm5.Comment: 37 pages, LaTeX, including 12 Figures and 1 Table. Accepted for
publication in the Astronomical Journa
Kinematics and Mass Profile of AWM 7
We have measured 492 redshifts (311 new) in the direction of the poor cluster
AWM~7 and have identified 179 cluster members (73 new). We use two independent
methods to derive a self-consistent mass profile, under the assumptions that
the absorption-line galaxies are virialized and that they trace an underlying
Navarro, Frenk & White (1997) dark matter profile: (1) we fit such an NFW
profile to the radial distribution of galaxy positions and to the velocity
dispersion profile; (2) we apply the virial mass estimator to the cluster. With
these assumptions, the two independent mass estimates agree to \sim 15% within
1.7 h^{-1} Mpc, the radial extent of our data; we find an enclosed mass \sim
(3+-0.5)\times 10^{14} h^{-1} M_\odot. The largest potential source of
systematic error is the inclusion of young emission-line galaxies in the mass
estimate.
We investigate the behavior of the surface term correction to the virial mass
estimator under several assumptions about the velocity anisotropy profile,
still within the context of the NFW model, and remark on the sensitivity of
derived mass profiles to outliers. We find that one must have data out to a
large radius in order to determine the mass robustly, and that the surface term
correction is unreliable at small radii.Comment: LaTeX, 5 tables, 7 figures, appeared as 2000 AJ 119 44; typos and Eq.
9 corrected; results are unaffecte
The 1.4 GHz light curve of GRB 970508
We report on Westerbork 1.4 GHz radio observations of the radio counterpart
to -ray burst GRB~970508, between 0.80 and 138 days after this event.
The 1.4 GHz light curve shows a transition from optically thick to thin
emission between 39 and 54 days after the event. We derive the slope of the
spectrum of injected electrons () in two
independent ways which yield values very close to . This is in agreement
with a relativistic dynamically near-adiabatic blast wave model whose emission
is dominated by synchrotron radiation and in which a significant fraction of
the electrons cool fast.Comment: Paper I. Accepted for publication in the Astrophysical Journal
Letter
Surveying the Dynamic Radio Sky with the Long Wavelength Demonstrator Array
This paper presents a search for radio transients at a frequency of 73.8 MHz
(4 m wavelength) using the all-sky imaging capabilities of the Long Wavelength
Demonstrator Array (LWDA). The LWDA was a 16-dipole phased array telescope,
located on the site of the Very Large Array in New Mexico. The field of view of
the individual dipoles was essentially the entire sky, and the number of
dipoles was sufficiently small that a simple software correlator could be used
to make all-sky images. From 2006 October to 2007 February, we conducted an
all-sky transient search program, acquiring a total of 106 hr of data; the time
sampling varied, being 5 minutes at the start of the program and improving to 2
minutes by the end of the program. We were able to detect solar flares, and in
a special-purpose mode, radio reflections from ionized meteor trails during the
2006 Leonid meteor shower. We detected no transients originating outside of the
solar system above a flux density limit of 500 Jy, equivalent to a limit of no
more than about 10^{-2} events/yr/deg^2, having a pulse energy density >~ 1.5 x
10^{-20} J/m^2/Hz at 73.8 MHz for pulse widths of about 300 s. This event rate
is comparable to that determined from previous all-sky transient searches, but
at a lower frequency than most previous all-sky searches. We believe that the
LWDA illustrates how an all-sky imaging mode could be a useful operational
model for low-frequency instruments such as the Low Frequency Array, the Long
Wavelength Array station, the low-frequency component of the Square Kilometre
Array, and potentially the Lunar Radio Array.Comment: 20 pages; accepted for publication in A
An XMM-Newton observation of the galaxy group MKW 4
We present an X-ray study of the galaxy group or poor cluster MKW 4. Working
with XMM data we examine the distribution and properties of the hot gas which
makes up the group halo. The inner halo shows some signs of structure, with
circular or elliptical beta models providing a poor fit to the surface
brightness profile. This may be evidence of large scale motion in the inner
halo, but we do not find evidence of sharp fronts or edges in the emission. The
temperature of the halo declines in the core, with deprojected spectral fits
showing a central temperature of ~1.3 keV compared to ~3 keV at 100 kpc. In the
central ~30 kpc of the group multi-temperature spectral models are required to
fit the data, but they indicate a lack of gas at low temperatures. Steady state
cooling flow models provide poor fits to the inner regions of the group and the
estimated cooling time of the gas is long except within the central dominant
galaxy, NGC 4073. Abundance profiles show a sharp increase in the core of the
group, with mean abundance rising by a factor of two in the centre of NGC 4073.
Fitting individual elements shows the same trend, with high values of Fe, Si
and S in the core. We estimate that ~50% of the Fe in the central 40 kpc was
injected by SNIa, in agreement with previous ASCA studies. Using our best
fitting surface brightness and temperature models, we calculate the mass, gas
fraction, entropy and mass-to-light ratio of the group. At 100 kpc (~0.1 virial
radii) the total mass and gas entropy of the system (~2x10^13 Msol and ~300 keV
cm^2) are quite comparable to those of other systems of similar temperature,
but the gas fraction is rather low (~1%). We conclude that MKW 4 is a fairly
relaxed group, which has developed a strong central temperature gradient but
not a large-scale cooling flow.Comment: 17 pages, 9 postscript figures, accepted for publication in MNRA
The Dynamical Status of the Cluster of Galaxies 1E0657-56
We present the results of a new spectroscopic and photometric survey of the
hot X-ray cluster 1E0657-56, at z=0.296. We determine the presence of a low
velocity dispersion subcluster, which is offset from the main cluster position
by 0.7 Mpc and about 600 km/s. We determine the virial masses and total
luminosities of the cluster and its subcluster, and solve for the two-body
dynamical model. With additional constraints from the results of the analysis
of the cluster X-ray emission by Markevitch et al. (2001), we find that the
subcluster passed through the cluster centre about 0.15 Gyr ago. Taken at face
value the mass of the subcluster is typical of a loose group. It is however
difficult to establish the pre-merger mass of the colliding system. We provide
tentative evidence that the subcluster is in fact the remnant core of a
moderately massive cluster, stripped by the collision with 1E0657-56. The main
cluster dynamics does not seem to have suffered from this collision. On the
contrary, the cluster X-ray properties seem to have been significantly
affected. We also discuss the effect of the subcluster collision in relation to
starburst events and the cluster radio halo.Comment: Accepted for publication on A&
- …