422 research outputs found
Stripped Spiral Galaxies as Promising Targets for the Determination of the Cepheid distance to the Virgo Cluster
The measurement of precise galaxy distances by Cepheid observations out to
the distance of the Virgo cluster is important for the determination of the
Hubble constant (). The Virgo cluster is thereby often used as an
important stepping stone. The first HST measurement of the distance of a Virgo
galaxy (M100) using Cepheid variables provided a value for
km/s/Mpc (Freedman et al. 1994). This measurement was preceeded by a ground
based study of the Virgo spiral NGC4571 (Pierce et al. 1994) formally providing
km/s/Mpc. These determinations rely on the accuracy with which
the position of this observed spiral galaxy can be located with respect to the
Virgo cluster center. This uncertainty introduces a major error in the
determination of , together with the uncertainty in the adopted Virgo
infall velocity of the Local Group. Here we propose the use of spiral galaxies
which show clear signs of being stripped off their interstellar medium by the
intracluster gas of the Virgo cluster as targets for the Cepheid distance
measurements. We show that the stripping process and the knowledge of the
intracluster gas distribution from ROSAT X-ray observations allow us to locate
these galaxies with an at least three times higher precision with respect to
M87 than in the case of other spirals like M100. The X-ray observations further
imply that M87 is well centered within the intracluster gas halo of the Virgo
cluster and that M86 is associated with a group of galaxies and a larger dark
matter halo. The combination of these informations could enable us to locate
the two stripped spiral galaxies quite precisely within the Virgo cluster and
could greatly improve the determination of the Virgo cluster distance.Comment: 21 pages, Latex(aaspp.sty), including 6 figures, accepted for
publication in ApJL (shortened abstract:
The Orbital Structure of Dark Matter Halos with Gas
With the success of the Chandra and XMM missions and the maturation of
gravitational lensing techniques, powerful constraints on the orbital structure
of cluster dark matter halos are possible. I show that the X-ray emissivity and
mass of a galaxy cluster uniquely specify the anisotropy and velocity
dispersion profiles of its dark matter halo. I consider hydrostatic as well as
cooling flow scenarios, and apply the formalism to the lensing cluster
CL0024+16 and the cooling flow cluster Abell 2199. In both cases, the model
predicts a parameter-free velocity dispersion profile that is consistent with
independent optical redshift surveys of the clusters.Comment: 17 pages, 12 figures; to appear in the Astrophysical Journa
OVI, NV and CIV in the Galactic Halo: II. Velocity-Resolved Observations with Hubble and FUSE
We present a survey of NV and OVI (and where available CIV) in the Galactic
halo, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the
Hubble Space Telescope (HST) along 34 sightlines. These ions are usually
produced in nonequilibrium processes such as shocks, evaporative interfaces, or
rapidly cooling gas, and thus trace the dynamics of the interstellar medium.
Searching for global trends in integrated and velocity-resolved column density
ratios, we find large variations in most measures, with some evidence for a
systematic trend of higher ionization (lower NV/OVI column density ratio) at
larger positive line-of-sight velocities. The slopes of log[N(NV)/N(OVI)] per
unit velocity range from -0.015 to +0.005, with a mean of
-0.0032+/-0.0022(r)+/-0.0014(sys) dex/(km/s). We compare this dataset with
models of velocity-resolved high-ion signatures of several common physical
structures. The dispersion of the ratios, OVI/NV/CIV, supports the growing
belief that no single model can account for hot halo gas, and in fact some
models predict much stronger trends than are observed. It is important to
understand the signatures of different physical structures to interpret
specific lines of sight and future global surveys.Comment: ApJ in press 43 pages, 22 fig
The ROSAT North Ecliptic Pole Survey: The Optical Identifications
The X-ray data around the North Ecliptic Pole (NEP) of the ROSAT All Sky
Survey have been used to construct a contiguous area survey consisting of a
sample of 445 individual X-ray sources above a flux of ~2x10^-14 erg cm^-2 s^-1
in the 0.5-2.0 keV energy band. The NEP survey is centered at RA (2000) = 18h
00m, DEC(2000) = +66deg 33arcmin and covers a region of 80.7 sq. deg at a
moderate Galactic latitude of b = 29.8deg. Hence, the NEP survey is as deep and
covers a comparable solid angle to the ROSAT serendipitous surveys, but is also
contiguous. We have identified 99.6% of the sources and determined redshifts
for the extragalactic objects. In this paper we present the optical
identifications of the NEP catalog of X-ray sources including basic X-ray data
and properties of the sources. We also describe with some detail the optical
identification procedure. The classification of the optical counterparts to the
NEP sources is very similar to that of previous surveys, in particular the
Einstein Extended Medium Sensitivity Survey (EMSS). The main constituents of
the catalog are active galactic nuclei (~49%), either type 1 or type 2
according to the broadness of their permitted emission lines. Stellar
counterparts are the second most common identification class (~34%). Clusters
and groups of galaxies comprise 14%, and BL Lacertae objects 2%. One non-AGN
galaxy, and one planetary nebula have also been found. The NEP catalog of X-ray
sources is a homogeneous sample of astronomical objects featuring complete
optical identification.Comment: Accepted for publication in the ApJS; 33 pages including 12
postscript figures and 3 tables; uses emulateapj.sty. On-line source catalog
at http://www.eso.org/~cmullis/research/nep-catalog.htm
Investigating Heating and Cooling in the BCS & B55 Cluster Samples
We study clusters in the BCS cluster sample which are observed by Chandra and
are more distant than redshift, z>0.1. We select from this subsample the
clusters which have both a short central cooling time and a central temperature
drop, and also those with a central radio source. Six of the clusters have
clear bubbles near the centre. We calculate the heating by these bubbles and
express it as the ratio r_heat/r_cool=1.34+/-0.20. This result is used to
calculate the average size of bubbles expected in all clusters with central
radio sources. In three cases the predicted bubble sizes approximately match
the observed radio lobe dimensions.
We combine this cluster sample with the B55 sample studied in earlier work to
increase the total sample size and redshift range. This extended sample
contains 71 clusters in the redshift range 0<z<0.4. The average distance out to
which the bubbles offset the X-ray cooling in the combined sample is at least
r_heat/r_cool=0.92+/-0.11. The distribution of central cooling times for the
combined sample shows no clusters with clear bubbles and t_cool>1.2Gyr. An
investigation of the evolution of cluster parameters within the redshift range
of the combined samples does not show any clear variation with redshift.Comment: 12 pages, 9 figures, accepted for publication in MNRA
X-ray and Radio Interactions in the Cores of Cooling Flow Clusters
We present high resolution ROSAT x-ray and radio observations of three
cooling flow clusters containing steep spectrum radio sources at their cores.
All three systems exhibit strong signs of interaction between the radio plasma
and the hot intracluster medium. Two clusters, A133 and A2626, show enhanced
x-ray emission spatially coincident with the radio source whereas the third
cluster, A2052, exhibits a large region of x-ray excess surrounding much of the
radio source. Using 3-D numerical simulations, we show that a perturbed jet
propagating through a cooling flow atmosphere can give rise to amorphous radio
morphologies, particularly in the case where the jet was ``turned off'' and
allowed to age passively. In addition, the simulated x-ray surface brightness
produced both excesses and deficits as seen observationally.Comment: 25 pages, 10 figures, accepted for publication in A
A 15-Kiloparsec X-Ray Disk in the Elliptical Galaxy NGC 1700
We present Chandra observations of the young elliptical galaxy NGC 1700. The
X-ray isophotes are highly flattened between semimajor axes of 30 and 80
arcseconds, reaching a maximum ellipticity of approximately 0.65 at 60 arcsec
(15 kpc). The surface brightness profile in the spectrally soft, flattened
region is shallower than that of the starlight, indicating that the emission
comes from hot gas rather than stellar sources. The flattening is so extreme
that the gas cannot be in hydrostatic equilibrium in any plausible potential. A
likely alternative is that the gas has significant rotational support. A simple
model, representing isothermal gas distributed about a particular angular
momentum, can reproduce the X-ray morphology while staying consistent with
stellar kinematics. The specific angular momentum of the gas matches that of
the stars in the most isophotally distorted outer part of the galaxy, and its
cooling time matches the time since the last major merger. We infer that the
gas was acquired in that merger, which involved a pre-existing elliptical
galaxy with a hot ISM. The hot gas carried the angular momentum of the
encounter, and has since gradually settled into a rotationally flattened,
cooling disk.Comment: 11 pages, 2 figures, AASTeX 5.0. Accepted for publication in The
Astrophysical Journa
The relationship between the optical Halpha filaments and the X-ray emission in the core of the Perseus cluster
NGC 1275 in the centre of the Perseus cluster of galaxies, Abell 426, is
surrounded by a spectacular filamentary Halpha nebula. Deep Chandra X-ray
imaging has revealed that the brighter outer filaments are also detected in
soft X-rays. This can be due to conduction and mixing of the cold gas in the
filaments with the hot, dense intracluster medium. We show the correspondence
of the filaments in both wavebands and draw attention to the relationship of
two prominent curved NW filaments to an outer, buoyant radio bubble seen as a
hole in the X-ray image. There is a strong resemblance in the shape of the hole
and the disposition of the filaments to the behaviour of a large air bubble
rising in water. If this is a correct analogy, then the flow is laminar and the
intracluster gas around this radio source is not turbulent. We obtain a limit
on the viscosity of this gas.Comment: Accepted for publication in MNRA
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