345 research outputs found
A Cold Front in A3667: Hydrodynamics and Magnetic Field in the Intracluster Medium
This conference presentation discusses a Chandra observation of the cold
front in Abell 3667. We first review our earlier results which include a
measurement of the front velocity, M~1, using the ratio of exterior and
interior gas pressures; observations of the hydrodynamic effects expected for a
transonic front motion (weak bow shock and gas compression near the leading
edge of the front); direct observation of the suppressed diffusion across the
front, and estimate of the magnetic field strength near the front from
suppression of the Kelvin-Helmholtz instabilities.
The new results include using the 2-dimensional brightness distribution
inside the cold front (a) to show that the front is stable and (b) to map the
mass distribution in the gas cloud. This analysis confirms the existence of a
dark matter subcluster traveling with the front. We also fix an algebraic error
in our published calculations for the growth rate of the KH instability and
discuss an additional effect which could stabilize the front against the
small-scale perturbations. These updates only strengthen our conclusions
regarding the importance of the magnetic fields for the front dynamics.Comment: Shortened version of the paper published in Astronomy Letters; based
on talk at conference "High Energy Astrophysics 2001", Moscow, Dec 200
A moving cold front in the intergalactic medium of A3667
We present results from a Chandra observation of the central region of the
galaxy cluster A3667, with emphasis on the prominent sharp X-ray brightness
edge spanning 0.5 Mpc near the cluster core. Our temperature map shows
large-scale nonuniformities characteristic of the ongoing merger, in agreement
with earlier ASCA results. The brightness edge turns out to be a boundary of a
large cool gas cloud moving through the hot ambient gas, very similar to the
"cold fronts" discovered by Chandra in A2142. The higher quality of the A3667
data allows the direct determination of the cloud velocity. At the leading edge
of the cloud, the gas density abruptly increases by a factor of 3.9+-0.8, while
the temperature decreases by a factor of 1.9+-0.2 (from 7.7 keV to 4.1 keV).
The ratio of the gas pressures inside and outside the front shows that the
cloud moves through the ambient gas at near-sonic velocity, M=1+-0.2 or
v=1400+-300 km/s. In front of the cloud, we observe the compression of the
ambient gas with an amplitude expected for such a velocity. A smaller surface
brightness discontinuity is observed further ahead, ~350 kpc in front of the
cloud. We suggest that it corresponds to a weak bow shock, implying that the
cloud velocity may be slightly supersonic. Given all the evidence, the cold
front appears to delineate the remnant of a cool subcluster that recently has
merged with A3667. The cold front is remarkably sharp. The upper limit on its
width, 3.5 arcsec or 5 kpc, is several times smaller than the Coulomb mean free
path. This is a direct observation of suppression of the transport processes in
the intergalactic medium, most likely by magnetic fields.Comment: Submitted to ApJ. 9 pages with embedded color figures, uses
emulateapj5. Postscript with higher quality figures is available at
http://hea-www.harvard.edu/~alexey/a3667-hydro.ps.g
Chandra estimate of the magnetic field strength near the cold front in A3667
We use the Chandra observation of the cold front in the intracluster gas of
A3667 to estimate the magnetic field strength near the front. The front is seen
in the Chandra data as a sharp discontinuity in the gas density which
delineates a large body of dense cool gas moving with the near-sonic velocity
through the less dense, hotter gas. Without a magnetic field, the front should
be quickly disturbed by the Kelvin-Helmholtz instability arising from
tangential motion of gas layers. However, Chandra image shows that the front is
stable within a +-30deg sector in the direction of the cloud motion, beyond
which it gradually disappears. We suggest that the Kelvin-Helmholtz instability
within the +-30deg sector is suppressed by surface tension of the magnetic
field whose field lines are parallel to the front. The required field strength
is B ~ 10 muG. Magnetic field near the front is expected to be stronger and
have very different structure compared to the bulk of the intergalactic medium,
because the field lines are stretched by the tangential gas motions. Such a
magnetic configuration, once formed, would effectively stop the plasma
diffusion and heat conduction across the front, and may inhibit gas mixing
during the subcluster merger. We note that even the increased magnetic field
near the front contributes only 10-20% to the total gas pressure, and therefore
magnetic pressure is unimportant for hydrostatic cluster mass estimates.Comment: Submitted to ApJ Letters. 4 pages with embedded color figures, uses
emulateapj5. Postscript with higher quality figures is available at
http://hea-www.harvard.edu/~alexey/a3667-mag.ps.g
A Differential X-Ray Gunn-Peterson Test Using a Giant Cluster Filament
Using CCD detectors onboard the forthcoming X-ray observatories Chandra and
XMM, it is possible to devise a measurement of the absolute density of heavy
elements in the hypothetical warm gas filling intercluster space. This gas may
be the largest reservoir of baryonic matter in the Universe, but even its
existence has not been proven observationally at low redshifts. The proposed
measurement would make use of a unique filament of galaxy clusters spanning
over 700 Mpc (0.1<z<0.2) along the line of sight in a small area of the sky in
Aquarius. The surface density of Abell clusters there is more than 6 times the
sky average. It is likely that the intercluster matter column density is
enhanced by a similar factor, making its detection feasible under certain
optimistic assumptions about its density and elemental abundances. One can
compare photoabsorption depth, mostly in the partially ionized oxygen edges, in
the spectra of clusters at different distances along the filament, looking for
a systematic increase of depth with the distance. The absorption can be
measured by the same detector and through the same Galactic column, hence the
differential test. A CCD moderate energy resolution (about 100 eV) is adequate
for detecting an absorption edge at a known redshift.Comment: Latex, 4 pages, 3 figures, uses emulateapj.sty. ApJ Letters in pres
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