33 research outputs found
Deep Chandra observations of NGC 1404 : cluster plasma physics revealed by an infalling early-type galaxy
The intracluster medium (ICM), as a magnetized and highly ionized fluid, provides an ideal laboratory to study plasma physics under extreme conditions that cannot yet be achieved on Earth. NGC 1404 is a bright elliptical galaxy that is being gas stripped as it falls through the ICM of the Fornax Cluster. We use the new {\sl Chandra} X-ray observations of NGC 1404 to study ICM microphysics. The interstellar medium (ISM) of NGC 1404 is characterized by a sharp leading edge, 8 kpc from the galaxy center, and a short downstream gaseous tail. Contact discontinuities are resolved on unprecedented spatial scales (0\farcs5=45\,pc) due to the combination of the proximity of NGC 1404, the superb spatial resolution of {\sl Chandra}, and the very deep (670 ksec) exposure. At the leading edge, we observe sub-kpc scale eddies generated by Kelvin-Helmholtz instability and put an upper limit of 5\% Spitzer on the isotropic viscosity of the hot cluster plasma. We also observe mixing between the hot cluster gas and the cooler galaxy gas in the downstream stripped tail, which provides further evidence of a low viscosity plasma. The assumed ordered magnetic fields in the ICM ought to be smaller than 5\,ÎŒG to allow KHI to develop. The lack of evident magnetic draping layer just outside the contact edge is consistent with such an upper limit
Capturing the 3D motion of an infalling galaxy via fluid dynamics
The Fornax Cluster is the nearest galaxy cluster in the southern sky. NGC 1404 is a bright elliptical galaxy falling through the intracluster medium of the Fornax Cluster. The sharp leading edge of NGC 1404 forms a classical "cold front" that separates 0.6 keV dense interstellar medium and 1.5 keV diffuse intracluster medium. We measure the angular pressure variation along the cold front using a very deep (670\,ksec) {\sl Chandra} X-ray observation. We are taking the classical approach -- using stagnation pressure to determine a substructure's speed -- to the next level by not only deriving a general speed but also directionality which yields the complete velocity field as well as the distance of the substructure directly from the pressure distribution. We find a hydrodynamic model consistent with the pressure jump along NGC 1404's atmosphere measured in multiple directions. The best-fit model gives an inclination of 33â and a Mach number of 1.3 for the infall of NGC 1404, in agreement with complementary measurements of the motion of NGC 1404. Our study demonstrates the successful treatment of a highly ionized ICM as ideal fluid flow, in support of the hypothesis that magnetic pressure is not dynamically important over most of the virial region of galaxy clusters
Chandra Observations of NGC 4438: An Environmentally Damaged Galaxy in the Virgo Cluster
We present results from a 25 ksec CHANDRA ACIS-S observation of galaxies
NGC4438 and NGC4435 in the Virgo Cluster. X-ray emission in NGC4438 is observed
in a ~700 pc nuclear region, a 2.3 kpc spherical bulge, and a network of
filaments extending 4-10 kpc to the W and SW of the galaxy. The X-ray emission
in all 3 regions is highly correlated to similar features observed in Halpha.
Spectra of the filaments and bulge are well represented by a 0.4 keV MEKAL
model with combined 0.3-2 keV intrinsic luminosity of 1.24x10^{40}erg/s,
electron densities ~ 0.02-0.04 cm^{-3}, cooling times of 400-700 Myr and X-ray
gas mass <~ 3.7x10^8 Msolar. In the nuclear region of NGC4438 X-ray emission is
seen from the nucleus and from two outflow bubbles extending 360(730) pc to the
NW(SE) of the nucleus. The spectrum of the NW outflow bubble plus nucleus is
well fitted by an absorbed (n_H=1.9x10^{21} cm^{-2}) 0.58 keV MEKAL plasma
model plus a heavily absorbed (n_H = 2.9 x10^{22} cm^{-2}) Gamma = 2, power law
component. The electron density, cooling time, and X-ray gas mass in the NW
outflow are ~0.5 cm^{-3}, 30 Myr and 3.5x10^6 Msolar. Weak X-ray emission is
observed in the central region of NGC4435 with the peak of the hard emission
coincident with the galaxy's optical center; while the peak of the soft X-ray
emission is displaced 316 pc to the NE. The spectrum of NGC 4435 is well fitted
by a non-thermal power law plus a thermal component from 0.2-0.3 keV diffuse
ISM gas. We argue that the X-ray properties of gas outside the nuclear region
in NGC4438 and in NGC4435 favor a high velocity, off-center collision between
these galaxies ~ 100 Myr ago; while the nuclear X-ray emitting outflow gas in
NGC4438 has been heated only recently (within ~ 1-2 Myr) by shocks (v_s ~ 600
kms^{-1}) possibly powered by a central AGN.Comment: 40 pages, 7 figures; minor changes to conform to published version,
improved spectral fits to NGC 4435, improved figures 3,5; new figures 6b,
Chandra Observations of Galaxy Cluster Abell 2218
We present results from two observations (combined exposure of ~17 ks) of
galaxy cluster A2218 using the Advanced CCD Imaging Spectrometer on board the
Chandra X-ray Observatory that were taken on October 19, 1999. Using a
Raymond-Smith single temperature plasma model corrected for galactic absorption
we find a mean cluster temperature of kT = 6.9+/-0.5 keV, metallicity of
0.20+/-0.13 (errors are 90 % CL) and rest-frame luminosity in the 2-10 keV
energy band of 6.2x10^{44} erg/s in a LambdaCDM cosmology with H_0=65 km/s/Mpc.
The brightness distribution within 4'.2 of the cluster center is well fit by a
simple spherical beta model with core radius 66".4 and beta = 0.705 . High
resolution Chandra data of the inner 2' of the cluster show the x-ray
brightness centroid displaced ~22" from the dominant cD galaxy and the presence
of azimuthally asymmetric temperature variations along the direction of the
cluster mass elongation. X-ray and weak lensing mass estimates are in good
agreement for the outer parts (r > 200h^{-1}) of the cluster; however, in the
core the observed temperature distribution cannot reconcile the x-ray and
strong lensing mass estimates in any model in which the intracluster gas is in
thermal hydrostatic equilibrium. Our x-ray data are consistent with a scenario
in which recent merger activity in A2218 has produced both significant
non-thermal pressure in the core and substructure along the line of sight; each
of these phenomena probably contributes to the difference between lensing and
x-ray core mass estimates.Comment: 33 pages, 6 figures, uses AASTeX 5.02, ApJ submitte
Hydrodynamical Simulations of the Lyman Alpha Forest: Model Comparisons
We investigate the properties of the Lyman alpha forest as predicted by
numerical simulations for a range of currently viable cosmological models. This
is done in order to understand the dependencies of the forest on cosmological
parameters. Focusing on the redshift range from two to four, we show that: (1)
most of the evolution in the distributions of optical depth, flux and column
density can be understood by simple scaling relations, (2) the shape of optical
depth distribution is a sensitive probe of the amplitude of density
fluctuations on scales of a few hundred kpc, (3) the mean of the b distribution
(a measure of the width of the absorption lines) is also very sensitive to
fluctuations on these scales, and decreases as they increase. We perform a
preliminary comparison to observations, where available. A number of other
properties are also examined, including the evolution in the number of lines,
the two-point flux distribution and the HeII opacity.Comment: 37 pages, 21 figures, submitted to Ap