3,335 research outputs found
Pre-Heated Isentropic Gas in Groups of Galaxies
We confirm that the standard assumption of isothermal, shock-heated gas in
cluster potentials is unable to reproduce the observed X-ray luminosity-
temperature relation of groups of galaxies. As an alternative, we construct a
physically motivated model for the adiabatic collapse of pre-heated gas into an
isothermal potential that improves upon the original work of Kaiser (1991). The
luminosity and temperature of the gas is calculated, assuming an appropriate
distribution of halo formation times and radiation due to both bremsstrahlung
and recombination processes. This model successfully reproduces the slope and
dispersion of the luminosity-temperature relation of galaxy groups. We also
present calculations of the temperature and luminosity functions for galaxy
groups under the prescription of this model. This model makes two strong
predictions for haloes with total masses M<10^13 M_sun, which are not yet
testable with current data: (1) the gas mass fraction will increase in direct
proportion to the halo mass; (2) the gas temperature will be larger than the
virial temperature of the mass. The second effect is strong enough that group
masses determined from gas temperatures will be overestimated by about an order
of magnitude if it is assumed that the gas temperature is the virial
temperature. The entropy required to match observations can be obtained by
heating the gas at the turnaround time, for example, to about 3 X 10^6 K at
z=1, which is too high to be generated by a normal rate of supernova
explosions. This model breaks down on the scale of low mass clusters, but this
is an acceptable limitation, as we expect accretion shocks to contribute
significantly to the entropy of the gas in such objects.Comment: Final, refereed version, accepted by MNRAS. One new figure and
several clarifying statements have been added. Uses mn.a4.sty (hacked
mn.sty). Also available from
http://astrowww.phys.uvic.ca/~balogh/entropy.ps.g
Star formation activity of intermediate redshift cluster galaxies out to the infall regions
We present a spectroscopic analysis of two galaxy clusters out to ~4Mpc at
z~0.2. The two clusters VMF73 and VMF74 identified by Vikhlinin et al. (1998)
were observed with MOSCA at the Calar Alto 3.5m telescope. Both clusters lie in
the ROSAT PSPC field R285 and were selected from the X-ray Dark Cluster Survey
(Gilbank et al. 2004) that provides optical V- and I-band data. VMF73 and VMF74
are located at respective redshifts of z=0.25 and z=0.18 with velocity
dispersions of 671 km/s and 442 km/s, respectively. The spectroscopic
observations reach out to ~2.5 virial radii. Line strength measurements of the
emission lines H_alpha and [OII]3727 are used to assess the star formation
activity of cluster galaxies which show radial and density dependences. The
mean and median of both line strength distributions as well as the fraction of
star forming galaxies increase with increasing clustercentric distance and
decreasing local galaxy density. Except for two galaxies with strong H_alpha
and [OII] emission, all of the cluster galaxies are normal star forming or
passive galaxies. Our results are consistent with other studies that show the
truncation in star formation occurs far from the cluster centre.Comment: 15 pages, 12 figures. A&A in pres
Magnetic and Transport Properties of Fe-Ag granular multilayers
Results of magnetization, magnetotransport and Mossbauer spectroscopy
measurements of sequentially evaporated Fe-Ag granular composites are
presented. The strong magnetic scattering of the conduction electrons is
reflected in the sublinear temperature dependence of the resistance and in the
large negative magnetoresistance. The simultaneous analysis of the magnetic
properties and the transport behavior suggests a bimodal grain size
distribution. A detailed quantitative description of the unusual features
observed in the transport properties is given
Magnetopause current as seen by Cluster
The four-spacecraft, magnetic field measurements on Cluster can be combined to produce an accurate determination of the electric current in the magnetopause boundary during stable magnetopause crossings. For events that are planar on the scale of the spacecraft configuration, the thickness of the current layer can be accurately estimated from its magnetic profile at each spacecraft and the corresponding boundary crossing times. The latter, give a determination of boundary motion relative to the Cluster array. We use the estimates of all these properties, for a range of spacecraft separation distances, to show, firstly, that the estimate of electric current density is representative even when the spatial scale of the configuration of Cluster spacecraft approaches the thickness of the current layer. Secondly, we show that the estimated current lies in the plane of the boundary and demonstrate this for crossings occurring during large-scale ripples on the magnetopause. Thirdly, we show that the magnitude of the current is accurately represented, averaged over the extent of the current layer, by comparing to the change in the boundary-parallel magnetic field component divided by the estimated current layer thickness. We demonstrate this last point using a range of crossings each having a different thickness and crossing speed, different changes in the magnetic field component and different current densities
The role of tidal interactions in driving galaxy evolution
We carry out a statistical analysis of galaxy pairs selected from chemical
hydrodynamical simulations with the aim at assessing the capability of
hierarchical scenarios to reproduce recent observational results for galaxies
in pairs. Particularly, we analyse the effects of mergers and interactions on
the star formation (SF) activity, the global mean chemical properties and the
colour distribution of interacting galaxies. We also assess the effects of
spurious pairs.Comment: to appear in "Groups of galaxies in the nearby Universe" ESO
Workshop, (Dec 2005) Santiago, Chil
Wavelet analysis of magnetic turbulence in the Earth's plasma sheet
Recent studies provide evidence for the multi-scale nature of magnetic
turbulence in the plasma sheet. Wavelet methods represent modern time series
analysis techniques suitable for the description of statistical characteristics
of multi-scale turbulence. Cluster FGM (fluxgate magnetometer) magnetic field
high-resolution (~67 Hz) measurements are studied during an interval in which
the spacecraft are in the plasma sheet. As Cluster passes through different
plasma regions, physical processes exhibit non-steady properties on
magnetohydrodynamic (MHD) and small, possibly kinetic scales. As a consequence,
the implementation of wavelet-based techniques becomes complicated due to the
statistically transitory properties of magnetic fluctuations and finite size
effects. Using a supervised multi-scale technique which allows existence test
of moments, the robustness of higher-order statistics is investigated. On this
basis the properties of magnetic turbulence are investigated for changing
thickness of the plasma sheet.Comment: 17 pages, 5 figure
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