57 research outputs found
Does magnetic pressure affect the ICM dynamics?
A possible discrepancy found in the determination of mass from gravitational
lensing data, and from X-rays observations, has been largely discussed in the
latest years (for instance, Miralda-Escude & Babul (1995)). Another important
discrepancy related to these data is that the dark matter is more centrally
condensed than the X-ray-emitting gas, and also with respect to the galaxy
distribution (Eyles et al. 1991). Could these discrepancies be consequence of
the standard description of the ICM, in which it is assumed hydrostatic
equilibrium maintained by thermal pressure? We follow the evolution of the ICM,
considering a term of magnetic pressure, aiming at answering the question
whether or not these discrepancies can be explained via non-thermal terms of
pressure. Our results suggest that the magnetic pressure could only affect the
dynamics of the ICM on scales as small as < 1kpc. Our models are constrained by
the observations of large and small scale fields and we are successful at
reproducing available data, for both Faraday rotation limits and inverse
Compton limits for the magnetic fields. In our calculations the radius (from
the cluster center) in which magnetic pressure reaches equipartition is smaller
than radii derived in previous works, as a consequence of the more realistic
treatment of the magnetic field geometry and the consideration of a sink term
in the cooling flow.Comment: 8 pages with 7 figures included. MNRAS accepted. Minor changes in the
section of discussions and conclusions. Also available at
http://www.iac.es/publicaciones/preprints.htm
The effects of magnetic fields in cold clouds in cooling flows
Large masses of absorbing material are inferred to exist in cooling flows in
clusters of galaxies from the excess X-ray absorption in the spectra of some
X-ray clusters. The absorbing material is probably in the form of cold clouds
pressure-confined by the surrounding, hot, X-ray emitting gas. The cold clouds
could remain relatively static until they are destroyed by evaporation or
ablation, or give rise to star formation. If the final fate of the clouds is
stars, the IMF of the stars formed over the whole cooling flow region ( kpc) should be biased to low masses, to avoid a very luminous, blue halo
for the central galaxy of the cooling flow. However, there is evidence for
bright star formation in the innermost (r < 10 kpc) regions of some cooling
flows, and, therefore, the biasing of the IMF towards low masses should not
occur or be less important at smaller radii. The consideration of magnetic
fields may shed light on these two points. If magnetic fields are present, the
magnetic critical mass should be considered, besides the Jeans mass, in
establishing a natural mass scale for star formation. When this new mass scale
is taken into account, we obtain the right variation of the biasing of the IMF
with the radius in addition to inhibition of high-mass star formation at large
radii. We also demonstrate that magnetic reconnection is a efficient than
ambipolar diffusion in removing magnetic fields in cold clouds.Comment: 9 pages, 1 figure, accepted for publication in MNRA
Lyman break galaxies as young spheroids
We investigate the nature of Lyman break galaxies (LBGs) using a
chemodynamical model for evolution of galaxies. Our models predict an early
(the first Gyr) stage of intense star formation in the evolution of massive
spheroids which could be identified to the LBGs, observed at redshift
with strong ongoing star formation. In particular, we are successful in
reproducing the properties of the LBG DSF 2237+116 C2 with a model describing a
young spheroid. The comparison of the predictions of our models with
the observations gives support to the scenario in which LBGs are the
progenitors of present-day massive spheroids, i.e. bulges of luminous early
type spirals or luminous elliptical galaxies.Comment: 8 pages, 3 figures, accepted for publication in MNRA
Premature dismissal of high-redshift elliptical galaxies
It has recently been argued that single-collapse high-redshift models for
elliptical galaxy formation can be rejected because they predict large numbers
of very red galaxies at intermediate redshifts which are not seen in deep
optical-infrared surveys. We argue, however, that this conclusion is premature
since, while much effort has been invested in refining the predictions of
hierarchical CDM models, only very simplistic models have been used to study
the evolution of galaxies in other cosmogonies (e.g. isocurvature models). We
demonstrate that the use of a more realistic multi-zone chemo-dynamical
single-collapse model, yields colours at intermediate redshifts which are much
bluer than inferred from the one-zone model, and indeed are comparable to those
predicted by hierarchical merging despite still allowing of the final
stellar mass of elliptical galaxies to be formed in the first Gyr of their
evolution. We, therefore, conclude that the one-zone model should be avoided to
predict the colours of high-redshift galaxies and that the use of realistic
multi-zone models allows the existence of ellipticals at high redshift, being
their dismissal premature.Comment: Submitted to MNRA
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