19 research outputs found
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
Self-Generated Magnetic Fields in Galactic Cooling Flows
Interstellar magnetic fields in elliptical galaxies are assumed to have their
origin in stellar fields that accompany normal mass loss from an evolving
population of old stars. The seed fields are amplified by interstellar
turbulence driven by stellar mass loss and supernova events. These disordered
fields are further amplified by time-dependent compression in the inward moving
galactic cooling flow and are expected to dominate near the galactic core.
Under favorable circumstances, fields similar in strength to those observed G can be generated solely from these natural
galactic processes. In general the interstellar field throughout elliptical
galaxies is determined by the outermost regions in the interstellar gas where
the turbulent dynamo process can occur. Because of the long hydrodynamic flow
times in galactic cooling flows, currently observed magnetic fields may result
from periods of intense turbulent field amplification that occurred in the
outer galaxy in the distant past. Particularly strong fields in ellipticals may
result from ancient galactic mergers or shear turbulence introduced at the
boundary between the interstellar gas and ambient cluster gas.Comment: 21 pages in AASTEX LaTeX with 2 figures; accepted by Astrophysical
Journa
The role of damped Alfven waves on magnetospheric accretion models of young stars
We examine the role of Alfven wave damping in heating the plasma in the
magnetic funnels of magnetospheric accretion models of young stars. We study
four different damping mechanisms of the Alfven waves: nonlinear, turbulent,
viscous-resistive and collisional. Two different possible origins for the
Alfven waves are discussed: 1) Alfven waves generated at the surface of the
star by the shock produced by the infalling matter; and 2) Alfven waves
generated locally in the funnel by the Kelvin-Helmholtz instability. We find
that, in general, the damping lengths are smaller than the tube length. Since
thermal conduction in the tube is not efficient, Alfven waves generated only at
the star's surface cannot heat the tube to the temperatures necessary to fit
the observations. Only for very low frequency Alfven waves ~10^{-5} the ion
cyclotron frequency, is the viscous-resistive damping length greater than the
tube length. In this case, the Alfven waves produced at the surface of the star
are able to heat the whole tube. Otherwise, local production of Alfven waves is
required to explain the observations. The turbulence level is calculated for
different frequencies for optically thin and thick media. We find that
turbulent velocities varies greatly for different damping mechanisms, reaching
\~100 km s^{-1} for the collisional damping of small frequency waves.Comment: 29 pages, 12 figures, to appear in The Astrophysical Journa
Emission-Line Properties of the Optical Filaments of NGC 1275
Extended nebular filaments are seen at optical wavelengths in NGC 1275, the
central galaxy in the Perseus cluster. The agents responsible for the
excitation of these filaments remain poorly understood. In this paper we
investigate possible mechanisms for powering the filaments, using measurements
from an extensive spectroscopic data set acquired at the Lick Observatory 3-m
Shane telescope. The results show that the filaments are in an extremely low
ionization and excitation state. The high signal-to-noise ratio of the spectra
allows us to measure or place sensitive upper limits on weak but important
diagnostic lines. We compare the observed line intensity ratios to the
predictions of various ionization models, including photoionization by an
active galactic nucleus, shock heating, stellar photoionization, and
photoionization by the intracluster medium. We also investigate possible roles
for cluster extreme-ultraviolet emission, and filtering of cluster soft X-ray
emission by an ionized screen, in the energetics of the filaments. None of
these mechanisms provides an entirely satisfactory explanation for the physical
state of the nebulae. Heating and ionization by reconnection of the
intracluster magnetic field remains a potentially viable alternative, which
merits further investigation through Faraday rotation studies.Comment: Accepted for publication in Ap
Cooling of X-ray Emitting Gas by Heat Conduction in the Center of Cooling Flow Clusters
We study the possibility that a large fraction of the gas at temperatures of
\~10^7 K in cooling flow clusters cools by heat conduction to lower
temperatures, rather than by radiative cooling. We argue that this process,
when incorporated into the so-called "moderate cooling flow model", where the
effective age of the intracluster medium is much lower than the age of the
cluster, reduces substantially the expected X-ray luminosity from gas residing
at temperatures of <10^7 K. In this model, the radiative mass cooling rate of
gas at ~10^7 K inferred from X-ray observations, which is <20 % of the mass
cooling rates cited in the past, is easily met. The heat conduction is
regulated by reconnection between the magnetic field lines in cold (~10^4 K)
clouds and the field lines in the intracluster medium. A narrow conduction
front is formed, which, despite the relatively low temperature, allows
efficient heat conduction from the hot ICM to the cold clouds. The reconnection
between the field lines in cold clouds and those in the intracluster medium
occurs only when the magnetic field in the ICM is strong enough. This occurs
only in the very inner regions of cooling flow clusters, at r~10-30 kpc. The
large ratio of the number of H\alpha photons to the number of cooling hydrogen
atoms is explained by this scenario.Comment: Updated version to be published in Astronomy and Astrophysics.
Original in "The Riddle of Cooling Flows in Galaxies and Clusters of
Galaxies", Charlottesville, VA, USA. May 31 -- June 4, 2003, Eds. Reiprich,
T. H., Kempner, J. C., and Soker, N. Website at
http://www.astro.virginia.edu/coolflow
The Origin of Primordial Magnetic Fields
(abridged) We suggest here that the large scale fields G, observed
in galaxies at both high and low redshifts by Faraday Rotation Measurements
(FRMs), have their origin in the electromagnetic fluctuations that naturally
occurred in the dense hot plasma that existed just after the QHPT. We evolve
the predicted fields to the present time. The size of the region containing a
coherent magnetic field increased due to the fusion (polymerization) of smaller
regions. Magnetic fields (MFs) over a comoving pc
region are predicted at redshift z . These fields are orders of
magnitude greater than those predicted in previous scenarios for creating
primordial magnetic fields. Line-of-sight average magnetic fields (MFs)
G, valid for FRMs, are obtained over a 1 Mpc comoving region at
the redshift z 10.
In the collapse to a galaxy (comoving size 30 kpc) at z 10, the
fields are amplified to G. This indicates that the MFs created
immediately after the QHPT, predicted by the Fluctuation-Dissipation Theorem,
could be the origin of the fields observed by FRMs in galaxies at
both high and low redshifts.Comment: 34 pages, 8 figure