5,061 research outputs found
A cluster expansion approach to renormalization group transformations
The renormalization group (RG) approach is largely responsible for the
considerable success which has been achieved in developing a quantitative
theory of phase transitions. This work treats the rigorous definition of the RG
map for classical Ising-type lattice systems in the infinite volume limit at
high temperature. A cluster expansion is used to justify the existence of the
partial derivatives of the renormalized interaction with respect to the
original interaction. This expansion is derived from the formal expressions,
but it is itself well-defined and convergent. Suppose in addition that the
original interaction is finite-range and translation-invariant. We will show
that the matrix of partial derivatives in this case displays an approximate
band property. This in turn gives an upper bound for the RG linearization.Comment: 13 page
HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies
It has been recently argued that the HCN J=1--0 line emission may not be an
unbiased tracer of dense molecular gas (\rm n\ga 10^4 cm^{-3}) in Luminous
Infrared Galaxies (LIRGs: ) and HCO J=1--0
may constitute a better tracer instead (Graci\'a-Carpio et al. 2006), casting
doubt into earlier claims supporting the former as a good tracer of such gas
(Gao & Solomon 2004; Wu et al. 2006). In this paper new sensitive HCN J=4--3
observations of four such galaxies are presented, revealing a surprisingly wide
excitation range for their dense gas phase that may render the J=1--0
transition from either species a poor proxy of its mass. Moreover the
well-known sensitivity of the HCO abundance on the ionization degree of the
molecular gas (an important issue omitted from the ongoing discussion about the
relative merits of HCN and HCO as dense gas tracers) may severely reduce
the HCO abundance in the star-forming and highly turbulent molecular gas
found in LIRGs, while HCN remains abundant. This may result to the decreasing
HCO/HCN J=1--0 line ratio with increasing IR luminosity found in LIRGs, and
casts doubts on the HCO rather than the HCN as a good dense molecular gas
tracer. Multi-transition observations of both molecules are needed to identify
the best such tracer, its relation to ongoing star formation, and constrain
what may be a considerable range of dense gas properties in such galaxies.Comment: 16 pages, 4 figures, Accepted for publication in the Astrophysical
Journa
Charged Rotating Black Holes on a 3-Brane
We study exact stationary and axisymmetric solutions describing charged
rotating black holes localized on a 3-brane in the Randall-Sundrum braneworld.
The charges of the black holes are considered to be of two types, the first
being an induced tidal charge that appears as an imprint of nonlocal
gravitational effects from the bulk space and the second is a usual electric
charge arising due to a Maxwell field trapped on the brane. We assume a special
ansatz for the metric on the brane taking it to be of the Kerr-Schild form and
show that the Kerr-Newman solution of ordinary general relativity in which the
electric charge is superceded by a tidal charge satisfies a closed system of
the effective gravitational field equations on the brane. It turns out that the
negative tidal charge may provide a mechanism for spinning up the black hole so
that its rotation parameter exceeds its mass. This is not allowed in the
framework of general relativity. We also find a new solution that represents a
rotating black hole on the brane carrying both charges. We show that for a
rapid enough rotation the combined influence of the rotational dynamics and the
local bulk effects of the "squared" energy momentum tensor on the brane distort
the horizon structure of the black hole in such a way that it can be thought of
as composed of non-uniformly rotating null circles with growing radii from the
equatorial plane to the poles. We finally study the geodesic motion of test
particles in the equatorial plane of a rotating black hole with tidal charge.
We show that the effects of negative tidal charge tend to increase the horizon
radius, as well as the radii of the limiting photon orbit, the innermost bound
and the innermost stable circular orbits for both direct and retrograde motions
of the particles.Comment: RevTeX 4, 33 pages, 4 figures, new references adde
Kinetic theory for nongeodesic particle motion: Selfinteracting equilibrium states and effective viscous fluid pressures
The particles of a classical relativistic gas are supposed to move under the
influence of a quasilinear (in the particle four-momenta), self-interacting
force inbetween elastic, binary collisions. This force which is completely
fixed by the equilibrium conditions of the gas, gives rise to an effective
viscous pressure on the fluid phenomenological level. Earlier results
concerning the possibility of accelerated expansion of the universe due to
cosmological particle production are reinterpreted. A phenomenon such as power
law inflation may be traced back to specific self-interacting forces keeping
the particles of a gas universe in states of generalized equilibrium.Comment: 16 pages, latex, uses ioplppt.sty. To appear in Class. Quantum Gra
Dynamics of a self-gravitating thin cosmic string
We assume that a self-gravitating thin string can be locally described by
what we shall call a smoothed cone. If we impose a specific constraint on the
model of the string, then its central line obeys the Nambu-Goto equations. If
no constraint is added, then the worldsheet of the central line is a totally
geodesic surface.Comment: 20 pages, latex, 1 figure, final versio
Late acceleration and crossing in induced gravity
We study the cosmological evolution on a brane with induced gravity within a
bulk with arbitrary matter content. We consider a Friedmann-Robertson-Walker
brane, invariantly characterized by a six-dimensional group of isometries. We
derive the effective Friedmann and Raychaudhuri equations. We show that the
Hubble expansion rate on the brane depends on the covariantly defined
integrated mass in the bulk, which determines the energy density of the
generalized dark radiation. The Friedmann equation has two branches,
distinguished by the two possible values of the parameter \ex=\pm 1. The
branch with \ex=1 is characterized by an effective cosmological constant and
accelerated expansion for low energy densities. Another remarkable feature is
that the contribution from the generalized dark radiation appears with a
negative sign. As a result, the presence of the bulk corresponds to an
effective negative energy density on the brane, without violation of the weak
energy condition. The transition from a period of domination of the matter
energy density by non-relativistic brane matter to domination by the
generalized dark radiation corresponds to a crossing of the phantom divide
.Comment: 7 pages, no figures, RevTex 4.0; (v2) new references are added, minor
corrections and expanded discussion; (v3) additional comments at the end of
section III, minor corrections and several new references are added, to match
published version in Phys. Rev.
CO Emission in Low Luminosity, HI Rich Galaxies
We present 12CO 1-0 observations of eleven low luminosity M_B > -18),
HI--rich dwarf galaxies. Only the three most metal-rich galaxies, with
12+log(O/H) ~ 8.2, are detected. Very deep CO spectra of six extremely
metal-poor systems (12+log(O/H) < 7.5) yield only low upper limits on the CO
surface brightness, I_CO < 0.1 K km/s. Three of these six have never before
been observed in a CO line, while the others now have much more stringent upper
limits. For the very low metallicity galaxy Leo A, we do not confirm a
previously reported detection in CO, and the limits are consistent with another
recent nondetection. We combine these new observations with data from the
literature to form a sample of dwarf galaxies which all have CO observations
and measured oxygen abundances. No known galaxies with 12+log(O/H) < 7.9 (Z <
0.1 solar) have been detected in CO. Most of the star-forming galaxies with
higher (12+log(O/H) > 8.1) metallicities are detected at similar or higher I_CO
surface brightnesses. The data are consistent with a strong dependence of the
I_CO/M_H_2 = X_CO conversion factor on ambient metallicity. The strikingly low
upper limits on some metal-poor galaxies lead us to predict that the conversion
factor is non-linear, increasing sharply below approximately 1/10 of the solar
metallicity (12+log(O/H) < 7.9).Comment: 25 pages, 4 figures, 3 tables. Accepted for publication in AJ Tables
replaced -- now formated for landscape orientatio
The Origin of the Dust Arch in the Halo of NGC 4631: An Expanding Superbubble?
We study the nature and the origin of the dust arch in the halo of the
edge-on galaxy NGC 4631 detected by Neininger & Dumke (1999). We present CO
observations made using the new On-The-Fly mapping mode with the FCRAO 14m
telescope, and find no evidence for CO emission associated with the dust arch.
Our examination of previously published HI data shows that if previous
assumptions about the dust temperature and gas/dust ratio are correct, then
there must be molecular gas associated with the arch, below our detection
threshold. If this is true, then the molecular mass associated with the dust
arch is between 1.5 x 10^8 M(sun)and 9.7 x 10^8 M(sun), and likely towards the
low end of the range. A consequence of this is that the maximum allowed value
for the CO-to-H_2 conversion factor is 6.5 times the Galactic value, but most
likely closer to the Galactic value. The kinematics of the HI apparently
associated with the dust arch reveal that the gas here is not part of an
expanding shell or outflow, but is instead two separate features (a tidal arm
and a plume of HI sticking out into the halo) which are seen projected together
and appear as a shell. Thus there is no connection between the dust "arch" and
the hot X-ray emitting gas that appears to surround the galaxy Wang et al.
(2001).Comment: 14 pages, including 4 figures. Accepted by A.J. for March 200
Star formation in the giant HII regions of M101
The molecular components of three giant HII regions (NGC 5461, NGC 5462, NGC
5471) in the galaxy M101 are investigated with new observations from the James
Clerk Maxwell Telescope, the NRAO 12-meter, and the Owens Valley millimeter
array. Of the three HII regions, only NGC 5461 had previously been detected in
CO emission.
We calculate preliminary values for the molecular mass of the GMCs in NGC
5461 by assuming a CO-to-H_2 factor (X factor) and then compare these values
with the virial masses. We conclude that the data in this paper demonstrate for
the first time that the value of X may decrease in regions with intense star
formation.
The molecular mass for the association of clouds in NGC 5461 is approximately
3x10^7 Mo and is accompanied by 1-2 times as much atomic mass. The observed CO
emission in NGC 5461 is an order of magnitude stronger than in NGC 5462, while
it was not possible to detect molecular gas toward NGC 5471 with the JCMT. An
even larger ratio of atomic to molecular gas in NGC 5471 was observed, which
might be attributed to efficient conversion of molecular to atomic gas.
The masses of the individual clouds in NGC 5461, which are gravitationally
bound, cover a range of (2-8) x 10^5 Mo, comparable with the masses of Galactic
giant molecular clouds (GMCs). Higher star forming efficiencies, and not
massive clouds, appear to be the prerequisite for the formation of the large
number of stars whose radiation is required to produce the giant HII regions in
M101.Comment: 32 pages, 5 figures, accepted for publication in the Astrophysical
Journa
On the dual interpretation of zero-curvature Friedmann-Robertson-Walker models
Two possible interpretations of FRW cosmologies (perfect fluid or dissipative
fluid)are considered as consecutive phases of the system. Necessary conditions
are found, for the transition from perfect fluid to dissipative regime to
occur, bringing out the conspicuous role played by a particular state of the
system (the ''critical point '').Comment: 13 pages Latex, to appear in Class.Quantum Gra
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