4,953 research outputs found
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
Atomic Carbon in Galaxies
We present new measurements of the ground state fine-structure line of atomic
carbon at 492 GHz in a variety of nearby external galaxies, ranging from spiral
to irregular, interacting and merging types. In comparison with CO(1-0), the
CI(1-0) intensity stays fairly comparable in the different environments, with
an average value of the ratio of the line integrated areas in Kkm/s of
CI(1-0)/CO(1-0) = 0.2 +/- 0.2. However, some variations can be found within
galaxies, or between galaxies. Relative to CO lines, CI(1-0) is weaker in
galactic nuclei, but stronger in disks, particularly outside star forming
regions. Also, in NGC 891, the CI(1-0) emission follows the dust continuum at
1.3mm extremely well along the full length of the major axis where molecular
gas is more abundant than atomic gas. Atomic carbon therefore appears to be a
good tracer of molecular gas in external galaxies, possibly more reliable than
CO. Atomic carbon can contribute significantly to the thermal budget of
interstellar gas. Cooling due to C and CO amounts typically to 2 x 10^{-5} of
the FIR continuum or 5% of the CII line. However, C and CO cooling reaches 30%
of the gas total, in Ultra Luminous InfraRed Galaxies, where CII is abnormally
faint. Together with CII/FIR, the emissivity ratio CI(1-0)/FIR can be used as a
measure of the non-ionizing UV radiation field in galaxies.Comment: 26 pages, 8 figure
Phantom energy traversable wormholes
It has been suggested that a possible candidate for the present accelerated
expansion of the Universe is ''phantom energy''. The latter possesses an
equation of state of the form , consequently violating
the null energy condition. As this is the fundamental ingredient to sustain
traversable wormholes, this cosmic fluid presents us with a natural scenario
for the existence of these exotic geometries. Due to the fact of the
accelerating Universe, macroscopic wormholes could naturally be grown from the
submicroscopic constructions that originally pervaded the quantum foam. One
could also imagine an advanced civilization mining the cosmic fluid for phantom
energy necessary to construct and sustain a traversable wormhole.
In this context, we investigate the physical properties and characteristics
of traversable wormholes constructed using the equation of state , with . We analyze specific wormhole geometries, considering
asymptotically flat spacetimes and imposing an isotropic pressure. We also
construct a thin shell around the interior wormhole solution, by imposing the
phantom energy equation of state on the surface stresses. Using the ''volume
integral quantifier'' we verify that it is theoretically possible to construct
these geometries with vanishing amounts of averaged null energy condition
violating phantom energy. Specific wormhole dimensions and the traversal
velocity and time are also deduced from the traversability conditions for a
particular wormhole geometry. These phantom energy traversable wormholes have
far-reaching physical and cosmological implications. For instance, an advanced
civilization may use these geometries to induce closed timelike curves,
consequently violating causality.Comment: 9 pages, Revtex4. V2: Considerable comments and references added, no
physics changes, now 10 pages. Accepted for publication in Physical Review
The Effect of Star Formation on Molecular Clouds in Dwarf Irregular Galaxies: IC 10 and NGC 6822
We have observed the 13CO J=2-1, 12CO J=2-1 and 12CO J=3-2 lines at a few
locations in the dwarf irregular galaxies IC 10 and NGC 6822 using the James
Clerk Maxwell Telescope. In addition, we report the first detection of the 13CO
J=3-2 transition in a Local Group galaxy. These low metallicity environments
appear to be porous to UV radiation and allow for more efficient heating of
molecular gas by nearby HII regions. The high 12CO J=3-2/J=2-1 ratio in NGC
6822 suggests that the 12CO emission is optically thin in this region. This
high line ratio is likely the result of its location inside a large HII region
with low metallicity and low gas content. In IC 10 we observe structures on a
variety of size scales that all appear to be gravitationally bound. This effect
may help explain the rather high star formation rate in IC 10.Comment: 20 pages with 6 ps figures, accepted for publication in The
Astrophysical Journa
Dynamical Masses in Luminous Infrared Galaxies
We have studied the dynamics and masses of a sample of ten nearby luminous
and ultraluminous infrared galaxies (LIRGS and ULIRGs), using 2.3 micron CO
absorption line spectroscopy and near-infrared H- and Ks-band imaging. By
combining velocity dispersions derived from the spectroscopy, disk
scale-lengths obtained from the imaging, and a set of likely model density
profiles, we calculate dynamical masses for each LIRG. For the majority of the
sample, it is difficult to reconcile our mass estimates with the large amounts
of gas derived from millimeter observations and from a standard conversion
between CO emission and H_2 mass. Our results imply that LIRGs do not have huge
amounts of molecular gas (10^10-10^11 Msolar) at their centers, and support
previous indications that the standard conversion of CO to H_2 probably
overestimates the gas masses and cannot be used in these environments. This in
turn suggests much more modest levels of extinction in the near-infrared for
LIRGs than previously predicted (A_V~10-20 versus A_V~100-1000). The lower gas
mass estimates indicated by our observations imply that the star formation
efficiency in these systems is very high and is triggered by cloud-cloud
collisions, shocks, and winds rather than by gravitational instabilities in
circumnuclear gas disks.Comment: 14 pages, 2 figures, accepted to Ap
Quantum gravity on a laptop: 1+1 Dimensional Causal Dynamical Triangulation simulation
AbstractThe quest for quantum gravity has been long and difficult. Causal Dynamical Triangulation is a new and straightforward approach to quantum gravity that recovers classical spacetime at large scales by enforcing causality at small scales. CDT combines quantum physics with general relativity in a Feynman sum-over-geometries and converts the sum into a discrete statistical physics problem. We solve this problem using a new Monte Carlo simulation to compute the spatial fluctuations of an empty universe with one space and one time dimensions. Our results compare favorably with theory and provide an accessible but detailed introduction to quantum gravity via a simulation that runs on a laptop computer
The return to quiescence of Aql X-1 following the 2010 outburst
Aql X-1 is the most prolific low mass X-ray binary transient hosting a
neutron star. In this paper we focus on the return to quiescence following the
2010 outburst of the source. This decay was monitored thanks to 11 pointed
observations taken with XMM-Newton, Chandra and Swift. The decay from outburst
to quiescence is very fast, with an exponential decay characteristic time scale
of ~2 d. Once in quiescence the X-ray flux of Aql X-1 remained constant, with
no further signs of variability or decay. The comparison with the only other
well-monitored outburst from Aql X-1 (1997) is tail-telling. The luminosities
at which the fast decay starts are fully compatible for the two outbursts,
hinting at a mechanism intrinsic to the system and possibly related to the
neutron star rotation and magnetic field (i.e., the propeller effect). In
addition, for both outbursts, the decay profiles are also very similar, likely
resulting from the shut-off of the accretion process onto the neutron star
surface. Finally, the quiescent neutron star temperatures at the end of the
outbursts are well consistent with one another, suggesting a hot neutron star
core dominating the thermal balance. Small differences in the quiescent X-ray
luminosity among the two outbursts can be attributed to a different level of
the power law component.Comment: MNRAS accepted (4 figures and 6 tables
Many accelerating black holes
We show how the Weyl formalism allows metrics to be written down which
correspond to arbitrary numbers of collinear accelerating neutral black holes
in 3+1 dimensions. The black holes have arbitrary masses and different
accelerations and share a common acceleration horizon. In the general case, the
black holes are joined by cosmic strings or struts that provide the necessary
forces that, together with the inter black hole gravitational attractions,
produce the acceleration. In the cases of two and three black holes, the
parameters may be chosen so that the outermost black hole is pulled along by a
cosmic string and the inner black holes follow behind accelerated purely by
gravitational forces. We conjecture that similar solutions exist for any number
of black holes.Comment: 12 pages, LaTe
The 1998 outburst of the X-ray transient XTE J2012+381 as observed with BeppoSAX
We report on the results of a series of X-ray observations of the transient
black hole candidate XTE J2012+381 during the 1998 outburst performed with the
BeppoSAX satellite. The observed broad-band energy spectrum can be described
with the superposition of an absorbed disk black body, an iron line plus a high
energy component, modelled with either a power law or a Comptonisation tail.
The source showed pronounced spectral variability between our five
observations. While the soft component in the spectrum remained almost
unchanged throughout our campaign, we detected a hard spectral tail which
extended to 200 keV in the first two observations, but became barely detectable
up to 50 keV in the following two. A further re-hardening is observed in the
final observation. The transition from a hard to a soft and then back to a hard
state occurred around an unabsorbed 0.1-200 keV luminosity of 10^38 erg/s (at
10 kpc). This indicates that state transitions in XTE 2012+281 are probably not
driven only by mass accretion rate, but additional physical parameters must
play a role in the evolution of the outburst.Comment: Paper accepted for publication on A&A (macro included, 9 pages, 5
figures
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