572 research outputs found
CO+ in M 82: A Consequence of Irradiation by X-rays
Based on its strong CO+ emission it is argued that the M 82 star-burst galaxy
is exposed to a combination of FUV and X-ray radiation. The latter is likely to
be the result of the star-burst superwind, which leads to diffuse thermal
emission at ~0.7 keV, and a compact hard, 2-10 keV, source (but not an AGN).
Although a photon-dominated region (FUV) component is clearly present in the
nucleus of M 82, and capable of forming CO+, only X-ray irradiated gas of
density 10^3-10^5 cm^-3 can reproduce the large, ~(1-4)x10^13 cm^-2, columns of
CO+ that are observed toward the proto-typical star-burst M 82. The total X-ray
luminosity produced by M 82 is weak, ~10^41 erg s^-1, but this is sufficient to
drive the formation of CO+.Comment: added discussion on more recent X-ray observation
HCN to HCO^+ Millimeter Line Diagnostics of AGN Molecular Torus I : Radiative Transfer Modeling
We explore millimeter line diagnostics of an obscuring molecular torus
modeled by a hydrodynamic simulation with three-dimensional nonLTE radiative
transfer calculations. Based on the results of high-resolution hydrodynamic
simulation of the molecular torus around an AGN, we calculate intensities of
HCN and HCO^{+} rotational lines as two representative high density tracers.
The three-dimensional radiative transfer calculations shed light on a
complicated excitation state in the inhomogeneous torus, even though a
spatially uniform chemical structure is assumed. Our results suggest that HCN
must be much more abundant than HCO^{+} in order to obtain a high ratio
() observed in some of the nearby galaxies. There is a
remarkable dispersion in the relation between integrated intensity and column
density, indicative of possible shortcomings of HCN(1-0) and HCO^{+}(1-0) lines
as high density tracers. The internal structures of the inhomogeneous molecular
torus down to subparsec scale in external galaxies will be revealed by the
forthcoming Atacama Large Millimeter/submillimeter Array (ALMA). The
three-dimensional radiative transfer calculations of molecular lines with
high-resolution hydrodynamic simulation prove to be a powerful tool to provide
a physical basis for molecular line diagnostics of the central regions of
external galaxies.Comment: 29 pages, 13 figures, Accepted for publication in ApJ, For high
resolution figures see http://alma.mtk.nao.ac.jp/~masako/MS72533v2.pd
The Effects of Dark Matter Decay and Annihilation on the High-Redshift 21 cm Background
The radiation background produced by the 21 cm spin-flip transition of
neutral hydrogen at high redshifts can be a pristine probe of fundamental
physics and cosmology. At z~30-300, the intergalactic medium (IGM) is visible
in 21 cm absorption against the cosmic microwave background (CMB), with a
strength that depends on the thermal (and ionization) history of the IGM. Here
we examine the constraints this background can place on dark matter decay and
annihilation, which could heat and ionize the IGM through the production of
high-energy particles. Using a simple model for dark matter decay, we show
that, if the decay energy is immediately injected into the IGM, the 21 cm
background can detect energy injection rates >10^{-24} eV cm^{-3} sec^{-1}. If
all the dark matter is subject to decay, this allows us to constrain dark
matter lifetimes <10^{27} sec. Such energy injection rates are much smaller
than those typically probed by the CMB power spectra. The expected brightness
temperature fluctuations at z~50 are a fraction of a mK and can vary from the
standard calculation by up to an order of magnitude, although the difference
can be significantly smaller if some of the decay products free stream to lower
redshifts. For self-annihilating dark matter, the fluctuation amplitude can
differ by a factor <2 from the standard calculation at z~50. Note also that, in
contrast to the CMB, the 21 cm probe is sensitive to both the ionization
fraction and the IGM temperature, in principle allowing better constraints on
the decay process and heating history. We also show that strong IGM heating and
ionization can lead to an enhanced H_2 abundance, which may affect the earliest
generations of stars and galaxies.Comment: submitted to Phys Rev D, 14 pages, 8 figure
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
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The Highly Deuterated Chemistry of the Early Universe
A comprehensive chemistry of the highly deuterated species D2, D+ 2, D2H+, and D+ 3 in the early universe is presented. Fractional abundances for each are calculated as a function of redshift z in the recombination era. The abundances of the isotopologues are found to display similar behavior. Fractionation enhances the abundances of most of the more highly deuterated species as the redshift decreases due to the closing of some reaction channels as the gas temperature cools. Rate coefficients for the majority of the reactions involving the deuterated species are uncertain resulting in a corresponding uncertainty in their predicted abundances.Astronom
The Effects of UV Continuum and Lyman alpha Radiation on the Chemical Equilibrium of T Tauri Disks
We show in this Letter that the spectral details of the FUV radiation fields
have a large impact on the chemistry of protoplanetary disks surrounding T
Tauri stars. We show that the strength of a realistic stellar FUV field is
significantly lower than typically assumed in chemical calculations and that
the radiation field is dominated by strong line emission, most notably Lyman
alpha radiation. The effects of the strong Lyman alpha emission on the chemical
equilibrium in protoplanetary disks has previously been unrecognized. We
discuss the impact of this radiation on molecular observations in the context
of a radiative transfer model that includes both direct attenuation and
scattering. In particular, Lyman alpha radiation will directly dissociate water
vapor and may contribute to the observed enhancements of CN/HCN in disks.Comment: 14 pages, 4 figures, accepted by ApJ Letter
Rate coefficients for rovibrational transitions in H_2 due to collisions with He
We present quantum mechanical and quasiclassical trajectory calculations of
cross sections for rovibrational transitions in ortho- and para-H_2 induced by
collisions with He atoms. Cross sections were obtained for kinetic energies
between 10^-4 and 3 eV, and the corresponding rate coefficients were calculated
for the temperature range 100<T<4000 K. Comparisons are made with previous
calculations.Comment: 21 pages, 2 figures, AAS, eps
Rovibrationally resolved photodissociation of HeH+
Accurate photodissociation cross sections have been obtained for the A-X
electronic transition of HeH+ using ab initio potential curves and dipole
transition moments. Partial cross sections have been evaluated for all
rotational transitions from the vibrational levels v"=0-11 and over the entire
accessible wavelength range 100-1129 Angstrom. Assuming a Boltzmann
distribution of the rovibrational levels of the X state, photodissociation
cross sections are presented for temperatures between 500 and 12,000 K. A
similar set of calculations was performed for the pure rovibrational
photodissociation in the X-X electronic ground state, but covering photon
wavelengths into the far infrared. Applications of the cross sections to the
destruction of HeH+in the early Universe and in UV-irradiated environments such
as primordial halos and protoplanetary disks are briefly discussed
First Detection of HCO+ Emission at High Redshift
We report the detection of HCO+(1-0) emission towards the Cloverleaf quasar
(z=2.56) through observations with the Very Large Array. This is the first
detection of ionized molecular gas emission at high redshift (z>2). HCO+
emission is a star formation indicator similar to HCN, tracing dense molecular
hydrogen gas (n(H_2) ~= 10^5 cm^{-3}) within star-forming molecular clouds. We
derive a lensing-corrected HCO+ line luminosity of L'(HCO+) = 3.5 x 10^9 K km/s
pc^2. Combining our new results with CO and HCN measurements from the
literature, we find a HCO+/CO luminosity ratio of 0.08 and a HCO+/HCN
luminosity ratio of 0.8. These ratios fall within the scatter of the same
relationships found for low-z star-forming galaxies. However, a HCO+/HCN
luminosity ratio close to unity would not be expected for the Cloverleaf if the
recently suggested relation between this ratio and the far-infrared luminosity
were to hold. We conclude that a ratio between HCO+ and HCN luminosity close to
1 is likely due to the fact that the emission from both lines is optically
thick and thermalized and emerges from dense regions of similar volumes. The
CO, HCN and HCO+ luminosities suggest that the Cloverleaf is a composite
AGN--starburst system, in agreement with the previous finding that about 20% of
the total infrared luminosity in this system results from dust heated by star
formation rather than heating by the AGN. We conclude that HCO+ is potentially
a good tracer for dense molecular gas at high redshift.Comment: 5 pages, 3 figures, ApJL, in press (accepted May 17, 2006
Millimeter Interferometric Investigations of the Energy Sources of Three Ultraluminous Infrared Galaxies, UGC 5101, Mrk 273, and IRAS 17208-0014, based on HCN to HCO+ Ratios
We present interferometric observations of three ultraluminous infrared
galaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3-mm
wavelength range, using the Nobeyama Millimeter Array. Both the HCN (J=1-0) and
HCO+ (J=1-0) molecular lines were observed simultaneously. HCN emission was
clearly detected at the nuclear positions of these ULIRGs, and HCO+ emission
was detected at the nuclear positions of UGC 5101 and IRAS 17208-0014. The HCN
to HCO+ brightness-temperature ratios toward the nuclei of the three ULIRGs
were derived and compared with those of lower luminosity galaxies known to be
dominated by active galactic nuclei (AGNs) or starbursts. In UGC 5101 and Mrk
273, where there is evidence for obscured AGNs from previous observations at
other wavelengths, we found high HCN/HCO+ ratios (>1.8) that are in the range
found for AGN-dominated galaxies. In IRAS 17208-0014, where the presence of a
powerful obscured AGN has been unclear, the ratio (1.7) is in between the
observed values for starburst- and AGN-dominated galaxies. The high HCN/HCO+
brightness-temperature ratios in UGC 5101 and Mrk 273 could be the consequence
of an HCN abundance enhancement, which is expected from chemical effects of the
central X-ray emitting AGN on the surrounding dense molecular gas. Our proposed
millimeter interferometric method based on HCN/HCO+ ratios may be an effective
tool for unveiling elusive buried AGNs at the cores of ULIRGs, especially
because of the negligible dust extinction at these wavelengths.Comment: 15 pages (emulateapj.sty), 8 figures (figures 1-5 resolution
reduced), Accepted for publication in Astronomical Journal, A PDF file with
high resolution is availble at
http://optik2.mtk.nao.ac.jp/~imanishi/Paper/HCN/HCN.pd
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