2,615 research outputs found
Electron Excitation of High Dipole Moment Molecules Reexamined
Emission from high-dipole moment molecules such as HCN allows determination
of the density in molecular clouds, and is often considered to trace the
"dense" gas available for star formation. We assess the importance of electron
excitation in various environments. The ratio of the rate coefficients for
electrons and H molecules, 10 for HCN, yields the requirements
for electron excitation to be of practical importance if $n({\rm H}_2) \leq\
10^{5.5} ~ \rm cm^{-3}X({\rm e}^-) \geq\ 10^{-5}n_{\rm{}c}({\rm H_2})X^*({\rm{}e}^-)X({\rm e}^-)^+X^*({\rm e}^-)_2^-^+^+$ ratios observed in the nuclear regions of luminous galaxies may
be in part a result of electron excitation of high dipole moment tracers. The
importance of electron excitation will depend on detailed models of the
chemistry, which may well be non-steady state and non-static.Comment: published in Ap
Low Virial Parameters in Molecular Clouds: Implications for High Mass Star Formation and Magnetic Fields
Whether or not molecular clouds and embedded cloud fragments are stable
against collapse is of utmost importance for the study of the star formation
process. Only "supercritical" cloud fragments are able to collapse and form
stars. The virial parameter, alpha=M_vir/M, which compares the virial to the
actual mass, provides one way to gauge stability against collapse.
Supercritical cloud fragments are characterized by alpha<2, as indicated by a
comprehensive stability analysis considering perturbations in pressure and
density gradients. Past research has suggested that virial parameters alpha>2
prevail in clouds. This would suggest that collapse towards star formation is a
gradual and relatively slow process, and that magnetic fields are not needed to
explain the observed cloud structure. Here, we review a range of very recent
observational studies that derive virial parameters <<2 and compile a catalogue
of 1325 virial parameter estimates. Low values of alpha are in particular
observed for regions of high mass star formation (HMSF). These observations may
argue for a more rapid and violent evolution during collapse. This would enable
"competitive accretion" in HMSF, constrain some models of "monolithic
collapse", and might explain the absence of high--mass starless cores.
Alternatively, the data could point at the presence of significant magnetic
fields ~1 mG at high gas densities. We examine to what extent the derived
observational properties might be biased by observational or theoretical
uncertainties. For a wide range of reasonable parameters, our conclusions
appear to be robust with respect to such biases.Comment: accepted to Ap
Enhancement of Vibronic and Ground-State Vibrational Coherences in 2D Spectra of Photosynthetic Complexes
A vibronic-exciton model is applied to investigate the mechanism of
enhancement of coherent oscillations due to mixing of electronic and nuclear
degrees of freedom recently proposed as the origin of the long-lived
oscillations in 2D spectra of the FMO complex [Christensson et al. J. Phys.
Chem. B 116 (2012) 7449]. We reduce the problem to a model BChl dimer to
elucidate the role of resonance coupling, site energies, nuclear mode and
energy disorder in the enhancement of vibronic-exciton and ground-state
vibrational coherences, and to identify regimes where this enhancement is
significant. For a heterodimer representing the two coupled BChls 3 and 4 of
the FMO complex, the initial amplitude of the vibronic-exciton and vibrational
coherences are enhanced by up to 15 and 5 times, respectively, compared to the
vibrational coherences in the isolated monomer. This maximum initial amplitude
enhancement occurs when there is a resonance between the electronic energy gap
and the frequency of the vibrational mode. The bandwidth of this enhancement is
about 100 cm-1 for both mechanisms. The excitonic mixing of electronic and
vibrational DOF leads to additional dephasing relative to the vibrational
coherences. We evaluate the dephasing dynamics by solving the quantum master
equation in Markovian approximation and observe a strong dependence of the
life-time enhancement on the mode frequency. Long-lived vibronic-exciton
coherences are found to be generated only when the frequency of the mode is in
the vicinity of the electronic resonance. Although the vibronic-exciton
coherences exhibit a larger initial amplitude compared to the ground-state
vibrational coherences, we conclude that both type have a similar magnitude at
long time for the present model. The ability to distinguish between
vibronic-exciton and ground-state vibrational coherences in the general case of
molecular aggregate is discussed.Comment: 16 pages, 6 figure
The Ages of Elliptical Galaxies in a Merger Model
The tightness of the observed colour-magnitude and Mg- velocity
dispersion relations for elliptical galaxies has often been cited as an
argument against a picture in which ellipticals form by the merging of spiral
disks. A common view is that merging would mix together stars of disparate ages
and produce a large scatter in these relations. Here I use semi-analytic models
of galaxy formation to derive the distribution of the mean ages, colours and
metallicities of the stars in elliptical galaxies formed by mergers in a flat
CDM universe. It is seen that most of the stars in ellipticals form at
relatively high redshift (z > 1.9) and that the predicted scatter in the
colour-magnitude and Mg_2 - sigma relations falls within observational bounds.
I conclude that the apparent homogeneity in the properties of the stellar
populations of ellipticals is not inconsistent with a merger scenario for the
origin of these systems.Comment: latex file, figures available upon reques
Does the Number Density of Elliptical Galaxies Change at z<1?
We have performed a detailed V/Vmax test for a sample of the Canada-France
Redshift Survey (CFRS) for the purpose of examining whether the comoving number
density of field galaxies changes significantly at redshifts of z<1. Taking
into account the luminosity evolution of galaxies which depends on their
morphological type through different history of star formation, we obtain
\sim 0.5 in the range of 0.3<z<0.8, where reliable redshifts were
secured by spectroscopy of either absorption or emission lines for the CFRS
sample. This indicates that a picture of mild evolution of field galaxies
without significant mergers is consistent with the CFRS data. Early-type
galaxies, selected by their (V-I)_{AB} color, become unnaturally deficient in
number at z>0.8 due to the selection bias, thereby causing a fictitious
decrease of . We therefore conclude that a reasonable choice of upper
bound of redshift z \sim 0.8 in the V/Vmax test saves the picture of passive
evolution for field ellipticals in the CFRS sample, which was rejected by
Kauffman, Charlot, & White (1996) without confining the redshift range.
However, about 10% of the CFRS sample consists of galaxies having colors much
bluer than predicted for irregular galaxies, and their \avmax is significantly
larger than 0.5. We discuss this population of extremely blue galaxies in terms
of starburst that has just turned on at their observed redshifts.Comment: 11 pages including 3 figures, to appear in ApJ Letter
The Properties of Satellite Galaxies in External Systems. I. Morphology and Structural Parameters
We present the first results of an ongoing project to study the
morphological, kinematical, dynamical, and chemical properties of satellite
galaxies of external giant spiral galaxies. The sample of objects has been
selected from the catalogue by Zaritsky et al. (1997). The paper analyzes the
morphology and structural parameters of a subsample of 60 such objects. The
satellites span a great variety of morphologies and surface brightness
profiles. About two thirds of the sample are spirals and irregulars, the
remaining third being early-types. Some cases showing interaction between pairs
of satellites are presented and briefly discussed.Comment: Accepted for publication in Astrophys. Journal Supp. Se
Spectral and morphological properties of quasar hosts in SPH simulations of AGN feeding by mergers
We present a method for generating virtual observations from
smoothed-particle-hydrodynamics (SPH) simulations. This method includes stellar
population synthesis models and the reprocessing of starlight by dust to
produce realistic galaxy images. We apply this method and simulate the merging
of two identical giant Sa galaxies. The merger remnant is an elliptical galaxy.
The merger concentrates the gas content of the two galaxies into the nuclear
region. The gas that flows into the nuclear region refuels the central black
holes of the merging galaxies. We follow the refuelling of the black holes
during the merger semi-analytically.
In the simulation presented in this article, the black holes grow from 3 x
10^7 to 1.8X 10^8 Solar masses, with a peak AGN luminosity of M_B ~ -23.7. We
study how the morphological and spectral properties of the system evolve during
the merger and work out the predictions of this scenario for the properties of
host galaxies during the active phase. The peak of AGN activity coincides with
the merging of the two galactic nuclei and occurs at a stage when the remnant
looks like a lenticular galaxy. The simulation predicts the formation of a
circumnuclear starburst ring/dusty torus with an opening angle of 30-40 degrees
and made of clouds with n_H=10^24 cm^-2. The average optical depth of the torus
is quite high, but the obscuring medium is patchy, so that there still exist
lines of sight where the AGN is visible in a nearly edge-on view. For the same
reason, there are lines of sight where the AGN is completely obscured in the
face-on view.Comment: 14 pages, 11 figure
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