2,440 research outputs found
Fractional bidromy in the vibrational spectrum of HOCl
We introduce the notion of fractional bidromy which is the combination of
fractional monodromy and bidromy, two recent generalizations of Hamiltonian
monodromy. We consider the vibrational spectrum of the HOCl molecule which is
used as an illustrative example to show the presence of nontrivial fractional
bidromy. To our knowledge, this is the first example of a molecular system
where such a generalized monodromy is exhibited.Comment: 9 pages, 2 figue
A New Solution of The Cosmological Constant Problems
We extend the usual gravitational action principle by promoting the bare
cosmological constant (CC) from a parameter to a field which can take many
possible values. Variation leads to a new integral constraint equation which
determines the classical value of the effective CC that dominates the wave
function of the universe. In a realistic cosmological model, the expected value
of the effective CC, is calculated from measurable quantities to be O(t_U), as
observed, where t_U is the present age of the universe in Planck units,. Any
application of our model produces a falsifiable prediction for in
terms of other measurable quantities. This leads to a specific falsifiable
prediction for the observed spatial curvature parameter of Omega_k0=-0.0055.
Our testable proposal requires no fine tunings or extra dark-energy fields but
does suggest a new view of time and cosmological evolution.Comment: 5 pages; v3: version accepted by Phys. Rev. Let
An Isocurvature CDM Cosmogony. I. A Worked Example of Evolution Through Inflation
I present a specific worked example of evolution through inflation to the
initial conditions for an isocurvature CDM model for structure formation. The
model invokes three scalar fields, one that drives power law inflation, one
that survives to become the present-day CDM, and one that gives the CDM field a
mass that slowly decreases during inflation and so ``tilts'' the primeval mass
fluctuation spectrum of the CDM. The functional forms for the potentials and
the parameter values that lead to an observationally acceptable model for
structure formation do not seem to be out of line with current ideas about the
physics of the very early universe. I argue in an accompanying paper that the
model offers an acceptable fit to main observational constraints.Comment: 11 pages, 3 postscript figures, uses aas2pp4.st
Retaining Black Holes with Very Large Recoil Velocities
Recent numerical simulations of binary black hole mergers show the
possibility of producing very large recoil velocities (> 3000 km/s). Kicks of
this magnitude should be sufficient to eject the final black hole from
virtually any galactic potential. This result has been seen as a potential
contradiction with observations of supermassive black holes residing in the
centers of most galaxies in the local universe. Using an extremely simplified
merger tree model, we show that, even in the limit of very large ejection
probability, after a small number of merger generations there should still be
an appreciable fraction (>50%) of galaxies with supermassive black holes today.
We go on to argue that the inclusion of more realistic physics ingredients in
the merger model should systematically increase this retention fraction,
helping to resolve a potential conflict between theory and observation. Lastly,
we develop a more realistic Monte Carlo model to confirm the qualitative
arguments and estimate occupation fractions as a function of the central
galactic velocity dispersion.Comment: 6 pages, 3 figures; Comments welcom
Starburst and AGN activity in ultraluminous infrared galaxies
(Abridged) We examine the power source of 41 local Ultraluminous Infrared
Galaxies using archival infrared and optical photometry. We fit the observed
Spectral Energy Distributions (SEDs) with starburst and AGN components; each
component being drawn from a family of templates. We find all of the sample
require a starburst, whereas only half require an AGN. In 90% of the sample the
starburst provides over half the IR emission, with a mean fractional luminosity
of 82%. When combined with other galaxy samples we find that starburst and AGN
luminosities correlate over 6 decades in IR luminosity suggesting that a common
factor governs both luminosities, plausibly the gas masses in the nuclear
regions. We find that the mid-IR 7.7 micron line-continuum ratio is no
indication of the starburst luminosity, or the fractional AGN luminosity, and
therefore that this ratio is not a reliable diagnostic of the power source in
ULIRGs. We propose that the scatter in the radio-IR correlation in ULIRGs is
due to a skewed starburst IMF and/or relic relativistic electrons from a
previous starburst, rather than contamination from an obscured AGN. We show
that most ULIRGs undergo multiple starbursts during their lifetime, and by
inference that mergers between more than two galaxies may be common amongst
ULIRGs. Our results support the evolutionary model for ULIRGs proposed by
Farrah et al 2001, where they can follow many different evolutionary paths of
starburst and AGN activity in transforming merging spiral galaxies into
elliptical galaxies, but that most do not go through an optical QSO phase. The
lower level of AGN activity in our local sample than in z~1 HLIRGs implies that
the two samples are distinct populations. We postulate that different galaxy
formation processes at high-z are responsible for this difference.Comment: 24 pages, 8 figures. Accepted for publication in MNRA
Principal Component Analysis and Radiative Transfer modelling of Spitzer IRS Spectra of Ultra Luminous Infrared Galaxies
The mid-infrared spectra of ultraluminous infrared galaxies (ULIRGs) contain
a variety of spectral features that can be used as diagnostics to characterise
the spectra. However, such diagnostics are biased by our prior prejudices on
the origin of the features. Moreover, by using only part of the spectrum they
do not utilise the full information content of the spectra. Blind statistical
techniques such as principal component analysis (PCA) consider the whole
spectrum, find correlated features and separate them out into distinct
components.
We further investigate the principal components (PCs) of ULIRGs derived in
Wang et al.(2011). We quantitatively show that five PCs is optimal for
describing the IRS spectra. These five components (PC1-PC5) and the mean
spectrum provide a template basis set that reproduces spectra of all z<0.35
ULIRGs within the noise. For comparison, the spectra are also modelled with a
combination of radiative transfer models of both starbursts and the dusty torus
surrounding active galactic nuclei. The five PCs typically provide better fits
than the models. We argue that the radiative transfer models require a colder
dust component and have difficulty in modelling strong PAH features.
Aided by the models we also interpret the physical processes that the
principal components represent. The third principal component is shown to
indicate the nature of the dominant power source, while PC1 is related to the
inclination of the AGN torus.
Finally, we use the 5 PCs to define a new classification scheme using 5D
Gaussian mixtures modelling and trained on widely used optical classifications.
The five PCs, average spectra for the four classifications and the code to
classify objects are made available at: http://www.phys.susx.ac.uk/~pdh21/PCA/Comment: 11 pages, 12 figures, accepted for publication in MNRA
Sub-millimetre observations of hyperluminous infrared galaxies
We present sub-mm photometry for 11 Hyperluminous Infrared Galaxies (HLIRGs)
and use radiative transfer models for starbursts and AGN to investigate the IR
emission. In all sources both a starburst and AGN are required to explain the
IR emission. The mean starburst fraction is 35%, with a range spanning 80%
starburst dominated to 80% AGN dominated. In all cases the starburst dominates
at rest-frame wavelengths >50 microns, with star formation rates >500 solar
masses per year. The trend of increasing AGN fraction with increasing IR
luminosity seen in IRAS galaxies peaks in HLIRGs, and is not higher than the
fraction seen in bright ULIRGs. The AGN and starburst luminosities correlate,
suggesting that a common physical factor, plausibly the dust masses, governs
their luminosities. Our results suggest that the HLIRG population is comprised
both of ULIRG-like galaxy mergers, and of young galaxies going through their
maximal star formation periods whilst harbouring an AGN. The coeval AGN and
starburst activity in our sources implies that starburst and AGN activity, and
the peak starburst and AGN luminosities, can be coeval in active galaxies
generally. When extrapolated to high-z our sources have comparable sub-mm
fluxes to sub-mm survey sources. At least some sub-mm survey sources are
therefore likely to be comprised of similar galaxy populations to those found
in the HLIRG population. It is also plausible from these results that high-z
sub-mm sources harbour heavily obscured AGN. The differences in X-ray and
sub-mm properties between HLIRGs at z~1 and sub-mm sources at z~3 implies
evolution between the two epochs. Either the mean AGN obscuration level is
greater at z~3 than at z~1, or the fraction of IR-luminous sources at z~3 that
contain AGN is smaller than that at z~1.Comment: 15 pages. Accepted for publication in MNRA
First Structure Formation: A Simulation of Small Scale Structure at High Redshift
We describe the results of a simulation of collisionless cold dark matter in
a LambdaCDM universe to examine the properties of objects collapsing at high
redshift (z=10). We analyze the halos that form at these early times in this
simulation and find that the results are similar to those of simulations of
large scale structure formation at low redshift. In particular, we consider
halo properties such as the mass function, density profile, halo shape, spin
parameter, and angular momentum alignment with the minor axis. By understanding
the properties of small scale structure formation at high redshift, we can
better understand the nature of the first structures in the universe, such as
Population III stars.Comment: 31 pages, 14 figures; accepted for publication in ApJ. Figure 1 can
also be viewed at http://cfa-www.harvard.edu/~hjang/research
Evolution of the Pairwise Peculiar Velocity Distribution Function in Lagrangian Perturbation Theory
The statistical distribution of the radial pairwise peculiar velocity of
galaxies is known to have an exponential form as implied by observations and
explicitly shown in N-body simulations. Here we calculate its statistical
distribution function using the Zel'dovich approximation assuming that the
primordial density fluctuations are Gaussian distributed. We show that the
exponential distribution is realized as a transient phenomena on megaparsec
scales in the standard cold-dark-matter model.Comment: 19 pages, 8 Postscript figures, AAS LaTe
Soft X-ray Absorption by High-Redshift Intergalactic Helium
The Lyman alpha absorption from intergalactic, once-ionized helium (HeII) has
been measured with HST in four quasars over the last few years, over the
redshift range 2.4 < z < 3.2. These observations have indicated that the HeII
reionization may not have been completed until z\simeq 2.8, and that large
fluctuations in the intensity of the HeII-ionizing background were present
before this epoch. The detailed history of HeII reionization at higher
redshifts is, however, model-dependent and difficult to determine from these
observations, because the IGM can be completely optically thick to Lya photons
when only a small fraction of the helium remains as HeII. In addition, finding
quasars in which the HeII Lya absorption can be observed becomes increasingly
difficult at higher redshift, owing to the large abundance of hydrogen Lyman
limit systems.
It is pointed out here that HeII in the IGM should also cause detectable
continuum absorption in the soft X-rays. The spectrum of a high-redshift source
seen behind the IGM when most of the helium was HeII should recover from the
HeII Lyman continuum absorption at an observed energy \sim 0.1 keV. Galactic
absorption will generally be stronger, but not by a large factor; the
intergalactic HeII absorption can be detected as an excess over the expected
Galactic absorption from the 21cm HI column density. In principle, this method
allows a direct determination of the fraction of helium that was singly ionized
as a function of redshift, if the measurement is done on a large sample of
high-redshift sources over a range of redshift.Comment: accepted to The Astrophysical Journal Letter
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