1,094 research outputs found
Toy Models for Galaxy Formation versus Simulations
We describe simple useful toy models for key processes of galaxy formation in
its most active phase, at z > 1, and test the approximate expressions against
the typical behaviour in a suite of high-resolution hydro-cosmological
simulations of massive galaxies at z = 4-1. We address in particular the
evolution of (a) the total mass inflow rate from the cosmic web into galactic
haloes based on the EPS approximation, (b) the penetration of baryonic streams
into the inner galaxy, (c) the disc size, (d) the implied steady-state gas
content and star-formation rate (SFR) in the galaxy subject to mass
conservation and a universal star-formation law, (e) the inflow rate within the
disc to a central bulge and black hole as derived using energy conservation and
self-regulated Q ~ 1 violent disc instability (VDI), and (f) the implied steady
state in the disc and bulge. The toy models provide useful approximations for
the behaviour of the simulated galaxies. We find that (a) the inflow rate is
proportional to mass and to (1+z)^5/2, (b) the penetration to the inner halo is
~50% at z = 4-2, (c) the disc radius is ~5% of the virial radius, (d) the
galaxies reach a steady state with the SFR following the accretion rate into
the galaxy, (e) there is an intense gas inflow through the disc, comparable to
the SFR, following the predictions of VDI, and (f) the galaxies approach a
steady state with the bulge mass comparable to the disc mass, where the
draining of gas by SFR, outflows and disc inflows is replenished by fresh
accretion. Given the agreement with simulations, these toy models are useful
for understanding the complex phenomena in simple terms and for
back-of-the-envelope predictions.Comment: Resubmitted to MNRAS after responding to referee's comments; Revised
figure
The Implications of Galaxy Formation Models for the TeV Observations of Current Detectors
This paper represents a step toward constraining galaxy formation models via
TeV gamm a ray observations. We use semi-analytic models of galaxy formation to
predict a spectral distribution for the intergalactic infrared photon field,
which in turn yields information about the absorption of TeV gamma rays from
extra-galactic sources. By making predictions for integral flux observations at
>200 GeV for several known EGRE T sources, we directly compare our models with
current observational upper limits obtained by Whipple. In addition, our
predictions may offer a guide to the observing programs for the current
population of TeV gamma ray observatories.Comment: 6 pages, 11 figures, to appear in the proceedings of the 6th TeV
Workshop at Snowbird, U
The SED of the TeV BLLac 1ES 1426+428 after correction for the TeV--IR absorption
The recent HEGRA detection and spectrum of 1ES 1426+428 at TeV energies, once
corrected for absorption using present estimates of the diffuse extragalactic
IR background, suggest that the high energy peak of the Spectral Energy
Distribution (SED) could be much higher than the synchrotron one
(), and lie at energies above 8-10 TeV. To see if such an SED could
be accounted for, we have applied a "finite injection time" SSC model, and
present here some preliminary results. Within this model, we found the need of
an external ("ambient") contribution to the energy density of seed photons, in
order to account for both the high Compton dominance and the hard spectrum.Comment: 4 pages, 5 figures, to appear in the proceedings of the conference
"Relativistic jets in the Chandra and XMM era", Bologna, 23-27/9/02 (New
Astr. Rev.
Constraining the IMF using TeV gamma ray absorption
Gamma rays of ~TeV energies from distant sources suffer attenuation due to
pair production off of ~1 micron EBL photons. We may exploit this process in
order to indirectly measure the EBL and constrain models of galaxy formation.
Here, using semi-analytic models of galaxy formation, we examine how gamma ray
absorption may be used as an indirect probe of the stellar initial mass
function (IMF), although there is a degeneracy with dust modeling. We point out
that with the new generation of gamma ray telescopes including STACEE, MAGIC,
HESS, VERITAS, and Milagro, we should soon possess a wealth of new data and a
new method for probing the nature of the IMF.Comment: contribution to "TeV Astrophysics of Extragalactic Sources" VERITAS
workshop, editors M. Catanese, J. Quinn, T. Weekes; 3 pages 1 figur
The Arbitrary Trajectory Quantization Method
The arbitrary trajectory quantization method (ATQM) is a time dependent
approach to quasiclassical quantization based on the approximate dual
relationship that exists between the quantum energy spectra and classical
periodic orbits. It has recently been shown however, that, for polygonal
billiards, the periodicity criterion must be relaxed to include closed
almost-periodic (CAP) orbit families in this relationship. In light of this
result, we reinvestigate the ATQM and show that at finite energies, a
smoothened quasiclassical kernel corresponds to the modified formula that
includes CAP families while the delta function kernel corresponding to the
periodic orbit formula is recovered at high energies. Several clarifications
are also provided.Comment: revtex, ps figure
The Milky Way as a Kiloparsec-Scale Axionscope
Very high energy gamma-rays are expected to be absorbed by the extragalactic
background light over cosmological distances via the process of
electron-positron pair production. Recent observations of cosmologically
distant gamma-ray emitters by ground based gamma-ray telescopes have, however,
revealed a surprising degree of transparency of the universe to very high
energy photons. One possible mechanism to explain this observation is the
oscillation between photons and axion-like-particles (ALPs). Here we explore
this possibility further, focusing on photon-ALP conversion in the magnetic
fields in and around gamma-ray sources and in the magnetic field of the Milky
Way, where some fraction of the ALP flux is converted back into photons. We
show that this mechanism can be efficient in allowed regions of the ALP
parameter space, as well as in typical configurations of the Galactic Magnetic
Field. As case examples, we consider the spectrum observed from two HESS
sources: 1ES1101-232 at redshift z=0.186 and H 2356-309 at z=0.165. We also
discuss features of this scenario which could be used to distinguish it from
standard or other exotic models.Comment: 7 pages, 4 figures. Matches published versio
Gamma Rays from Ultra-High Energy Cosmic Rays in Cygnus A
Ultra-high energy cosmic rays (UHECRs) accelerated in the jets of active
galactic nuclei can accumulate in high magnetic field, ~100 kpc-scale regions
surrounding powerful radio galaxies. Photohadronic processes involving UHECRs
and photons of the extragalactic background light make ultra-relativistic
electrons and positrons that initiate electromagnetic cascades, leading to the
production of a gamma-ray synchrotron halo. We calculate the halo emission in
the case of Cygnus A and show that it should be detectable with the Fermi Gamma
ray Space Telescope and possibly detectable with ground-based gamma-ray
telescopes if radio galaxies are the sources of UHECRs.Comment: 9 pages, 2 figures, extended calculations, added references; ApJL, in
pres
An apprach to generate large and small leptonic mixing angles
We take up the point of view that Yukawa couplings can be either 0 or 1, and
the mass patterns of fermions are generated purely from the structure of the
Yukawa matrices. We utilize such neutrino as well as charged leptonic textures
which lead to (maximal) mixing angles of in each sector for relevant
transitions. The combined leptonic CKM mixing angles are
which lead to very small relevant to solar neutrino and LSND
experiments. We propose that on the other hand the absence of the charged
leptonic partner of the sterile neutrino maintains the angle from the
neutrino sector for the transition and hence
atmospheric neutrino anomaly is explained through maximal mixing
Cluster Cores, Gravitational Lensing, and Cosmology
Many multiply--imaged quasars have been found over the years, but none so far
with image separation in excess of 8\arcsec. The absence of such large
splittings has been used as a test of cosmological models: the standard Cold
Dark Matter model has been excluded on the basis that it predicts far too many
large--separation double images. These studies assume that the lensing
structure has the mass profile of a singular isothermal sphere. However, such
large splittings would be produced by very massive systems such as clusters of
galaxies, for which other gravitational lensing data suggest less singular mass
profiles. Here we analyze two cases of mass profiles for lenses: an isothermal
sphere with a finite core radius (density , and a Hernquist profile (). We find that small core radii
kpc, as suggested by the cluster data, or large a \gsim 300 h^{-1} kpc, as
needed for compatibility with gravitational distortion data, would reduce the
number of large--angle splittings by an order of magnitude or more. Thus, it
appears that these tests are sensitive both to the cosmological model (number
density of lenses) and to the inner lens structure, which is unlikely to depend
sensitively on the cosmology, making it difficult to test the cosmological
models by large--separation quasar lensing until we reliably know the structure
of the lenses themselves.Comment: 17 pages, uuencoded compressed tarred postscript file including text
and 1 figure. To appear in January 20, 1996 issue of ApJ Letter
Quantal Consequences of Perturbations Which Destroy Structurally Unstable Orbits in Chaotic Billiards
Non-generic contributions to the quantal level-density from parallel segments
in billiards are investigated. These contributions are due to the existence of
marginally stable families of periodic orbits, which are structurally unstable,
in the sense that small perturbations, such as a slight tilt of one of the
segments, destroy them completely. We investigate the effects of such
perturbation on the corresponding quantum spectra, and demonstrate them for the
stadium billiard
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