2,439 research outputs found
Understanding contextualised rational action - author's response
Understanding contextualised rational action - author's respons
Probing Galaxy Formation with TeV Gamma Ray Absorption
We present here the extragalactic background light (EBL) predicted by
semi-analytic models of galaxy formation, and show how measurements of the
absorption of gamma rays of TeV energies via pair production on the EBL
can probe cosmology and the formation of galaxies. Semi-analytic models permit
a physical treatment of the key processes of galaxy formation -- including
gravitational collapse and merging of dark matter halos, gas cooling and
dissipation, star formation, supernova feedback and metal production -- and
have been shown to reproduce key observations at low and high redshift. Using
this approach, we investigate the consequences of variations in input
assumptions such as the stellar initial mass function and the underlying
cosmology. We conclude that observational studies of the absorption of TeV gamma rays will help to constrain the star formation
history of the universe, and the nature and extent of the extinction of
starlight due to dust and reradiation of the absorbed energy at infrared
wavelengths.Comment: 17 pages, 8 figures, presented at the VERITAS Workshop on TeV
Astrophysics of Extragalactic Sources, eds. M. Catanese and T. Weekes, to be
published in Astroparticle Physic
A Reanalysis of Small Scale Velocity Dispersion in the CfA1 Survey
The velocity dispersion of galaxies on scales of Mpc,
, may be estimated from the anisotropy of the galaxy-galaxy
correlation function in redshift space. We present a reanalysis of the CfA1
survey, correct an error in the original analysis of Davis and Peebles (1983),
and find that is extremely sensitive to the details of how
corrections for infall into the Virgo cluster are applied. We conclude that a
robust value of cannot be obtained from this survey. We also
discuss results from other redshift surveys, including the effect of removing
clusters.Comment: 12 pages, uuencoded(latex file + 2 Postscript figures), uses aas
macro
Extragalactic Background Light and Gamma-Ray Attenuation
Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN)
and gamma ray bursts (GRBs) give upper limits on the extragalactic background
light (EBL) from the UV to the mid-IR that are only a little above the lower
limits from observed galaxies. These upper limits now rule out some EBL models
and purported observations, with improved data likely to provide even stronger
constraints. We present EBL calculations both based on multiwavelength
observations of thousands of galaxies and also based on semi-analytic models,
and show that they are consistent with these lower limits from observed
galaxies and with the gamma-ray upper limit constraints. Such comparisons
"close the loop" on cosmological galaxy formation models, since they account
for all the light, including that from galaxies too faint to see. We compare
our results with those of other recent works, and discuss the implications of
these new EBL calculations for gamma ray attenuation. Catching a few GRBs with
groundbased atmospheric Cherenkov Telescope (ACT) arrays or water Cherenkov
detectors could provide important new constraints on the high-redshift star
formation history of the universe.Comment: 12 pages, 8 multi-panel figures, Invited talk at the 25th Texas
Symposium on Relativistic Astrophysics, Heidelberg December 6-10, 201
The nature of the ISM in galaxies during the star-formation activity peak of the Universe
We combine a semi-analytic model of galaxy formation, tracking atomic and
molecular phases of cold gas, with a three-dimensional radiative-transfer and
line tracing code to study the sub-mm emission from atomic and molecular
species (CO, HCN, [CI], [CII], [OI]) in galaxies. We compare the physics that
drives the formation of stars at the epoch of peak star formation (SF) in the
Universe (z = 2.0) with that in local galaxies. We find that normal
star-forming galaxies at high redshift have much higher CO-excitation peaks
than their local counterparts and that CO cooling takes place at higher
excitation levels. CO line ratios increase with redshift as a function of
galaxy star-formation rate, but are well correlated with H2 surface density
independent of redshift. We find an increase in the [OI]/[CII] line ratio in
typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to
counterparts at z = 0. Our model results suggest that typical star-forming
galaxies at high redshift consist of much denser and warmer star-forming clouds
than their local counterparts. Galaxies belonging to the tail of the SF
activity peak at z = 1.2 are already less dense and cooler than counterparts
during the actual peak of SF activity (z = 2.0). We use our results to discuss
how future ALMA surveys can best confront our predictions and constrain models
of galaxy formation.Comment: 19 pages, 14 figures, accepted for publication in MNRA
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