899 research outputs found
Nonlinear density evolution from an improved spherical collapse model
We investigate the evolution of non-linear density perturbations by taking
into account the effects of deviations from spherical symmetry of a system.
Starting from the standard spherical top hat model in which these effects are
ignored, we introduce a physically motivated closure condition which specifies
the dependence of the additional terms on the density contrast, . The
modified equation can be used to model the behaviour of an overdense region
over a sufficiently large range of . The key new idea is a Taylor
series expansion in () to model the non-linear epoch. We show that
the modified equations quite generically lead to the formation of stable
structures in which the gravitational collapse is halted at around the virial
radius. The analysis also allows us to connect up the behaviour of individual
overdense regions with the non-linear scaling relations satisfied by the two
point correlation function.Comment: 11 pages, 6 figures. Final version, contains added discussion and
modified figures to match the accepted versio
Stock Option Expense, Forward-Looking Information, and Implied Volatilities of Traded Options
Prior research generally finds that firms underreport option expense by managing
assumptions underlying option valuation (e.g. they shorten the expected option lives), but it fails to document management of a key assumption, the one concerning expected stock-price volatility. Using a new methodology, we address two questions: (1) To what extent do companies follow the guidance in FAS 123 and use forward looking information in addition to the readily available historical volatility in estimating expected volatility? (2) What determines
the cross-sectional variation in the reliance on forward looking information? We find that firms use both historical and forward-looking information in deriving expected volatility. We also find, however, that the reliance on forward-looking information is limited to situations where this reliance results in reduced expected volatility and thus smaller option expense. We interpret this finding as managers opportunistically use the discretion in estimating expected volatility afforded by FAS 123. In support of this interpretation, we also find that managerial incentives
play a key role in this opportunism
Statistical properties of Faraday rotation measure from large-scale magnetic fields in intervening disc galaxies
To constrain the large-scale magnetic field strengths in cosmologically
distant galax- ies, we derive the probability distribution function of Faraday
rotation measure (RM) when random lines of sight pass through a sample of disc
galaxies, with axisymmetric large-scale magnetic fields. We find that the width
of the RM distribution of the galaxy sample is directly related to the mean
large-scale field strength of the galaxy population, provided the dispersion
within the sample is lower than the mean value. In the absence of additional
constraints on parameters describing the magneto-ionic medium of the
intervening galaxies, and in the situation where RMs produced in the
intervening galaxies have already been statistically isolated from other RM
contributions along the lines of sight, our simple model of the magneto-ionic
medium in disc galaxies suggests that the mean large-scale magnetic field of
the population can be measured to within ~ 50% accuracy.Comment: 4 pages, Proceedings of FM8 "New Insights in Extragalactic Magnetic
Fields", XXXth General Assembly of the IAU, Vienna, August 20-31, 201
The Gas Accretion Rate of Star-forming Galaxies over the last 4 Gyr
Star-forming galaxies are believed to replenish their atomic gas reservoir,
which is consumed in star-formation, through accretion of gas from their
circumgalactic mediums (CGMs). However, there are few observational constraints
today on the gas accretion rate in external galaxies. Here, we use our recent
measurement of the scaling relation between the atomic hydrogen (HI) mass
and the stellar mass in star-forming galaxies at , with the relations between the star-formation rate (SFR) and , and
the molecular gas mass and , and the assumption that
star-forming galaxies evolve along the main sequence, to determine the
evolution of the neutral gas reservoir and the average net gas accretion rate
onto the disks of star-forming galaxies over the past 4 Gyr. For galaxies with
today, we find that both and in the
disk have increased, while has decreased, since . The
average gas accretion rate onto the disk over the past 4 Gyr is similar to the
average SFR over this period, implying that main-sequence galaxies have
maintained a stable HI reservoir, despite the consumption of gas in
star-formation. We obtain an average net gas accretion rate (over the past 4
Gyr) of for galaxies with the stellar mass of the
Milky Way. At low redshifts, , the reason for the decline in
the cosmic SFR density thus appears to be the inefficiency in the conversion of
atomic gas to molecular gas, rather than insufficient gas accretion from the
CGM.Comment: Accepted for publication in The Astrophysical Journal Letter
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