1,707 research outputs found
Spin and orbital Hall effects for diffracting optical beams in gradient-index media
We examine the evolution of paraxial beams carrying intrinsic spin and
orbital angular momenta (AM) in gradient-index media. A parabolic-type equation
is derived which describes the beam diffraction in curvilinear coordinates
accompanying the central ray. The center of gravity of the beam experiences
transverse AM-dependent deflections -- the spin and orbital Hall effects. The
spin Hall effect generates a transverse translation of the beam as a whole, in
precise agreement with recent geometrical optics predictions. At the same time,
the orbital Hall effect is significantly affected by the diffraction in the
inhomogeneous medium and is accompanied by changes in the intrinsic orbital AM
and deformations of the beam.Comment: 4 pages, 2 figures, to appear in Phys. Rev.
Escape of Ionizing Radiation from High Redshift Galaxies
We model the escape of ionizing radiation from high-redshift galaxies using
high-resolution Adaptive Mesh Refinement N-body + hydrodynamics simulations.
Our simulations include time-dependent and spatially-resolved transfer of
ionizing radiation in three dimensions, including effects of dust absorption.
For galaxies of total mass M > 10^11 Msun and star formation rates SFR ~ 1-5
Msun/yr, we find angular averaged escape fractions of 0.01-0.03 over the entire
redshift interval studied (3<z<9). In addition, we find that the escape
fraction varies by more than an order of magnitude along different
lines-of-sight within individual galaxies, from the largest values near
galactic poles to the smallest along the galactic disk. The escape fraction
declines steeply at lower masses and SFR. We show that the low values of escape
fractions are due to a small fraction of young stars located just outside the
edge of HI disk. We compare our predicted escape fraction of ionizing photons
with previous results, and find a general agreement with both other simulation
results and available direct detection measurements at z ~ 3. We also compare
our simulations with a novel method to estimate the escape fraction in galaxies
from the observed distribution of neutral hydrogen column densities along the
lines of sights to long duration gamma-ray bursts. Using this method we find
escape fractions of the GRB host galaxies of 2-3%, consistent with our
theoretical predictions. [abridged]Comment: submitted to Ap
Towards a complete accounting of energy and momentum from stellar feedback in galaxy formation simulations
Stellar feedback plays a key role in galaxy formation by regulating star
formation, driving interstellar turbulence and generating galactic scale
outflows. Although modern simulations of galaxy formation can resolve scales of
10-100 pc, star formation and feedback operate on smaller, "subgrid" scales.
Great care should therefore be taken in order to properly account for the
effect of feedback on global galaxy evolution. We investigate the momentum and
energy budget of feedback during different stages of stellar evolution, and
study its impact on the interstellar medium using simulations of local star
forming regions and galactic disks at the resolution affordable in modern
cosmological zoom-in simulations. In particular, we present a novel subgrid
model for the momentum injection due to radiation pressure and stellar winds
from massive stars during early, pre-supernova evolutionary stages of young
star clusters. Early injection of momentum acts to clear out dense gas in star
forming regions, hence limiting star formation. The reduced gas density
mitigates radiative losses of thermal feedback energy from subsequent supernova
explosions, leading to an increased overall efficiency of stellar feedback. The
detailed impact of stellar feedback depends sensitively on the implementation
and choice of parameters. Somewhat encouragingly, we find that implementations
in which feedback is efficient lead to approximate self-regulation of global
star formation efficiency. We compare simulation results using our feedback
implementation to other phenomenological feedback methods, where thermal
feedback energy is allowed to dissipate over time scales longer than the formal
gas cooling time. We find that simulations with maximal momentum injection
suppress star formation to a similar degree as is found in simulations adopting
adiabatic thermal feedback.Comment: ApJ submitted. For a high-resolution version of the paper, see
http://kicp.uchicago.edu/~agertz
Simulations of disk galaxies with cosmic ray driven galactic winds
We present results from high-resolution hydrodynamic simulations of isolated
SMC- and Milky Way-sized galaxies that include a model for feedback from
galactic cosmic rays (CRs). We find that CRs are naturally able to drive winds
with mass loading factors of up to ~10 in dwarf systems. The scaling of the
mass loading factor with circular velocity between the two simulated systems is
consistent with \propto v_c^{1-2} required to reproduce the faint end of the
galaxy luminosity function. In addition, simulations with CR feedback reproduce
both the normalization and the slope of the observed trend of wind velocity
with galaxy circular velocity. We find that winds in simulations with CR
feedback exhibit qualitatively different properties compared to SN driven
winds, where most of the acceleration happens violently in situ near star
forming sites. In contrast, the CR-driven winds are accelerated gently by the
large-scale pressure gradient established by CRs diffusing from the
star-forming galaxy disk out into the halo. The CR-driven winds also exhibit
much cooler temperatures and, in the SMC-sized system, warm (T~10^4 K) gas
dominates the outflow. The prevalence of warm gas in such outflows may provide
a clue as to the origin of ubiquitous warm gas in the gaseous halos of galaxies
detected via absorption lines in quasar spectra.Comment: ApJL accepted. Replaced with accepted version. Minor revision in
response to referee comment
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