2,408 research outputs found
[C II] emission from galactic nuclei in the presence of X-rays
The luminosity of [C II] is used to probe the star formation rate in
galaxies, but the correlation breaks down in some active galactic nuclei
(AGNs). Models of the [C II] emission from galactic nuclei do not include the
influence of X-rays on the carbon ionization balance, which may be a factor in
reducing the [C II] luminosity. We calculate the [C II] luminosity in galactic
nuclei under the influence of bright sources of X-rays. We solve the balance
equation of the ionization states of carbon as a function of X-ray flux,
electron, atomic hydrogen, and molecular hydrogen density. These are input to
models of [CII] emission from the interstellar medium (ISM) in galactic nuclei.
We also solve the distribution of the ionization states of oxygen and nitrogen
in highly ionized regions. We find that the dense warm ionized medium (WIM) and
dense photon dominated regions (PDRs) dominate the [C II] emission when no
X-rays are present. The X-rays in galactic nuclei can affect strongly the C
abundance in the WIM converting some fraction to C and higher ionization
states and thus reducing its [C II] luminosity. For an X-ray luminosity >
10 erg/s the [C II] luminosity can be suppressed by a factor of a few,
and for very strong sources, >10 erg/s, such as found for many AGNs by
an order of magnitude. Comparison of the model with extragalactic sources shows
that the [C II] to far-infrared ratio declines for an X-ray luminosity
>10 erg/s, in reasonable agreement with our model.Comment: 16 pages and 14 figures, accepted for publication in A&
Breakdown of the operator product expansion in the 't Hooft model
We consider deep inelastic scattering in the 't Hooft model. Being solvable,
this model allows us to directly compute the moments associated with the cross
section at next-to-leading order in the 1/Q^2 expansion. We perform the same
computation using the operator product expansion. We find that all the terms
match in both computations except for one in the hadronic side, which is
proportional to a non-local operator. The basics of the result suggest that a
similar phenomenon may occur in four dimensions in the large N_c limit.Comment: 4 page
Bridging the Gap: Observations and Theory of Star Formation Meet on Large and Small Scales
The drive to understand galaxy formation and evolution over the lifetime of the universe has justified vast space-based and ground-based telescope facilities, as well as the development of new technologies. The details of star formation, and the modes by which that activity couples to the broader galactic environment, occurs on small spatial scales. These scales can only be traced with great sophistication in the local universe, as witnessed by observations using the Spitzer Space Telescope, the Herschel Space Observatory, the Stratospheric Observatory for Infrared Astronomy (SOFIA), and the Atacama Large Millimeter Array (ALMA). A critical realization of the last decade, however, is that the large scales and small scales are strongly coupled, and cannot be treated in isolation. At the same time, the scales studied in the local universe are "sub-grid"
for the purpose of cosmological simulations that make valiant efforts to include the physics of star formation and its feedback to the local environment. The fundamental uncertainties in how this sub-grid physics is incorporated into the larger picture are by far the greatest limitation in understanding galaxy formation and evolution.
The main goal of cosmological simulations is to understand the formation and evolution of the universe over a wide range of scales going from the Hubble volume to sub-light-year scales within galaxies. However, due to computational limitations, the physics of star formation in galaxies and the effects this process has on the formation and evolution of galaxies are often greatly simplified.
For instance, accounting for the effect of stellar feedback in cosmological simulations is a crucial factor in galaxy formation and evolution. Without stellar feedback in galaxies, the gas would rapidly cool and collapse, converting all available gas into stars within a dynamical time. This consequence is in sharp conflict with observations—there are vastly fewer stars in our universe
than the models would predict. Until recently, numerical simulations have been unable to regulate star formation efficiently enough to reproduce observations, with many consequences: models with too many stars also predict they form at the wrong times in the universe’s history, that there are the wrong abundances of heavy elements, that galaxies look nothing like the Milky Way, and even that the observable properties of dark matter are (apparently) discordant with new
precision-cosmology measurements. All of these problems may, in fact, be due to the fundamental problem of understanding how small and large scales interact
Deep inelastic scattering and factorization in the 't Hooft Model
We study in detail deep inelastic scattering in the 't Hooft model. We are
able to analytically check current conservation and to obtain analytic
expressions for the matrix elements with relative precision O(1/Q^2) for 1-x >>
\beta^2/Q^2. This allows us to compute the electron-meson differential cross
section and its moments with 1/Q^2 precision. For the former we find maximal
violations of quark-hadron duality, as it is expected for a large N_c analysis.
For the latter we find violations of the operator product expansion at
next-to-leading order in the 1/Q^2 expansion.Comment: 55 pages, 16 figure
L1599B: Cloud Envelope and C+ Emission in a Region of Moderately Enhanced Radiation Field
We study the effects of an asymmetric radiation field on the properties of a
molecular cloud envelope. We employ observations of carbon monoxide (12CO and
13CO), atomic carbon, ionized carbon, and atomic hydrogen to analyze the
chemical and physical properties of the core and envelope of L1599B, a
molecular cloud forming a portion of the ring at approximately 27 pc from the
star Lambda Ori. The O III star provides an asymmetric radiation field that
produces a moderate enhancement of the external radiation field. Observations
of the [CII] fine structure line with the GREAT instrument on SOFIA indicate a
significant enhanced emission on the side of the cloud facing the star, while
the [Ci], 12CO and 13CO J = 1-0 and 2-1, and 12CO J = 3-2 data from the PMO and
APEX telescopes suggest a relatively typical cloud interior. The atomic, ionic,
and molecular line centroid velocities track each other very closely, and
indicate that the cloud may be undergoing differential radial motion. The HI
data from the Arecibo GALFA survey and the SOFIA/GREAT [CII] data do not
suggest any systematic motion of the halo gas, relative to the dense central
portion of the cloud traced by 12CO and 13CO.Comment: 9 Figure
The Magnetic Field in Taurus Probed by Infrared Polarization
We present maps of the plane-of-sky magnetic field within two regions of the
Taurus molecular cloud: one in the dense core L1495/B213 filament, the other in
a diffuse region to the west. The field is measured from the polarization of
background starlight seen through the cloud. In total, we measured 287
high-quality near-infrared polarization vectors in these regions. In
L1495/B213, the percent polarization increases with column density up to Av ~ 9
mag, the limits of our data. The Radiative Torques model for grain alignment
can explain this behavior, but models that invoke turbulence are inconsistent
with the data. We also combine our data with published optical and
near-infrared polarization measurements in Taurus. Using this large sample, we
estimate the strength of the plane-of-sky component of the magnetic field in
nine subregions. This estimation is done with two different techniques that use
the observed dispersion in polarization angles. Our values range from 5-82
microgauss and tend to be higher in denser regions. In all subregions, the
critical index of the mass-to-magnetic flux ratio is sub-unity, implying that
Taurus is magnetically supported on large scales (~2 pc). Within the region
observed, the B213 filament makes a sharp turn to the north and the direction
of the magnetic field also takes a sharp turn, switching from being
perpendicular to the filament to becoming parallel. This behavior can be
understood if we are observing the rim of a bubble. We argue that it has
resulted from a supernova remnant associated with a recently discovered nearby
gamma-ray pulsar.Comment: Accepted into the Astrophysical Journal. 20 pages in emulateapj
format including 10 figures and 4 table
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