11,053 research outputs found
Radiative Transfer and Radiative driving of Outflows in AGN and Starbursts
To facilitate the study of black hole fueling, star formation, and feedback
in galaxies, we outline a method for treating the radial forces on interstellar
gas due to absorption of photons by dust grains. The method gives the correct
behavior in all of the relevant limits (dominated by the central point source;
dominated by the distributed isotropic source; optically thin; optically thick
to UV/optical; optically thick to IR) and reasonably interpolates between the
limits when necessary. The method is explicitly energy conserving so that
UV/optical photons that are absorbed are not lost, but are rather redistributed
to the IR where they may scatter out of the galaxy. We implement the radiative
transfer algorithm in a two-dimensional hydrodynamical code designed to study
feedback processes in the context of early-type galaxies. We find that the
dynamics and final state of simulations are measurably but only moderately
affected by radiative forces on dust, even when assumptions about the
dust-to-gas ratio are varied from zero to a value appropriate for the Milky
Way. In simulations with high gas densities designed to mimic ULIRGs with a
star formation rate of several hundred solar masses per year, dust makes a more
substantial contribution to the dynamics and outcome of the simulation. We find
that, despite the large opacity of dust to UV radiation, the momentum input to
the flow from radiation very rarely exceeds L/c due to two factors: the low
opacity of dust to the re-radiated IR and the tendency for dust to be destroyed
by sputtering in hot gas environments. We also develop a simplification of our
radiative transfer algorithm that respects the essential physics but is much
easier to implement and requires a fraction of the computational cost.Comment: 25 pages, 17 figures, submitted to MNRA
Simulating multiple merger pathways to the central kinematics of early-type galaxies
Two-dimensional integral field surveys such as ATLAS^3D are producing rich
observational data sets yielding insights into galaxy formation. These new
kinematic observations have highlighted the need to understand the evolutionary
mechanisms leading to a spectrum of fast-rotators and slow-rotators in
early-type galaxies. We address the formation of slow and fast rotators through
a series of controlled, comprehensive hydrodynamical simulations sampling
idealized galaxy merger scenarios constructed from model spiral galaxies.
Idealized and controlled simulations of this sort complement the more
'realistic' cosmological simulations by isolating and analyzing the effects of
specific parameters, as we do in this paper. We recreate minor and major binary
mergers, binary merger trees with multiple progenitors, and multiple sequential
mergers. Within each of these categories of formation history, we correlate
progenitor gas fraction, mass ratio, orbital pericenter, orbital ellipticity,
and spin with remnant kinematic properties. We create kinematic profiles of
these 95 simulations comparable to ATLAS^3D data. By constructing remnant
profiles of the projected specific angular momentum (lambda_R = /
, triaxiality, and measuring the incidences of kinematic
twists and kinematically decoupled cores, we distinguish between varying
formation scenarios. We find that binary mergers nearly always form fast
rotators. Slow rotators can be formed from zero initial angular momentum
configurations and gas-poor mergers, but are not as round as the ATLAS^3D
galaxies. Remnants of binary merger trees are triaxial slow rotators.
Sequential mergers form round slow rotators that most resemble the ATLAS^3D
rotators.Comment: MNRAS, in press, 12 pages, 15 figure
Anisotropy of the Microwave Sky at 90 GHz: Results from Python II
We report on additional observations of degree scale anisotropy at 90~GHz
from the Amundsen-Scott South Pole Station in Antarctica. Observations during
the first season with the Python instrument yielded a statistically significant
sky signal; in this paper we report the confirmation of that signal with data
taken in the second year, and on results from an interleaving set of fields.Comment: 10 pages, plus 2 figures. Postscript and uufiles versions available
via anonymous ftp at ftp://astro.uchicago.edu/pub/astro/ruhl/pyI
Galaxy size trends as a consequence of cosmology
We show that recently documented trends in galaxy sizes with mass and
redshift can be understood in terms of the influence of underlying cosmic
evolution; a holistic view which is complimentary to interpretations involving
the accumulation of discreet evolutionary processes acting on individual
objects. Using standard cosmology theory, supported with results from the
Millennium simulations, we derive expected size trends for collapsed cosmic
structures, emphasising the important distinction between these trends and the
assembly paths of individual regions. We then argue that the observed variation
in the stellar mass content of these structures can be understood to first
order in terms of natural limitations of cooling and feedback. But whilst these
relative masses vary by orders of magnitude, galaxy and host radii have been
found to correlate linearly. We explain how these two aspects will lead to
galaxy sizes that closely follow observed trends and their evolution, comparing
directly with the COSMOS and SDSS surveys. Thus we conclude that the observed
minimum radius for galaxies, the evolving trend in size as a function of mass
for intermediate systems, and the observed increase in the sizes of massive
galaxies, may all be considered an emergent consequence of the cosmic
expansion.Comment: 14 pages, 13 figures. Accepted by MNRA
Radius Dependent Luminosity Evolution of Blue Galaxies in GOODS-N
We examine the radius-luminosity (R-L) relation for blue galaxies in the Team
Keck Redshift Survey (TKRS) of GOODS-N. We compare with a volume-limited, Sloan
Digital Sky Survey sample and find that the R-L relation has evolved to lower
surface brightness since z=1. Based on the detection limits of GOODS this can
not be explained by incompleteness in low surface-brightness galaxies. Number
density arguments rule out a pure radius evolution. It can be explained by a
radius dependent decline in B-band luminosity with time. Assuming a linear
shift in M_B with z, we use a maximum likelihood method to quantify the
evolution. Under these assumptions, large (R_{1/2} > 5 kpc), and intermediate
sized (3 < R_{1/2} < 5 kpc) galaxies, have experienced Delta M_B =1.53
(-0.10,+0.13) and 1.65 (-0.18, +0.08) magnitudes of dimming since z=1. A simple
exponential decline in star formation with an e-folding time of 3 Gyr can
result in this amount of dimming. Meanwhile, small galaxies, or some subset
thereof, have experienced more evolution, 2.55 (+/- 0.38) magnitudes. This
factor of ten decline in luminosity can be explained by sub-samples of
starbursting dwarf systems that fade rapidly, coupled with a decline in burst
strength or frequency. Samples of bursting, luminous, blue, compact galaxies at
intermediate redshifts have been identified by various previous studies. If
there has been some growth in galaxy size with time, these measurements are
upper limits on luminosity fading.Comment: 34 Total pages, 15 Written pages, 19 pages of Data Table, 13 Figures,
accepted for publication in Ap
Shapes of Stellar Systems and Dark Halos from Simulations of Galaxy Major Mergers
Using a sample of 89 snapshots from 58 hydrodynamic binary galaxy major
merger simulations, we find that stellar remnants are mostly oblate while dark
matter halos are mostly prolate or triaxial. The stellar minor axis and the
halo major axis are almost always nearly perpendicular. This can be understood
by considering the influence of angular momentum and dissipation during the
merger. If binary mergers of spiral galaxies are responsible for the formation
of elliptical galaxies or some subpopulation thereof, these galaxies can be
expected to be oblate and inhabit their halos with the predicted shapes and
orientations. These predictions are potentially relevant to observational
studies of weak gravitational lensing, where one must stack many optically
aligned galaxies in order to determine the shape of the resulting stacked mass
distribution. The simple relationship between the dark and luminous matter
presented here can be used to guide the stacking of galaxies to minimize the
information lost.Comment: 4 pages, 5 figures. Minor changes to match published versio
First Results from SPARO: Evidence for Large-Scale Toroidal Magnetic Fields in the Galactic Center
We have observed the linear polarization of 450 micron continuum emission
from the Galactic center, using a new polarimetric detector system that is
operated on a 2 m telescope at the South Pole. The resulting polarization map
extends ~ 170 pc along the Galactic plane and ~ 30 pc in Galactic latitude, and
thus covers a significant fraction of the central molecular zone. Our map shows
that this region is permeated by large-scale toroidal magnetic fields. We
consider our results together with radio observations that show evidence for
poloidal fields in the Galactic center, and with Faraday rotation observations.
We compare all of these observations with the predictions of a magnetodynamic
model for the Galactic center that was proposed in order to explain the
Galactic Center Radio Lobe as a magnetically driven gas outflow. We conclude
that the observations are basically consistent with the model.Comment: 11 pages, 2 figures, 1 table, submitted to ApJ Let
Heating of the molecular gas in the massive outflow of the local ultraluminous-infrared and radio-loud galaxy 4C12.50
We present a comparison of the molecular gas properties in the outflow vs. in
the ambient medium of the local prototype radio-loud and ultraluminous-infrared
galaxy 4C12.50 (IRAS13451+1232), using new data from the IRAM Plateau de Bure
interferometer and 30m telescope, and the Herschel space telescope. Previous
H_2 (0-0) S(1) and S(2) observations with the Spitzer space telescope had
indicated that the warm (~400K) molecular gas in 4C12.50 is made up of a
1.4(+-0.2)x10^8 M_sun ambient reservoir and a 5.2(+-1.7)x10^7 M_sun outflow.
The new CO(1-0) data cube indicates that the corresponding cold (25K) H_2 gas
mass is 1.0(+-0.1)x10^10 M_sun for the ambient medium and <1.3x10^8 M_sun for
the outflow, when using a CO-intensity-to-H_2-mass conversion factor alpha of
0.8 M_sun /(K km/s pc^2). The combined mass outflow rate is high, 230-800
M_sun/yr, but the amount of gas that could escape the galaxy is low. A
potential inflow of gas from a 3.3(+-0.3)x10^8 M_sun tidal tail could moderate
any mass loss. The mass ratio of warm-to-cold molecular gas is >= 30 times
higher in the outflow than in the ambient medium, indicating that a
non-negligible fraction of the accelerated gas is heated to temperatures at
which star formation is inefficient. This conclusion is robust against the use
of different alpha factor values, and/or different warm gas tracers (H_2 vs.
H_2 plus CO): with the CO-probed gas mass being at least 40 times lower at 400K
than at 25K, the total warm-to-cold mass ratio is always lower in the ambient
gas than in the entrained gas. Heating of the molecular gas could facilitate
the detection of new outflows in distant galaxies by enhancing their emission
in intermediate rotational number CO lines.Comment: A&A, in pres
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