10,221 research outputs found
Optical and Near-Infrared Imaging of the IRAS 1-Jy Sample of Ultraluminous Infrared Galaxies: II. The Analysis
The present paper discusses the results from an analysis of the images
presented in Paper I (astro-ph/0207373) supplemented with new spectroscopic
data obtained at Keck. All but one object in the 1-Jy sample show signs of a
strong tidal interaction/merger. Multiple mergers involving more than two
galaxies are seen in no more than 5 of the 118 (< 5%) systems. None of the 1-Jy
sources is in the first-approach stage of the interaction, and most (56%) of
them harbor a single disturbed nucleus and are therefore in the later stages of
a merger. Seyfert galaxies (especially those of type 1), warm ULIGs
(f_{25}/f_{60} > 0.2) and the more luminous systems (> 10^{12.5} L_sun) all
show a strong tendency to be advanced mergers with a single nucleus. An
analysis of the surface brightness profiles of the host galaxies in
single-nucleus sources reveals that about 73% of the R and K' surface
brightness profiles are fit adequately by an elliptical-like R^{1/4}-law. These
elliptical-like 1-Jy systems have luminosities, half-light radii, and R-band
axial ratio distribution that are similar to those of normal (inactive)
intermediate-luminosity ellipticals and follow with some scatter the same mu_e
- r_e relation. These elliptical-like hosts are most common among merger
remnants with Seyfert 1 nuclei (83%), Seyfert 2 optical characteristics (69%)
or mid-infrared (ISO) AGN signatures (80%). In general, the results from the
present study are consistent with the merger-driven evolutionary sequence
``cool ULIGs --> warm ULIGs --> quasars,'' although there are many exceptions.
(abridged)Comment: Correction to D.-C. Kim's affiliations. 42 pages + 3 tables + 3
multi-page jpeg figures; see
http://www.astro.umd.edu/~veilleux/pubs/paper2.tar.gz for original figure
Three-dimensional modeling of the HI kinematics of NGC 2915
The nearby blue compact dwarf, NGC 2915, has its stellar disc embedded in a
large, extended (~ 22 B-band scale-lengths) HI disc. New high-resolution HI
synthesis observations of NGC 2915 have been obtained with the Australia
Telescope Compact Array. These observations provide evidence of extremely
complex HI kinematics within the immediate vicinity of the galaxy's
star-forming core. We identify and quantify double-peaked HI line profiles near
the centre of the galaxy and show that the HI energetics can be accounted for
by the mechanical energy output of the central high-mass stellar population
within time-scales of 10^6-10^7 yr. Full three-dimensional models of the HI
data cube are generated and compared to the observations to test various
physical scenarios associated with the high-mass star-forming core of NGC 2915.
Purely circular HI kinematics are ruled out together with the possibility of a
high-velocity-dispersion inter-stellar medium at inner radii. Radial velocities
of ~ 30 km/s are required to describe the central-most HI kinematics of the
system. Our results lend themselves to the simple physical scenario in which
the young stellar core of the galaxy expels the gas outwards from the centre of
the disc, thereby creating a central HI under-density. These kinematics should
be thought of as being linked to a central HI outflow rather than a large-scale
galactic blow-out or wind.Comment: 11 pages, 6 figures, accepted for publication in MNRA
Radio spectral study of the cluster of galaxies Abell 2255
Spectral index studies of halos, relics, and radio galaxies provide useful
information on their origin and connection with merger processes. We present
WSRT multi-wavelength observations of the galaxy cluster Abell 2255 at 25 cm,
85 cm, and 2 m. The spectral index images allowed us to study the integrated
spectrum of halo and relic and to investigate the physical properties of the
Beaver head-tail radio galaxy belonging to the cluster. In the radio halo, the
spectral index is steeper at the center and flatter at the locations of the
radio filaments, clearly detected at 25 cm. In the relics, the spectral index
flattens, moving away from the cluster center. For the Beaver radio galaxy, the
spectrum severely steepens from the head towards the end of the tail, because
of the energy losses suffered by the relativistic particles. In the 2 m map,
which is the first high-sensitivity image presented in the literature at such a
long wavelength, a new Mpc-size emission region is detected between the known
radio halo and the NW relic. Not detecting this feature in the more sensitive
85 cm observations implies that it must have a very steep spectrum (alpha <=
-2.6). The observational properties of the radio halo suggest that either we
are looking at a superposition of different structures (filaments in the
foreground plus real halo in the background) seen in projection across the
cluster center or that the halo is intrinsically peculiar. The newly detected
extended region to the NW of the halo could be considered as an asymmetric
extension of the halo itself. However, since radio halos are known in the
literature as structures showing a regular morphology, the new feature could
represent the first example of steep Mpc-size diffuse structures (MDS),
detected around clusters at very low frequencies.Comment: 23 pages, 18 figures. A&A, in pres
Gravitational Wave Tests of General Relativity with the Parameterized Post-Einsteinian Framework
Gravitational wave astronomy has tremendous potential for studying extreme
astrophysical phenomena and exploring fundamental physics. The waves produced
by binary black hole mergers will provide a pristine environment in which to
study strong field, dynamical gravity. Extracting detailed information about
these systems requires accurate theoretical models of the gravitational wave
signals. If gravity is not described by General Relativity, analyses that are
based on waveforms derived from Einstein's field equations could result in
parameter biases and a loss of detection efficiency. A new class of
"parameterized post-Einsteinian" (ppE) waveforms has been proposed to cover
this eventuality. Here we apply the ppE approach to simulated data from a
network of advanced ground based interferometers (aLIGO/aVirgo) and from a
future spaced based interferometer (LISA). Bayesian inference and model
selection are used to investigate parameter biases, and to determine the level
at which departures from general relativity can be detected. We find that in
some cases the parameter biases from assuming the wrong theory can be severe.
We also find that gravitational wave observations will beat the existing bounds
on deviations from general relativity derived from the orbital decay of binary
pulsars by a large margin across a wide swath of parameter space.Comment: 16 pages, 10 figures. Modified in response to referee comment
LAMOST Experiment for Galactic Understanding and Exploration (LEGUE) The survey science plan
We describe the current plans for a spectroscopic survey of millions of stars
in the Milky Way galaxy using the Guo Shou Jing Telescope (GSJT, formerly the
Large Area Multi-Object Spectroscopic Telescope - LAMOST). The survey will
obtain spectra for 2.5 million stars brighter than during dark/grey
time, and 5 million stars brighter than or on nights that are
moonlit or have low transparency. The survey will begin in fall of 2012, and
will run for at least four years. The telescope design constrains the optimal
declination range for observations to , and site
conditions lead to an emphasis on stars in the direction of the Galactic
anticenter. The survey is divided into three parts with different target
selection strategies: disk, anticenter, and spheroid. The resulting dataset
will be used to study the merger history of the Milky Way, the substructure and
evolution of the disks, the nature of the first generation of stars through
identification of the lowest metallicity stars, and star formation through
study of open clusters and the OB associations. Detailed design of the LEGUE
survey will be completed after a review of the results of the pilot survey in
summer 2012.Comment: 19 pages, 1 figure, accepted for publication in RA
Insights into symmetric and asymmetric vortex mergers using the core growth model
We revisit the two vortex merger problem (symmetric and asymmetric) for the
Navier-Stokes equations using the core growth model for vorticity evolution
coupled with the passive particle field and an appropriately chosen
time-dependent rotating reference frame. Using the combined tools of analyzing
the topology of the streamline patterns along with careful tracking of passive
fields, we highlight the key features of the stages of evolution of vortex
merger, pinpointing deficiencies in the low-dimensional model with respect to
similar experimental/numerical studies. The model, however, reveals a far
richer and delicate sequence of topological bifurcations than has previously
been discussed in the literature for this problem, and at the same time points
the way towards a method of improving the model.Comment: 17 pages, 16 figure
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