743 research outputs found
Dynamical Masses in Luminous Infrared Galaxies
We have studied the dynamics and masses of a sample of ten nearby luminous
and ultraluminous infrared galaxies (LIRGS and ULIRGs), using 2.3 micron CO
absorption line spectroscopy and near-infrared H- and Ks-band imaging. By
combining velocity dispersions derived from the spectroscopy, disk
scale-lengths obtained from the imaging, and a set of likely model density
profiles, we calculate dynamical masses for each LIRG. For the majority of the
sample, it is difficult to reconcile our mass estimates with the large amounts
of gas derived from millimeter observations and from a standard conversion
between CO emission and H_2 mass. Our results imply that LIRGs do not have huge
amounts of molecular gas (10^10-10^11 Msolar) at their centers, and support
previous indications that the standard conversion of CO to H_2 probably
overestimates the gas masses and cannot be used in these environments. This in
turn suggests much more modest levels of extinction in the near-infrared for
LIRGs than previously predicted (A_V~10-20 versus A_V~100-1000). The lower gas
mass estimates indicated by our observations imply that the star formation
efficiency in these systems is very high and is triggered by cloud-cloud
collisions, shocks, and winds rather than by gravitational instabilities in
circumnuclear gas disks.Comment: 14 pages, 2 figures, accepted to Ap
Extended Emission Line Gas in Radio Galaxies - PKS0349-27
PKS0349-27 is a classical FRII radio galaxy with an AGN host which has a
spectacular, spiral-like structure in its extended emission line gas (EELG). We
have measured the velocity field in this gas and find that it splits into 2
cloud groups separated by radial velocities which at some points approach 400
km/s Measurements of the diagnostic emission line ratios [OIII]5007/H-beta,
[SII]6716+6731/H-alpha, and [NII]6583/H-alpha in these clouds show no evidence
for the type of HII region emission associated with starburst activity in
either velocity system. The measured emission line ratios are similar to those
found in the nuclei of narrow-line radio galaxies, but the extended
ionization/excitation cannot be produced by continuum emission from the active
nucleus alone. We present arguments which suggest that the velocity
disturbances seen in the EELG are most likely the result of a galaxy-galaxy
collision or merger but cannot completely rule out the possibility that the gas
has been disrupted by the passage of a radio jet.Comment: 12 pages, 3 fig pages, to appear in the Astrophys.
Log-Networks
We introduce a growing network model in which a new node attaches to a
randomly-selected node, as well as to all ancestors of the target node. This
mechanism produces a sparse, ultra-small network where the average node degree
grows logarithmically with network size while the network diameter equals 2. We
determine basic geometrical network properties, such as the size dependence of
the number of links and the in- and out-degree distributions. We also compare
our predictions with real networks where the node degree also grows slowly with
time -- the Internet and the citation network of all Physical Review papers.Comment: 7 pages, 6 figures, 2-column revtex4 format. Version 2: minor changes
in response to referee comments and to another proofreading; final version
for PR
A Study of the Direct-Fitting Method for Measurement of Galaxy Velocity Dispersions
We have measured the central stellar velocity dispersions of 33 nearby spiral
and elliptical galaxies, using a straightforward template-fitting algorithm
operating in the pixel domain. The spectra, obtained with the Double
Spectrograph at Palomar Observatory, cover both the Ca triplet and the Mg b
region, and we present a comparison of the velocity dispersion measurements
from these two spectral regions. Model fits to the Ca triplet region generally
yield good results with little sensitivity to the choice of template star. In
contrast, the Mg b region is more sensitive to template mismatch and to details
of the fitting procedure such as the order of a polynomial used to match the
continuum shape of the template to the object. As a consequence of the
correlation of the [Mg/Fe] ratio with velocity dispersion, it is difficult to
obtain a satisfactory model fit to the Mg b lines and the surrounding Fe blends
simultaneously, particularly for giant elliptical galaxies with large velocity
dispersions. We demonstrate that if the metallicities of the galaxy and
template star are not well matched, then direct template-fitting results are
improved if the Mg b lines themselves are excluded from the fit and the
velocity dispersion is determined from the surrounding weaker lines.Comment: 14 pages. To appear in A
Analysis of the 23 June 2001 M w = 8.4 Peru underthrusting earthquake and its aftershocks
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95298/1/grl16079.pd
Towards the characterization of individual users through Web analytics
We perform an analysis of the way individual users navigate in the Web. We
focus primarily in the temporal patterns of they return to a given page. The
return probability as a function of time as well as the distribution of time
intervals between consecutive visits are measured and found to be independent
of the level of activity of single users. The results indicate a rich variety
of individual behaviors and seem to preclude the possibility of defining a
characteristic frequency for each user in his/her visits to a single site.Comment: 8 pages, 4 figures. To appear in Proceeding of Complex'0
Pictor A (PKS 0518-45) - From Nucleus to Lobes
We present radio and optical imaging and kinematic data for the radio galaxy
Pictor A, including HST continuum and [OIII], emission-line images (at a
resolution of 25 - 100 mas) and ground-based imaging and spectroscopy (at a
resolution of ~ 1.5". The radio data include 3 cm Australia Telescope images of
the core, at a resolution comparable to that of the optical, ground-based
images, and a VLBI image of a jet in the compact core (at a resolution of 2 -
25 mas), which seems to align with a continuum ``jet'' found in the HST images.
The core radio jet, the HST optical continuum ``jet'', and the NW H-alpha
filaments all appear to point toward the optical-synchrotron hot-spot in the NW
lobe of this object and are associated with a disrupted velocity field in the
extended ionized gas. The ground-based spectra which cover this trajectory also
yield line ratios for the ionized gas which have anomalously low [NII] (6564),
suggesting either a complex, clumpy structure in the gas with a higher
cloud-covering factor at larger radii and with denser clouds than is found in
the nuclear regions of most NLRG and Seyfert 2 galaxies, or some other,
unmodeled, mechanism for the emergent spectrum from this region. The H-alpha
emission-line filaments to the N appear to be associated with a 3 cm radio
continuum knot which lies in a gap in the filaments ~ 4" from the nucleus.
Altogether, the data in this paper provide good circumstantial evidence for
non-disruptive redirection of a radio jet by interstellar gas clouds in the
host galaxy.Comment: 19 pages, 6 ps.gz fig pages, to appear in the Ap.J. Supp
Numerical Study of Order in a Gauge Glass Model
The XY model with quenched random phase shifts is studied by a T=0 finite
size defect energy scaling method in 2d and 3d. The defect energy is defined by
a change in the boundary conditions from those compatible with the true ground
state configuration for a given realization of disorder. A numerical technique,
which is exact in principle, is used to evaluate this energy and to estimate
the stiffness exponent . This method gives in
2d and in 3d, which are considerably larger than
previous estimates, strongly suggesting that the lower critical dimension is
less than three. Some arguments in favor of these new estimates are given.Comment: 4 pages, 2 figures, revtex. Submitted to Phys. Rev. Let
Numerical Study of Spin and Chiral Order in a Two-Dimensional XY Spin Glass
The two dimensional XY spin glass is studied numerically by a finite size
scaling method at T=0 in the vortex representation which allows us to compute
the exact (in principle) spin and chiral domain wall energies. We confirm
earlier predictions that there is no glass phase at any finite T. Our results
strongly support the conjecture that both spin and chiral order have the same
correlation length exponent . We obtain preliminary results
in 3d.Comment: 4 pages, 2 figures, revte
Anomalous finite-size effect in superconducting Josephson junction arrays
We report large-scale simulations of the resistively-shunted Josephson
junction array in strip geometry. As the strip width increases, the voltage
first decreases following the dynamic scaling ansatz proposed by Minnhagen {\it
et al.} [Phys. Rev. Lett. {\bf 74}, 3672 (1995)], and then rises towards the
asymptotic value predicted by Ambegaokar {\it et al.} [Phys. Rev. Lett. {\bf
40}, 783 (1978)]. The nonmonotonic size-dependence is attributed to shortened
life time of free vortices in narrow strips, and points to the danger of
single-scale analysis applied to a charge-neutral superfluid state.Comment: 4 pages, 2 figure
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