1,437 research outputs found
The Kinematically Measured Pattern Speeds of NGC 2523 and NGC 4245
We have applied the Tremaine-Weinberg continuity equation method to derive
the bar pattern speed in the SB(r)b galaxy NGC 2523 and the SB(r)0/a galaxy NGC
4245 using the Calcium Triplet absorption lines. These galaxies were selected
because they have strong inner rings which can be used as independent tracers
of the pattern speed. The pattern speed of NGC 2523 is 26.4 6.1 km
s kpc, assuming an inclination of 49.7 and a distance
of 51.0 Mpc. The pattern speed of NGC 4245 is 75.5 31.3 km s
kpc, assuming an inclination of 35.4 and a distance of 12.6
Mpc. The ratio of the corotation radius to the bar radius of NGC 2523 and NGC
4245 is 1.4 0.3 and 1.1 0.5, respectively. These values place the
bright inner rings near and slightly inside the corotation radius, as predicted
by barred galaxy theory. Within the uncertainties, both galaxies are found to
have fast bars that likely indicate dark halos of low central concentration.
The photometric properties, bar strengths, and disk stabilities of both
galaxies are also discussed.Comment: Accepted for publication in The Astronomical Journal, 11 figures, 2
table
The Distribution of Bar and Spiral Strengths in Disk Galaxies
The distribution of bar strengths in disk galaxies is a fundamental property
of the galaxy population that has only begun to be explored. We have applied
the bar/spiral separation method of Buta, Block, and Knapen to derive the
distribution of maximum relative gravitational bar torques, Q_b, for 147 spiral
galaxies in the statistically well-defined Ohio State University Bright Galaxy
Survey (OSUBGS) sample. Our goal is to examine the properties of bars as
independently as possible of their associated spirals. We find that the
distribution of bar strength declines smoothly with increasing Q_b, with more
than 40% of the sample having Q_b <= 0.1. In the context of recurrent bar
formation, this suggests that strongly-barred states are relatively short-lived
compared to weakly-barred or non-barred states. We do not find compelling
evidence for a bimodal distribution of bar strengths. Instead, the distribution
is fairly smooth in the range 0.0 <= Q_b < 0.8. Our analysis also provides a
first look at spiral strengths Q_s in the OSU sample, based on the same torque
indicator. We are able to verify a possible weak correlation between Q_s and
Q_b, in the sense that galaxies with the strongest bars tend also to have
strong spirals.Comment: Accepted for publication in the Astronomical Journal, August 2005
issue (LaTex, 23 pages + 11 figures, uses aastex.cls
Integral Field Spectroscopy of 23 Spiral Bulges
We have obtained Integral Field Spectroscopy for 23 spiral bulges using
INTEGRAL on the William Herschel Telescope and SPIRAL on the Anglo-Australian
Telescope. This is the first 2D survey directed solely at the bulges of spiral
galaxies. Eleven galaxies of the sample do not have previous measurements of
the stellar velocity dispersion (sigma*). These data are designed to complement
our Space Telescope Imaging Spectrograph program for estimating black hole
masses in the range 10^6-10^8M_sun using gas kinematics from nucleated disks.
These observations will serve to derive the stellar dynamical bulge properties
using the traditional Mgb and CaII triplets. We use both Cross Correlation and
Maximum Penalized Likelihood to determine projected sigma* in these systems and
present radial velocity fields, major axis rotation curves, curves of growth
and sigma* fields. Using the Cross Correlation to extract the low order 2D
stellar dynamics we generally see coherent radial rotation and irregular
velocity dispersion fields suggesting that sigma* is a non-trivial parameter to
estimate.Comment: 11 pages, 30 figures, accepted for publication in ApJ
The properties of the Galactic bar implied by gas kinematics in the inner Milky Way
Longitude-velocity (l-V) diagrams of H I and CO gas in the inner Milky Way
have long been known to be inconsistent with circular motion in an axisymmetric
potential. Several lines of evidence suggest that the Galaxy is barred, and gas
flow in a barred potential could be consistent with the observed ``forbidden''
velocities and other features in the data. We compare the H I observations to
l-V diagrams synthesized from 2-D fluid dynamical simulations of gas flows in a
family of barred potentials. The gas flow pattern is very sensitive to the
parameters of the assumed potential, which allows us to discriminate among
models. We present a model that reproduces the outer contour of the H I l-V
diagram reasonably well; this model has a strong bar with a semimajor axis of
3.6 kpc, an axis ratio of approximately 3:1, an inner Lindblad resonance (ILR),
and a pattern speed of 42 km/s/kpc, and matches the data best when viewed from
34\deg to the bar major axis. The behavior of the models, combined with the
constraint that the shocks in the Milky Way bar should resemble those in
external barred galaxies, leads us to conclude that wide ranges of parameter
space are incompatible with the observations. In particular we suggest that the
bar must be fairly strong, must have an ILR, and cannot be too end-on, with the
bar major axis at 35\deg +/- 5\deg to the line of sight. The H I data exhibit
larger forbidden velocities over a wider longitude range than are seen in
molecular gas; this important difference is the reason our favored model
differs so significantly from other recently proposed models.Comment: 23 pages, 14 figures, 1 table, uses emulateapj and psfig, 640 kb.
Submitted to Ap
Trees with Given Stability Number and Minimum Number of Stable Sets
We study the structure of trees minimizing their number of stable sets for
given order and stability number . Our main result is that the
edges of a non-trivial extremal tree can be partitioned into stars,
each of size or , so that every vertex is included in at most two
distinct stars, and the centers of these stars form a stable set of the tree.Comment: v2: Referees' comments incorporate
Kinematics of Spiral Arm Streaming in M51
We use CO and H alpha velocity fields to study the gas kinematics in the
spiral arms and interarms of M51 (NGC 5194), and fit the 2D velocity field to
estimate the radial and tangential velocity components as a function of spiral
phase (arm distance). We find large radial and tangential streaming velocities,
which are qualitatively consistent with the predictions of density wave theory
and support the existence of shocks. The streaming motions are complex, varying
significantly across the galaxy as well as along and between arms. Aberrations
in the velocity field indicate that the disk is not coplanar, perhaps as far in
as 20\arcsec\ (800 pc) from the center. Velocity profile fits from CO and H
alpha are typically similar, suggesting that most of the H alpha emission
originates from regions of recent star formation. We also explore vortensity
and mass conservation conditions. Vortensity conservation, which does not
require a steady state, is empirically verified. The velocity and density
profiles show large and varying mass fluxes, which are inconsistent with a
steady flow for a single dominant global spiral mode. We thus conclude that the
spiral arms cannot be in a quasi-steady state in any rotating frame, and/or
that out of plane motions may be significant.Comment: 50 pages, including 20 figures; Accepted for publication in ApJ. PDF
version with high resolution figures available at
http://www.astro.umd.edu/~shetty/Research
Molecular Gas Kinematics in Barred Spiral Galaxies
To quantify the effect that bar driven mass inflow can have on the evolution
of a galaxy requires an understanding of the dynamics of the inflowing gas. In
this paper we study the kinematics of the dense molecular gas in a set of seven
barred spiral galaxies to determine which dynamical effects dominate. The
kinematics are derived from observations of the CO J=(1-0) line made with the
Berkeley-Illinois-Maryland Association (BIMA) millimeter array. We compare the
observed kinematics to those predicted by ideal gas hydrodynamic and ballistic
cloud-based models of gas flow in a barred potential. The hydrodynamic model is
in good qualitative agreement with both the current observations of the dense
gas and previous observations of the kinematics of the ionized gas. The
observed kinematics indicate that the gas abruptly changes direction upon
entering the dust lanes to flow directly down the dust lanes along the leading
edge of the bar until the dust lanes approach the nuclear ring. Near the
location where the dust lanes intersect the nuclear ring, we see two velocity
components: a low velocity component, corresponding to gas on circular orbits,
and a higher velocity component, which can be attributed to the fraction of gas
flowing down the bar dust lane which sprays past the contact point toward the
other half of the bar. The ballistic cloud-based model of the ISM is not
consistent with the observed kinematics. The kinematics in the dust lanes
require large velocity gradients which cannot be reproduced by an ISM composed
of ballistic clouds with long mean-free-paths. Therefore, even the dense ISM
responds to hydrodynamic forces.Comment: To be published in the Astrophysical Journal, Nov. 20, 199
A burst search for gravitational waves from binary black holes
Compact binary coalescence (CBC) is one of the most promising sources of
gravitational waves. These sources are usually searched for with matched
filters which require accurate calculation of the GW waveforms and generation
of large template banks. We present a complementary search technique based on
algorithms used in un-modeled searches. Initially designed for detection of
un-modeled bursts, which can span a very large set of waveform morphologies,
the search algorithm presented here is constrained for targeted detection of
the smaller subset of CBC signals. The constraint is based on the assumption of
elliptical polarisation for signals received at the detector. We expect that
the algorithm is sensitive to CBC signals in a wide range of masses, mass
ratios, and spin parameters. In preparation for the analysis of data from the
fifth LIGO-Virgo science run (S5), we performed preliminary studies of the
algorithm on test data. We present the sensitivity of the search to different
types of simulated CBC waveforms. Also, we discuss how to extend the results of
the test run into a search over all of the current LIGO-Virgo data set.Comment: 12 pages, 4 figures, 2 tables, submitted for publication in CQG in
the special issue for the conference proceedings of GWDAW13; corrected some
typos, addressed some minor reviewer comments one section restructured and
references updated and correcte
Maximum Disk Mass Models for Spiral Galaxies
We present axisymmetric maximum disk mass models for a sample of 74 spiral
galaxies taken from the southern sky Fabry-Perot Tully-Fisher survey (Schommer
et al. 1993). The sample contains galaxies spanning a large range of
morphologies and having rotation widths from 180 km/sec to 680 km/secs. For
each galaxy we have an I-band image and a 2-d H_alpha velocity field. The
distribution of mass is modeled as a sum of disk and bulge components with
distinct, constant mass-to-light ratios. No dark matter halo is included in the
fits. The models reproduce the overall structure of the rotation curves in the
majority of galaxies, providing good fits to galaxies which exhibit pronounced
structural differences in their surface brightness profiles. 75% of galaxies
for which the rotation curve is measured to R_23.5 or beyond are well fit by a
mass-traces-light model for the entire region within R_23.5. The models for
about 20% of the galaxies do not fit well; the failure of most of these models
is traced directly to non-axisymmetric structures, primarily bars but also
strong spiral arms. The median I-band M/L of the disk plus bulge is 2.4+/-0.9
h_75 in solar units, consistent with normal stellar populations. These results
require either that the mass of dark matter within the optical disk of spiral
galaxies is small, or that its distribution is very precisely coupled to the
distribution of luminous matter.Comment: accepted by AJ, 29 pages, 8 figures, uses emulateapj.st
Decreased Frequency of Strong Bars in S0 Galaxies: Evidence for Secular Evolution?
Using data from the Near-Infrared S0 Survey (NIRS0S) of nearby, early-type
galaxies, we examine the distribution of bar strengths in S0 galaxies as
compared to S0/a and Sa galaxies, and as compared to previously published bar
strength data for Ohio State University Bright Spiral Galaxy Survey (OSUBSGS)
spiral galaxies. Bar strengths based on the gravitational torque method are
derived from 2.2 micron Ks-band images for a statistical sample of 138 (98 S0,
40 S0/a,Sa) galaxies having a mean total blue magnitude <= 12.5 and
generally inclined less than 65 degrees. We find that S0 galaxies have weaker
bars on average than spiral galaxies in general, even compared to their closest
spiral counterparts, S0/a and Sa galaxies. The differences are significant and
cannot be due entirely to uncertainties in the assumed vertical scale-heights
or in the assumption of constant mass-to-light ratios. Part of the difference
is likely due simply to the dilution of the bar torques by the higher mass
bulges seen in S0s. If spiral galaxies accrete external gas, as advocated by
Bournaud & Combes, then the fewer strong bars found among S0s imply a lack of
gas accretion according to this theory. If S0s are stripped former spirals, or
else are evolved from former spirals due to internal secular dynamical
processes which deplete the gas as well as grow the bulges, then the weaker
bars and the prevalence of lenses in S0 galaxies could further indicate that
bar evolution continues to proceed during and even after gas depletionComment: Accepted for publication in the Astrophysical Journal, September 2010
issue (LaTex, 29 pages + 3 figures, uses aastex.cls
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