117 research outputs found
Modeling Non-Circular Motions in Disk Galaxies: Application to NGC 2976
We present a new procedure to fit non-axisymmetric flow patterns to 2-D
velocity maps of spiral galaxies. We concentrate on flows caused by bar-like or
oval distortions to the total potential that may arise either from a
non-axially symmetric halo or a bar in the luminous disk. We apply our method
to high-quality CO and Halpha data for the nearby, low-mass spiral NGC 2976
previously obtained by Simon et al., and find that a bar-like model fits the
data at least as well as their model with large radial flows. We find
supporting evidence for the existence of a bar in the baryonic disk. Our model
suggests that the azimuthally averaged central attraction in the inner part of
this galaxy is larger than estimated by these authors. It is likely that the
disk is also more massive, which will limit the increase to the allowed dark
halo density. Allowance for bar-like distortions in other galaxies may either
increase or decrease the estimated central attraction.Comment: 12 pages, 6 figures, accepted for publication in ApJ. v2: minor
changes to match proofs. For version with high-resolution figures, see
http://www.physics.rutgers.edu/~spekkens/papers/noncirc.pd
Impact of dark matter subhalos on extended HI disks of galaxies: Possible formation of HI fine structures and stars
Recent observations have discovered star formation activities in the extreme
outer regions of disk galaxies. However it remains unclear what physical
mechanisms are responsible for triggering star formation in such low-density
gaseous environments of galaxies. In order to understand the origin of these
outer star-forming regions, we numerically investigate how the impact of dark
matter subhalos orbiting a gas-rich disk galaxy embedded in a massive dark
matter halo influences the dynamical evolution of outer HI gas disk of the
galaxy. We find that if the masses of the subhalos () in a galaxy
with an extended HI gas disk are as large as , where
is the total mass of the galaxy's dark halo, local fine structures
can be formed in the extended HI disk. We also find that the gas densities of
some apparently filamentary structures can exceed a threshold gas density for
star formation and thus be likely to be converted into new stars in the outer
part of the HI disk in some models with larger . These results thus
imply that the impact of dark matter subhalos (``dark impact'') can be
important for better understanding the origin of recent star formation
discovered in the extreme outer regions of disk galaxies. We also suggest that
characteristic morphologies of local gaseous structures formed by the dark
impact can indirectly prove the existence of dark matter subhalos in galaxies.
We discuss the origin of giant HI holes observed in some gas-rich galaxies
(e.g., NGC 6822) in the context of the dark impact.Comment: 8 pages, 4 figures, accepted by ApJ
Identifying the Environment and Redshift of GRB Afterglows from the Time-Dependence of Their Absorption Spectra
The discovery of Gamma-Ray Burst (GRB) afterglows revealed a new class of
variable sources at optical and radio wavelengths. At present, the environment
and precise redshift of the detected afterglows are still unknown. We show that
if a GRB source resides in a compact (<100pc) gas-rich environment, the
afterglow spectrum will show time-dependent absorption features due to the
gradual ionization of the surrounding medium by the afterglow radiation.
Detection of this time-dependence can be used to constrain the size and density
of the surrounding gaseous system. For example, the MgII absorption line
detected in GRB970508 should have weakened considerably during the first month
if the absorption occurred in a gas cloud of size <100pc around the source. The
time-dependent HI or metal absorption features provide a precise determination
of the GRB redshift.Comment: 13 pages, 4 figures, submitted to ApJ
The Structure of Dark Matter Haloes in Dwarf Galaxies
Recent observations indicate that dark matter haloes have flat central
density profiles. Cosmological simulations with non-baryonic dark matter
predict however self similar haloes with central density cusps. This
contradiction has lead to the conclusion that dark matter must be baryonic.
Here it is shown that the dark matter haloes of dwarf spiral galaxies represent
a one parameter family with self similar density profiles. The observed global
halo parameters are coupled with each other through simple scaling relations
which can be explained by the standard cold dark matter model if one assumes
that all the haloes formed from density fluctuations with the same primordial
amplitude. We find that the finite central halo densities correlate with the
other global parameters. This result rules out scenarios where the flat halo
cores formed subsequently through violent dynamical processes in the baryonic
component. These cores instead provide important information on the origin and
nature of dark matter in dwarf galaxies.Comment: uuencoded Z-compressed postscript file, 10 pages, 3 figures included,
to appear in ApJ Letter
Rotational Widths for Use in the Tully-Fisher Relation. II. The Impact of Surface Brightness
Using a large sample of spiral galaxies for which 21 cm single-dish and/or
long-slit optical spectra are available, we make a detailed comparison between
various estimates of rotational widths. Different optical width estimators are
considered and their limitations discussed, with emphasis on biases associated
with rotation curve properties (shape and extent) and disk central surface
brightness. The best match with HI rotational velocities is obtained with
Polyex widths, which are measured at the optical radius (encompassing a fixed
fraction of the total light of the galaxy) from a model fit to the rotation
curve. In contrast with Polyex widths, optical rotational velocities measured
at 2.15 disk scale lengths r_d deviate from HI widths by an amount that
correlates with the central surface brightness of the disk. This bias occurs
because the rotation curves of galaxies are in general still rising at 2.15
r_d, and the fraction of total mass contained within this radius decreases with
increasing disk surface brightness. Statistical corrections, parameterized by
the radial extent of the observed rotation curve, are provided to reduce Polyex
and HI width measurements into a homogeneous system. This yields a single
robust estimate of rotational velocity to be used for applications of disk
scaling relations.Comment: 13 pages, 8 figures. To appear in the Astronomical Journal (August
2007
NGC 4138 - A Case Study in Counterrotating Disk Formation
The Sa(r) galaxy NGC 4138 has been recently found to contain an extensive
counterrotating disk which appears to be still forming. Up to a third of the
stars in the disk system may be on retrograde orbits. A counterrotating ring of
H II regions, along with extended counterrotating H I gas, suggests that the
retrograde material has been recently acquired in the gas phase and is still
trickling in. Using numerical simulations, we have attempted to model the
process by which the counterrotating mass has been accreted by this galaxy. We
investigate two possibilities: continuous retrograde infall of gas, and a
retrograde merger with a gas-rich dwarf galaxy. Both processes are successful
in producing a counterrotating disk of the observed mass and dimensions without
heating up the primary significantly. Contrary to our experience with a
fiducial cold, thin primary disk, the gas-rich merger works well for the
massive, compact primary disk of NGC 4138 even though the mass of the dwarf
galaxy is a significant fraction of the mass of the primary disk. Although we
have restricted ourselves mainly to coplanar infall and mergers, we report on
one inclined infall simulation as well. We also explore the possibility that
the H-alpha ring seen in the inner half of the disk is a consequence of
counterrotating gas clouds colliding with corotating gas already present in the
disk and forming stars in the process.Comment: To appear in ApJ, 21 pages, LaTeX (aaspp4) format, 17 figs (gzipped
tar file) also available at ftp://bessel.mps.ohio-state.edu/pub/thakar/cr2/
or at http://www-astronomy.mps.ohio-state.edu/~thakar
Simple Models for Turbulent Self-Regulation in Galaxy Disks
We propose that turbulent heating, wave pressure and gas exchanges between
different regions of disks play a dominant role in determining the preferred,
quasi-equilibrium, self-similar states of gas disks on large-scales. We present
simple families of analytic, thermohydrodynamic models for these global states,
which include terms for turbulent pressure and Reynolds stresses. Star
formation rates, phase balances, and hydrodynamic forces are all tightly
coupled and balanced. The models have stratified radial flows, with the cold
gas slowly flowing inward in the midplane of the disk, and with the warm/hot
phases that surround the midplane flowing outward.
The models suggest a number of results that are in accord with observation,
as well as some novel predictions, including the following. 1) The large-scale
gas density and thermal phase distributions in galaxy disks can be explained as
the result of turbulent heating and spatial couplings. 2) The turbulent
pressures and stresses that drive radial outflows in the warm gas also allow a
reduced circular velocity there. This effect was observed by Swaters, Sancisi
and van der Hulst in NGC 891, a particularly turbulent edge-on disk. The models
predict that the effect should be universal in such disks. 3) They suggest that
a star formation rate like the phenomenological Schmidt Law is the natural
result of global thermohydrodynamical balance, and may not obtain in disks far
from equilibrium. (Abridged)Comment: 37 pages, 1 gif figure, accepted for publication in the Astrophysical
Journa
Loss of mass and stability of galaxies in MOND
The self-binding energy and stability of a galaxy in MOND-based gravity are
curiously decreasing functions of its center of mass acceleration towards
neighbouring mass concentrations. A tentative indication of this breaking of
the Strong Equivalence Principle in field galaxies is the RAVE-observed escape
speed in the Milky Way. Another consequence is that satellites of field
galaxies will move on nearly Keplerian orbits at large radii (100 - 500 kpc),
with a declining speed below the asymptotically constant naive MOND prediction.
But consequences of an environment-sensitive gravity are even more severe in
clusters, where member galaxies accelerate fast: no more Dark-Halo-like
potential is present to support galaxies, meaning that extended axisymmetric
disks of gas and stars are likely unstable. These predicted reappearance of
asymptotic Keplerian velocity curves and disappearance of "stereotypic
galaxies" in clusters are falsifiable with targeted surveys.Comment: 4 pages, 2 figures, ApJ Letter
Effects of Dust on Gravitational Lensing by Spiral Galaxies
Gravitational lensing of an optical QSO by a spiral galaxy is often
counteracted by dust obscuration, since the line-of-sight to the QSO passes
close to the center of the galactic disk. The dust in the lens is likely to be
correlated with neutral hydrogen, which in turn should leave a Lyman-alpha
absorption signature on the QSO spectrum. We use the estimated dust-to-gas
ratio of the Milky-Way galaxy as a mean and allow a spread in its values to
calculate the effects of dust on lensing by low redshift spiral galaxies. Using
a no-evolution model for spirals at z<1 we find (in Lambda=0 cosmologies) that
the magnification bias due to lensing is stronger than dust obscuration for QSO
samples with a magnitude limit B<16. The density parameter of neutral hydrogen,
Omega_HI, is overestimated in such samples and is underestimated for fainter
QSOs.Comment: 18 pages, 4 figures, ApJ, in pres
The Nature of Nearby Counterparts to Intermediate Redshift Luminous Compact Blue Galaxies I. Optical/H I Properties and Dynamical Masses
We present single-dish H I spectra obtained with the Green Bank Telescope,
along with optical photometric properties from the Sloan Digital Sky Survey, of
20 nearby (D < 70 Mpc) Luminous Compact Blue Galaxies (LCBGs). These ~L*, blue,
high surface brightness, starbursting galaxies were selected with the same
criteria used to define LCBGs at higher redshifts. We find these galaxies are
gas-rich, with M(HI) ranging from 5*10^8 to 8*10^9 M_sun, and M(HI)/L_B ranging
from 0.2 to 2 M_sun/L_sun, consistent with a variety of morphological types of
galaxies. We find the dynamical masses (measured within R_25) span a wide
range, from 3*10^9 to 1*10^11 M_sun. However, at least half have dynamical
mass-to-light ratios smaller than nearby galaxies of all Hubble types, as found
for LCBGs at intermediate redshifts. By comparing line widths and effective
radii with local galaxy populations, we find that LCBGs are consistent with the
dynamical mass properties of Magellanic (low luminosity) spirals, and the more
massive irregulars and dwarf ellipticals, such as NGC 205.Comment: 33 pages, 8 figures, accepted by Ap
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