221 research outputs found
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
Star Clusters in the Nearby Late-Type Galaxy NGC 1311
Ultraviolet, optical and near infrared images of the nearby (D ~ 5.5 Mpc) SBm
galaxy NGC 1311, obtained with the Hubble Space Telescope, reveal a small
population of 13 candidate star clusters. We identify candidate star clusters
based on a combination of their luminosity, extent and spectral energy
distribution. The masses of the cluster candidates range from ~1000 up to
~100000 Solar masses, and show a strong positive trend of larger mass with
increasing with cluster age. Such a trend follows from the fading and
dissolution of old, low-mass clusters, and the lack of any young super star
clusters of the sort often formed in strong starbursts. The cluster age
distribution is consistent with a bursting mode of cluster formation, with
active episodes of age ~10 Myr, ~100 Myr and ~1 Gyr. The ranges of age and mass
we probe are consistent with those of the star clusters found in quiescent
Local Group dwarf galaxies.Comment: 21 pages, 11 figures, accepted by A
Discovery of Large-Scale Gravitational Infall in a Massive Protostellar Cluster
We report Mopra (ATNF), Anglo-Australian Telescope, and Atacama Submillimeter
Telescope Experiment observations of a molecular clump in Carina, BYF73 =
G286.21+0.17, which give evidence of large-scale gravitational infall in the
dense gas. From the millimetre and far-infrared data, the clump has mass ~ 2 x
10^4 Msun, luminosity ~ 2-3 x 10^4 Lsun, and diameter ~ 0.9 pc. From radiative
transfer modelling, we derive a mass infall rate ~ 3.4 x 10^-2 Msun yr-1. If
confirmed, this rate for gravitational infall in a molecular core or clump may
be the highest yet seen. The near-infrared K-band imaging shows an adjacent
compact HII region and IR cluster surrounded by a shell-like photodissociation
region showing H2 emission. At the molecular infall peak, the K imaging also
reveals a deeply embedded group of stars with associated H2 emission. The
combination of these features is very unusual and we suggest they indicate the
ongoing formation of a massive star cluster. We discuss the implications of
these data for competing theories of massive star formation.Comment: v1: 23 pages single-column, 6 figures (some multipart) at end v2: 14
pages 2-column, 6 figures interspersed v3: edited to referee's comments with
new sections and new figures; accepted to MNRAS, 20 pages 2-column, 8 figures
(some multipart) intersperse
Peanuts at an angle: detecting and measuring the three-dimensional structure of bars in moderately inclined galaxies
We show that direct detection and measurement of the vertically thickened parts of bars (so called ‘boxy’ or ‘peanut- shaped’ bulges) are possible not only for edge-on galaxies but also for galaxies with moderate inclinations (i < 70◦), and that examples are relatively common in the nearby Universe. The analysis of a sample of 78 nearby, moderately inclined (i � 65◦) early-type (S0–Sb) barred galaxies shows that the isophotal signature of the box/peanut can usually be detected for inclinations as low as i ∼ 40◦ – and in exceptional cases down to i ∼ 30◦. In agreement with the predictions from N-body simulations, the signature is most easily detectable when the bar’s position angle is within ∼50◦ of the galaxy major axis; in particular, galaxies where the bar lies very close to the minor axis do not show the signature clearly or at all. For galaxies with i = 40◦–65◦ and relative angles <45◦, we find evidence for the signature ≈2/3 of the time; the true frequency of box/peanut structures in bars may be higher. Comparison with N-body models also allows us to link observed photometric morphology with 3D physical structures, and thus estimate the relative sizes of box/peanut structures and bars. For our local sample, we find that box/peanut structures range in radial size (measured along the bar major axis) from 0.4 to 3.8 kpc (mean =1.5 ± 0.9 kpc) and span 0.26–0.58 of the bar length (mean of 0.38 ± 0.08). This is a clear observational confirmation that when bars thicken, it is not the entire bar which does so, but only the inner part. This technique can also be used to identify galaxies with bars which have not vertically thickened. We suggest that NGC 3049 and IC 676 may be particularly good examples, and that the fraction of S0–Sb bars which lack box/peanut structures is at least ∼13 per cent
An Imaging Survey of Early-Type Barred Galaxies
This paper presents the results of a high-resolution imaging survey, using
both ground-based and Hubble Space Telescope images, of a complete sample of
nearby barred S0--Sa galaxies in the field, with a particular emphasis on
identifying and measuring central structures within the bars: secondary bars,
inner disks, nuclear rings and spirals, and off-plane dust. A discussion of the
frequency and statistical properties of the various types of inner structures
has already been published. Here, we present the data for the individual
galaxies and measurements of their bars and inner structures. We set out the
methods we use to find and measure these structures, and how we discriminate
between them. In particular, we discuss some of the deficiencies of ellipse
fitting of the isophotes, which by itself cannot always distinguish between
bars, rings, spirals, and dust, and which can produce erroneous measurements of
bar sizes and orientations.Comment: LaTeX, 66 pages (including 42 figures, 36 in color). To appear in The
Astrophysical Journal Supplement. Full-resolution and text-only versions
available at http://www.iac.es/galeria/erwin/research
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Internal Tides and Turbulence along the 3000-m Isobath of the Hawaiian Ridge
Full-depth velocity and density profiles taken along the 3000-m isobath characterize the semidiurnal
internal tide and bottom-intensified turbulence along the Hawaiian Ridge. Observations reveal baroclinic
energy fluxes of 21 ± 5 kW m⁻¹ radiating from French Frigate Shoals, 17 ± 2.5 kW m⁻¹ from Kauai
Channel west of Oahu, and 13 ± 3.5 kW m⁻¹ from west of Nihoa Island. Weaker fluxes of 1–4 ± 2 kWm⁻¹
radiate from the region near Necker Island and east of Nihoa Island. Observed off-ridge energy fluxes
generally agree to within a factor of 2 with those produced by a tidally forced numerical model. Average
turbulent diapycnal diffusivity K is (0.5–1) x 10⁻⁴ m² s⁻¹ above 2000 m, increasing exponentially to 20 x 10⁻⁴ m² s⁻¹ near the bottom. Microstructure values agree well with those inferred from a finescale internal
wave-based parameterization. A linear relationship between the vertically integrated energy flux and vertically
integrated turbulent dissipation rate implies that dissipative length scales for the radiating internal
tide exceed 1000 km
Molecular Gas and Star Formation in Bars of Nearby Spiral Galaxies
We compare the distribution of molecular gas and star formation activity in
the bar region of six spirals (NGC 2903, NGC 3627, NGC 4321, NGC 5457, NGC
6946, & IC 342) from the BIMA Survey of Nearby Galaxies (SONG). The molecular
gas, traced using the CO (J=1--0) emission line, is brightest along the leading
edge of the stellar bar in the bar dust lanes. The star formation activity,
traced using the Halpha emission line, is offset towards the leading side of
the CO emission. A cross-correlation analysis shows that a) the HII regions are
offset 0--800 pc on the leading side of the CO emission, b) the largest offsets
are found in the strongest bars, and c) there is a wide range in offsets in a
single bar with no systematic pattern as a function of the galacto-centric
radius. The CO-Halpha offset constrains how stars may form depending on the gas
flow. We examine possible star formation scenarios in context of the two main
classes of bar gas flow simulations, the N-body/sticky particle and
hydrodynamic models. Though both model gas flows are generally consistent with
the observed offsets, we suggest the inclusion of a two- or multi-phase medium
to improve the agreement between models and observation.Comment: Accepted for publication in the Astronomical Journal, schedule for
November 2002. 28 pages, 12 figures in PNG format. Paper available in
emulateapj style (gzipped-postscript format) at
http://www.astro.caltech.edu/~kartik/papers/haco.ps.g
The Mechanochemistry of Endocytosis
An integrated theoretical model reveals how the chemical and the mechanical aspects of endocytosis are coordinated coherently in yeast cells, driving progression through the endocytic pathway and ensuring efficient vesicle scission in vivo
WASP family proteins and formins compete in pseudopod- and bleb-based migration
Actin pseudopods induced by SCAR/WAVE drive normal migration and chemotaxis in eukaryotic cells. Cells can also migrate using blebs, in which the edge is driven forward by hydrostatic pressure instead of actin. In Dictyostelium discoideum, loss of SCAR is compensated by WASP moving to the leading edge to generate morphologically normal pseudopods. Here we use an inducible double knockout to show that cells lacking both SCAR and WASP are unable to grow, make pseudopods or, unexpectedly, migrate using blebs. Remarkably, amounts and dynamics of actin polymerization are normal. Pseudopods are replaced in double SCAR/WASP mutants by aberrant filopods, induced by the formin dDia2. Further disruption of the gene for dDia2 restores cells’ ability to initiate blebs and thus migrate, though pseudopods are still lost. Triple knockout cells still contain near-normal F-actin levels. This work shows that SCAR, WASP, and dDia2 compete for actin. Loss of SCAR and WASP causes excessive dDia2 activity, maintaining F-actin levels but blocking pseudopod and bleb formation and migration
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