33 research outputs found
Ram pressure stripping and galaxy orbits: The case of the Virgo cluster
We investigate the role of ram pressure stripping in the Virgo cluster using
N-body simulations. Radial orbits within the Virgo cluster's gravitational
potential are modeled and analyzed with respect to ram pressure stripping. The
N-body model consists of 10000 gas cloud complexes which can have inelastic
collisions. Ram pressure is modeled as an additional acceleration on the clouds
located at the surface of the gas distribution in the direction of the galaxy's
motion within the cluster. We made several simulations changing the orbital
parameters in order to recover different stripping scenarios using realistic
temporal ram pressure profiles. We investigate systematically the influence of
the inclination angle between the disk and the orbital plane of the galaxy on
the gas dynamics. We show that ram pressure can lead to a temporary increase of
the central gas surface density. In some cases a considerable part of the total
atomic gas mass (several 10^8 M_solar) can fall back onto the galactic disk
after the stripping event. A quantitative relation between the orbit parameters
and the resulting HI deficiency is derived containing explicitly the
inclination angle between the disk and the orbital plane. The comparison
between existing HI observations and the results of our simulations shows that
the HI deficiency depends strongly on galaxy orbits. It is concluded that the
scenario where ram pressure stripping is responsible for the observed HI
deficiency is consistent with all HI 21cm observations in the Virgo cluster.Comment: 29 pages with 21 figures. Accepted for publication in Ap
High Latitude Radio Emission in a Sample of Edge-On Spiral Galaxies
We have mapped 16 edge-on galaxies at 20 cm using the VLA. For 5 galaxies, we
could form spectral index, energy and magnetic field maps. We find that all but
one galaxy show evidence for non-thermal high latitude radio continuum
emission, suggesting that cosmic ray halos are common in star forming galaxies.
The high latitude emission is seen over a variety of spatial scales and in
discrete and/or smooth features. In general, the discrete features emanate from
the disk, but estimates of CR diffusion lengths suggest that diffusion alone is
insufficient to transport the particles to the high latitudes seen (> 15 kpc in
one case). Thus CRs likely diffuse through low density regions and/or are
assisted by other mechanisms (e.g. winds). We searched for correlations between
the prevalence of high latitude radio emission and a number of other
properties, including the global SFR, supernova input rate per unit star
forming, and do not find clear correlations with any of these properties.Comment: 40 pages of text, 3 figures, 6 tables, and an appendix of 21 jpeg
figures (which is a radio continuum catalogue of 17 galaxies). to appear in
A. J. (around January 1999
Galactic-Scale Outflow and Supersonic Ram-Pressure Stripping in the Virgo Cluster Galaxy NGC 4388
The Hawaii Imaging Fabry-Perot Interferometer (HIFI) on the University of
Hawaii 2.2m telescope was used to map the Halpha and [O III] 5007 A
emission-line profiles across the entire disk of the edge-on Sb galaxy NGC
4388. We confirm a rich complex of highly ionized gas that extends ~4 kpc above
the disk of this galaxy. Low-ionization gas associated with star formation is
also present in the disk. Evidence for bar streaming is detected in the disk
component and is discussed in a companion paper (Veilleux, Bland-Hawthorn, &
Cecil 1999; hereafter VBC). Non-rotational blueshifted velocities of 50 - 250
km/s are measured in the extraplanar gas north-east of the nucleus. The
brighter features in this complex tend to have more blueshifted velocities. A
redshifted cloud is also detected 2 kpc south-west of the nucleus. The velocity
field of the extraplanar gas of NGC 4388 appears to be unaffected by the
inferred supersonic (Mach number M ~ 3) motion of this galaxy through the ICM
of the Virgo cluster. We argue that this is because the galaxy and the high-|z|
gas lie behind a Mach cone with opening angle ~ 80 degrees. The shocked ICM
that flows near the galaxy has a velocity of ~ 500 km/s and exerts insufficient
ram pressure on the extraplanar gas to perturb its kinematics. We consider
several explanations of the velocity field of the extraplanar gas. Velocities,
especially blueshifted velocities on the N side of the galaxy, are best
explained as a bipolar outflow which is tilted by > 12 degrees from the normal
to the disk. The observed offset between the extraplanar gas and the radio
structure may be due to buoyancy or refractive bending by density gradients in
the halo gas. Velocity substructure in the outflowing gas also suggests an
interaction with ambient halo gas.Comment: 29 pages including 5 figures, Latex, requires aaspp4.sty, to appear
in ApJ, 520 (July 20, 1999 issue
Arm Structure in Anemic Spiral Galaxies
Anemic galaxies have less prominent star formation than normal galaxies of
the same Hubble type. Previous studies showed they are deficient in total
atomic hydrogen but not in molecular hydrogen. Here we compare the combined
surface densities of HI and H2 at mid-disk radii with the Kennicutt threshold
for star formation. The anemic galaxies are below threshold, which explains
their lack of prominent star formation, but they are not much different than
other early type galaxies, which also tend to be below threshold. The spiral
wave amplitudes of anemic and normal galaxies were also compared, using images
in B and J passbands from the OSU Bright Spiral Galaxy Survey. Anemic galaxies
have normal spiral wave properties too, with the same amplitudes and radial
dependencies as other galaxies of the same arm class. Because of the lack of
gas, spiral waves in early type galaxies and anemics do not have a continuous
supply of stars with low velocity dispersions to maintain a marginally stable
disk. As a result, they are either short-lived, evolving toward lenticulars and
S0 types in only a few rotations at mid-disk, or they are driven by the
asymmetries associated with gas removal in the cluster environment.Comment: 15 pages, 3 figures, accepted by A
The Taurus Spitzer Survey: New Candidate Taurus Members Selected Using Sensitive Mid-Infrared Photometry
We report on the properties of pre-main-sequence objects in the Taurus
molecular clouds as observed in 7 mid- and far-infrared bands with the Spitzer
Space Telescope. There are 215 previously-identified members of the Taurus
star-forming region in our ~44 square degree map; these members exhibit a range
of Spitzer colors that we take to define young stars still surrounded by
circumstellar dust (noting that ~20% of the bonafide Taurus members exhibit no
detectable dust excesses). We looked for new objects in the survey field with
similar Spitzer properties, aided by extensive optical, X-ray, and ultraviolet
imaging, and found 148 candidate new members of Taurus. We have obtained
follow-up spectroscopy for about half the candidate sample, thus far confirming
34 new members, 3 probable new members, and 10 possible new members, an
increase of 15-20% in Taurus members. Of the objects for which we have
spectroscopy, 7 are now confirmed extragalactic objects, and one is a
background Be star. The remaining 93 candidate objects await additional
analysis and/or data to be confirmed or rejected as Taurus members. Most of the
new members are Class II M stars and are located along the same cloud filaments
as the previously-identified Taurus members. Among non-members with Spitzer
colors similar to young, dusty stars are evolved Be stars, planetary nebulae,
carbon stars, galaxies, and AGN.Comment: Accepted to ApJS. Two large online-only figures available with the
preprint here: http://web.ipac.caltech.edu/staff/rebull/research.htm
Model of SNARE-Mediated Membrane Adhesion Kinetics
SNARE proteins are conserved components of the core fusion machinery driving diverse membrane adhesion and fusion processes in the cell. In many cases micron-sized membranes adhere over large areas before fusion. Reconstituted in vitro assays have helped isolate SNARE mechanisms in small membrane adhesion-fusion and are emerging as powerful tools to study large membrane systems by use of giant unilamellar vesicles (GUVs). Here we model SNARE-mediated adhesion kinetics in SNARE-reconstituted GUV-GUV or GUV-supported bilayer experiments. Adhesion involves many SNAREs whose complexation pulls apposing membranes into contact. The contact region is a tightly bound rapidly expanding patch whose growth velocity increases with SNARE density . We find three patch expansion regimes: slow, intermediate, fast. Typical experiments belong to the fast regime where depends on SNARE diffusivities and complexation binding constant. The model predicts growth velocities s. The patch may provide a close contact region where SNAREs can trigger fusion. Extending the model to a simple description of fusion, a broad distribution of fusion times is predicted. Increasing SNARE density accelerates fusion by boosting the patch growth velocity, thereby providing more complexes to participate in fusion. This quantifies the notion of SNAREs as dual adhesion-fusion agents
A Polarised Population of Dynamic Microtubules Mediates Homeostatic Length Control in Animal Cells
An analysis of cells grown on micro-patterned lines, and of cells during zebrafish development, identifies a population of microtubules that align along the long axis of cells to mediate homeostatic length control
SN 2009kn - the twin of the Type IIn supernova 1994W
We present an optical and near-infrared photometric and spectroscopic study
of supernova (SN) 2009kn spanning ~1.5 yr from the discovery. The optical
spectra are dominated by the narrow (full width at half-maximum ~1000 km s^-1)
Balmer lines distinctive of a Type IIn SN with P Cygni profiles. Contrarily,
the photometric evolution resembles more that of a Type IIP SN with a large
drop in luminosity at the end of the plateau phase. These characteristics are
similar to those of SN 1994W, whose nature has been explained with two
different models with different approaches. The well-sampled data set on SN
2009kn offers the possibility to test these models, in the case of both SN
2009kn and SN 1994W. We associate the narrow P Cygni lines with a swept-up
shell composed of circumstellar matter and SN ejecta. The broad emission line
wings, seen during the plateau phase, arise from internal electron scattering
in this shell. The slope of the light curve after the post-plateau drop is
fairly consistent with that expected from the radioactive decay of 56Co,
suggesting an SN origin for SN 2009kn. Assuming radioactivity to be the main
source powering the light curve of SN 2009kn in the tail phase, we infer an
upper limit for 56Ni mass of 0.023 M_sun. This is significantly higher than
that estimated for SN 1994W, which also showed a much steeper decline of the
light curve after the post-plateau drop. We also observe late-time
near-infrared emission which most likely arises from newly formed dust produced
by SN 2009kn. As with SN 1994W, no broad lines are observed in the spectra of
SN 2009kn, not even in the late-time tail phase.Comment: 21 pages, 19 figures. Minor changes in text and figures from v1.
Published in MNRA
The Atlas3D project -- XIII. Mass and morphology of HI in early-type galaxies as a function of environment
We present the Atlas3D HI survey of 166 nearby early-type galaxies (ETGs)
down to M(HI)~10^7 M_sun. We detect HI in ~40% of all ETGs outside the Virgo
cluster and in ~10% of all ETGs inside it. This demonstrates that it is common
for non-cluster ETGs to host HI. The HI morphology varies from regular
discs/rings (the majority of the detections) to unsettled gas distributions.
The former are either small discs (M(HI)<10^8 M_sun) confined within the
stellar body and sharing the same kinematics of the stars, or large discs/rings
(M(HI) up to 5x10^9 M_sun) extending to tens of kpc from the host galaxy and
frequently kinematically decoupled from the stars. Neutral hydrogen provides
material for star formation in ETGs. Galaxies with central HI exhibit
signatures of star formation in ~70% of the cases, ~5 times more frequently
than galaxies without central HI. The central ISM is dominated by molecular
gas. In ETGs with a small gas disc the conversion of HI into H_2 is as
efficient as in spirals. The ETG HI mass function has M*~2x10^9 M_sun and
slope=-0.7. ETGs host much less HI than spirals as a family. However, a
significant fraction of them is as HI-rich as spirals. The main difference
between ETGs and spirals is that the former lack the high-column-density HI
typical of the bright stellar disc of the latter. We find an envelope of
decreasing M(HI) with increasing environment density. The gas-richest ETGs live
in the poorest environments (where star-formation is more common), galaxies in
the centre of Virgo have the lowest HI content, and the cluster outskirts are a
transition region. We find an HI morphology-density relation. At low
environment density HI is mostly distributed on large discs/rings. More
disturbed HI morphologies dominate environment densities typical of rich
groups, confirming the importance of processes occurring on a galaxy-group
scale for the evolution of ETGs.Comment: Accepted for publication on MNRA