423 research outputs found
Origin of Disc Lopsidedness in the Eridanus Group of Galaxies
The HI surface density maps for a sample of 18 galaxies in the Eridanus group
are Fourier analysed. This analysis gives the radial variation of the
lopsidedness in the HI spatial distribution. The lopsidedness is quantified by
the Fourier amplitude of the component normalized to the average
value. It is also shown that in the radial region where the stellar disc and HI
overlap, their coefficients are comparable. All the galaxies studied show
significant lopsidedness in HI. The mean value of in the inner regions of
the galaxies (1.5 - 2.5 scale lengths) is . This value of is
twice the average value seen in the field galaxies. Also, the lopsidedness is
found to be smaller for late-type galaxies, this is opposite to the trend seen
in the field galaxies. These two results indicate a different physical origin
for disc lopsidedness in galaxies in a group environment compared to the field
galaxies. Further, a large fraction ( 30%) shows a higher degree of
lopsidedness (). It is also seen that the disk lopsidedness
increases with the radius as demonstrated in earlier studies, but over a radial
range that is two timeslarger than done in the previous studies. The average
lopsidedness of the halo potential is estimated to be %, assuming that
the lopsidedness in HI disc is due to its response to the halo asymmetry.Comment: 10 pages, 5 figures, Accepted for publication in MNRA
A Magellanic Origin for the Warp of the Galaxy
We show that a Magellanic Cloud origin for the warp of the Milky Way can
explain most quantitative features of the outer HI layer recently identified by
Levine, Blitz & Heiles (2005). We construct a model similar to that of Weinberg
(1998) that produces distortions in the dark matter halo, and we calculate the
combined effect of these dark-halo distortions and the direct tidal forcing by
the Magellanic Clouds on the disk warp in the linear regime. The interaction of
the dark matter halo with the disk and resonances between the orbit of the
Clouds and the disk account for the large amplitudes observed for the vertical
m=0,1,2 harmonics. The observations lead to six constraints on warp forcing
mechanisms and our model reasonably approximates all six. The disk is shown to
be very dynamic, constantly changing its shape as the Clouds proceed along
their orbit. We discuss the challenges to MOND placed by the observations.Comment: 4 pages, 3 figures, submitted to ApJ Letters. Additional graphics, 3d
visualizations and movies available at
http://www.astro.umass.edu/~weinberg/lm
Isotropic reconstruction of 3D fluorescence microscopy images using convolutional neural networks
Fluorescence microscopy images usually show severe anisotropy in axial versus
lateral resolution. This hampers downstream processing, i.e. the automatic
extraction of quantitative biological data. While deconvolution methods and
other techniques to address this problem exist, they are either time consuming
to apply or limited in their ability to remove anisotropy. We propose a method
to recover isotropic resolution from readily acquired anisotropic data. We
achieve this using a convolutional neural network that is trained end-to-end
from the same anisotropic body of data we later apply the network to. The
network effectively learns to restore the full isotropic resolution by
restoring the image under a trained, sample specific image prior. We apply our
method to synthetic and real datasets and show that our results improve
on results from deconvolution and state-of-the-art super-resolution techniques.
Finally, we demonstrate that a standard 3D segmentation pipeline performs on
the output of our network with comparable accuracy as on the full isotropic
data
Spatial and Kinematical Lopsidedness of Atomic Hydrogen in the Ursa Major Group of Galaxies
We have carried out the harmonic analysis of the atomic hydrogen (HI) surface
density maps and the velocity fields for 11 galaxies belonging to the Ursa
Major group, over a radial range of 4-6 disc scalelengths in each galaxy. This
analysis gives the radial variation of spatial lopsidedness, quantified by the
Fourier amplitude A of the m=1 component normalised to the average value.
The kinematical analysis gives a value for the elongation of the potential to
be . The mean amplitude of spatial lopsidedness is found to be
in the inner disc, similar to the field galaxies, and is smaller by
a factor of compared to the Eridanus group galaxies. It is also shown
that the the average value of A does not increase with the Hubble type,
contrary to what is seen in field galaxies. We argue that the physical origin
of lopsidedness in the Ursa Major group of galaxies is tidal interactions,
albeit weaker and less frequent than in Eridanus. Thus systematic studies of
lopsidedness in groups of galaxies can provide dynamical clues regarding the
interactions and evolution of galaxies in a group environment.Comment: 12 pages, 7 figures, 3 tables. Accepted for publication by MNRA
Global Star Formation Rates in Disk Galaxies and Circumnuclear Starbursts from Cloud Collisions
We invoke star formation triggered by cloud-cloud collisions to explain
global star formation rates of disk galaxies and circumnuclear starbursts.
Previous theories based on the growth rate of gravitational perturbations
ignore the dynamically important presence of magnetic fields. Theories based on
triggering by spiral density waves fail to explain star formation in systems
without such waves. Furthermore, observations suggest gas and stellar disk
instabilities are decoupled. Following Gammie, Ostriker & Jog (1991), the cloud
collision rate is set by the shear velocity of encounters with initial impact
parameters of a few tidal radii, due to differential rotation in the disk.
This, together with the effective confinement of cloud orbits to a two
dimensional plane, enhances the collision rate above that for particles in a
three dimensional box. We predict Sigma_{SFR}(R) proportional to Sigma_{gas}
Omega (1-0.7 beta). For constant circular velocity (beta = 0), this is in
agreement with recent observations (Kennicutt 1998). We predict a B-band
Tully-Fisher relation: L_{B} proportional to v_{circ}^{7/3}, also consistent
with observations. As additional tests, we predict enhanced star formation in
regions with relatively high shear rates, and lower star formation efficiencies
in clouds of higher mass.Comment: 27 pages including 3 figures and 2 tables. Accepted to ApJ. Expanded
statistical analysis of cloud SF efficiency test. Stylistic changes. Data for
figures available electronically at
http://astro.berkeley.edu/~jt/disksfr.htm
Global axisymmetric stability analysis for a composite system of two gravitationally coupled scale-free discs
In a composite system of gravitationally coupled stellar and gaseous discs,
we perform linear stability analysis for axisymmetric coplanar perturbations
using the two-fluid formalism. The background stellar and gaseous discs are
taken to be scale-free with all physical variables varying as powers of
cylindrical radius with compatible exponents. The unstable modes set in as
neutral modes or stationary perturbation configurations with angular frequency
.Comment: 7 pages using AAS styl
Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies
New high resolution interferometer data of 10 IR ultraluminous galaxies shows
the molecular gas is in rotating nuclear rings or disks with radii 300 to 800
pc. Most of the CO flux comes from a moderate-density, warm, intercloud medium
rather than self-gravitating clouds. Gas masses of ~ 5 x 10^9 Msun, 5 times
lower than the standard method are derived from a model of the molecular disks.
The ratio of molecular gas to dynamical mass, is M_gas/M_dyn ~ 1/6 with a
maximum ratio of gas to total mass surface density of 1/3. For the galaxies
VIIZw31, Arp193, and IRAS 10565+24, there is good evidence for rotating
molecular rings with a central gap. In addition to the rotating rings a new
class of star formation region is identified which we call an Extreme
Starburst. They have a characteristic size of only 100 pc., about 10^9 Msun of
gas and an IR luminosity of ~3 x 10^11 Lsun. Four extreme starbursts are
identified in the 3 closest galaxies in the sample Arp220, Arp193 and Mrk273.
They are the most prodigious star formation events in the local universe, each
representing about 1000 times as many OB stars as 30 Doradus. In Arp220, the CO
and 1.3 mm continuum maps show the two ``nuclei'' embedded in a central ring or
disk and a fainter structure extending 3 kpc to the east, normal to the nuclear
disk. There is no evidence that these sources really are the pre-merger nuclei.
They are compact, extreme starburst regions containing 10^9 Msun of dense
molecular gas and new stars, but no old stars. Most of the dust emission and
HCN emission arises in the two extreme starbursts. The entire bolometric
luminosity of Arp~220 comes from starbursts, not an AGN. In Mrk231, the disk
geometry shows that the molecular disk cannot be heated by the AGN; the far IR
luminosity of Mrk~231 is powered by a starburst, not the AGN. (Abridged)Comment: 97 pages Latex with aasms.sty, including 29 encapsulated Postscript
figures. Figs 18 and 23 are GIFs. 31 figures total. Text and higher quality
versions of figures available at
http://sbastk.ess.sunysb.edu/www/RINGS_ESB_PREPRINT.html To be published in
Ap. J., 10 Nov. 199
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