52 research outputs found
Small-scale star formation at low metallicity
Massive star formation in a low metallicity environment is investigated by studying the morphology of small HII regions in the Small Magellanic Cloud. A classification scheme based upon the symmetry of form in the light of H-alpha is proposed to make possible an examination of the properties of blister candidates with respect to nebulas embedded in a more uniform medium. A new diagnostic of size is developed to derive quantitative information about the ionized gas and ionizing stars. The asymmetrical surface-brightness distribution of many HII regions demonstrates that massive stars often form at the edge of dense neutral clouds. However, the existence of many symmetrical nebulas with similar sizes, luminosities, and surface brightnesses shows that massive star formation often occurs within these clouds. Nevertheless, the statistics of the two different forms indicate that the rate of massive star formation declines less steeply with radius across host clouds than in the Milky Way, suggesting that external triggering may play a larger role in initiating star formation
Image-Processing Techniques for the Creation of Presentation-Quality Astronomical Images
The quality of modern astronomical data, the power of modern computers and
the agility of current image-processing software enable the creation of
high-quality images in a purely digital form. The combination of these
technological advancements has created a new ability to make color astronomical
images. And in many ways it has led to a new philosophy towards how to create
them. A practical guide is presented on how to generate astronomical images
from research data with powerful image-processing programs. These programs use
a layering metaphor that allows for an unlimited number of astronomical
datasets to be combined in any desired color scheme, creating an immense
parameter space to be explored using an iterative approach. Several examples of
image creation are presented.
A philosophy is also presented on how to use color and composition to create
images that simultaneously highlight scientific detail and are aesthetically
appealing. This philosophy is necessary because most datasets do not correspond
to the wavelength range of sensitivity of the human eye. The use of visual
grammar, defined as the elements which affect the interpretation of an image,
can maximize the richness and detail in an image while maintaining scientific
accuracy. By properly using visual grammar, one can imply qualities that a
two-dimensional image intrinsically cannot show, such as depth, motion and
energy. In addition, composition can be used to engage viewers and keep them
interested for a longer period of time. The use of these techniques can result
in a striking image that will effectively convey the science within the image,
to scientists and to the public.Comment: 104 pages, 38 figures, submitted to A
The Fragmenting Superbubble Associated with the HII Region W4
New observations at high latitudes above the HII region W4 show that the
structure formerly identified as a chimney candidate, an opening to the
Galactic halo, is instead a superbubble in the process of fragmenting and
possibly evolving into a chimney. Data at high Galactic latitudes (b > 5
degrees) above the W3/W4 star forming region at 1420 and 408 MHz Stokes I
(total power) and 1420 MHz Stokes Q and U (linear polarization) reveal an
egg-shaped structure with morphological correlations between our data and the
H-alpha data of Dennison, Topasna, & Simonetti. Polarized intensity images show
depolarization extending from W4 up the walls of the superbubble, providing
strong evidence that the radio continuum is generated by thermal emission
coincident with the H-alpha emission regions. We conclude that the parts of the
HII region hitherto known as W4 and the newly revealed thermal emission are all
ionized by the open cluster OCl 352. Assuming a distance of 2.35 kpc, the ovoid
structure is 164 pc wide and extends 246 pc above the mid-plane of the Galaxy.
The shell's emission decreases in total-intensity and polarized intensity in
various locations, appearing to have a break at its top and another on one
side. Using a geometric analysis of the depolarization in the shell's walls, we
estimate that a magnetic field line-of-sight component of 3 to 5 uG exists in
the shell. We explore the connection between W4 and the Galactic halo,
considering whether sufficient radiation can escape from the fragmenting
superbubble to ionize the kpc-scale H-alpha loop discovered by Reynolds,
Sterling & Haffner.Comment: 42 pages, 14 figures; Accepted for publication in Ap
CHANG-ES XXVII: A Radio/X-ray Catalogue of Compact Sources in and around Edge-on Galaxies
We present catalogues of discrete, compact radio sources in and around the
discs of 35 edge-on galaxies in the Continuum Halos in Nearby Galaxies -- an
EVLA Survey (CHANG-ES). The sources were extracted using the PyBDSF program at
both 1.6 GHz (L-band) and 6.0 GHz (C-band) from matching resolution (
3 arcsec) data. We also present catalogues of X-ray sources from Chandra data
sets for 27 of the galaxies. The sources at the two radio frequency bands were
positionally cross-correlated with each other, and the result cross-correlated
with the X-ray sources. All catalogues are included for download with this
paper. We detect a total of 2507 sources at L-band and 1413 sources at C-band.
Seventy-five sources have been successfully cross-correlated in both radio
bands plus X-ray. Three new nuclear sources are candidates for Low Luminosity
Active Galactic Nuclei in NGC~3877, NGC~4192, and NGC~5792; the one in NGC~3877
also appears to be variable. We also find new nuclear sources in two companion
galaxies: NGC~4435 (companion to NGC~4438) and NGC~4298 (companion to
NGC~4302). We have also discovered what appears to be a foreground double-star;
each star has X-ray emission and there is radio emission at both L-band and
C-band in between them. This could be a colliding wind binary system.
Suggestions for follow-up studies are offered.Comment: Accepted to MNRAS, 57 pages of which pages 30 to 57 are figures. 5
ancillary files containing tabular information, as explained in the
appendice
CHANG-ES VII: Magnetic outflows from the Virgo cluster galaxy NGC 4388
We investigate the effects of ram pressure on the ordered magnetic field of a
galaxy hosting a radio halo and strong nuclear outflows. New radio images in
total and polarized intensity of the edge-on Virgo galaxy NGC\,4388 were
obtained within the CHANG-ES EVLA project. The unprecedented noise level
reached allows us to detect striking new features of the ordered magnetic
field. The nuclear outflow extends far into the halo to about 5\,kpc from the
center and is spatially correlated with the and X-ray emission.
For the first time, the southern outflow is detected. Above and below both
spiral arms we find extended blobs of polarized emission with an ordered field
oriented perpendicular to the disk. The synchrotron lifetime of the cosmic ray
electrons (CREs) in these regions yields a mean outflow velocity of
(270\pm70)\kms, in agreement with a galactic wind scenario. The observed
symmetry of the polarized halo features in NGC 4388 excludes a compression of
the halo gas by the ram pressure of the intra-cluster medium (ICM). The
assumption of equilibrium between the halo pressure and the ICM ram pressure
yields an estimate of the ICM density that is consistent with both the ICM
density derived from X-ray observations and the recent \textit{Planck}
Sunyaev-Zel'dovich measurements. The detection of a faint radio halo around
cluster galaxies could thus be used for an estimate of ICM ram pressure
The Vela Cloud: A Giant HI Anomaly in the NGC 3256 Group
We present Australia Telescope Compact Array (ATCA) observations of a
galaxy-sized intergalactic HI cloud (the Vela Cloud) in the NGC 3256 galaxy
group. The group contains the prominent merging galaxy NGC 3256, which is
surrounded by a number of HI fragments, the tidally disturbed galaxy NGC 3263,
and several other peculiar galaxies. The Vela Cloud, with an HI mass of 3-5 *
10**9 solar masses, resides southeast of NGC 3256 and west of NGC 3263, within
an area of 9' x 16' (100 kpc x 175 kpc for an adopted distance of 38 Mpc). In
our ATCA data the Vela Cloud appears as 3 diffuse components and contains 4
density enhancements. The Vela Cloud's properties, together with its group
environment, suggest that it has a tidal origin. Each density enhancement
contains ~10**8 solar masses of HI gas which is sufficient material for the
formation of globular cluster progenitors. However, if we represent the
enhancements as Bonnor-Ebert spheres, then the pressure of the surrounding HI
would need to increase by at least a factor of 6 in order to cause the collapse
of an enhancement. Thus we do not expect them to form massive bound stellar
systems like super star clusters or tidal dwarf galaxies. Since the HI density
enhancements have some properties in common with High Velocity Clouds, we
explore whether they may evolve to be identified with these starless clouds
instead.Comment: 47 pages, 13 figures (incl. a & b), accepted by AJ, changes are minor
additions, rearranging, and clarifications esp. in sections 6 &
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