75 research outputs found
Multi-Scale Analysis of Magnetic Fields in Filamentary Molecular Clouds in Orion A
New visible and K-band polarization measurements on stars surrounding
molecular clouds in Orion A and stars in the BN vicinity are presented. Our
results confirm that magnetic fields located inside the Orion A molecular
clouds and in their close neighborhood are spatially connected. On and around
the BN object, we measured the angular offsets between the K-band polarization
data and available submm data. We find high values of the polarization degree,
P_{K}, and of the optical depth, \tau_{K}, close to an angular offset position
of 90^{\circ} whereas lower values of P_{K} and \tau_{K} are observed for
smaller angular offsets. We interpret these results as evidence for the
presence of various magnetic field components toward lines of sight in the
vicinity of BN. On a larger scale, we measured the distribution of angular
offsets between available H-band polarization data and the same submm data set.
Here we find an increase of with angular offset which we interpret as a
rotation of the magnetic field by \lesssim 60^{\circ}. This trend generalizes
previous results on small scale toward and around lines of sight to BN and is
consistent with a twist of the magnetic field on a larger scale towards OMC-1.
A comparison of our results with several other studies suggests that a
two-component magnetic field, maybe helical, could be wrapping the OMC-1
filament.Comment: 53 pages, 21 figures, 7 tables, Accepted in the Astrophysical Journa
Point Sources from a Spitzer IRAC Survey of the Galactic Center
We have obtained Spitzer/IRAC observations of the central 2.0 x 1.4 degrees
(~280 x 200 pc) of the Galaxy at 3.6-8.0 microns. A point source catalog of
1,065,565 objects is presented. The catalog includes magnitudes for the point
sources at 3.6, 4.5, 5.8, and 8.0 microns, as well as JHK photometry from
2MASS. The point source catalog is confusion limited with average limits of
12.4, 12.1, 11.7, and 11.2 magnitudes for [3.6], [4.5], [5.8], and [8.0],
respectively. We find that the confusion limits are spatially variable because
of stellar surface density, background surface brightness level, and extinction
variations across the survey region. The overall distribution of point source
density with Galactic latitude and longitude is essentially constant, but
structure does appear when sources of different magnitude ranges are selected.
Bright stars show a steep decreasing gradient with Galactic latitude, and a
slow decreasing gradient with Galactic longitude, with a peak at the position
of the Galactic center. From IRAC color-magnitude and color-color diagrams, we
conclude that most of the point sources in our catalog have IRAC magnitudes and
colors characteristic of red giant and AGB stars.Comment: 44 pages, 13 figures, ApJS in pres
The Mid-Infrared Colors of the ISM and Extended Sources at the Galactic Center
A mid-infrared (3.6-8 um) survey of the Galactic Center has been carried out
with the IRAC instrument on the Spitzer Space Telescope. This survey covers the
central 2x1.4 degree (~280x200 pc) of the Galaxy. At 3.6 and 4.5 um the
emission is dominated by stellar sources, the fainter ones merging into an
unresolved background. At 5.8 and 8 um the stellar sources are fainter, and
large-scale diffuse emission from the ISM of the Galaxy's central molecular
zone becomes prominent. The survey reveals that the 8 to 5.8 um color of the
ISM emission is highly uniform across the surveyed region. This uniform color
is consistent with a flat extinction law and emission from polycyclic aromatic
hydrocarbons (PAHs). Models indicate that this broadband color should not be
expected to change if the incident radiation field heating the dust and PAHs is
<10^4 times that of the solar neighborhood. The few regions with unusually red
emission are areas where the PAHs are underabundant and the radiation field is
locally strong enough to heat large dust grains to produce significant 8 um
emission. These red regions include compact H II regions, Sgr B1, and wider
regions around the Arches and Quintuplet Clusters. In these regions the
radiation field is >10^4 times that of the solar neighborhood. Other regions of
very red emission indicate cases where thick dust clouds obscure deeply
embedded objects or very early stages of star formation.Comment: 37 pages, 15 Postscript figures (low resolution). Accepted for
publication in the Ap
High Resolution Mid-Infrared Imaging of Infrared Luminous Starburst Galaxies
Observations for seven infrared luminous starburst galaxies are reported in the mid-infrared from 8 - 18 microns using the Keck Telescopes with spatial resolution approaching the diffraction limit. All of the galaxies observed show evidence of strong interactions based on optical morphologies. For these galaxies, a substantial fraction, usually more than 50%, of the infrared luminosity is generated in regions ranging in sizes from 100pc -- 1 Kpc. Nuclear starbursts often dominate the infrared luminosity, but this is not always true. In some galaxies, most notably NGC 6090, substantial infrared luminosity greatly in excess of the nuclear luminosity is generated in regions associated with the physical interaction between two galaxies. The radio emission is a good tracer of the location of high luminosity young stars. The visual/ultraviolet radiation output of the nearby star forming galaxies is dominated by emission from regions that are generally not producing the copious infrared luminosity of the systems. The regions of high infrared luminosity in local starburst galaxies are significantly smaller than the galaxies as a whole. The integrated spectral energy distributions (SEDs) of these galaxies are very different from the SEDs of the regions of star formation. If the SEDs of star-forming regions in these galaxies reflect the SEDs found in forming galaxies at high redshift, the distant galaxies should be dominated by the mid and far-infrared luminosity output far more than the integrated luminous output of nearby starburst galaxies would suggest
Methanol Maser Emission from Galactic Center Sources with Excess 4.5 {\mu}m Emission
We present a study of signatures of on-going star formation in a sample of
protostellar objects with enhanced 4.5 {\mu}m emission ('green' sources) near
the Galactic center. To understand how star formation in the Galactic center
region compares to that of the Galactic disk, we used the Expanded Very Large
Array to observe radiatively excited Class II 6.7 GHz CH3OH masers and
collisionally excited Class I 44 GHz CH3OH masers, both tracers of high-mass
star formation, toward a sample of 34 Galactic center and foreground 'green'
sources. We find that 33\pm15% of Galactic center sources are coincident with
6.7 GHz masers, and that 44\pm17% of foreground sources are coincident with 6.7
GHz masers. For 44 GHz masers, we find correlation rates of 27\pm13% and
25\pm13% for Galactic center green sources and foreground green sources,
espectively. Based on these CH3OH maser detection rates, as well as
correlations of green sources with other tracers of star formation, such as 24
{\mu}m emission and infrared dark clouds (IRDCs), we find no significant
difference between the green sources in the Galactic center and those
foreground to it. This suggests that once the star formation process has begun,
the environmental differences between the Galactic center region and the
Galactic disk have little effect on its observational signatures. We do find,
however, some evidence that may support a recent episode of star formation in
the Galactic center region.Comment: 73 pages, 34 figures, 5 tables. Accepted for publication in Ap
The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared
High spatial resolution observations in the 1 to 3.5 micron region of the
Galactic Center source known historically as IRS 13 are presented. They include
ground-based adaptive optics images in the H, Kp (2.12/0.4 micron) and L bands,
NICMOS data in filters between 1.1 and 2.2 micron, and integral field
spectroscopic data from BEAR, an Imaging FTS, in the HeI 2.06 micron and the
Br line regions. Analysis of all these data provides a completely new
picture of the main component, IRS 13E, which appears as a cluster of seven
individual stars within a projected diameter of ~0.5'' (0.02 pc). The brightest
sources, 13E1, 13E2, 13E3 (a binary), and 13E4, are all massive stars, 13E1 a
blue object, with no detected emission line while 13E2 and 13E4 are high-mass
emission line stars. 13E2 is at the WR stage and 13E4 a massive O-type star.
13E3A and B are extremely red objects, proposed as other examples of dusty WR
stars. All these sources have a common westward proper motion. 13E5, is a red
source similar to 13E3A/B. This concentration of comoving massive hot stars,
IRS 13E, is proposed as the remaining core of a massive star cluster, which
could harbor an intermediate-mass black hole (IMBH) of ~1300 M_sol. This
detection plays in favor of a scenario in which the helium stars and the other
hot stars in the central pc originate from the stripping of a massive cluster
formed several tens of pc from the center. The detection of a discrete X-ray
emission (Baganoff et al. 2003) at the IRS~13 position is examined in this
context.Comment: 14 pages, 6 figures (3 in color), LaTeX2e, accepted in A&
IRAC Observations of M81
IRAC images of M81 show three distinct morphological constituents: a smooth
distribution of evolved stars with bulge, disk, and spiral arm components; a
clumpy distribution of dust emission tracing the spiral arms; and a pointlike
nuclear source. The bulge stellar colors are consistent with M-type giants, and
the disk colors are consistent with a slightly younger population. The dust
emission generally follows the blue and ultraviolet emission, but there are
large areas that have dust emission without ultraviolet and smaller areas with
ultraviolet but little dust emission. The former are presumably caused by
extinction, and the latter may be due to cavities in the gas and dust created
by supernova explosions. The nucleus appears fainter at 8 um than expected from
ground-based 10 um observations made four years ago.Comment: ApJS in press (Spitzer special issue); 15 pages, 3 figures. Changes:
unused references removed, numbers and labels in Table 1 change
GLIMPSE: I. A SIRTF Legacy Project to Map the Inner Galaxy
GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire), a SIRTF
Legacy Science Program, will be a fully sampled, confusion-limited infrared
survey of the inner two-thirds of the Galactic disk with a pixel resolution of
\~1.2" using the Infrared Array Camera (IRAC) at 3.6, 4.5, 5.8, and 8.0
microns. The survey will cover Galactic latitudes |b| <1 degree and longitudes
|l|=10 to 65 degrees (both sides of the Galactic center). The survey area
contains the outer ends of the Galactic bar, the Galactic molecular ring, and
the inner spiral arms. The GLIMPSE team will process these data to produce a
point source catalog, a point source data archive, and a set of mosaicked
images. We summarize our observing strategy, give details of our data products,
and summarize some of the principal science questions that will be addressed
using GLIMPSE data. Up-to-date documentation, survey progress, and information
on complementary datasets are available on the GLIMPSE web site:
www.astro.wisc.edu/glimpse.Comment: Description of GLIMPSE, a SIRTF Legacy project (Aug 2003 PASP, in
press). Paper with full res.color figures at
http://www.astro.wisc.edu/glimpse/glimpsepubs.htm
The Bubbling Galactic Disk
A visual examination of the images from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) has revealed 322 partial and closed rings that we propose represent partially or fully enclosed three-dimensional bubbles. We argue that the bubbles are primarily formed by hot young stars in massive star formation regions. We have found an average of about 1.5 bubbles per square degree. About 25% of the bubbles coincide with known radio H II regions, and about 13% enclose known star clusters. It appears that B4-B9 stars (too cool to produce detectable radio H II regions) probably produce about three-quarters of the bubbles in our sample, and the remainder are produced by young O-B3 stars that produce detectable radio H II regions. Some of the bubbles may be the outer edges of H II regions where PAH spectral features are excited and may not be dynamically formed by stellar winds. Only three of the bubbles are identified as known SNRs. No bubbles coincide with known planetary nebulae or W-R stars in the GLIMPSE survey area. The bubbles are small. The distribution of angular diameters peaks between 1' and 3' with over 98% having angular diameters less than 10' and 88% less than 4'. Almost 90% have shell thicknesses between 0.2 and 0.4 of their outer radii. Bubble shell thickness increases approximately linearly with shell radius. The eccentricities are rather large, peaking between 0.6 and 0.7; about 65% have eccentricities between 0.55 and 0.85
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