109 research outputs found
Infrared polarization studies of protostars
The thesis to be proved is that the mechanism of optical interstellar polarization, preferential extinction by magnetically aligned
aspherical grains, can be extrapolated successfully to explain the
large infrared polarizations observed in molecular cloud protostellar
sources. Observations of the linear polarization of 13 sources between 1.65 μm and 4.8 μm and of the circular polarization of 9 sources
at 2.2 μm have been obtained. Analogously to the optical interstellar
case, the data have been modelled using the Rayleigh approximation to
calculate extinction and phase -lag efficiencies for a number of grain
models. The models successfully account for the high infrared ellipticities (ratio of linear to circular polarization) observed in the
molecular clouds, with twists in the grain alignment of about 40°;
smaller than those required to explain optical interstellar circular
polarization. A shortcoming in the model linear polarization at
X < 3 μm is attributable to a failure in the Rayleigh approximation.
A dissimilarity in the polarization through the ice band between two
of the protostellar sources can be understood by differing grain
compositions. A correlation between the position angles of polarization of the protostars and the nearby interstellar field stars can
be interpreted by saying the Galactic magnetic field permeates the dense
molecular clouds. There are indications that the polarization
mechanism even operates in the enhanced density regions of the clouds;
then a more efficient alignment mechanism than paramagnetic relaxation is required. It is suggested that this may be "pinwheeling,"
possibly accompanied by super-paramagnetism of the grains. The
twists in the magnetic field lines implied by the model may arise in
the collapse process of the rotating, magnetized clouds
Resolution of the Compact Radio Continuum Sources in Arp220
We present 2 cm and 3.6 cm wavelength very long baseline interferometry
images of the compact radio continuum sources in the nearby ultra-luminous
infrared galaxy Arp220. Based on their radio spectra and variability
properties, we confirm these sources to be a mixture of supernovae (SNe) and
supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both
wavelengths. The SNe generally only have upper size limits. In contrast all the
SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent
with them having just entered their Sedov phase while embedded in an
interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the
diameter-luminosity correlation for SNRs (and so also on the diameter-surface
brightness relation) and extend these correlations to very small sources. The
data are consistent with the relation L {\propto} D^{-9/4}. Revised
equipartition arguments adjusted to a magnetic field to relativistic particle
energy density ratio of 1% combined with a reasonable synchrotron-emitting
volume filling factor of 10% give estimated magnetic field strengths in the SNR
shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from
compression of ambient ISM fields and must instead be internally generated. We
set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in
relativistic particles, 2%-20% of the explosion kinetic energy, is consistent
with estimates from models that fit the IR-radio correlation in compact
starburst galaxies.Comment: 16 pages, 5 figure
Characteristics of UGC galaxies detected by IRAS
Infrared Astronomy Satellite (IRAS) detection rates at 60 microns were determined for the Uppsala General Catalog of Galaxies (Nilson 1973; the UCG). Late-type spirals, characterized by a normal IR/B ratio of approximately 0.6, are detected to a velocity of approximately 6000 km/s for L sub B = L sub *. Contrary to the situation for IRAS-selected galaxy samples, little evidence was found for a correlation between IR/B and 60/100 microns in this large optically-selected sample. Thus a significant fraction of the IRAS-measured far-infrared flux from normal spirals must originate in the diffuse interstellar medium, heated by the interstellar radiation field. Support was not found for Burstein and Lebofsky's (1986) conclusion that spiral disks are optically thick in the far-infrared
Continuum and spectral line observations of the OH Megamaser galaxy Arp 220
We present MERLIN observations of the continuum (both 1.6 and 5 GHz) and OH
maser emission towards Arp220. the correct spatial configuration of the various
componnents of the galaxy is revealed. In the eastern component the masers are
shown to be generally coincident with the larger scale continuum emission; in
the west, the masers and continuum do not generally arise from the same
location. A velocity gradient (0.32+/-0.03km/s/pc) is found in the eastern
nuclear region in MERLIN scales; this gradient is three times smaller than seen
in OH and implies that the OH gas lies inside the HI. A re-analysis of
previously presented global VLBI data (Lonsdale et al. 1998) reveals a very
high velocity gradient (18.67+/-0.12km/s/pc) in one component, possibly the
site of a heavily obscured AGN.Comment: 10 pages, 11 figures, accepted by MNRA
Global properties of the nearby spiral M101
M101 (NGC 5457) is a classic Sc I spiral galaxy located suffiently nearby, 6.8 Mpc, that its structure can be studied even with the coarse angular resolution of the Infrared Astronomy Satellite (IRAS). The global infrared properties of M101 are addressed including the radial dependence of its infrared emission
Multiwavelength radio observations of the compact starburst in Arp 220
We report the first detection at multiple radio wavelengths (13, 6, and 3.6 cm) of 18 compact
sources within both nuclei of the Ultra Luminous Infra-Red Galaxy (ULIRG) Arp 220. In just
over half of the sources we find that the observed spectra are consistent with the standard model
of powerful Type IIn supernovae interacting with their pre-explosion stellar wind. The rate of appearance
of new radio sources ascribed to these supernova events suggests that a large fraction of
core-collapse supernovae in Arp 220 are highly luminous, possibly implying a radically different
stellar initial mass function (IMF) or stellar evolution compared to galactic disks. A second group
of sources, consisting of the brightest and longest monitored sources at 18 cm, do not easily fit
the radio supernova model. We propose that these are young supernova remnants that have just
begun interacting with their surrounding dense ISM
VLBI Images of 49 Radio Supernovae in Arp 220
We have used a Very Long Baseline Interferometry (VLBI) array at 18cm
wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1 pc
linear resolution, and with very high sensitivity. The resulting map has an rms
of 5.5 microJy/beam, and careful image analysis results in 49 confirmed point
sources ranging in flux density from 1.2 mJy down to ~60 microJy. Comparison
with high sensitivity data from 12 months earlier reveals at least four new
sources. The favored interpretation of these sources is that they are radio
supernovae, and if all new supernovae are detectable at this sensitivity, a
resulting estimate of the supernova rate in the Arp 220 system is 4 +/- 2 per
year. The implied star formation rate is sufficient to power the entire
observed far-infrared luminosity of the galaxy. The two nuclei of Arp 220
exhibit striking similarities in their radio properties, though the western
nucleus is more compact, and appears to be ~3 times more luminous than the
eastern nucleus. There are also some puzzling differences, and differential
free-free absorption, synchrotron aging and expansion losses may all be playing
a role. Comparison with the nearby starburst galaxy M82 supports the hypothesis
that the activity in Arp 220 is essentially a scaled-up version of that in M82.Comment: 24 pages, 3 figures, 1 table. Accepted for publication in Ap.
- …