3,772 research outputs found
The Extreme Compact Starburst in MRK 273
Images of neutral Hydrogen 21cm absorption and radio continuum emission at
1.4 GHz from Mrk 273 were made using the Very Long Baseline Array and Very
Large Array. These images reveal a gas disk associated with the northern
nuclear region with a diameter of 0.5'' (370 pc), at an inclination angle of
53deg. The radio continuum emission is composed of a diffuse component plus a
number of compact sources. This morphology resembles those of nearby, lower
luminosity starburst galaxies. These images provide strong support for the
hypothesis that the luminosity of the northern source is dominated by an
extreme compact starburst. The HI 21cm absorption shows an east-west gradient
in velocity of 450 km/s across 0.3'' (220 pc), implying an enclosed mass of 2e9
M_solar, comparable to the molecular gas mass. The brightest of the compact
sources may indicate radio emission from an active nucleus (AGN), but this
source contributes only 3.8% to the total flux density of the northern nuclear
region. The HI 21cm absorption toward the southeast radio nucleus suggests
infall at 200 km/s on scales < 40 pc, and the southwest near IR nucleus is not
detected in high resolution radio continuum images.Comment: standard AAS format, 23 pages, 5 figures, fixed figure. To appear in
ApJ Letter
A radio continuum survey of edge-on spiral galaxies at 90 cm
Accurate spectral indices of the radio emission from both the thin disk and thick disk or halo components are critical to understanding the propagation mechanisms of electrons within spiral galaxies. The spectral indices give information of relative importance of diffusion and synchrotron energy loss in the propagation of electrons in the disk. Our goal of this survey is to locate a larger sample of spiral galaxies that exhibit halo phenomena so that a statistical analysis will be possible
Discovery of High-Latitude CO in a HI Supershell in NGC 5775
We report the discovery of very high latitude molecular gas in the edge-on
spiral galaxy, NGC 5775. Emission from both the J=1-0 and 2-1 lines of 12CO is
detected up to 4.8 kpc away from the mid-plane of the galaxy. NGC 5775 is known
to host a number of HI supershells. The association of the molecular gas
M(H2,F2) = 3.1x10^7 solar masses reported here with one of the HI supershells
(labeled F2) is clear, which suggests that molecular gas may have survived the
process which originally formed the supershell. Alternatively, part of the gas
could have been formed in situ at high latitude from shock-compression of
pre-existing HI gas. The CO J=2-1/J=1-0 line ratio of 0.34+-40% is
significantly lower than unity, which suggests that the gas is excited
subthermally, with gas density a few times 100 cubic cm. The molecular gas is
likely in the form of cloudlets which are confined by magnetic and cosmic rays
pressure. The potential energy of the gas at high latitude is found to be
2x10^56 ergs and the total (HI + H2) kinetic energy is 9x10^53 ergs. Based on
the energetics of the supershell, we suggest that most of the energy in the
supershell is in the form of potential energy and that the supershell is on the
verge of falling and returning the gas to the disk of the galaxy.Comment: Accept by ApJL, 4 pages, 3 ps figure
Clusters of Extragalactic Ultra Compact HII Regions
We report on the detection of optically thick free-free radio sources in the
galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret
these sources as being young, embedded star birth regions, which are likely to
be clusters of ultracompact HII regions. All 35 of the sources presented in
this article have positive radio spectral indices alpha>0 suggesting an
optically thick thermal bremsstrahlung emission arising in the HII region
surrounding hot stars. Energy requirements indicate a range of a several to
>500 O7V star equivalents powering each HII region. Assuming a Salpeter IMF,
this corresponds to integrated stellar masses of 0.1--60,000 Msun. For roughly
half of the sources in our sample, there is no obvious optical counterpart,
giving further support for their deeply embedded nature. Their luminosities and
radio spectral energy distributions are consistent with HII regions having
electron densities from 1500 cm^-3 to 15000 cm^-3 and radii of 1 - 7 pc. We
suggest that the less luminous of these sources are extragalactic ultracompact
HII region complexes, those of intermediate luminosity are similar to W49 in
the Galaxy, while the brightest will be counterparts to 30 Doradus. These
objects constitute the lower mass range of extragalactic ``ultradense HII
regions'' which we argue are the youngest stages of massive star cluster
formation yet observed. This sample is beginning to fill in the continuum of
objects between small associations of ultracompact HII regions and the massive
extragalactic clusters that may evolve into globular clusters.Comment: 37 pages, uses AASTeX; scheduled to appear in ApJ v. 559 October
2001. Full postscript version available from
http://www.astro.wisc.edu/~chip/Papers/Johnson_Kobulnicky_etal_ApJ559.ps.g
High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz
We present a wide field, sub-arcminute resolution VLA image of the Galactic
Center region at 330 MHz. With a resolution of ~ 7" X 12" and an RMS noise of
1.6 mJy/beam, this image represents a significant increase in resolution and
sensitivity over the previously published VLA image at this frequency. The
improved sensitivity has more than tripled the census of small diameter sources
in the region, has resulted in the detection of two new Non Thermal Filaments
(NTFs), 18 NTF candidates, 30 pulsar candidates, reveals previously known
extended sources in greater detail, and has resulted in the first detection of
Sagittarius A* in this frequency range.
A version of this paper containing full resolution images may be found at
http://lwa.nrl.navy.mil/nord/AAAB.pdf.Comment: Astronomical Journal, Accepted 62 Pages, 21 Figure
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
Spatially-resolved Thermal Continuum Absorption against the Supernova Remnant W49B
We present sub-arcminute resolution imaging of the Galactic supernova remnant
W49B at 74 MHz (25") and 327 MHz (6"), the former being the lowest frequency at
which the source has been resolved. While the 327 MHz image shows a shell-like
morphology similar to that seen at higher frequencies, the 74 MHz image is
considerably different, with the southwest region of the remnant almost
completely attenuated. The implied 74 MHz optical depth (~ 1.6) is much higher
than the intrinsic absorption levels seen inside two other relatively young
remnants, Cas A and the Crab Nebula, nor are natural variations in the
relativistic electron energy spectra expected at such levels. The geometry of
the absorption is also inconsistent with intrinsic absorption. We attribute the
absorption to extrinsic free-free absorption by a intervening cloud of thermal
electrons. Its presence has already been inferred from the low-frequency
turnover in the integrated continuum spectrum and from the detection of radio
recombination lines toward the remnant. Our observations confirm the basic
conclusions of those measurements, and our observations have resolved the
absorber into a complex of classical HII regions surrounded either partially or
fully by low-density HII gas. We identify this low-density gas as an extended
HII region envelope (EHE), whose statistical properties were inferred from low
resolution meter- and centimeter-wavelength recombination line observations.
Comparison of our radio images with HI and H_2CO observations show that the
intervening thermal gas is likely associated with neutral and molecular
material as well.Comment: 18 pages, LaTeX with AASTeX-5, 5 figures in 7 PostScript files;
accepted for publication in the Ap
Near-infrared [Fe II] emission from supernova remnants and the supernova rate of starburst galaxies
In an effort to better calibrate the supernova rate of starburst galaxies as
determined from near-IR [Fe II] features, we report on a [Fe II] 1.644 microns
line-imaging survey of a sample of 42 optically-selected SNRs in M33. A wide
range of [Fe II] luminosities are observed within our sample (from less than 6
to 695 L_sun). Our data suggest that the bright [Fe II] SNRs are entering the
radiative phase and that the density of the local ISM largely controls the
amount of [Fe II] emission. We derive the following relation between the [Fe
II] 1.644 microns line luminosity of radiative SNRs and the electronic density
of the postshock gas, n_e: L_[Fe II] (L_sun) ~ 1.1 n_e (cm^-3). We also find a
correlation in our data between L_[Fe II] and the metallicity of the
shock-heated gas, but the physical interpretation of this result remains
inconclusive, as our data also show a correlation between the metallicity and
n_e. The dramatically higher level of [Fe II] emission from SNRs in the central
regions of starburst galaxies is most likely due to their dense environments,
although metallicity effects might also be important. The typical [Fe
II]-emitting lifetime of a SNR in the central regions of starburst galaxies is
found to be of the order of 10^4 yr. On the basis of these results, we provide
a new empirical relation allowing the determination of the current supernova
rate of starburst galaxies from their integrated near-IR [Fe II] luminosity.Comment: 13 pages, 5 figures; accepted for publication in MNRA
High Resolution, High Sensitivity Imaging of the Galactic Center at 330 MHz
We present results derived from a wide field, sub-arcminute resolution VLA image of the Galactic Center region at 330 MHz (λ = 90 cm). With a resolution of ∼7″ × 12″ and an rms noise of 1.6 mJy beam−1, this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency (eg. LaRosa et al. 2000). The improvement in sensitivity has significantly increased the census of small diameter sources in the region, resulted in the detection of two new Non-Thermal Filaments (NTFs) and 18 new NTF candidates, and resulted in the lowest frequency (tentative) detection of Sgr A*
Radio continuum observations of the Virgo cluster spiral NGC 4522 The signature of ram pressure
Radio continuum observations at 20 and 6 cm of the highly inclined Virgo
spiral galaxy NGC 4522 are presented. Both, 20 and 6 cm total emission
distributions are asymmetric with an extended component to the west where
extraplanar atomic gas and Halpha emission are found. The 6 cm polarized
emission is located at the eastern edge of the galactic disk. Its peak is
located about 1 kpc to the east of the total emission peak. We argue that this
phenomena is a characteristic feature for cluster galaxies which are
experiencing significant pressure from the intracluster medium. The degree of
polarization decreases from the east to the west. The flattest spectral index
between 20 and 6 cm coincides with the peak of the 6 cm polarized emission.
These findings are consistent with a picture of a large scale shock due to ram
pressure located at the east of the galaxy where cosmic rays are accelerated.
We conclude that it is likely that the galaxy experiences active ram pressure.Comment: 11 pages, 11 figures. Accepted for publication in A
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