- Dense and Warm Molecular Gas in Arp 220
We present the results from the observations of Arp 220 in
multiple CO spectral line transitions of J = 6-5, J = 3-2, J = 2-1 and
the isotope 13CO at J = 2-1 at 690, 340 and 230 GHz using the
Submillimeter Array (SMA) with the angular resolutions of sub-arcsec to
a few arcsec. Based on the measured line ratios, we modeled the excitation
conditions of the molecular clouds in the nuclear region of Arp 220,
suggesting that the CO J = 6-5 emission arises from dense
(n(H2)~105 cm-3) and warm
(Tk~60 K) cloud components. Each of the four
CO J = 6-5 clumps contains at least a few times 108
M molecular gas.
The dense and warm molecular clouds are likely the
consequence of inelastic collisions between the molecular
clouds in the counterrotating nuclear disks.
Properties determined from these high-density regions suggest
that they are in the early stage of star formation.
The study of Arp 220 could provide us detailed information on
the astrophysical processes in the galaxies at high red-shifts.
- High-Density, Compact H II Regions in the Starburst Galaxies NGC 3628 and IC 694: High-Resolution VLA Observations of the H92 alpha Radio Recombination Line
Using the Very Large Array (VLA), 1 we have observed the H92 alpha line in the nuclear regions
of NGC 3628 and Arp 299 (IC 694 + NGC 3690) with an angular resolution of 1". The radio
recombination lines could provide quite important constraints to the physical conditions
for the ionized gas in the nuclear regions. For the central nuclear components in NGC 3628,
IC 694, and NGC 3690, we find that the electron density of the ionized gas is in the range
5 x 102 to 5 x 104 cm-3,
with corresponding geometrical sizes ranging
from 25 to 0.01 pc. For the regions with a large
line-to-continuum ratio, such as the anomalous kinematic
components in IC 694 and NGC 3628, the electron density
of the H II regions is well
constrained. The derived parameters suggest the existence of ~100 high-density, compact
H II regions (ne ~ 104 cm-3 and l ~ 1--2 pc) in these two components. The large number
of high-density, compact H II regions deduced for these components can be regarded as
evidence for enhanced massive star formation in anomalous kinematic structures that are
often produced in interacting systems. Our new H92 alpha line observations reveal the
presence of rotating circumnuclear ionized gas disks in both NGC 3628 and IC 694. From
the analysis of the nuclear kinematics, we find that a total dynamical mass of
3 x 108 Msun exists within a radius R = 120 pc in NGC 3628 and 7 x
within R = 200 pc in IC 694. The kinematics of the ionized gas and the models with a
collection of H II regions infer that the ratio of H II to dynamical mass in the
starburst nuclei varies from 1 x 104 cm-3 (for the high-density, compact H II region model)
to 1 x 102 cm-3(for the low-density, large H II region model).
Radio Recombination Lines from the Nuclear Regions of Starburst Galaxies
Using the Very Large Array (VLA) with an angular resolution of 3", we have detected
the hydrogen recombination line H92alpha from the galaxies Arp 220, M83, and NGC 2146.
The line emission arises from the nuclear regions with a line-to-continuum ratio of 1%
or less. In order to fit both the observed H92alpha line and continuum data in the
nuclear regions, we have considered two types of models. First, we utilize a model
with a collection of H II regions. A large number of compact H II regions are required
in this model. With electron temperatures in the range 5 x 103 -
1 x 104 K
and a range of electron densities, this model can account for both the line intensity
and the continuum spectrum. In most cases, the H92alpha line is dominated by internal
stimulated emission due to free-free continuum arising within the H II regions.
In a low-density case (ne = 50 cm-3) for Arp 220,
about half the line emission
comes from external stimulated emission due to the background nonthermal source.
Typical rates of ionizing photons predicted from these models are
~5 x 1052 s-1
for M83, ~4 x 1053 s-1 for NGC 2146,
and 5 x 1054 s-1
for Arp 220. We infer
that 105 O5 stars are required in Arp 220,
which is an order of magnitude greater
than in NGC 2146 and 2 orders of magnitude greater than in M83.
uniform slab models with Te >= 5 x 103
and ne in the range of 50 - 1 x 104
appear to fit both the H92alpha line and continuum data of Arp 220 and M83. In the
low-density models, stimulated emission by the background nonthermal radiation
appears to be dominant at low frequencies, and the lines at higher frequencies
arise primarily from spontaneous emission. The uniform slab model requires a higher
ionizing photon rate than the H II region model. No slab models with reasonable Te
can fit the data observed in M83 and NGC 2146. Combining previous published data
with these new observations, a sample of 13 galaxies has been observed for radio
recombination lines (RRLs) with the VLA. Nine out of the 13 galaxies have been
detected in the H92alpha line. While the H9alpha line luminosity appears to be correlated
with the Bralpha line luminosity, we find that nearly all the RRL galaxies show
a significant excess in H92alpha line compared to the expected LTE value. The excess
in the H92alpha line flux suggests that non-LTE effects are important for the H92alpha
line in these starburst nuclei. A strong correlation between H92alpha and the molecular
lines of HCN/HCO+ is also found, indicating that the RRL emitters may be spatially
associated with the dense molecular cores. The inferred high electron density also
suggests an intimate relation between the RRLs and the dense molecular medium
in these galaxies.
Detection of H92 alpha Recombination Line from the Galaxies NGC 3628, IC 694, and NGC 1365
Using the Very Large Array with an angular resolution of 3", we
have detected the hydrogen recombination line H92alpha
(nurest = 8309.38 MHz) from the starburst nuclei in the
galaxies NGC 3628 and IC 694 and also from the Seyfert II nucleus in
NGC 1365. In each case the line-emitting region extends over a few
hundred parsecs. The detected lines have peak intensities in the range
0.5-1.5 mJy and widths (FWHM) 200- 400 km s-1. The line
was not detected in three other galaxies (NGC 262, NGC 1068, and NGC
3079) that were observed to a similar sensitivity level.
We present a model in which a collection of H II regions in the
nuclear region accounts for the observed H92alpha line. The required
number of H II regions, their temperature, electron density, and
linear size are constrained by the observed line flux density, line
width, continuum spectrum, and size of the line-emitting region. If
the temperature of the H II regions is above 5000 K, then electron
densities in the range 5-50 x 103 cm-3 are
permitted by the available constraints. Several hundred H II regions
of a few parsecs in size, with a total mass of a few times
105 Msun, are required to account for the
observed line flux density. The rate of production of Lyman continuum
photons required to maintain the ionization is a few times
1054 s-1. Much of the line emission comes from
internal stimulated emission due to the continuum generated within the
H II regions which account for 5%-30% of the observed total continuum
at 5 GHz. Predictions are made for the expected recombination line and
continuum flux density as a function of frequency from the nuclear
region of the galaxies.
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