177 research outputs found
13CO at the centre of M82
Using the IRAM interferometer, we have observed the nearby starburst galaxy
M82 with a 4.2" resolution (~70 pc) in the 1->0 line of 13CO and in the 2.6-mm
continuum.
The spatial distribution of the 13CO line shows the same gross features as
the 12CO(1->0) map of Shen & Lo (1995), namely two lobes and a compact central
source, though with different relative intensities. The lobes are more
conspicuous and the central source is fainter in 13CO than in 12CO.
The velocity field observed around the nucleus shows a very steep gradient
(140 km/s over 75 pc), which is very probably caused by the stellar bar visible
in the near infrared. The dynamical centre coincides with the IR peak and is
shifted 6" north-east of the compact 13CO source. The two CO lobes appear to be
associated with the ends of the bar and not with a molecular ring, as usually
assumed. They are probably shaped by the strong UV radiation from the central
region. 13CO must be more photodissociated than the self-shielded 12CO
molecules in the central ~250 pc region, which may explain the relative
weakness of the 13CO central source.
A 130 pc-wide bubble of molecular gas has been identified, which happens to
host the most luminous compact radio source in M82. It lies 120 pc west of the
IR peak between the central source and the western lobe and seems characterized
by warmer gas, strong UV radio free-free radiation, and an enhanced cosmic ray
production rate.Comment: Accepted by A&A; 9 pages, 9 ps figures, needs LaTeX 2e A&A macro and
psfig Styl
First detection of ammonia in M82
We report the detection of the (J,K) = (1,1), (2,2), and (3,3) inversion
lines of ammonia (NH3) towards the south--western molecular lobe in M82. The
relative intensities of the ammonia lines are characterized by a rotational
temperature of T_rot=29+/-5 K which implies an average kinetic temperature of
T_kin~60 K. A Gaussian decomposition of the observed spectra indicates
increasing kinetic temperatures towards the nucleus of M82, consistent with
recent findings based on CO observations. The observations imply a very low NH3
abundance relative to H2, X(NH3)~5x10^(-10). We present evidence for a
decreasing NH3 abundance towards the central active regions in M82 and
interpret this abundance gradient in terms of photodissociation of NH3 in PDRs.
The low temperature derived here from NH3 also explains the apparent
underabundance of complex molecules like CH_3OH and HNCO, which has previously
been reported.Comment: 4 pages, 4 figures, accepted by ApJ
Cold dust in a selected sample of nearby galaxies. I. The interacting galaxy NGC4631
We have observed the continuum emission of the interacting galaxy NGC4631 at
0.87 and 1.23mm using the Heinrich-Hertz-Telescope on Mt. Graham and the IRAM
30-m telescope on Pico Veleta. We have obtained fully sampled maps which cover
the optical emission out to a radius of about 7' at both wavelengths. For a
detailed analysis, we carefully subtracted the line contributions and
synchrotron and free-free emission from the data, which added up to 6% at
1.23mm and 10% at 0.87mm. We combined the flux densities with FIR data to
obtain dust spectra and calculate dust temperatures, absorption cross sections,
and masses. Assuming a ``standard'' dust model, which consists of two
populations of big grains at moderate and warm temperatures, we obtained
temperatures of 18K and 50K for the both components. However, such a model
suffers from an excess of the radiation at 1.23mm, and the dust absorption
cross section seems to be enhanced by a factor 3 compared to previous results
and theoretical expectations. At large galactocentric radii, where the galaxy
shows disturbances as a result of gravitational interaction, this effect seems
to be even stronger. Some possibilities to resolve these problems are
discussed. The data could be explained by a very cold dust component at a
temperature of 4-6K, an increased abundance of very small grains, or a
component of grains with unusual optical properties. We favour the latter
possibility, since the first two lead to inconsistencies.Comment: 12 pages, 6 figures, accepted for publication in Astronomy &
Astrophysics. Updated version with minor errors corrected (typos, LaTeX
formatting, missing citation
The influence of the cluster environment on the large-scale radio continuum emission of 8 Virgo cluster spirals
The influence of the environment on the polarized and total power radio
continuum emission of cluster spiral galaxies is investigated. We present deep
scaled array VLA 20 and 6 cm observations including polarization of 8 Virgo
spiral galaxies. These data are combined with existing optical, HI, and Halpha
data. Ram pressure compression leads to sharp edges of the total power
distribution at one side of the galactic disk. These edges coincide with HI
edges. In edge-on galaxies the extraplanar radio emission can extend further
than the HI emission. In the same galaxies asymmetric gradients in the degree
of polarization give additional information on the ram pressure wind direction.
The local total power emission is not sensitive to the effects of ram pressure.
The radio continuum spectrum might flatten in the compressed region only for
very strong ram pressure. This implies that neither the local star formation
rate nor the turbulent small-scale magnetic field are significantly affected by
ram pressure. Ram pressure compression occurs mainly on large scales (>=1 kpc)
and is primarily detectable in polarized radio continuum emission.Comment: 16 pages, 10 figures, accepted for publication in A&
Molecular gas in the Andromeda galaxy
We present a new 12CO(J=1-0)-line survey of the Andromeda galaxy, M31,
covering the bright disk with the highest resolution to date (85 pc along the
major axis), observed On-the-Fly (in italics) with the IRAM 30-m telescope. We
discuss the distribution of the CO emission and compare it with the
distributions of HI and emission from cold dust traced at 175mum. Our main
results are: 1. Most of the CO emission comes from the radial range R=3-16 kpc,
but peaks near R=10 kpc. The emission is con- centrated in narrow, arm-like
filaments defining two spiral arms with pitch angles of 7d-8d. The average
arm-interarm brightness ratio along the western arms reaches 20 compared to 4
for HI. 2. For a constant conversion factor Xco, the molecular fraction of the
neutral gas is enhanced in the arms and decreases radially. The apparent
gas-to-dust ratios N(HI)/I175 and (N(HI)+2N(H2))/I175 increase by a factor of
20 between the centre and R=14 kpc, whereas the ratio 2N(H2)/I175 only
increases by a factor of 4. Implications of these gradients are discussed. In
the range R=8-14 kpc total gas and cold dust are well correlated; molecular gas
is better correlated with cold dust than atomic gas.Comment: 21 pages, 16 figures. Accepted for publication in A&
M-Band Spectra of Dust Embedded Sources at the Galactic Center
The goal of the present paper is to investigate the circumstellar material
around the brightest dust-enshrouded sources in the central stellar cluster of
the Milky Way. Observations have been carried out at the European Southern
Observatory's Very Large Telescope on Paranal, Chile. We have used the long
wavelength (LWS3) low resolution (LR) spectroscopic mode of the ISAAC camera at
the VLT in the spectral range of the M filter from 4.4micron to 5.1micron. The
use of a slit width of 0.6" implied a spectral resolution of R=l/Dl=800 (Dv=375
km/s). These observations resulted in M-band spectra of 15 bright sources in
the central stellar cluster of the Milky Way. In addition to gaseous 12^CO
(4.666 micron) and 13^CO (4.77 micron) vibration-rotational absorptions, we
detect a strong absorption due to a mixture of polar and apolar CO ice
(centered at 4.675 micron). In the shorter wavelength absorption wing there is
an absorption feature due to XCN at 4.62 micron. The XCN absorption is
strongest toward the M2 supergiant IRS7. We find that the extinctions due to
material traced by the CO ice and the CO gas absorptions may be of comparable
importance. Using the spectra of IRS2L and IRS16C we perform a first order
correction of the line of sight absorption due to CO-ice and 13^CO gas. In
combination with published hydrogen number density estimates from sub-mm
CO(7-6) and FIR [OI] line data we obtain gas masses of the circumstellar shells
of the order of 10^-3 and 10^-2 solar masses. This implies that in future
spectra taken at high spectral and angular resolution the bright and dust
embedded Galactic Center sources should show a substantial line absorption due
to source intrinsic absorption.Comment: 23 pages, 9 figures, Accepted for publication in Ap
Formation of a Massive Black Hole at the Center of the Superbubble in M82
We performed 12CO(1-0), 13CO(1-0), and HCN(1-0) interferometric observations
of the central region (about 450 pc in radius) of M82 with the Nobeyama
Millimeter Array, and have successfully imaged a molecular superbubble and
spurs. The center of the superbubble is clearly shifted from the nucleus by 140
pc. This position is close to that of the massive black hole (BH) of >460 Mo
and the 2.2 micron secondary peak (a luminous supergiant dominated cluster),
which strongly suggests that these objects may be related to the formation of
the superbubble. Consideration of star formation in the cluster based on the
infrared data indicates that (1) energy release from supernovae can account for
the kinetic energy of the superbubble, (2) the total mass of stellar-mass BHs
available for building-up the massive BH may be much higher than 460 Mo, and
(3) it is possible to form the middle-mass BH of 100-1000 Mo within the
timescale of the superbubble. We suggest that the massive BH was produced and
is growing in the intense starburst region.Comment: 9 pages, 3 figures, to appear in ApJ Lette
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