657 research outputs found
The Turbulence Power Spectrum in Optically Thick Interstellar Clouds
The Fourier power spectrum is one of the most widely used statistical tools
to analyze the nature of magnetohydrodynamic turbulence in the interstellar
medium. Lazarian & Pogosyan (2004) predicted that the spectral slope should
saturate to -3 for an optically thick medium and many observations exist in
support of their prediction. However, there have not been any numerical studies
to-date testing these results. We analyze the spatial power spectrum of MHD
simulations with a wide range of sonic and Alfv\'enic Mach numbers, which
include radiative transfer effects of the CO transition. We confirm
numerically the predictions of Lazarian & Pogosyan (2004) that the spectral
slope of line intensity maps of an optically thick medium saturates to -3.
Furthermore, for very optically thin supersonic CO gas, where the density or CO
abundance values are too low to excite emission in all but the densest shock
compressed gas, we find that the spectral slope is shallower than expected from
the column density. Finally, we find that mixed optically thin/thick CO gas,
which has average optical depths on order of unity, shows mixed behavior: for
super-Alfv\'enic turbulence, the integrated intensity power spectral slopes
generally follow the same trend with sonic Mach number as the true column
density power spectrum slopes. However, for sub-Alfv\'enic turbulence the
spectral slopes are steeper with values near -3 which are similar to the very
optically thick regime.Comment: accepted to Ap
[CII] 158m and [NII] 205m emission from IC 342 - Disentangling the emission from ionized and photo-dissociated regions
Aims: We investigate how much of the [CII] emission in the nucleus of the
nearby spiral galaxy IC 342 is contributed by PDRs and by the ionized gas. We
examine the spatial variations of starburst/PDR activity and study the
correlation of the [CII] line with the [NII] 205{\textmu}m emission line coming
exclusively from the HII regions. Methods: We present small maps of [CII] and
[NII] lines recently observed with the GREAT receiver on board SOFIA. In
particular we present a super-resolution method to derive how unresolved,
kinematically correlated structures in the beam contribute to the observed line
shapes. Results: We find that the emission coming from the ionized gas shows a
kinematic component in addition to the general Doppler signature of the
molecular gas. We interpret this as the signature of two bi-polar lobes of
ionized gas expanding out of the galactic plane. We then show how this requires
an adaptation of our understanding of the geometrical structure of the nucleus
of IC~342. Examining the starburst activity we find ratios
between 400 and 1800 in energy units.
Applying predictions from numerical models of HII and PDR regions to derive the
contribution from the ionized phase to the total [CII] emission we find that
35-90% of the observed [CII] intensity stems from the ionized gas if both
phases contribute. Averaged over the central few hundred parsec we find for the
[CII] contribution a HII-to-PDR ratio of 70:30. Conclusions: The ionized gas in
the center of IC 342 contributes more strongly to the overall [CII] emission
than is commonly observed on larger scales and than is predicted. Kinematic
analysis shows that the majority of the [CII] emission is related to the strong
but embedded star formation in the nuclear molecular ring and only marginally
emitted from the expanding bi-polar lobes of ionized gas.Comment: 20 pages spectra available online:
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ submitted to and accepted by
A&
A 492 GHz cooled Schottky receiver for radio-astronomy
We developed a 492 GHz cooled GaAs Schottky receiver driven by a solid state local oscillator with a DSB noise temperature of 550 K measured at the telescope. The receiver-bandwidth is approx. equal to 1.0 GHz. Quasi-optical mirrors focus the sky and local oscillator radiation into the mixer. Stability analysis via the Allan variance method shows that the total system including a 1 GHz bandwidth acousto-optical spectrometer built in Cologne allows integration times up to 100 sec per half switching cycle. We successfully used the receiver at the KOSMA 3 m telescope on Gornergrat (3150m) located in the central Swiss Alps near Zermatt during January-February 1992 for observations of the 492 GHz, (CI) (3)P1 to (3)P0 fine structure line in several galactic sources. These observations confirm that Gornergrat is an excellent winter submillimeter site in accordance with previous predictions based on the atmospheric opacity from KOSMA 345 GHz measurements
Detection of a large fraction of atomic gas not associated with star-forming material in M17 SW
We probe the column densities and masses traced by the ionized and neutral
atomic carbon with spectrally resolved maps, and compare them to the diffuse
and dense molecular gas traced by [C I] and low- CO lines toward the
star-forming region M17SW. We mapped a 4.1pc x 4.7pc region in the [C I] 609
m line using the APEX telescope, as well as the CO isotopologues with the
IRAM 30m telescope. We analyze the data based on velocity channel maps that are
1 km/s wide. We correlate their spatial distribution with that of the [C II]
map obtained with SOFIA/GREAT. Optically thin approximations were used to
estimate the column densities of [C I] and [C II] in each velocity channel. The
spatial distribution of the [C I] and all CO isotopologues emission was found
to be associated with that of [C II] in about 20%-80% of the mapped region,
with the high correlation found in the central (15-23 km/s ) velocity channels.
The excitation temperature of [C I] ranges between 40 K and 100 K in the inner
molecular region of M17 SW. Column densities in 1 km/s channels between
~10 and ~10 cm were found for [C I]. Just ~20% of the
velocity range (~40 km/s) that the [C II] line spans is associated with the
star-forming material traced by [C I] and CO. The total gas mass estimated from
the [C II] emission gives a lower limit of ~4.4x10 . At least
64% of this mass is not associated with the star-forming material in M17SW. We
also found that about 36%, 17%, and 47% of the [C II] emission is associated
with the HII, HI, and H_2 regimes, respectively. Comparisons with the
H41 line shows an ionization region mixed with the neutral and part of
the molecular gas, in agreement with the clumped structure and dynamical
processes at play in M17SW. These results are also relevant to extra-galactic
studies in which [C II] is often used as a tracer of star-forming material.Comment: 21 pages + 6 pages of appendix, 32 figures in total, accepted for
publication on A&A (10/12/2014) Relevant calibrated data cubes are available
on CD
CII, CI, and CO in the massive star forming region W3 Main
We have used the KOSMA 3m telescope to map the core 7'x5' of the Galactic
massive star forming region W3Main in the two fine structure lines of atomic
carbon and four mid-J transitions of CO and 13CO. In combination with a map of
singly ionized carbon (Howe et al. 1991), and FIR fine structure line data
observed by ISO/LWS at the center position, these data sets allow to study in
detail the physical structure of the photon dominated cloud interface regions
(PDRs) where the occurance of carbon changes from CII to CI, and to CO.Comment: 4 pages, 4 figures, to appear in "Proceedings of the 4th
Cologne-Bonn-Zermatt-Symposium, The dense interstellar medium in galaxies",
eds. S. Pfalzner, C. Kramer, C. Straubmeier, and A. Heithausen (Springer
Verlag
The ionized and hot gas in M17 SW: SOFIA/GREAT THz observations of [C II] and 12CO J=13-12
With new THz maps that cover an area of ~3.3x2.1 pc^2 we probe the spatial
distribution and association of the ionized, neutral and molecular gas
components in the M17 SW nebula. We used the dual band receiver GREAT on board
the SOFIA airborne telescope to obtain a 5'.7x3'.7 map of the 12CO J=13-12
transition and the [C II] 158 um fine-structure line in M17 SW and compare the
spectroscopically resolved maps with corresponding ground-based data for low-
and mid-J CO and [C I] emission. For the first time SOFIA/GREAT allow us to
compare velocity-resolved [C II] emission maps with molecular tracers. We see a
large part of the [C II] emission, both spatially and in velocity, that is
completely non-associated with the other tracers of photon-dominated regions
(PDR). Only particular narrow channel maps of the velocity-resolved [C II]
spectra show a correlation between the different gas components, which is not
seen at all in the integrated intensity maps. These show different morphology
in all lines but give hardly any information on the origin of the emission. The
[C II] 158 um emission extends for more than 2 pc into the M17 SW molecular
cloud and its line profile covers a broader velocity range than the 12CO
J=13-12 and [C I] emissions, which we interpret as several clumps and layers of
ionized carbon gas within the telescope beam. The high-J CO emission emerges
from a dense region between the ionized and neutral carbon emissions,
indicating the presence of high-density clumps that allow the fast formation of
hot CO in the irradiated complex structure of M17 SW. The [C II] observations
in the southern PDR cannot be explained with stratified nor clumpy PDR models.Comment: 4 pages, 4 figures, letter accepted for the SOFIA/GREAT A&A 2012
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