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Journal Article•DOI•

A 12 co j = 6-5 map of m82: the significance of warm molecular gas

John Ward1, John Ward2, Jonas Zmuidzinas1, Andrew I. Harris3, K. G. Isaak4, K. G. Isaak3 •
California Institute of Technology1, Jet Propulsion Laboratory2, University of Maryland, College Park3, University of Cambridge4
10 Apr 2003-The Astrophysical Journal (IOP Publishing)-Vol. 587, Iss: 1, pp 171-185
TL;DR: In this article, the authors presented a 12 CO J ¼ 6−5 map of the nuclear regions of the starburst galaxy M82 at a resolution of 14 00 taken at the Caltech Submillimeter Observatory (CSO).
Abstract: We present a 12 CO J ¼ 6–5 map of the nuclear regions of the starburst galaxy M82 at a resolution of 14 00 taken at the Caltech Submillimeter Observatory (CSO). Hot spots were found on either side of the dynamical center. We compare our results with a high-resolution 12 CO J ¼ 2–1 interferometer map, and present a 12 CO J ¼ 6–5/ 12 CO J ¼ 2–1 line ratio map obtained using a novel deconvolution technique. This line ratio is highest at the two J ¼ 6–5 integrated intensity peaks, reaching 0.4 and 0.5 in the northeast and southwest peaks, respectively, and is typically 0.2 elsewhere in the nuclear region. We also present measurements of 12 CO J ¼ 4–3, 12 CO J ¼ 3–2, and 13 CO J ¼ 3–2, and an upper limit for 13 CO J ¼ 6–5. We analyze these observations in the context of a two-component large velocity gradient (LVG) excitation model. Likelihood density curves were calculated for each of the model parameters and a variety of related physical quantities for the northeast and southwest peaks based on the measured line intensities and their associated uncertainties. This approach reveals in an unbiased way how well various quantities can be constrained by the CO observations. We find that the beam-averaged 12 CO and 13 CO column densities, the isotopomer abundance ratio, and the area filling factors are among the best constrained quantities, while the cool component H2 density and pressure are less well constrained. The results of this analysis suggest that the warm gas is less dense than the cool gas, and that over half of the total molecular gas mass in these nuclear regions is warmer than 50 K. Subject headings: galaxies: individual (M82) — galaxies: ISM — galaxies: starburst — submillimeter

Summary (2 min read)

Jump to: [1. INTRODUCTION] – [2. OBSERVATIONS] – [3. RESULTS] and [4. CONCLUSIONS]

1. INTRODUCTION

  • M82, also known as NGC 3034, is a nearby irregular starburst galaxy in UrsaMajor, and is seen nearly edge-on.
  • The CO J ¼ 6–5 line was first detected toward M82 by Harris et al. (1991), who showed that molecular gas in starburst galaxies is substantially warmer than in typical disk clouds.
  • CO excitation analyses in the large velocity gradient (LVG) approximation can now be calculated in a few milliseconds on a typical modern desktop computer.
  • The authors have also calculated likelihood functions for a variety of physical quantities derived from these parameters, such as the pressure and the beamaveraged column density.
  • This technique uses all spatial and velocity information in both maps, and can calculate the line ratio at a higher spatial resolution than the numerator map.

2. OBSERVATIONS

  • All observations were performed at the Caltech Submillimeter Observatory (CSO) in 1998 January and 1999 January using SIS receivers operating in double-sideband mode.
  • The use of a broadband tunerless receiver reduces calibration errors associated with receiver sideband imbalance.
  • A reference point was chosen where the line connecting the hot spots crossed the 200 km s 1 velocity contour.
  • The coordinates of the 12CO J ¼ 6–5 map were then adjusted to match the reference point to the coordinates of a point found the same way in the interferometer maps.
  • Zero-order baselines were subtracted from all reported measurements except 12CO J ¼ 4–3, which was corrected for a first-order baseline, and all reported results are given on a main beam brightness temperature scale (TMB).

3. RESULTS

  • Figure 2 shows two peaks in integrated intensity, the southwest hot spot being stronger than the northeast.
  • Likelihood curves of the free parameters are shown in Figure 7; the curves for derived quantities are shown in Figure 8.
  • The results from the two different cloud models differ in detail but do not change the basic results of this paper; the authors therefore summarize the differences rather than present figures and tables of the spherical model results.
  • These results suggest that the southwest lobe has higher beam-averaged 13CO column density and lower cool-component H2 density.
  • In part because of the nature of the model and the somewhat arbitrary distinction between ‘‘ warm ’’ and ‘‘ cool ’’ gas, the ratio of warm gas mass to cool gas mass is not well constrained.

4. CONCLUSIONS

  • Results of this model suggest that the nuclear region contains a large number of small, cool molecular clouds, and a small number of large, warm, low-density clouds.
  • The density of warm molecular gas in the nuclear region of M82 is low, with median likelihood estimates around 103 cm 3.
  • Furthermore, with this method, a Lagrange multiplier provides a natural way to adjust the resolution of the resulting ratio map.
  • To aid this selection, the authors also calculated residuals by finding the difference between the measured J ¼ 6–5 data and T6 5ðr; vÞ computed in equation (A1).

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A
12
CO J = 6–5 MAP OF M82: THE SIGNIFICANCE OF WARM MOLECULAR GAS
John S. Ward
1
and Jonas Zmuidzinas
Department of Physics, 320-47, California Institute of Technology, Pasadena, CA 91125;
john.ward@jpl.nasa.gov, jonas@submm.caltech.edu
and
Andrew I. Harris and Kate G. Isaak
2
Astronomy Department, University of Maryland, College Park, MD 20742;
harris@astro.umd.edu, isaak@mrao.cam.ac.uk
Received 2001 November 17; accepted 2002 December 23
ABSTRACT
We present a
12
CO J ¼ 6–5 map of the nuclear regions of the starburst galaxy M82 at a resolution of 14
00
taken at the Caltech Submillimeter Observatory (CSO). Hot spots were found on either side of the dynamical
center. We compare our results with a high-resolution
12
CO J ¼ 2–1 interferometer map, and present a
12
CO
J ¼ 6–5/
12
CO J ¼ 2–1 line ratio map obtained using a novel deconvolution technique. This line ratio is
highest at the two J ¼ 6–5 integrated intensity peaks, reaching 0.4 and 0.5 in the northeast and southwest
peaks, respectively, and is typically 0.2 elsewhere in the nuclear region. We also present measurements of
12
CO
J ¼ 4–3,
12
CO J ¼ 3–2, and
13
CO J ¼ 3–2, and an upper limit for
13
CO J ¼ 6–5. We analyze these
observations in the context of a two-component large velocity gradient (LVG) excitation model. Likelihood
density curves were calculated for each of the model parameters and a variety of related physical quantities for
the northeast and southwest peaks based on the measured line intensities and their associated uncertainties.
This approach reveals in an unbiased way how well various quantities can be constrained by the CO observa-
tions. We find that the beam-averaged
12
CO and
13
CO column densities, the isotopomer abundance ratio, and
the area filling factors are among the best constrained quantities, while the cool component H
2
density and
pressure are less well constrained. The results of this analysis suggest that the warm gas is less dense than the
cool gas, and that over half of the total molecular gas mass in these nuclear regions is warmer than 50 K.
Subject headings: galaxies: individual (M82) — galaxies: ISM — galaxies: starburst — submillimeter
1. INTRODUCTION
M82, also known as NGC 3034, is a nearby irregular star-
burst galaxy in Ursa Major, and is seen nearly edge-on. Dust
obscures the nuclear region at optical wavelengths. M82 is
the brightest IR galaxy in the sky, and has a total IR
luminosity of 3:8 10
10
L
(Colbert et al. 1999). It is
believed that the starburst was triggered by a close encounter
with M81 about 10
8
yr ago (Yun, Ho, & Lo 1993). The bright
far-infrared (FIR) fine-structure lines seen in the inner kilo-
parsec of M82 imply a far-UV radiation field about 10
3
times
larger than the local solar value (Colbert et al. 1999). This
strong UV field is generated by a population of massive
young stars that also heat the dust which produces the large
IR luminosity. The nucleus has a large molecular gas mass, a
few 10
8
M
, giving rise to strong CO emission (Wild et al.
1992; Weiss et al. 2001). At an estimated current star forma-
tion rate of 1 M
yr
1
, this gas will be consumed in about
2 10
8
yr (Lord et al. 1996). Maps of both molecular line
emission and thermal dust continuum show a double-peaked
structure in the central kiloparsec (Neininger et al. 1998;
Hughes, Gear, & Robson 1994).
The CO J ¼ 6–5 line was first detected toward M82 by
Harris et al. (1991), who showed that molecular gas in star-
burst galaxies is substantially warmer than in typical disk
clouds. The CO J ¼ 6–5 transition probes higher excitation
temperatures (116 K above the ground state) than the more
accessible CO J ¼ 1–0 (5.5 K), CO J ¼ 2–1 (17 K), and CO
J ¼ 3–2 (33 K) trans itions. The higher J spectral lines thus
provide important information needed to understand the
large mass of T 100 K molecular gas that is heated by
massive young stars in a starburst galaxy. Recent improve-
ments in receiver techn ology now allow high-quality maps
to be made of the
12
CO J ¼ 6–5 and J ¼ 7–6 rotational lines
in nearby galaxies. Mao et al. (2000) recently mapped the
12
CO J ¼ 7–6 line. We present here a map of
12
CO J ¼ 6–5.
As improving receiver technology provides astronomers
with ever higher quality data, advances in digital computers
open up possibilities for analyzing observations in new and
better ways. For instance, CO excitation analyses in the
large velocity gradient (LVG) approximation can now be
calculated in a few milliseconds on a typic al modern desk-
top computer. This allows us to evaluate multicomponent
models with many parameters, examining huge volumes of
parameter space to find all combinations of the model
parameters consistent with the measured data rather than
only calculating some narrow range of possible solutions.
Using measured line intensities, we have calculated likeli-
hood density functions for each of the parameters of a two-
component LVG model for the northeast and southwest
CO emission peaks. We have also calculated likelihood
functions for a variety of physical quantities derived from
these parameters, such as the pressure and the beam-
averaged column density. These likelihood curves were used
to find median likelihood estimators and 95% confidence
intervals for the quantities of interest.
We have also developed a novel deconvolution
technique to compute line ratio maps when a high-quality,
1
Current address: Mail Stop 168-314, Jet Propulsion Laboratory, 4800
Oak Grove Drive, Pasadena, CA 91109-8099.
2
Current address: Cavendish Astrophysics, University of Cambridge,
Cambridge CB3 0HE, UK.
The Astrophysical Journal, 587:171–185, 2003 April 10
# 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.
171
high-resolution map is available for the denominator and
both maps are of the same species. This technique uses all
spatial and velocity information in both maps, and can
calculate the line ratio at a higher spatial resolution than the
numerator map. With this technique, information is not
thrown away, as with the common method of integrating
away the velocity information and smoothing the higher
resolution map before dividing.
2. OBSERVATIONS
All observations were performed at the Caltech Sub-
millimeter Observatory (CSO) in 1998 January and 1999
January using SIS receivers operating in double-sideband
mode. The
12
CO J ¼ 6–5 map was made in excellent
weather (
225 GHz
d0:04), with typical single-sideband
system temperatures (including the atmosphere) around
3000 K. The receiver consisted of a quasi-optical SIS mixer
with an integrated HEMT low noise amplifier (LNA); an
upgraded version of this receiver was described by Ward et
al. (2000). The use of a broadband tunerless receiver
reduces calibration errors associated with receiver sideband
imbalance. A pair of 1024 channel, 500 MHz acousto-
optical spectrometers were used as back ends. The chopping
secondary was switched at a 1 Hz rate with a 60
00
throw in
azimuth. Spectral intensities were calibrated on an ambient-
temperature chopper wheel and the sky periodically during
the observations. Beam maps of Saturn were used to deter-
mine the beam FWHP of approximately 14
00
, and the main
beam efficiency of 30%.
We do not have information about the error beam, which
would contain most of the missing power; as a result, we
are using a conservative estimate of 30% for the total
calibration uncertainty.
The map of M82 consists of 36 positions on 7>5 centers,
and covers 70
00
along the major axis of the galaxy and 50
00
along the minor axis (Fig. 1). At a distance of 3.6 Mpc
(Freedman et al. 1994), this corresponds to an area of
1200 880 pc mapped with a 250 pc FWHP beam. The
coordinates of the J ¼ 6–5 map were corrected for system-
atic pointing offsets by comparing line velocity structure to
12
CO J ¼ 2–1 and C
18
O J ¼ 1–0 Plateau de Bure Interfer-
ometer data cubes from Weiss et al. (2001), according to the
following procedure. A line connecting the antenna temper-
ature peaks of the bright hot spots on either side of the
dynamical center was found. This line was then drawn on
top of a contour map of line center velocity,
v
c
¼
R
T
A
vdv
R
T
A
dv
: ð1Þ
The contours of constant velocity crossed the line at nearly
right angles. A reference point was chosen where the line
connecting the hot spots crossed the 200 km s
1
velocity
contour. The coordinat es of the
12
CO J ¼ 6–5 map were
then adjusted to match the reference point to the coordi-
nates of a point found the same way in the interferometer
maps. The correction was 4>5 in right ascension and 7>2in
declination, for a total shift of about 0.6 of the FWHP
beam. This shift may have arisen because the telescope fixed
pointing offsets were not determined immediately preceding
the observations. The total pointing uncertainty is about 4
00
in each axis. Maps of integrated intensity and peak antenna
temperature are shown in Figures 2 and 3, respectively.
Position-velocity plots are shown in Figure 4.
Fig. 1.—Spectra of
12
CO J ¼ 6–5 in M82. The map has been rotated such that the horizontal offsets are approximately along the major axis. Offsets are in
arcsec from an arbitrary center. The vertical scale ranges from T
MB
of 1 to 4.5 K, and the horizontal scale ranges from 80 to 520 km s
1
.
172 WARD ET AL. Vol. 587
In addition to the
12
CO J ¼ 6–5 map, we have also
observed 17 positions of the
12
CO J ¼ 4–3 line, nine
positions of
12
CO J ¼ 3–2, and five positions of
13
CO
J ¼ 3–2, all with the CSO facility SIS receivers. Most of
these positions were observed as cuts along the major axis of
the galaxy. The beam sizes were 16>5, 24>4, and 25>5
FWHP at 461, 346, and 331 GHz, respectively, with corre-
sponding main beam efficiencies of 0.45, 0.50, and 0.50. The
CO J ¼ 3–2 observations were made with a 4 GHz wide-
band analog correlator spectrometer (WASP) similar to
that described by Harris & Zmuidzinas (2001). The
13
CO
J ¼ 3–2 spectra were scaled up by 14% to account for the
difference in atmospheric transmission in the two sidebands.
We also integrated on the
13
CO J ¼ 6–5 line for a single
Fig. 2.—Integrated intensity of
12
CO J ¼ 6–5 in M82. Contours are 50, 100, 150, 200, 250, 300, 350, and 400 K km s
1
.
Fig. 3.—Peak antenna temperature of
12
CO J ¼ 6–5 in M82. Contours are for T
MB
of 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 K.
No. 1, 2003
12
CO J = 6–5 MAP OF M82 173
pointing in 1998 January and 1999 January for a total of 96
minutes unde r conditions similar to those for the
12
CO
J ¼ 6–5 observations, but did not detect the line.
Zero-order baselines were subtracted from all reported
measurements except
12
CO J ¼ 4–3, which was corrected
for a first-order baseline, and all reported results (including
both the table s and the figures) are given on a main beam
brightness temperature scale (T
MB
).
3. RESULTS
Figure 2 shows two peaks in integrated intensity, the
southwest hot spot being stronger than the northeast. Com-
paring the integrated intensity to Figure 3, we see that the
two hot spots are much more distinct in peak antenna tem-
perature than in integrated intensity. This is due in part to a
spatial smearing effect caused by the antenna beam width
being comparable to the spacing between the hot spots, i.e.,
a14
00
FWHP beam compared to 19
00
peak separation in
Figure 3. The locations of the hot spots in the peak antenna
temperature plot are less affected by smearing, since the hot
spots differ in center velocity by 200 km s
1
.
Figure 5 shows the
12
CO J ¼ 6–5 integrated intensity
plotted over the
12
CO J ¼ 2–1 map from Weiss et al. (2001).
The hot spots in the CO J ¼ 6–5 map are closer together
than in the lower J map. This is consistent with the finding
of Mao et al. (2000) that the angular separation of the lobes
is smaller for a ‘‘ high CO excitation component ’’ than is
seen in the low-J CO lines. It can be seen in the figure that
the J ¼ 6–5 southwe st hot spot is located between the lower
J southwest and center hot spots. This suggests that the
southwest hot spot of the J ¼ 6–5 map includes unresolved
emission from both of these regions. The southwest hot spot
appears to be somewhat extended in the direction of the
minor axis in the J ¼ 6–5 map. It is unlikely that this shape
is real because it is not seen in any other published observa-
tions, including HCN J ¼ 1–0 (Brouillet & Schilke 1993)
and CO J ¼ 7–6 (Mao et al. 2000) maps. It is also unlikely
to be a calibration effect since the weather and receiver were
very stable during the observations. It is possible that the
telescope pointing drifted a few seconds of arc during the
observations, causing this artifact.
The total luminosity of the
12
CO J ¼ 6–5 line was found
to be 9:0 10
5
L
. This was calculated from the spatially
and spectrally integrated intensity of 2:8 10
5
Kkms
1
arcsec
2
after zeroth-order baseline subtraction, assuming
spherically symmetric emission and a distance of 3.6 Mpc.
The
13
CO J ¼ 6–5 line was not detected after 96 minutes
of integration time near the central pointing. The 3 upper
limit of the integrated intensity for a 14
00
beam calculated
from the rms channel noise is 7.9 K km s
1
. A reliable upper
limit may be somewhat larger, however, since uncertainties
in zeroth-order baseline subtraction and a small amount of
baseline ripple could dominate over the channel noise.
Nonetheless, with an implied J ¼ 6–5 antenna temperature
ratio of I(
12
CO)/I(
13
COÞe40, it is clear that
13
CO J ¼ 6–5
is very weak. Although
13
CO J ¼ 6–5 emission was not
detected, 450 lm continuum was observed at a level of
2:4 0:6Jybeam
1
. Thi s is consistent with the bolometer
value of 3.5 Jy beam
1
measured at the same location with a
narrower 9
00
FWHP beam by Hughes et al. (1994).
3.1. A
12
CO J ¼ 6 5/
12
CO J ¼ 2 1 Line Ratio Map
Figure 6 shows a map of the
12
CO J ¼ 6–5/
12
CO J ¼ 2–1
line ratio. The J ¼ 2–1 data were observed with the Plateau
de Bure Interferometer and include short-spa cing correc-
tions based on obs ervations with the IRAM 30 m telescope
(Weiss et al. 2001). The ratio map was calculated using a
Fig. 4.—Position-velocity diagram along the major axis of M82 for
12
CO J ¼ 4–3 and
12
CO J ¼ 6–5. The southwest hot spot peaks at different velocities in
the two transitions, and the overall velocity gradient is steeper in CO J ¼ 6–5. Note that the broad, lower intensity spectrum between the two hot spots explains
the difference in integrated intensity and peak temperature maps. The positions are offset from arbitrary centers along the major axis of the galaxy. Contours
are for T
MB
from 0.5 to 4 K in steps of 0.5 K.
174 WARD ET AL. Vol. 587
novel deconvolution technique that takes advantage of
velocity information in the J ¼ 6–5 map along with the high
spatial resolution of the J ¼ 2–1 map to improve the resolu-
tion and accuracy of the ratio map. A Lagrange multiplier
was used to set the relative weighting between minimizing
the resulting
2
and maximizing the smoothness of the
resulting ratio map. Details of the calculation are given in
the Appendix. It is immediately apparent that the brightness
temperature ratio is highest at the two integrated intensity
peaks, reaching 0.4 and 0.5 in the northeast and southwest
peaks, respectively, and is typically 0.2 elsewhere in the
nuclear region.
Fig. 5.—M82
12
CO J ¼ 6–5 integrated intensity contours superimposed on
12
CO J ¼ 2–1 integrated intensity from Weiss et al. (2001). Contours are 50,
100, 150, 200, 250, 300, 350, and 400 K km s
1
.
Fig. 6.—Deconvolved
12
CO J ¼ 6–5/
12
CO J ¼ 2–1 brightness temperature ratio map. Contours are from 0.1 to 0.45 by 0.05. The squares indicate the
locations of the J ¼ 6–5 intensity peaks, and the triangles the locations of the J ¼ 2–1 integrated intensity peaks.
No. 1, 2003
12
CO J = 6–5 MAP OF M82 175
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10 Dec 2011-The Astrophysical Journal
TL;DR: In this paper, the Herschel Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (Herschel SPIRE-FTS) observations of Arp 220, a nearby ultra-luminous infrared galaxy, were presented.
Abstract: We present Herschel Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (Herschel SPIRE-FTS) observations of Arp 220, a nearby ultra-luminous infrared galaxy. The FTS provides continuous spectral coverage from 190 to 670 {mu}m, a wavelength region that is either very difficult to observe or completely inaccessible from the ground. The spectrum provides a good measurement of the continuum and detection of several molecular and atomic species. We detect luminous CO (J = 4-3 to 13-12) and water rotational transitions with comparable total luminosity {approx}2 Multiplication-Sign 10{sup 8} L{sub Sun }; very high-J transitions of HCN (J = 12-11 to 17-16) in absorption; strong absorption features of rare species such as OH{sup +}, H{sub 2}O{sup +}, and HF; and atomic lines of [C I] and [N II]. The modeling of the continuum shows that the dust is warm, with T = 66 K, and has an unusually large optical depth, with {tau}{sub dust} {approx} 5 at 100 {mu}m. The total far-infrared luminosity of Arp 220 is L{sub FIR} {approx} 2 Multiplication-Sign 10{sup 12} L{sub Sun }. Non-LTE modeling of the extinction corrected CO rotational transitions shows that the spectral line energy distribution of CO is fit well by two temperature components:more » cold molecular gas at T {approx} 50 K and warm molecular gas at T {approx} 1350{sup +280}{sub -100} K (the inferred temperatures are much lower if CO line fluxes are not corrected for dust extinction). These two components are not in pressure equilibrium. The mass of the warm gas is 10% of the cold gas, but it dominates the CO luminosity. The ratio of total CO luminosity to the total FIR luminosity is L{sub CO}/L{sub FIR} {approx} 10{sup -4} (the most luminous lines, such as J = 6-5, have L{sub CO,J=6-5}/L{sub FIR} {approx} 10{sup -5}). The temperature of the warm gas is in excellent agreement with the observations of H{sub 2} rotational lines. At 1350 K, H{sub 2} dominates the cooling ({approx}20 L{sub Sun} M{sup -1}{sub Sun }) in the interstellar medium compared to CO ({approx}0.4 L{sub Sun} M{sup -1}{sub Sun }). We have ruled out photodissociation regions, X-ray-dominated regions, and cosmic rays as likely sources of excitation of this warm molecular gas, and found that only a non-ionizing source can heat this gas; the mechanical energy from supernovae and stellar winds is able to satisfy the large energy budget of {approx}20 L{sub Sun} M{sup -1}{sub Sun }. Analysis of the very high-J lines of HCN strongly indicates that they are solely populated by infrared pumping of photons at 14 {mu}m. This mechanism requires an intense radiation field with T > 350 K. We detect a massive molecular outflow in Arp 220 from the analysis of strong P Cygni line profiles observed in OH{sup +}, H{sub 2}O{sup +}, and H{sub 2}O. The outflow has a mass {approx}> 10{sup 7} M{sub Sun} and is bound to the nuclei with velocity {approx}< 250 km s{sup -1}. The large column densities observed for these molecular ions strongly favor the existence of an X-ray luminous AGN (10{sup 44} erg s{sup -1}) in Arp 220.« less

335 citations

Journal Article•DOI•
Superconducting detectors and mixers for millimeter and submillimeter astrophysics

[...]

Jonas Zmuidzinas1, Paul L. Richards2•
California Institute of Technology1, University of California, Berkeley2
20 Sep 2004
TL;DR: The scientific opportunities, the basic physics of these devices, the techniques for radiation coupling, and the recent progress in direct detectors are described, as well as the work on tunnel junction (superconductor-insulator-super Conductor) and hot-electron mixers.
Abstract: Superconducting detectors will play an increasingly significant role in astrophysics, especially at millimeter through far-IR wavelengths, where the scientific opportunities include key problems in astronomy and cosmology. Superconducting detectors offer many benefits: outstanding sensitivity, lithographic fabrication, and large array sizes, especially through the recent development of multiplexing techniques. This paper describes the scientific opportunities, the basic physics of these devices, the techniques for radiation coupling, and reviews the recent progress in direct detectors, such as transition-edge bolometers, and the work on tunnel junction (superconductor-insulator-superconductor) and hot-electron mixers.

307 citations

References
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Journal Article•DOI•
Detection of absorption by H2 in molecular clouds: A direct measurement of the H2:CO ratio

[...]

John H. Lacy, Roger F. Knacke1, Thomas R. Geballe2, Alan T. Tokunaga2•
Pennsylvania State University1, University of Hawaii at Manoa2
20 Jun 1994-The Astrophysical Journal
TL;DR: In this article, the authors search for the vibrational absorption of H2 and CO toward infrared sources embedded in molecular clouds and find that the results are consistent with the H2 ortho-para ratio being equilibrated at the cloud temperature.
Abstract: Vibrational absorption by H2 and CO has been searched for toward infrared sources embedded in molecular clouds. H2 was detected toward NGC 2024 IRS 2 and possibly toward NGC 2264 (GL 989). CO was detected toward both sources. The results are consistent with the H2 ortho:para ratio being equilibrated at the cloud temperature. Toward NGC 2024, H2:CO = (3700(sub -2600)(sup +3100)) (2 sigma limits), and toward NGC 2264, H2:CO less than 6000. Approximately one-third of all carbon is in gas-phase CO.

289 citations

Journal Article•DOI•
COBE Far Infrared Absolute Spectrophotometer Observations of Galactic Lines

[...]

D. J. Fixsen1, Charles L. Bennett2, John C. Mather2•
STX Corporation1, Goddard Space Flight Center2
20 Nov 1999-The Astrophysical Journal
TL;DR: In this article, the COBE Far Infrared Absolute Spectrophotometer (FIRAS) observations constitute an unbiased survey over the wavelength range from 100 μm to 1 cm over 99% of the sky.
Abstract: The COBE Far Infrared Absolute Spectrophotometer (FIRAS) observations constitute an unbiased survey over the wavelength range from 100 μm to 1 cm over 99% of the sky. Improved calibration of the FIRAS instrument and the inclusion of all of the FIRAS data allow an improved signal-to-noise ratio determination of the spectral lines by a factor of ~2 over our previous results. The resolution is low (0.45 cm-1), so only the strongest lines are observable. The CO chain from J = 1-0 to J = 8-7 is observed toward the Galactic center. The line ratios are roughly consistent with a 40 K excitation temperature. The 157.7 μm C II and 205.3 μm N II lines are observable over most of the sky. The 370.4 and 609.1 μm lines of C I and the 121.9 μm line of N II are observed in the Galactic plane. The line ratios at the Galactic center are consistent with a density of n0 ~ 30 cm-3 and a UV flux of G0 ≈ 15 μW m-2 sr-1 (10 Habing units). The 269 μm H2O line is observed toward the Galactic center in absorption.

246 citations

Journal Article•DOI•
Radiative Transfer, Excitation, and Cooling of Molecular Emission Lines (co and Cs)

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N.Z. Scoville, P.M. Solomon
15 Jan 1974-The Astrophysical Journal

245 citations

Journal Article•DOI•
The effect of violent star formation on the state of the molecular gas in M 82

[...]

A. Weiß1, N. Neininger1, S. Hüttemeister1, Ulrich Klein1•
University of Bonn1
01 Jan 2001-Astronomy and Astrophysics
TL;DR: In this article, the authors present the results of a high angular resolution, multi-transition analysis of the molecular gas in M 82, based on the two lowest transitions of and the ground transition of the rare isotopes and measured with the PdBI, the BIMA array and the IRAM 30 m telescope.
Abstract: We present the results of a high angular resolution, multi-transition analysis of the molecular gas in M 82. The analysis is based on the two lowest transitions of and the ground transition of the rare isotopes and measured with the PdBI, the BIMA array and the IRAM 30 m telescope. In order to address the question of how the intrinsic molecular cloud properties are influenced by massive star formation we have carried out radiative transfer calculations based on the observed CO line ratios. The calculations suggest that the kinetic temperature of the molecular gas is high in regions with strong star formation and drops towards the outer molecular lobes with less ongoing star formation. The location of the highest kinetic temperature is coincident with that of the mid infrared (MIR) peaks which trace emission from hot dust. The hot gas is associated with low H2 densities while the cold gas in the outer molecular lobes has high H2 densities. We find that CO intensities do not trace H2 , column densities well. Most of the molecular gas is distributed in a double-lobed distribution which surrounds the starburst. A detailed analysis of the conversion factor from CO intensity to H2 column density shows that X CO depends on the excitation conditions. We find , as expected for virialized clouds.

233 citations

Journal Article•DOI•
The effect of violent star formation on the state of the molecular gas in M82

[...]

Axel Weiss, N. Neininger, S. Huettemeister, Uli Klein
26 Oct 2000-arXiv: Astrophysics
TL;DR: In this paper, a high angular resolution, multi-transition analysis of the molecular gas in M82 is presented based on the two lowest transitions of 12CO and the ground transition of the rare isotopes 13CO and C18O measured with the PdBI, the BIMA array and the IRAM 30m telescope.
Abstract: We present the results of a high angular resolution, multi-transition analysis of the molecular gas in M82. The analysis is based on the two lowest transitions of 12CO and the ground transition of the rare isotopes 13CO and C18O measured with the PdBI, the BIMA array and the IRAM 30m telescope. In order to address the question of how the intrinsic molecular cloud properties are influenced by massive star formation we have carried out radiative transfer calculations based on the observed CO line ratios. The calculations suggest that the kinetic temperature of the molecular gas is high in regions with strong star formation and drops towards the outer molecular lobes with less ongoing star formation. The location of the highest kinetic temperature is coincident with that of the mid infrared peaks which trace emission from hot dust. The hot gas is associated with low H2 densities while the cold gas in the outer molecular lobes has high H2 densities. We find that CO intensities do not trace H2 column densities well. Most of the molecular gas is distributed in a double-lobed distribution which surrounds the starburst. A detailed analysis of the conversion factor from CO intensity to H2 column density shows that X_CO depends on the excitation conditions. We find X_CO ~ Sqrt(n_H2)/T_kin, as expected for virialized clouds.

209 citations

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Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "A 12co j = 6–5 map of m82: the significance of warm molecular gas" ?

The authors present a 12CO J 1⁄4 6–5 map of the nuclear regions of the starburst galaxy M82 at a resolution of 1400 taken at the Caltech Submillimeter Observatory ( CSO ). The authors compare their results with a high-resolution 12CO J 1⁄4 2–1 interferometer map, and present a 12CO J 1⁄4 6–5/12CO J 1⁄4 2–1 line ratio map obtained using a novel deconvolution technique. The authors also present measurements of 12CO J 1⁄4 4–3, 12CO J 1⁄4 3–2, and 13CO J 1⁄4 3–2, and an upper limit for 13CO J 1⁄4 6–5. The authors analyze these observations in the context of a two-component large velocity gradient ( LVG ) excitation model. The results of this analysis suggest that the warm gas is less dense than the cool gas, and that over half of the total molecular gasmass in these nuclear regions is warmer than 50K.