High-density molecular gas in external galaxies - Formaldehyde and carbon monosulfide

2007

ApJ

Using the IRAM 30 m telescope and the 15 m JCMT, we explore the value of paraformaldehyde (p-H 2 CO) as a tracer of density and temperature of the molecular gas in external galaxies. The target of our observations are the lobes of the molecular ring around the center of the nearby prototypical starburst galaxy M82. It is shown that p-H 2 CO provides one of the rare direct molecular thermometers. Reproducing the measured line intensities with a large velocity gradient (LVG) model, we find densities of n H 2 $ 7 ; 10 3 cm À3 and kinetic temperatures of T kin $ 200 K. The derived kinetic temperature is significantly higher than the dust temperature or the temperature deduced from ammonia (NH 3 ) lines, but our results agree well with the properties of the high-excitation component seen in CO. We also present the serendipitous discovery of the 4 2 ! 3 1 line of methanol (CH 3 OH ) in the northeastern lobe, which shows -unlike CO and H 2 CO-significantly different line intensities in the two lobes.

Section: Line Selectionmentioning

confidence: 73%

Section: Target Selection: M82mentioning

confidence: 83%

Section: Formaldehyde As a Molecular Gas Tracermentioning

confidence: 99%

See 1 more Smart Citation

Formaldehyde as a Tracer of Extragalactic Molecular Gas. I. Para‐H₂CO Emission from M82

Mühle

Seaquist

2007

ApJ

“…In contrast, formaldehyde (H 2 CO) is a more ubiquitous molecule in the interstellar medium (Downes et al 1980;Bieging et al 1982;Cohen et al 1983;Baan et al 1986Baan et al , 1990Baan et al , 1993Henkel et al 1991;Zylka et al 1992;Mangum et al 2008Mangum et al , 2013aZhang et al 2012;Ao et al 2013;Tang et al 2013;Ginsburg et al 2015Ginsburg et al , 2016Guo et al 2016). Unlike ammonia, the fractional abundance of H 2 CO is stable at various stages of star formation (Mangum et al 1990;Caselli et al 1993;Johnstone et al 2003;Gerner et al 2014;Tang et al 2017a,b,c).…”

Section: Introductionmentioning

confidence: 99%

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

Tang

Menten

et al. 2017

A&A

We mapped the kinetic temperature structure of the Orion molecular cloud 1 (OMC-1) with para-H 2 CO (J KaKc = 3 03 -2 02 , 3 22 -2 21 , and 3 21 -2 20 ) using the APEX 12 m telescope. This is compared with the temperatures derived from the ratio of the NH 3 (2,2)/(1,1) inversion lines and the dust emission. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured averaged line ratios of para-H 2 CO 3 22 -2 21 /3 03 -2 02 and 3 21 -2 20 /3 03 -2 02 . The gas kinetic temperatures derived from the para-H 2 CO line ratios are warm, ranging from 30 to >200 K with an average of 62 ± 2 K at a spatial density of 10 5 cm −3 . These temperatures are higher than those obtained from NH 3 (2,2)/(1,1) and CH 3 CCH (6-5) in the OMC-1 region. The gas kinetic temperatures derived from para-H 2 CO agree with those obtained from warm dust components measured in the mid infrared (MIR), which indicates that the para-H 2 CO (3-2) ratios trace dense and warm gas. The cold dust components measured in the far infrared (FIR) are consistent with those measured with NH 3 (2,2)/(1,1) and the CH 3 CCH (6-5) line series. With dust at MIR wavelengths and para-H 2 CO (3-2) on one side and dust at FIR wavelengths, NH 3 (2,2)/(1,1), and CH 3 CCH (6-5) on the other, dust and gas temperatures appear to be equivalent in the dense gas (n(H 2 ) 10 4 cm −3 ) of the OMC-1 region, but provide a bimodal distribution, one more directly related to star formation than the other. The non-thermal velocity dispersions of para-H 2 CO are positively correlated with the gas kinetic temperatures in regions of strong non-thermal motion (Mach number 2.5) of the OMC-1, implying that the higher temperature traced by para-H 2 CO is related to turbulence on a ∼0.06 pc scale. Combining the temperature measurements with para-H 2 CO and NH 3 (2,2)/(1,1) line ratios, we find direct evidence for the dense gas along the northern part of the OMC-1 10 km s −1 filament heated by radiation from the central Orion nebula.

“…Formaldehyde (H 2 CO) is a ubiquitous molecule in the Galactic interstellar medium (ISM) of our and external galaxies (Downes et al 1980;Cohen & Few 1981;Bieging et al 1982;Cohen et al 1983;Baan et al 1986Baan et al , 1990Baan et al , 1993Henkel et al 1991;Zylka et al 1992;Hüttemeister et al 1997;Heikkilä et al 1999;Wang et al 2004Wang et al , 2009Mangum et al 2008;Zhang et al 2012;Mangum et al 2013b;Ao et al 2013;Tang et al 2013;Ginsburg et al 2015Ginsburg et al , 2016Guo et al 2016). H 2 CO is thought to be formed on the surface of dust grains by successive hydrogenation of CO (Watanabe & Kouchi 2002;Woon 2002;Hidaka et al 2004): CO → HCO → H 2 CO. Variations of the fractional abundance of H 2 CO do not exceed one order of magnitude.…”

Section: Introductionmentioning

confidence: 99%

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

Tang

Chen

et al. 2017

A&A

Context. The kinetic temperature of molecular clouds is a fundamental physical parameter affecting star formation and the initial mass function. The Large Magellanic Cloud (LMC), the closest star forming galaxy with low metallicity, provides an ideal laboratory to study star formation in such an environment. Aims. The classical dense molecular gas thermometer NH 3 is rarely available in a low metallicity environment because of photoionization and a lack of nitrogen atoms. Our goal is to directly measure the gas kinetic temperature with formaldehyde toward six star-forming regions in the LMC. Methods. Three rotational transitions (J K A K C = 3 03 -2 02 , 3 22 -2 21 , and 3 21 -2 20 ) of para-H 2 CO near 218 GHz were observed with the Atacama Pathfinder EXperiment (APEX) 12 m telescope toward six star forming regions in the LMC. Those data are complemented by C 18 O 2-1 spectra. Results. Using non-LTE modeling with RADEX, we derive the gas kinetic temperature and spatial density, using as constraints the measured para-H 2 CO 3 21 -2 20 /3 03 -2 02 and para-H 2 CO 3 03 -2 02 /C 18 O 2-1 ratios. Excluding the quiescent cloud N159S, where only one para-H 2 CO line could be detected, the gas kinetic temperatures derived from the preferred para-H 2 CO 3 21 -2 20 /3 03 -2 02 line ratios range from 35 to 63 K with an average of 47 ± 5 K (errors are unweighted standard deviations of the mean). Spatial densities of the gas derived from the para-H 2 CO 3 03 -2 02 /C 18 O 2-1 line ratios yield 0.4 -2.9 × 10 5 cm −3 with an average of 1.5 ± 0.4 × 10 5 cm −3 . Temperatures derived from the para-H 2 CO line ratio are similar to those obtained with the same method from Galactic star forming regions and agree with results derived from CO in the dense regions (n(H 2 ) > 10 3 cm −3 ) of the LMC. A comparison of kinetic temperatures derived from para-H 2 CO with those from the dust also shows good agreement. This suggests that the dust and para-H 2 CO are well mixed in the studied star forming regions. A comparison of kinetic temperatures derived from para-H 2 CO 3 21 -2 20 /3 03 -2 02 and NH 3 (2,2)/(1,1) shows, however, a drastic difference. In the star forming region N159W, the gas temperature derived from the NH 3 (2,2)/(1,1) line ratio is ∼16 K (Ott et al. 2010), which is only half the temperature derived from para-H 2 CO and the dust. Furthermore, ammonia shows a very low abundance in a 30 ′′ beam. Apparently, ammonia only survives in the most shielded pockets of dense gas not yet irradiated by UV photons, while formaldehyde, less affected by photodissociation, is more widespread and is also sampling regions more exposed to the radiation of young massive stars. A correlation between the gas kinetic temperatures derived from para-H 2 CO and infrared luminosity, represented by the 250 µm flux, suggests that the kinetic temperatures traced by para-H 2 CO are correlated with the ongoing massive star formation in the LMC.