Extragalactic CO(3-2) observations

Coordinator: L.E.B. Johansson and R.S. Booth

Program is available and data products can be downloaded

Scientific justification:
The observations proposed here for the LMC are part of a larger program tostudy physical and chemical properties of molecular clouds, including theinterplay between gas and dust in a low metallicity environment. Here we focuson regions near recent star-formation where the existing young stellar clustersstrongly modify the environment via the enhanced UV radiation. A low dustcontent would have an impact on the physical and chemical properties of the gascomponent, e.g., lower shielding enhancing molecular dissociation rates. Thishas a direct consequence for the chemical state but also for the physicalproperties of the gas. In the latter case we would expect less extendedemission and a higher degree of clumping, at least for trace molecules.
Therefore, one preresquite to understand the physics of molecularclouds is knowledge of the total amount of dust. Combining data fromobservations of the 250 GHz continuum flux (SIMBA data) and CO line emission,Johansson et al. (2005) have derived gas-to-dust ratios for a sample ofmolecular clouds in the LMC. The results do indicate significantly lowerdust contents than present in Galactic clouds. However, to accurately definethe amount of dust, proper account for other emission processes is requiredas well as a good knowledge of the actual temperature of the dust. Informationon gas and dust temperatures can be achieved by mapping the clouds indifferent transitions and isotopes of CO.
Such data (1-0 and 2-1 transitions) exist for the present sample, i.e.,30Dor-10 and N159-W (Johansson et al. 1998; Bolatto et al. 2000). Extendingthe data set to CO(3-2) would put further constraints on the gas parametersand provide a higher spatial resolution. Smaller maps of CO(3-2) have beendone with SEST towards these objects, however, the intensity calibration israther uncertain. On the other hand, the CO distribution and line shapesshould be accurately defined, properties which can be used to estimatethe relative pointing errors between APEX and SEST and, thus, betterassociate the CO emission from different transitions.
Similar CO excitation arguments can be applied to the nearby galaxiesNGC 4945, Circinus and NGC 3256. SEST data exist for the three lowestJ-transitions. The line width is about 400 km/s in the cental parts(for allgalaxies) and would therefore provide a good test for the broad lineperformance of the APEX receivers and correlator. For both NGC 3256 andCircinus wehave high quality 1mm continuum data (Olsson and Aalto 2005)and forall three galaxies we have also conducted extensive studies of the ISMproperties(e.g. Aalto et al 1991ab; Bergman et al 1992; Aalto et al 1995; Curranet al 2001)where the merger NGC 3256 appears to be in an most extreme phase of itsevolutionwith global CO/13CO 1-0 line ratio of 35.
To derive temperatures we intend to apply the Mean Escape Probability (MEP),a LTE-like approximation of the radiative transfer equations (Sch%Gï¿¿%@ier et al.2005) as well as a non-LTE 1D Monte-Carlo model (Juvela 1998).

Observing time request and strategy
To cover the SEST CO(3-2) observations with 10" spacing, we need maps ofsizes 13x13, 7x7 and 9x9 in N159-W, 30Dor-10 and NGC 4945, respectively.The resolution and noise r.m.s. needed should be 500 KHz and 0.1K for theLMC clouds and 10 MHz and 0.02K for NGC 4945, NGC 3256 and Circinus. Withan equivalent SSB system temperature of about 400K we would need 2 and 4minutes per position, respectively. Adding 30% for overhead, the LMC clouds wouldrequire about 10 hours and NGC 4945, NGC 3256, Circinus 7 hours each. TheNGC 4945 map should be tilted by 45 degrees.
We note that the LMC is basically a day time object in July and shouldbe observed as late as possible in the Science Verification schedule.NGC 3256 is an early evening object in July.

Source list
Name RA(B1950.0) DEC (B1950.0) Vlsr (km/s)
N159-W 05:40:03.0 -69:47:03 237
30Dor-10 05:39:11.4 -69:06:00 249
NGC 4945 13:02:32.2 -49:12:02 560
NGC 3256 10:25:00.0 -43:38:00 2800
Circinus 14:09:17.7 -65:06:18 434

Aalto et al, 1991a, A&A, 247, 291
Aalto et al, 1991b, A&A, 249, 323
Aalto et al, 1995, A&A, 300, 369
Bergman et al, 1992, A&A, 265, 403
Bolatto et al. 2000, ApJ 545, 234
Curran et al, 2001, A&A, 367, 457
Johansson L.E.B. et al., 1998, A&A 331, 857
Johansson L.E.B., Nikolic S., Rantakyro F. et al., 2005, in prep.
Juvela M., 1998, A&A 329, 659
Schoier F.L., van der Tak F.F.S., van Dishoeck E.F., Black J.H.,2005, A&A 432, 369