APEX SV projects

EXTRAGALACTIC CO 3-2 OBSERVATIONS

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

Data:
Program is available and data products can be downloaded

Scientific justification:
The observations proposed here for the LMC are part of a larger program to study physical and chemical properties of molecular clouds, including the interplay between gas and dust in a low metallicity environment. Here we focus on regions near recent star-formation where the existing young stellar clusters strongly modify the environment via the enhanced UV radiation. A low dust content would have an impact on the physical and chemical properties of the gas component, e.g., lower shielding enhancing molecular dissociation rates. This has a direct consequence for the chemical state but also for the physical properties of the gas. In the latter case we would expect less extended emission and a higher degree of clumping, at least for trace molecules.
Therefore, one preresquite to understand the physics of molecular clouds is knowledge of the total amount of dust. Combining data from observations 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 of molecular clouds in the LMC. The results do indicate significantly lower dust contents than present in Galactic clouds. However, to accurately define the amount of dust, proper account for other emission processes is required as well as a good knowledge of the actual temperature of the dust. Information on gas and dust temperatures can be achieved by mapping the clouds in different 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). Extending the data set to CO(3-2) would put further constraints on the gas parameters and provide a higher spatial resolution. Smaller maps of CO(3-2) have been done with SEST towards these objects, however, the intensity calibration is rather uncertain. On the other hand, the CO distribution and line shapes should be accurately defined, properties which can be used to estimate the relative pointing errors between APEX and SEST and, thus, better associate the CO emission from different transitions.
Similar CO excitation arguments can be applied to the nearby galaxies NGC 4945, Circinus and NGC 3256. SEST data exist for the three lowest J-transitions. The line width is about 400 km/s in the cental parts (for all galaxies) and would therefore provide a good test for the broad line performance of the APEX receivers and correlator. For both NGC 3256 and Circinus wehave high quality 1mm continuum data (Olsson and Aalto 2005) and for all three galaxies we have also conducted extensive studies of the ISM properties (e.g. Aalto et al 1991ab; Bergman et al 1992; Aalto et al 1995; Curran et al 2001) where the merger NGC 3256 appears to be in an most extreme phase of its evolution with 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%G%@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 of sizes 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 the LMC clouds and 10 MHz and 0.02K for NGC 4945, NGC 3256 and Circinus. With an equivalent SSB system temperature of about 400K we would need 2 and 4 minutes per position, respectively. Adding 30% for overhead, the LMC clouds would require about 10 hours and NGC 4945, NGC 3256, Circinus 7 hours each. The NGC 4945 map should be tilted by 45 degrees.
We note that the LMC is basically a day time object in July and should be 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

References
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