1. Molecular Clouds in in the LMC at High Resolution: The Importance of Short ALMA Baselines T. Wong 1,2,4, J. B. Whiteoak 1, M. Hunt 2, J. Ott 1, Y.-N. Chin 3 1 CSIRO Australia Telescope National Facility 2 School of Physics, University of New South Wales, Australia 3 Tamkang University, Taiwan 4 Contact: Tony.Wong@csiro.au

2. Australia Telescope Compact Array Six 22m antennas near Narrabri, NSW, five moveable on rail tracks. National Facility open to proposers worldwide. Operates in 5 frequency bands from 1-25 GHz. 3mm (85-105+ GHz) upgrade in progress for 5 antennas (due late June). Wide-bandwidth (2 GHz x 4 IF) correlator under development (mid-2006). Longitude 150° E, Latitude 30° S

3. The ATCA Millimetre Upgrade 3 elements currently have dual linear polarization 3mm receivers, 5 by July. 2 observing bands: 84.9-87.3 and 88.5- 91.3 GHz. Full coverage of 85-105 GHz expected by July, extension to 115 GHz planned.  A ~0.35, T sys ~300 K (above atmosphere). (above) 3mm low- noise amplifiers based on InP MMIC technology (left) Both 3mm and 12mm systems are housed in a single dewar.

4. Molecular Clouds in the LMC A unique nearby, low-metallicity star formation environment. Contours: CO at 2.6’ resolution from NANTEN (Mizuno et al.) SEST spectra (Chin et al. 1997) N113 H II region

5. Mapping of N113 in 4 transitions We observed N113 in HCO +, HCN, HNC, C 2 H, and 87 GHz continuum. Observations conducted in 2003 July & August in two E-W configurations of 3 antennas (baselines 30-135 m). RMS noise ~30 mJy in a 2 km s -1 channel. Reference pointing on SiO maser R Dor, phase calibration using PKS B0537-441 (25° away). Integrated intensity images for 4 lines. Contour levels: 0.5 Jy bm -1 km s -1 for top panels and 0.2 for bottom panels. HCO + (1-0)HCN (1-0) C 2 H (N=1-0)HNC (1-0)

6. Comparison in u-v plane Assuming a SEST gain of 25 Jy/K, the total HCO + and HCN fluxes are 80 and 60 Jy km s -1 (Chin et al. 1997) respectively. Thus only ~15% of flux is detected on the shortest (30m) ATCA baseline. The relative sizes of the emission regions differ: HCO + is more extended than HCN, which in turn is more extended than HNC. Possible explanations: HCO + associated with an extended PDR, HNC/HCN enhanced in dense cores.

7. Implications for ALMA DRSP Emission from abundant species like HCO + and HCN is heavily resolved, even on a 30m baseline. Information on abundances & cloud structure will require observations from an array of smaller dishes (ACA). Lack of small-scale emission implies high-resolution observations will require much greater sensitivity. Note that ATCA’s field of view at 90 GHz (36”, see figure) is similar to ALMA’s at 230 GHz (~10 pc at LMC distance) — mosaicking clearly needed.