Molecular Clouds in in the LMC at High Resolution: The

Importance of Short ALMA Baselines

T. Wong1,2,4, J. B. Whiteoak1, M. Hunt2, J. Ott1, Y.-N. Chin3

1CSIRO Australia Telescope National Facility2School of Physics, University of New South Wales, Australia3Tamkang University, Taiwan4Contact: Tony.Wong@csiro.au

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

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, Tsys~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.

Molecular Clouds in the LMCA 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 HII region

Mapping of N113 in 4 transitions

• We observed N113 in HCO+, HCN, HNC, C2H, 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)

C2H (N=1-0) HNC (1-0)

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.

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.