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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: [email protected]
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.