Science Highlight




W51 in Band 1

Science Target Overview

The W51 giant molecular cloud located at a distance of 5 kpc is one of the most massive and active star-forming regions in our Galaxy, and is also the closest region containing a high-mass protocluster. In particular, inside the cloud, in its W51A region, there are several strong radio sources W51 IRS1, IRS2, e2 and e8 and extended (>0.5') free-free emission around IRS2 distributed within 2 arcminutes, comparable to the ALMA Band 1 field of view at 7 mm, and this region has been observed in the continuum and a variety of lines (including CO, CH3OH, SiO, SO2, NH3, etc) with VLA at similar and lower frequencies and with ALMA and SMA at higher frequencies. Thus, it is an excellent target to demonstrate and check the ALMA Band 1 imaging capability and fidelity by comparison with these previous observations.


ALMA Data Overview

The observations were carried out on Mar 25 and Apr 4, 2023 with total on-source time about 70 min. 18 antennas and 20 antennas participated to X4af7 and X8895, respectively. Typical PWV during observations was 2.6mm for X4a7f and 1.8mm for X8995. Maximum baseline length was about 1 km. In total 6 spectral windows (spw) are used to cover CH3OH, CS, SiO, and SO2 lines as well as the continuum emission.

Calibration was done using standard calibration script for QA2. Some additional flagging was added in the step=10 to flag out spurious-like features. J1924-2914 is used for bandpass calibration and flux scaling. The flux model for J1924-2914 was used from the measurement in Band 1 on Mar 30, 2023. In addition to Tsys and WVR correction, temporal gain time variation was calibrated using J1924+1540.

The data after the standard calibration were concatenated, and lines were flagged to generate channel-averaged continuum data. The self calibration was conducted on the channel-averaged continuum data, and then the solutions were also applied to the concatenated data without channel averaging and line flagging. Five runs of phase-only calibration and one run of amplitude+phase calibration were done. During this process, the thresholds of tclean and solution intervals were gradually reduced in each run. The final amplitude+phase self calibration was done with a solution interval of one scan. After the self calibration, the RMS in the continuum image decreases by ~75%.

The continuum subtraction was conducted in individual narrow spectral windows for lines using uvcontsub. There are several bright continuum sources with different spectral profiles in the field, and uvcontsub was not able to properly subtract the continuum from the wide spectral windows of a ~2GHz bandwidth, so the data product does not include continuum-subtracted wide spectral windows. Users may select a part of the channels of the wide spectral window to do continuum subtraction or conduct continuum subtraction in the image domain.

All the reference images were generated using tclean with auto-masking. Robust parameters of Briggs weighting and parameters for auto-masking were adjusted to better present the structures of the emission. The RMS achieved is ~0.8 mJy in the continuum image, and it is limited by the dynamical range. The maximum S/N is ~840. The RMS achieved in the line images range from 5 to 12 mJy per 0.4 km/s channel, depending on the line frequency and weighting parameter.


Using the data for publication

The following statement should be included in the acknowledgement of papers using the dataset listed above:

"This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00012.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ."


Obtaining the Data

The data products are contained in the following directory:

Parent directory for download

The above directory contains README, uncalibrated data, calibrated data, data reduction scripts, and reference images.