SPT 0027-50 in Band 2

Science Target Overview

SPT 0027-50 is a strongly lensed, dusty star-forming galaxy from the South Pole Telescope (SPT) survey at a redshift of 3.44 (Reuter et al. 2020). At this redshift two CO transitions, CO (4-3) and CO (3-2), can be observed within the frequency range of Band 2, thus demonstrating the capability of Band 2 for unique redshift identification via more than one CO transition. Previous observations with ALMA Band 3 (project codes 2015.1.00504.S, 2022.1.00172.S and 2022.1.00526.S) are available for comparison.

ALMA Data Overview

Observations were carried out on the 27th of November 2025 using 25 antennas, for a total on-source time of about 40 min. The spectral scan mode was used, covering the frequency range from 75 to 110 GHz at a spectral resolution of 488.281 kHz (20 spectral windows of bandwidth 1.875 GHz were needed for the full scan). The typical PWV during observations was 0.7-0.9 mm. The baseline lengths range from 15 m to 2.4 km. Calibration and imaging were performed using the ALMA Pipeline in a standard manner. Auto-masking was used for generating the images unless indicated otherwise. Self-calibration was not performed. Continuum subtraction was performed using Pipeline’s findcont.

Additional flagging and intervention in the pipeline were needed as follows: we performed spectral binning in the cubes before imaging since the native spectral resolution (about 1.5 km/s) is oversampling the targeted lines (with widths of about 250-300 km/s) resulting in noisy spectra. Two antennas were flagged in a few frequency ranges. Finally, a few Tsys spikes, due to WVR leakages, propagating to the bandpass solutions were also flagged in the Tsys table. Details on the specific applied flags are available in the README file.

Note: the flux calibration for Band 2 data is still being refined. The flux.csv table provided in the calibration folders needs to be used, as it provides the best values retrieved through a curvature fitting of the calibrator source at the other ALMA bands. In addition, some sidelobe residuals are present in the image cubes due to the UV-coverage of these observations being poorer than for standard observations because of the small number of antennas used during science verification. A large number of iterations and fine tuning of the threshold parameter in the cleaning procedure may improve the image quality. The continuum subtraction may also be optimized if images are manually improved.

The continuum emission is clearly detected and the lens is spatially resolved into two components. The achieved rms in the continuum image is 0.017 mJy/beam using 34.2 GHz bandwidth. The CO (4-3), CO (3-2) transitions are also clearly detected, co-spatial and consistent with a redshift of 3.44.

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#2011.0.00025.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSTC 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 and imaging scripts, and reference images.