Arp 220 in Band 2
Science Target Overview
The nearest ultraluminous infrared galaxy Arp 220 has been key to study the luminous phase of galaxy evolution after a major merger. It consists of two merger nuclei separated by about 1″ (∼400 pc) on the sky, each having a ∼100 pc scale rotating disk of molecular gas. Each nucleus consists of a central compact core and a more extended structure revealed by the high, ~0.03″, resolution observations. Arp 220 is very rich in line emission, including strong lines of dense tracers that can be observed together with CO isotopologues in Band 3.
ALMA observed Arp 220 numerous times. Project 2015.1.00113.S performed spectral scans in Bands 3, 6 and 7 with a spectral resolution of 2-5 km/s (Sakamoto et al. 2017, 2021 and Martin et al. 2016) and includes very high spatial resolution SBs (0.06”) and lower resolution ones (~0.4”). Project 2015.1.00167.S observed the source in Bands 3 and 4 with a spectral resolution of 7-12 km/s (Brown & Wilson 2019) at a resolution of ~0.5”. Thus, the low-resolution data of both projects are suitable for comparison to the SV data. This makes the source ideal to check ALMA Band 2 imaging capability for spectral lines and spectral line analysis and demonstrates the ALMA Band 2 capability of observing dense gas at the same time as CO isotopologues (C18O, 13CO, and 12CO J=1–0), that Band 2 will extend to sources of z = 0.3.
ALMA data overview
Observations have been carried out with three different MOUS on 20 November 2025 using 24 antennas.
- MOUS *X36 has a total on-source time of about 15 min. The spectral scan mode covering the frequency range from 75 to 110 GHz has been used with a total of 20 spectral windows.
- MOUS *X4e has a total on-source time of about 18 mins. Four spectral windows have been used to cover the 67-74GHz range.
- MOUS *X52 has a total on-source time of about 8 mins. Four spectral windows have been used to cover the 109-116GHz.
Typical PWV during observations was 2.3-2.6 mm. The baseline lengths range from 15 m to 2.4 km. All spectral windows have a spectral resolution of 488.281 kHz and a bandwidth of 1.875 GHz.
The calibration and imaging was initially performed using the ALMA Pipeline in a standard manner without additional flagging. The native spectral resolution (about 1.5 km/s) is oversampling the targeted lines resulting in noisy spectra. Therefore, we spectrally binned the cube using nbins=10 in the imaging parameters. The line widths are ~250-300 km/s, so the lines are well resolved.
However, 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, manual imaging was also performed in a few selected frequency ranges after flagging the longest baselines to reduce image artifacts. We also note that MOUS *X52 shows an astrometric offset derived from the observation using a distant phase calibrator. Details are provided in the README files.
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.
The continuum emission is clearly detected and the fluxes are consistent to previously reported fluxes in Band 3. Numerous lines are detected at high significance including dense gas tracers and CO isotopologues previously detected in Band 3.
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#2011.0.00026.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:
The above directory contains README, uncalibrated data, calibrated data, data reduction and imaging scripts, and reference images.