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Figure 3.1: ALMA imaging of the SDP.81 continuum in Band 4 (left), Band 6 (centre) and Band 7 (right), using Briggs weighting, robust parameter = 1. The beam size is 56×50, 39×30 and 31×23 mas for Bands 4, 6 and 7, respectively, and is indicated by the black ellipses in the lower left corners of each image. Note the faint emission close to the centre of the lens due to the AGN in the lensing galaxy. Image credit: ALMA Partnership et al., 2015b
3.2. 2014 ALMA Long Baseline Campaign 37 a nominal angular resolution of 10 mas. As a part of the ALMA Science Verification programme, the data was made publicly available in February 2015, after standard checks and calibration by the ALMA team. Given the unprecedented quality of the data, and the immediate availability to a wide scientific community, the LBC dataset presented an ideal target for a demonstration of our visibility-fitting lens modelling technique (Chapter 2).
A detailed description of the observational setup and data processing was given in ALMA Partnership et al. (2015a) and ALMA Partnership et al. (2015b), which respectively cover the 2014 Long Baseline Campaign in general and the SDP.81 observations in particular. Here, we provide a brief summary of these observations.
As one of the LBC targets, observations of SDP.81 were taken over several ob-serving blocks during 2014 October and November. These observations used the most extended ALMA configuration up to date: with 31 to 36 of the 12-meter an-tennas in the array and baseline lengths ranging from 15 m to 15 km. About 10%
of the baselines were shorter than 200 m. This gave an array that was sensitive to structures on angular scales between 19 arcsec and 16 mas (at 236 GHz).
The total time on-source for individual observations was as follows: 5.9 hours for Band 4, 4.4 h for Band 6 and 5.6 h for Band 7. The integration time varied between 2 and 6 seconds. Both theXX and YY polarizations were observed.
The data were taken in Bands 4, 6 and 7 and comprised both continuum imaging datasets centred at 2.0, 1.3 and 1.0 mm (156, 236 and 290 GHz), and spectral line datasets centred on the CO (5-4)1 (rest-frame frequencyf0 = 576.267GHz), CO (8-7) (f0 = 921.799 GHz), CO (10-9) (f0 = 1151.985 GHz) and H2O(202 → 111) (f0 = 987.927 GHz) emission lines. Of these, CO (5-4) fell into the Band 4; CO (8-7) and H2O(202 → 111) into Band 6 and CO (10-9) into Band 7. An attempt to observe the continuum emission in Band 3 was abandoned after preliminary tests revealed the source to be too faint in this particular Band (ALMA Partnership et al., 2015b). Figure 3.1 shows theCleaned images of the full-resolution continuum data as presented in ALMA Partnership et al. (2015b).
In each Band, four spectral windows (SPWs) were used; three of which were configured in the continuum mode with a channel width of 15.6 MHz. The spectral setups for the line observations was as follows: for the CO (5-4) line, a SPW centered on the line with a channel width of 0.976 MHz was used for all executions. In Band 6, a SPW centered on the H2O(202 → 111) line with 0.976 or 1.95 MHz resolutions was used (the spectral resolution varied between individual executions); the second line in this Band, the CO (8-7) line was covered by one of the continuum SPWs.
Finally, the CO (10-9) line was observed at a 1.95 MHz spectral resolution in Band 7.
The observing conditions were better than average for all three Bands - the precipitable water vapor values at zenith were between 0.6 3.2, 0.5 3.1 and 0.3 -0.7 mm for Bands 4, 6, and 7, respectively.
1In this work, we refer exclusively to the rotational CO transitions, following the CO (Jupper− Jlower) notation.
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3.2.2 Data inspection and selection
From our imaging tests, we found that the recovered structure of the source is highly dependent on the baseline data that are used. Namely, after continuum imaging using only baselines longer than 5 Mλ, only a handful of very compact regions were detected. On the other hand, continuum imaging based on baselines shorter than 0.5 Mλ revealed a faint Einstein ring. Figure 3.2 presents the imaging data for Bands 6 and 7 using a cut in the uv-plane at baseline lengths of 0.5 and 2 Mλ. The 0.5 Mλ images show that at mm-wavelengths the observed system comprises four images in a cusp configuration with evidence for a low-surface brightness Einstein ring. The 2 Mλ image shows that the large arc is composed of several structures with varying surface brightness, and although the Einstein ring is effectively resolved out at these scales, there is still evidence of emission from this part of the source.
For the visibilities at >1 Mλ (i.e. removing the shortest baselines), the extended arc was mostly resolved out, which demonstrates that most of the emission at mm-wavelengths is extended on scales larger than 0.2 arcsec. Only the most compact components within the individual images were detected with the >2.5km baselines.
We carried out additional tests to determine the scales on which most of the compact structures are resolved out. We found that they are detected up to around 5 Mλ, that is, around 30 mas-scales. Based on these results, we restrict our lens modelling to the visibility data at uv-distances ≤2 Mλ for the continuum, as there is little or no extended structure detected on baselines longer than this. The CO and H2O lines suffer from an even lower SNR at long baselines than the continuum, as we will describe later, to obtain a robust reconstruction, only baselines shorter than 1 Mλ`m were considered for the line datasets.
The visibility data for the final observing block of the Band 6 dataset had a larger rms noise compared to the rest of the observations; consequently, this observing block was removed from the dataset. Also, the absolute flux-density calibration of the first spectral window of the Band 6 dataset was higher by ∼ 28% relative to the other SPWs in the same Band, an offset that we corrected for during our analysis. The post-reduction quality of the Band 7 dataset was excellent; no additional corrections were required for this dataset.
A low-level continuum emission, co-spatial with the centre of the lensing galaxy, was detected prominently in the Band 7 continuum data. While reminiscent of a potential central lensed image, comparison of the continuum spectrum of this region with that of the four images showed that this is in fact an AGN hosted by the lensing galaxy (Wong et al., 2015). Due to its low flux-density and a very compact size, we could safely disregard this emission from our analysis.
The insufficient signal-to-noise ratio on the longer baselines of the data did not allow this data to be self-calibrated (ALMA Partnership et al., 2015b). Although we tried to apply self-calibration at least to the baselines within the innermost 2 Mλ, we did not manage to obtain a robust solution for the phase errors. As a result, no additional phase-corrections were applied. While this does not present a major issue for the source reconstruction, as shown by Hezaveh et al. (2016), the uncorrected phase errors can effectively mimic perturbations to the Einstein arc caused by a substructure in the lens potential.
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Figure 3.2: ALMA continuum imaging of SDP.81 of Band 6 (upper) and Band 7 con-tinuum (lower) (1.3 and 1.0 mm, respectively). The images have been produced at two angular resolutions using a cut in the uv-data of 0.5 Mλ (left) and 2 Mλ (right), and weighting was applied (natural and Briggs, robust parameter = 0) to emphasise the struc-ture seen on different scales. The synthesised restoring beam is shown as the white ellipse in the bottom left corner of each map.
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