ALMA Extends to 15-Kilometre Baselines: Submillimetre Science Down to 20-Milliarcsecond Resolution

Telescopes and Instrumentation ALMA Extends to 15-kilometre Baselines: Submillimetre Science down to 20-Milliarcsecond Resolution Catherine Vlahakis1,2 operate the world’s largest millimetre/ antenna stations on the Chajnantor site Leonardo Testi2 submillimetre array, located on the that have been built for this purpose, Paola Andreani2,3 Chajnantor Plateau in the north of Chile stretching out to almost ten kilometres at an elevation of 5000 metres above sea from the centre of the array. The 2014 level. ALMA, inaugurated in 2013, has a ALMA Long Baseline Campaign (LBC) 1 Joint ALMA Observatory, Santiago, Chile full complement of 66 antennas. The Call carried out the first ever tests using these 2 ESO for Proposals for Early Science Cycle 3 distant antenna stations; it was the first 3 ESO ALMA Regional Centre has recently closed, with a record num- time the whole observatory infrastructure ber of proposals received. had been used for antenna separations above about three kilometres. The LBC From September to late November A key new capability offered in Cycle 3 tests were carried out from September to 2014, the Atacama Large Millimeter/ is observing at high angular resolution November 2014 and consisted of exten- submillimeter Array (ALMA) carried out using baselines of up to ten kilometres. sive testing with 23 antennas on long a Long Baseline Campaign to test and This mode was tested and verified during baselines of up to 15 kilometres, together verify baselines up to ~ 15 kilometres a dedicated campaign held in the last with additional antennas on short (about for the first time. The tests were suc- quarter of 2014. A summary of this cam- 300 metres) baselines in order to provide cessful, demonstrating the power of paign — the 2014 ALMA Long Baseline the short spacings that are necessary ALMA to produce submillimetre-wave Campaign — is presented here. Activities for imaging extended objects. A total of imaging at resolutions of tens of milli- continue to extend and upgrade the capa- 22–36 antennas were employed for the arcseconds. Science Verification obser- bilities, including new receivers, digital LBC tests, with the number depending vations of five targets, including an electronics, data handling and analysis on observing band; most tests were car- asteroid, a protoplanetary disc, an software, and new observing modes ried out in Band 3 (100 GHz). A paper evolved star and a gravitationally lensed (such as millimetre-wave very long base- (ALMA Partnership, Fomalont et al., 2015; high-redshift galaxy, were obtained. line interferometry), with the goal of main- hereafter ALMA I) provides details of the The results demonstrate that the scien- taining ALMA at the forefront of astro- technical and calibration aspects of the tific leap for which ALMA was planned physical research for many decades to LBC, and a brief summary is given here. has been made. A brief overview of the come. 2014 Long Baseline Campaign and its ALMA calibration normally uses the phase scientific results are presented. referencing observing mode, whereby The 2014 ALMA Long Baseline Campaign short scans of the science target and a nearby quasar calibrator target are alter- ALMA is a global project with partners Imaging at resolutions of tens of milliarc- nated over the duration of the observation, in Europe, North America (USA and seconds (mas) is a major goal of ALMA. such that the phase measured on the Canada) and East Asia (Japan, Taiwan This requires baselines of up to almost calibrator can be transferred to the tar- and the Republic of Korea), to build and 16 kilometres, achieved using the distant get. The accuracy with which this can be )/ESO wingsforscience.com Clem & Adri Bacri-Normier ( Bacri-Normier Adri & Clem Figure 1. The ALMA Chajnantor site from the air in early 2013. 2 The Messenger 160 – June 2015 achieved was a critical factor for the out- term phase root mean square (rms) is the peak/image rms of a factor of two come of the long baseline observations, < 30 degrees. The cycle time needed for to six compared to imaging with phase and was addressed with a test strategy phase referencing calibration to produce referencing alone. For HL Tau, the that included: a) establishing the phase good image quality was found to be improvement was up to a factor of two. coherence of the long baseline array by 60–90 seconds in almost all cases. The For SDP.81, self calibration was not pos- carrying out test observations of quasars; most reliable method of determining sible at all. Conversely, for the quasar b) determining the observing strategy whether phase conditions are suitable for 3C 138, which has emission that is domi- needed to achieve good imaging results; carrying out a given observation of a nated by a strong compact core and other and c) carrying out Science Verification science target is with short observations relatively simple structures, the improve- (SV) observations to demonstrate the of a strong quasar (the “Go/noGo” pro- ment ranged from a factor of 15 to 30. end-to-end process from observation to cedure), rather than predictions. The See ALMA I for further details. Thus, calibration to imaging. In order to address water vapour radiometer (WVR) correc- more sophisticated methods of self cali- the former two points, the LBC tests tion, which takes into account estimated bration may need to be implemented in included: variations in the amount of precipitable future for some extended sources. 1) determination of the statistics of the water vapour along the line of sight to Another finding from the SV imaging was temporal phase variation as a function each antenna (see Nikolic et al., 2011; that in many cases the integration time of baseline length using observations of 2013), typically improves the phase noise on source may be driven not by the need a single bright quasar; by a factor of about two for clear skies, to reach a given signal rms, but by the 2) determination of the accuracy of the and further phase fluctuations are need to obtain sufficient coverage of the phase transfer using alternating obser- thought to be due to variations in the dry uv (Fourier domain) plane. vations of two quasars that are close atmosphere. together on the sky; 3) development of a quasi real-time The lack of an accurate dry atmospheric Science Verification observations at long method to estimate the feasibility of delay model, due to pressure variation baselines given long baseline observations; across the array over distances of several 4) determination of the optimal time inter- kilometres and the current single pres- Continuum and spectral line observations val (cycle time) between calibrator sure measurement at the centre of the of five SV targets were carried out to scans; array, is the dominant cause of system- demonstrate the long baseline potential 5) determination of antenna positions and atic phase differences (longer-term varia- of ALMA, chosen from a broad range of delay model errors using observations tions, i.e., over timescales of minutes science topics. The targets were the of many quasars distributed over the to hours) between observations of the asteroid Juno, the asymptotic giant branch sky; phase calibrator and the science target. (AGB) star Mira, the circumstellar disc 6) flux density measurement for candidate The addition of further weather stations around the young star HL Tau, the quasar weak phase calibrators; in the coming months, providing several 3C 138, and the gravitationally lensed 7) investigation of potential significant pressure measurements across the submillimetre galaxy SDP.81. The base- structure in calibrator sources when array, will allow improvement of the delay lines typically ranged from 15 metres observed at long baselines; and model and will be the main focus of the to ~15 kilometres, and observations were 8) measurement of the positional accuracy further long baseline testing later in 2015. taken in two or three bands (selected at long baselines, by phase referencing It is also expected that the addition of from Bands 3, 4, 6 and 7). The fully veri- among many quasars that are close further pressure sensors will improve the fied SV datasets are publicly available on together on the sky. astrometric accuracy. the ALMA Science Portal1. Details of these experiments, the analysis and conclusions are described in ALMA I. The survey for weak calibrators will The results of the LBC are presented continue in order to increase the number in a collection of four ApJ Letters by the An enormous amount was learned about of sources in the ALMA calibrator cata- ALMA Partnership. A summary of the ALMA observing at long baselines during logue (Fomalont et al., 2014). However, in main test results and SV observations is the highly successful LBC and some of order to find the faintest usable calibra- presented in ALMA I. Initial results on the the main results are briefly summarised tors, and thereby ensure that a suitable SV targets Juno, HL Tau and SDP.81 are here (see ALMA I for details). Further calibrator can be found close enough presented in ALMA Partnership, Hunter investigations are also ongoing, and addi- to a science target, it is likely that a differ- et al. (2015; hereafter ALMA II), ALMA tional long baseline testing will take place ent observing strategy may need to be Partnership, Brogan et al. (2015; hereafter in the last quarter of 2015 in order to fur- adopted in future. ALMA III) and ALMA Partnership, Vlahakis ther improve the observing mode and et al. (2015; hereafter ALMA IV), respec- test long baseline imaging at higher fre- For some of the SV targets (see the next tively. quencies. section for details), the emission was sufficiently strong that it was possible to Juno It was found that calibration via phase use the self-calibration technique to ALMA’s combination of excellent contin- referencing should, for most sources, improve the image quality.

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