ALMA achieves its highest resolution observations

The Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, has achieved the highest resolution observations since it began operations. During a technical test, a team of experts from the Joint ALMA Observatory (JAO), the National Astronomical Observatory of Japan (NAOJ), National Radio Astronomy Observatory (NRAO) in the USA, and ESO, imaged an evolved star with a resolution of 5 milliarcseconds. This shows ALMA can be used by astronomers to observe objects in detail equivalent to seeing a 10-metre-long bus on the Moon.

ALMA consists of 66 antennas which can be arranged in different positions across the high-altitude Chajnantor plateau in Chile. Each is equipped with receivers that allow it to observe radio waves in different frequency ranges, or bands. ALMA’s resolution increases both as the maximum separation between antennas increases and as the frequency of the observations increases. The new images were obtained with the most extended configuration possible for the ALMA array, with a maximum separation between its antennas of 16 km. They were made using the Band 10 receivers, which allow ALMA to observe at frequencies as high as 950 GHz, the highest possible for the array.

Since the observations push ALMA’s capabilities to the extreme, they were incredibly challenging to conduct. While Band 10 receivers have been available at ALMA since 2014, astronomers had to wait for the validation of a novel calibration technique, called band-to-band, to be able to conduct the new observations. They did so during a technical test in 2021 when they observed an evolved Milky Way star, R Leporis, using a bright galactic core as a calibrator, which, while distant, appears nearby R Leporis in the sky. The results are published today in the Astrophysical Journal.

This result has been achieved with significant support from ESO staff, who were involved in the test observations, the previous experiments in the lead up to this final technical achievement, and the development of the new calibration technique.

More Information

This result was presented in a paper titled “ALMA High-frequency Long Baseline Campaign in 2021: Highest Angular Resolution Submillimeter Wave Images for the Carbon-rich Star R Lep” to appear in the Astrophysical Journal (doi:10.3847/1538-4357/acf619).

The team is composed of Y. Asaki (JAO; NAOJ; SOKENDAI), L. Maud (ESO; Leiden University), H. Francke (JAO), H. Nagai (NAOJ), D. Petry (ESO), E. B. Fomalont (NRAO), E. Humphreys (JAO; ESO), A. M. S. Richards (University of Manchester), K. T. Wong (IRAM; Uppsala University), W. Dent (JAO), A. Hirota (JAO; NAOJ), J. M. Fernandez (Lowell Observatory), S. Takahashi (NAOJ), and A. S. Hales (JAO; NRAO).

A previous technical study, leading to the 2021 Campaign can be found in: “ALMA High-frequency Long-baseline Campaign in 2019: Band 9 and 10 In-band and Band-to-band Observations Using ALMA’s Longest Baselines”, published in the Astrophysical Journal in August 2023 (doi: 10.3847/1538-4365/acd6f1).

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.

Courtesy of European Southern Observatory