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Published January 20, 2019 | Accepted Version + Published
Journal Article Open

The Size, Shape, and Scattering of Sagittarius A* at 86 GHz: First VLBI with ALMA


The Galactic center supermassive black hole Sagittarius A* (Sgr A*) is one of the most promising targets to study the dynamics of black hole accretion and outflow via direct imaging with very long baseline interferometry (VLBI). At 3.5 mm (86 GHz), the emission from Sgr A* is resolvable with the Global Millimeter VLBI Array (GMVA). We present the first observations of Sgr A* with the phased Atacama Large Millimeter/submillimeter Array (ALMA) joining the GMVA. Our observations achieve an angular resolution of ~87 μas, improving upon previous experiments by a factor of two. We reconstruct a first image of the unscattered source structure of Sgr A* at 3.5 mm, mitigating the effects of interstellar scattering. The unscattered source has a major-axis size of 120 ± 34 μas (12 ± 3.4 Schwarzschild radii) and a symmetrical morphology (axial ratio of 1.2_(-0.2)^(+0.3)), which is further supported by closure phases consistent with zero within 3σ. We show that multiple disk-dominated models of Sgr A* match our observational constraints, while the two jet-dominated models considered are constrained to small viewing angles. Our long-baseline detections to ALMA also provide new constraints on the scattering of Sgr A*, and we show that refractive scattering effects are likely to be weak for images of Sgr A* at 1.3 mm with the Event Horizon Telescope. Our results provide the most stringent constraints to date for the intrinsic morphology and refractive scattering of Sgr A*, demonstrating the exceptional contribution of ALMA to millimeter VLBI.

Additional Information

© 2019. The American Astronomical Society. Received 2018 October 19; revised 2018 December 7; accepted 2018 December 7; published 2019 January 21. This work is supported by the ERC Synergy Grant "BlackHoleCam: Imaging the Event Horizon of Black Holes," grant 610058. We thank the National Science Foundation (AST-1126433, AST-1716536) and the Gordon and Betty Moore Foundation (GBMF-5278) for financial support of this work. This work was supported in part by the black hole initiative at Harvard University, which is supported by a grant from the John Templeton Foundation. M.K. acknowledges the financial support of JSPS KAKENHI grant Nos. JP18K03656 and JP18H03721. R.-S.L. is supported by the National Youth Thousand Talents Program of China and the Max-Planck Partner Group. L.L. acknowledges the financial support of DGAPA, UNAM (project IN112417), and CONACyT, México. I.C. acknowledges the financial support of the National Research Foundation of Korea (NRF) via a Global PhD Fellowship Grant (NRF-2015H1A2A1033752). This paper makes use of the following ALMA data: ADS/JAO.ALMA2016.1.00413.V. ALMA is a partnership of the ESO (representing its member states), NSF (USA), and NINS (Japan), together with the NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by the ESO, AUI/NRAO, and NAOJ. This research has made use of data obtained with the Global Millimeter VLBI Array (GMVA), which consists of telescopes operated by the Max-Planck-Institut für Radioastronomie (MPIfR), IRAM, Onsala, Metsahovi, Yebes, the Korean VLBI Network, the Green Bank Observatory, and the Long Baseline Observatory (LBO). The VLBA is an instrument of the LBO, which is a facility of the National Science Foundation operated by Associated Universities, Inc. The data were correlated at the DiFX correlator of the MPIfR in Bonn, Germany. This work is partly based on observations with the 100 m telescope of the MPIfR at Effelsberg. This work made use of the Swinburne University of Technology software correlator (Deller et al. 2011), developed as part of the Australian Major National Research Facilities Programme and operated under license. Software: AIPS (Greisen 2003), DiFX (Deller et al. 2011), HOPS (Whitney et al. 2004; L. Blackburn et al. 2019, in preparation), KORAL (Sa̧dowski et al. 2013, 2014, 2017), PolConvert (Martí-Vidal et al. 2016), eht-imaging library (Chael et al. 2016), Stochastic Optics (Johnson 2016).

Attached Files

Accepted Version - 1901.06226.pdf

Published - Issaoun_2019_ApJ_871_30.pdf


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August 22, 2023
August 22, 2023