Revealing the Physical Conditions around Sgr A* Using Bayesian Inference. I. Observations and Radiative Transfer

The Astrophysical Journal, vol. 916, pp. 69

Abstract

Abstract We report subarcsecond Atacama Large Millimeter/submillimeter Array (ALMA) observations between 272 and 375 GHz toward Sgr A*'s circumnuclear disk (CND). Our data comprise eight individual pointings, with significant SiO (87–76) and SO (7–6) emission detected toward 98 positions within these pointings. Additionally, we identify H2CS (91,9–81,8), OCS (25–24), and CH3OH (21,1–20,2) toward a smaller subset of positions. By using the observed peak line flux density, together with a Bayesian inference technique informed by radiative transfer models, we systematically recover the physical gas conditions toward each of these positions. We estimate that the bulk of the surveyed gas has temperatureTkin< 500 K and densitynH⪅ 106cm−3, consistent with previous studies of similar positions as traced by HCN clumps. However, we identify an uncharacteristically hot (Tkin≈ 600 K) and dense (nH≈ 106cm−3) source in the Northeastern Arm. This position is found to be approximately consistent with a gravitationally bound region dominated by turbulence. We also identify a nearby cold (Tkin≈ 60 K) and extremely dense (nH≈ 107cm−3) position that is again potentially bound and dominated by turbulence. We also determine that the total gas mass contained within the CND isMCND≲ 4 × 104M⊙. Furthermore, we qualitatively note that the observed chemical enrichment across large scales within the CND is consistent with bulk grain processing, though multiple desorption mechanisms are plausibly responsible. Further chemical modeling is required to identify the physical origin of the grain processing, as well as the localized H2CS and OCS emission.

Authors 5

1. Tomas James corresponding

   University College London

   Affiliation as printed

   Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK; tjames@star.ucl.ac.uk, t.james.17@ucl.ac.uk

   Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

2. S. Viti Aachen Leiden Observatory

   Leiden University · University College London · Leiden Observatory

   Affiliation as printed

   Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK; tjames@star.ucl.ac.uk, t.james.17@ucl.ac.uk

   Leiden Observatory, Leiden University, PO Box 9513, NL-2300RA, Leiden, The Netherlands

   Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

3. Farhad Yusef-Zadeh

   Northwestern University

   Affiliation as printed

   Department of Physics and Astronomy and CIERA, Northwestern University, Evanston, IL 60208, USA

4. M. Royster

   Northwestern University

   Affiliation as printed

   Department of Physics and Astronomy and CIERA, Northwestern University, Evanston, IL 60208, USA

5. M. Wardle

   Macquarie University

   Affiliation as printed

   Department of Physics and Astronomy and Research Centre for Astronomy, Astrophysics and Astrophotonics, Macquarie University, Sydney, NSW 2109, Australia