ALMA Observations of the Terahertz Spectrum of Sagittarius A∗

Geoffrey C. Bower, Jason Dexter, Keiichi Asada, Christiaan D. Brinkerink, Heino Falcke, Paul Ho, Makoto Inoue, Sera Markoff, Daniel P. Marrone, Satoki Matsushita, Monika Moscibrodzka, Masanori Nakamura, Alison Peck, Ramprasad Rao

Research output: Contribution to journalArticlepeer-review

40 Scopus citations


We present Atacama Large Millimeter/submillimeter Array (ALMA) observations at 233, 678, and 870 GHz of the Galactic Center black hole, Sagittarius A∗. These observations reveal a flat spectrum over this frequency range with spectral index α≈-0.3, where the flux density S∝ν α. We model the submillimeter and far-infrared spectrum with a one-zone synchrotron model of thermal electrons. We infer electron densities n=(2-5) 106 cm-3, electron temperatures T e=(1-3) 1011 K, and magnetic field strength B=10-50 G. The parameter range can be further constrained using the observed quiescent X-ray luminosity. The flat submillimeter spectrum results in a high electron temperature and implies that the emitting electrons are efficiently heated. We also find that the emission is most likely optically thin at 233 GHz. These results indicate that millimeter and submillimeter wavelength very long baseline interferometry of SgrA∗ including those of the Event Horizon Telescope should see a transparent emission region down to event horizon scales.

Original languageEnglish (US)
Article numberL2
JournalAstrophysical Journal Letters
Issue number1
StatePublished - Aug 10 2019


  • Galaxy:center
  • accretion, accretion disks
  • black hole physics
  • galaxies: nuclei

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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