Upper Limit on Brackett-γ Emission from the Immediate Accretion Flow onto the Galactic Black Hole

We present the first observational constraint on the Brackett-gamma (Br gamma) recombination line emission associated with the supermassive black hole at the center of our Galaxy, known as Sgr A*. By combining 13 yr of data with the adaptive optics fed integral field spectrograph OSIRIS at the W. M. Keck Observatory obtained as part of the Galactic Center Orbits Initiative, we extract the near-infrared spectrum within similar to 0.2 '' of the black hole and we derive an upper limit on the Br gamma flux. The aperture was set to match the size of the disk-like structure that was recently reported based on millimeter-wave Atacama Large Millimeter/submillimeter Array (ALMA) observations of the hydrogen recombination line, H30 alpha. Our stringent upper limit is at least a factor of 80 (and up to a factor of 245) below what would be expected from the ALMA measurements and strongly constrains possible interpretation of emission from this highly underluminous supermassive black hole.

Automated summary

We present the first observational constraint on the Brackett-gamma (Br gamma) recombination line emission associated with the supermassive black hole at the center of our Galaxy, known as Sgr A*. We derived a stringent upper limit on the Br gamma flux based on 13 years of data combined with the OSIRIS integral field spectrograph at the W. M. Keck Observatory, strongly constraining possible interpretations of emission from this highly underluminous supermassive black hole.