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

TL;DR

This study provides the first observational upper limit on Br-gamma emission from Sgr A*, constraining models of accretion flow around the Galactic supermassive black hole using 13 years of Keck Observatory data.

Contribution

It offers the first direct observational constraint on Br-gamma emission near Sgr A*, challenging previous expectations based on ALMA observations.

Findings

Upper limit on Br-gamma flux is 80-245 times lower than expected from ALMA data.

Constraints suggest emission from the accretion flow is much weaker than prior models predicted.

Results significantly limit interpretations of the accretion environment around Sgr A*.

Abstract

We present the first observational constraint on the Br-gamma recombination line emission associated with the supermassive black hole at the center of our Galaxy, known as Sgr A*. By combining 13 years 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 ~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 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 under-luminous supermassive black hole.