The gas orbiting the supermassive black hole Sagittarius A* (Sgr A*) at the center of the Milky Way contains an unexpectedly large amount of heavy elements. This is the conclusion reached by researchers from the Trottier Institute for Research on Exoplanets at McGill University. Their work is published in The Astronomical Journal.
In astronomy, "metals" refers to any elements heavier than helium and hydrogen, such as carbon, oxygen, iron or gold. Their presence affects how quickly gas cools, and therefore how easily it can be captured by the black hole.
According to the authors, the enrichment of the gas with metals most likely came from stars that exploded and gradually ejected material into the vicinity of the black hole. In addition, the composition of the gas turned out to be similar to that of many stars orbiting Sgr A*. This suggests that the gas and the stars may have formed together.
"We know that at the center of our Galaxy there is a black hole that pulls matter toward itself, but it absorbs far less matter and more slowly than other supermassive black holes," says Nicole Ford, a graduate student in Professor Daryl Haggard's group and the first author of the paper. "To understand why, we need to study the properties of the material available to it, including gas, dust and orbiting stars."
The data for the study were obtained using the James Webb Space Telescope (JWST), launched five years ago. It measures infrared light over a wide range of wavelengths with unprecedented spectral and spatial resolution. This allowed scientists to distinguish the thermal signatures of several gas clouds near the black hole and to precisely measure the metal content in each of them.
The results complement another recent work led by postdoc Mayura Balakrishnan, published in The Astrophysical Journal. It tested whether the gas retained traces of past bursts of black hole activity. It turned out that the gas emission is determined by local conditions, so isolating the influence of the black hole itself is difficult. This does not rule out past bursts, but shows that the surrounding gas cannot serve as a reliable "chronicle" of the black hole's activity.
Ford notes that to understand how the black hole pulls in matter, these observations must be compared with models describing the evolution of gas over long periods of time.