Gas Around the Milky Way's Black Hole Is Richer in Heavy Elements Than the Sun

Gas swirling near the supermassive black hole at the center of the Milky Way contains more heavy elements than the Sun does, according to a new study reported by Universe Today. The researchers suggest that exploding stars in the past probably seeded the gas, and that may help explain why the black hole feeds so slowly.

The work, published in The Astronomical Journal, used data from the James Webb telescope's JWST mid-infrared spectrograph. The lead author is Nicole Ford, a PhD student in Professor Daryl Haggard's research group in the Department of Physics at McGill University in Montreal.

The central numbers

• About 4 million solar masses. This is the mass of Sagittarius A* (Sgr A*), the black hole at the heart of the galaxy, according to Universe Today. It is considered quiescent rather than an active galactic nucleus, meaning it is fed by a trickle of material instead of a heavy flow.
• 0.1 parsec. The team studied the central 0.1 parsec of the Milky Way, which let them separate out different structures of glowing gas in the mid-infrared.
• Three gas features. The study measured the makeup and ionization state of the Bar, the Northern Arm and fast-moving clumps the authors call BC (Blue-shifted Compact) structures. All sit within a gas structure called the Galactic Center Minispiral.
• About 1.8 times solar. At scales smaller than a parsec in the Minispiral, the authors infer a metal abundance of roughly 1.8 times that of the Sun.

What "metals" means here

In astronomy, "metals" are not the metals of everyday life. The term covers every element heavier than hydrogen and helium. Those heavier elements did not exist until stars made them, so a higher share of them in a region points to a history of stellar activity there.

Where the extra metals probably came from

The researchers say the enrichment is probably due to past supernova explosions. Universe Today notes that a supernova remnant called Sgr A East overlaps Sgr A* in the sky.

A separate recent paper in The Astrophysical Journal, led by Mayura Balakrishnan, isolated that remnant from the black hole itself. According to Universe Today, that work found evidence that the supernova enriched the gas with metals and also helped keep it hot. Its authors concluded that Sgr A East has been a major source of energy that has kept the surroundings of Sgr A* hot and turbulent.

Why a slow-feeding black hole is puzzling

Gas can fall into Sgr A* only if it is cool and compressed. Metal-rich gas cools faster, which would normally speed up that flow. But Universe Today reports that a level of 1.8 times solar is only modest and cannot really drive much faster cooling. Together with the heating from the supernova remnant, this fits with Sgr A* receiving only a trickle of material.

Ford said in a press release that the black hole is consuming far less material than other supermassive black holes, and doing so more slowly. She said that understanding why requires looking at what is available for it to consume, including gas, dust and orbiting stars.

Why it matters

The finding does not change anything about daily life on Earth. Its value is in understanding how galaxies work. Sgr A* is much closer to Earth than the supermassive black holes in other large galaxies, so astronomers can study its fuel supply in unusual detail. Knowing what its surroundings are made of and how hot they are helps explain why this black hole is quiet rather than a bright, actively feeding one.

The result comes from one team's analysis of one region, and the link to past supernovae is an inference rather than a direct observation. The study ties it to the separate Sgr A East work, which points the same way.

Around Prague, based on reporting by Universe Today