A supermassive black hole (SMBH) likely resides at the center of the Milky Way, and in the centers of other galaxies like it. It’s never been seen though. It was discovered by watching a cluster of stars near the galactic center, called S stars.

S stars’ motions indicated the presence of a massive object in the Milky Way’s center and the scientific community mostly agreed that it must be an SMBH. It’s named Sagittarius A*.

But some scientists wonder if it really is a black hole. And one of the S stars could answer that question and a few others about black holes.

Scientists have been monitoring and studying the S stars for over 20 years. They’ve gathered precise astrometric data for the group of stars, and the measurements of the stars’ positions and movements around the galactic center have shown that there’s a massive object there. One particular star in the group—named S2 (or S0-2)—could help astronomers determine more clearly the nature of that massive object.

A new research letter is taking a close look at S2’s behaviour and asking a potentially uncomfortable question. The study is titled “What does lie at the Milky Way centre? Insights from the S2 star orbit precession.” It’s available on the pre-print site arxiv.org. The first author is C. R. Argüelles from the Fac. de Ciencias Astron. y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque.

The question of what’s at the Milky Way’s Center has largely been settled. The 2020 Nobel Prize in Physics was awarded to three scientists. Two of them, Andrea Ghez and Reinhard Genzel, received it for their research into the object at the center of the galaxy. The press release from the Nobel Prize Organization reads “Reinhard Genzel and Andrea Ghez discovered that an invisible and extremely heavy object governs the orbits of stars at the centre of our galaxy. A supermassive black hole is the only currently known explanation.”

Now there’s some uncertainty.

It’s possible that the massive object at the center of our galaxy is actually dark matter. In this case, fermionic dark matter. According to the authors, “It has been recently demonstrated that both, a classical Schwarzschild black hole (BH), and a dense concentration of self-gravitating fermionic dark matter (DM) placed at the Galaxy centre, can explain the precise astrometric data (positions and radial velocities) of the S-stars orbiting Sgr A*.”

There’s another cosmic actor playing a role in this question. It’s not a star, but a cloud of gas called G2. In 2014 G2 passed close by the Milky Way’s center. Astronomers think it came to within 36 light hours of Sgr A*. Prior to its closest approach, astronomers simulated what the encounter might look like. They thought that the black hole would tear gas from the cloud, leading to a pronounced brightening from Sgr A* as it accreted mass from G2. But it never happened.

Why weren’t there any fireworks when G2 passed so close to Sgr A*? As it turns out, G2 may not have been a gas cloud. Instead, G2 is made up of two components: a cold cloud of low-mass gas, and a dusty stellar object. So G2’s close passage by Sgr A* didn’t end up shedding much light on the nature of the object in the galactic center.

Back to S2.

As the authors outline, there are two competing models that can explain the nature of the massive object. And they differ in one important way. “While the Schwarzschild BH scenario predicts a unique prograde precession for S2, in the DM scenario it can be either retrograde or prograde…” they write. Whether S2’s precession is either prograde or retrograde depends on the amount of DM mass inside S2’s orbit.

The authors point out that even with all the observations of the S2 star and all of the publicly available data, researchers are still unable to determine which of the models are correct. But there’s an opportunity to observe S2 coming up that should determine if the Milky Way’s center is inhabited by a black hole or dark matter. “… upcoming S2 astrometry close to next apocentre passage could potentially establish if Sgr A* is governed by a classical BH or by a quantum DM system,” they write.

Most of the matter in the Universe is dark matter, not baryonic matter. So should we register surprise when some of the most massive objects around may be made of dark matter rather than regular matter? Definitely. Because confirming that Sgr A* is not a black hole but a mass of dark matter would be a huge deal.

But we’ll have to wait and see. S2 has an orbital period of 16 years, and the last closest approach to Sgr A* was in 2018. The next best opportunity to observe S2 as it passes by Sgr A* is in 2034.

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