210: The most powerful black holes in the universe may finally have an explanation



Quasars, the most extreme phenomena in the universe, are triggered when galactic collisions deliver gas to feeding black holes, new research suggests.

An illustration of the quasar P172+18 as it blasts out powerful jets of matter and radiation. (Image credit: ESO-M Kornmesser)

Scientists may have solved a 60-year-old mystery by discovering that quasars —  energetic objects that are powered by ravenous supermassive black holes and can outshine trillions of stars combined — form when galaxies collide and merge.

The findings indicate that the Milky Way could host a quasar of its own when it collides with the neighboring Andromeda galaxy several billion years from now.

Scientists have previously tracked quasars’ bright, energetic emissions to regions at the hearts of galaxies that span roughly the width of the solar system — meaning quasars must come from incredibly compact objects. The leading theory suggests that quasars are supermassive black holes heating huge amounts of surrounding gas, thus releasing tremendous amounts of radiation before the material falls onto the black hole’s surface.

Since their discovery six decades ago, quasars have puzzled scientists — mainly because it’s unclear just how supermassive black holes can draw in enough raw material to fuel such powerful emissions. While supermassive black holes dwell at the centers of most galaxies, the gas needed to fuel quasars tends to orbit on the outskirts of galaxies. Thus, there must be some delivery service moving gas toward the hearts of galaxies.

Now, new research published in the journal Monthly Notices of the Royal Astronomical Society uses deep imaging observations from the Isaac Newton Telescope in Spain’s Canary Islands to finally solve this puzzle.

“To understand how quasars are ignited we need to determine how gas can fall into the center of the host galaxies at sufficiently high rates,” lead study author Clive Tadhunter, a professor in the Department of Physics and Astronomy  at the University of Sheffield in the U.K., told Live Science via email. “One idea is that the necessary radial infall is caused by collisions between galaxies, whose associated gravitational forces can perturb the gas from its usual circular orbits.”

Two spiral galaxies collide, triggering a burst of star formation (red). (Image credit: NASA)

When comparing  48 nearby galaxies hosting quasars to 100 non-quasar galaxies, the researchers discovered the presence of distorted structures at the edges of the quasar-hosting galaxies. These structures also indicate a past or ongoing collision and merger with another galaxy, Tadhunter said.

“We found a high rate of such structures in quasar-hosting galaxies, three times that measured for a carefully matched control sample of non-quasar galaxies that were imaged with the same techniques,” Tadhunter said. “This provides strong evidence that quasars are indeed triggered in galaxy collisions.”

The team’s research isn’t the first time galactic mergers have been linked to quasars. Tadhunter pointed out, however, that attempts to test this hypothesis by hunting for distorted structures at the outer parts of galaxies that are characteristic of such collisions had previously proved ambiguous.

“Some studies have found the expected structures but others have not,” he continued. “We believe that much of the past ambiguity in this field is due to the fact that many of the previous imaging studies did not have sufficient depth to detect the sometimes faint distorted structures in the outer parts of the galaxies that host the quasars.”

Quasars can have a large influence on the evolution of galaxies that host them; better understanding how quasars ignite could help scientists hone their models of galaxy evolution and the evolution of the universe as a whole.

“It’s important to understand how, when, and where quasars are triggered, as once triggered, the enormous radiative power generated by a quasar can have a major, damaging effect on the surrounding host galaxy,” Tadhunter said. “For example, the pressure of the radiation can expel the remaining gas in the remnant galaxy system. Since gas is required to form new stars, this will cut off any future star formation activity, effectively the death throes of the galaxy.”

Tadhunter also pointed out that understanding the connection between galactic collisions and quasars is vital in determining the future of our own corner of the cosmos.

“The nearest large galaxy  —  the Andromeda Spiral  —  is coming directly towards us at a high velocity, and will collide and merge with the Milky Way in around 5 billion years,” he said. “When this happens, it’s likely that a quasar will be triggered as gas falls into the center of the remnant system.”

The team intends to follow up on this research by examining other quasars that are at a wider range of distances and that have been detected using other methods, to see if they have the same features that connect them to galactic collisions.

Live Science
published 29.04.2023


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56: Black holes may be swallowing invisible matter that slows the movement of stars



Scientists watched as two stars slowed in their orbits around black holes, concluding it was the result of “drag” generated by dark

An illustration of a supermassive black hole surrounded by a blazing accretion disk and wall of cosmic dust. Invisible dark matter may also find a home around cosmic giants like these, new research suggests. (Image credit: NASA/JPL-Caltech)

For the first time, scientists may have discovered indirect evidence that large amounts of invisible dark matter surround black holes. The discovery, if confirmed, could represent a major breakthrough in dark matter research.

Dark matter makes up around 85% of all matter in the universe, but it is almost completely invisible to astronomers. This is because, unlike the matter that comprises stars, planets and everything else around us, dark matter doesn’t interact with light and can’t be seen.

Fortunately, dark matter does interact gravitationally, enabling researchers to infer the presence of dark matter by looking at its gravitational effects on ordinary matter “proxies.” In the new research, a team of scientists from The Education University of Hong Kong (EdUHK) used stars orbiting black holes in binary systems as these proxies.

The team watched as the orbits of two stars decayed, or slightly slowed, by about 1 millisecond per year while moving around their companion black holes, designated A0620–00 and XTE J1118+480. The team concluded that the slow-down was the result of dark matter surrounding the black holes which generated significant friction and a drag on the stars as they whipped around their high-mass partners.

Using computer simulations of the black hole systems, the team applied a widely held model in cosmology called the dark matter dynamical friction model, which predicts a specific loss of momentum on objects interacting gravitationally with dark matter. The simulations revealed that the observed rates of orbital decay matched the predictions of the friction model. The observed rate of orbital decay is around 50 times greater than the theoretical estimation of about 0.02 milliseconds of orbital decay per year for binary systems lacking dark matter.

“This is the first-ever study to apply the ‘dynamical friction model’ in an effort to validate and prove the existence of dark matter surrounding black holes,” Chan Man Ho (opens in new tab), the team leader and an associate professor in the Department of Science and Environmental Studies at EdUHK, said in a statement (opens in new tab).

The team’s results, published Jan. 30 in The Astrophysical Journal Letters (opens in new tab), help to confirm a long-held theory in cosmology that black holes can swallow dark matter that comes close enough to them. This results in the dark matter being redistributed around the black holes, creating a “density spike” in their immediate vicinity that can subtly influence the orbit of surrounding objects.

Chan explained that previous attempts to study dark matter around black holes have relied on the emission of high-energy light in the form of gamma rays, or ripples in space known as gravitational waves. These emissions result from the collision and resulting merger of black holes – a rare event in the universe that can leave astronomers waiting a long time for sufficient data.

This research gives scientists a new way to study dark matter distributed around black holes that may help them to be more proactive in their search. The EdUHK team intends to hunt for similar black hole binary systems to study in the future.

“The study provides an important new direction for future dark matter research,” Chan said. “In the Milky Way Galaxy alone, there are at least 18 binary systems akin to our research subjects, which can provide rich information to help unravel the mystery of dark matter.”

Live Science
published 24.03.2023

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