Ejected Black Holes Drag Clusters of Stars With Them

This artist’s conception shows a rogue black hole that has been kicked out from the center of two merging galaxies. The black hole is surrounded by a cluster of stars that were ripped from the galaxies. Credit: Space Telescope Science Institute

The tight cluster of stars surrounding a supermassive black hole after it has been violently kicked out of a galaxy represents a new kind of astronomical object which may provide telltale clues to how the ejection event occurred. “Hypercompact stellar systems” result when a supermassive black hole is violently ejected from a galaxy, following a merger with another supermassive black hole. The evicted black hole rips stars from the galaxy as it is thrown out. The stars closest to the black hole move in tandem with the massive object and become a permanent record of the velocity at which the kick occurred.

“You can measure how big the kick was by measuring how fast the stars are moving around the black hole,” said David Merritt, professor of physics at the Rochester Institute of Technolyg. “Only stars orbiting faster than the kick velocity remain attached to the black hole after the kick. These stars carry with them a kind of fossil record of the kick, even after the black hole has slowed down. In principle, you can reconstruct the properties of the kick, which is nice because there would be no other way to do it.”

In a paper published in the July 10 issue of The Astrophysical Journal, Merritt and his colleagues discusses the theoretical properties of these objects and suggests that hundreds of these faint star clusters might be detected at optical wavelengths in our immediate cosmic environment. Some of these objects may already have been picked up in astronomical surveys. .

“Finding these objects would be like discovering DNA from a long-extinct species,” said team member Stefanie Komossa, from the Max-Planck-Institut for Extraterrestrial Physics in Germany.

The astronomers say the best place to find hypercompact stellar systems is in cluster of galaxies like the nearby Coma and Virgo clusters. These dense regions of space contain thousands of galaxies that have been merging for a long time. Merging galaxies result in merging black holes, which is a prerequisite for the kicks.

“Even if the black hole gets kicked out of one galaxy, it’s still going to be gravitationally bound to the whole cluster of galaxies,” Merritt says. “The total gravity of all the galaxies is acting on that black hole. If it was ever produced, it’s still going to be there somewhere in that cluster.”

Merritt and his co-authors think that scientists may have already seen hypercompact stellar systems and not realized it. These objects would be easy to mistake for common star systems like globular clusters. The key signature making hypercompact stellar systems unique is a high internal velocity. This is detectable only by measuring the velocities of stars moving around the black hole, a difficult measurement that would require a long time exposure on a large telescope.

From time to time, a hypercompact stellar system will make its presence known in a much more dramatic way, when one of the stars is tidally disrupted by the supermassive black hole. In this case, gravity stretches the star and sucks it into the black hole. The star is torn apart, causing a beacon-like flare that signals a black hole. The possibility of detecting one of these “recoil flares” was first discussed in an August 2008 paper by co-authors Merritt and Komossa.

“The only contact of these floating black holes with the rest of the universe is through their armada of stars,” Merritt says, “with an occasional display of stellar fireworks to signal ‘here we are.’”

Source: Rochester Institute of Technology

Beautiful Chaos

Stephen's Quintet. X-ray (NASA/CXC/CfA/E.O'Sullivan); Optical (Canada-France-Hawaii-Telescope/Coelum)

Can you imagine living in this region of space? Just think of the beautiful views you’d have in the sky – that is, if you survived the chaos as one galaxy is passing through the core of three other galaxies at ridiculous (ludicrous?) speeds (3.2 million km per hour / 2 million miles per hour) generating a shock wave of gas and X-rays.

This is Stephen’s Quintet, A compact group of galaxies, discovered about 130 years ago, located about 280 million light years from Earth. The curved, light blue ridge running down the center of the image shows X-ray data from the Chandra X-ray Observatory. The galaxy in the middle, NGC 7318b is passing through the core of the other galaxies at high speed and is thought to be causing the ridge of X-ray emission by generating a shock wave that heats the gas. The most prominent galaxy in front (NGC 7320) is actually far away from the other galaxies and is not part of the group.

(See the Chandra webpage for a roll-over labeled version)

Additional heating by supernova explosions and stellar winds has also probably taken place in Stephan’s Quintet. A larger halo of X-ray emission – not shown here – detected by ESA’s XMM-Newton could be evidence of shock-heating by previous collisions between galaxies in this group. Some of the X-ray emission is likely also caused by binary systems containing massive stars that are losing material to neutron stars or black holes.

Stephan’s Quintet provides a rare opportunity to observe a galaxy group in the process of evolving from an X-ray faint system dominated by spiral galaxies to a more developed system dominated by elliptical galaxies and bright X-ray emission. Being able to witness the dramatic effect of collisions in causing this evolution is important for increasing our understanding of the origins of the hot, X-ray bright halos of gas in groups of galaxies.

Source: Chandra

Why are Distant Galaxies Moving Away Faster?

Question: Why are more distant galaxies moving away faster?

Answer: As you know, the Universe is expanding after the Big Bang. That means that every part of the Universe was once crammed into a tiny spot smaller than a grain of sand. Then it began expanding, and here we are, 13.7 billion years later with a growing Universe.

The expansive force of dark energy is actually accelerating the expansion even faster. But we won’t bring that in to make things even more complex.

As we look out into the Universe, we see galaxies moving away from us faster and faster. The more distant a galaxy is, the more quickly it’s moving away.

To understand why this is happening, go and get a balloon (or blow one up in your mind). Once you’ve got it blown up a little, draw a bunch of dots on the surface of the balloon; some close and others much further away. Then blow up the balloon more and watch how the dots expand away from each other.

From the perspective of any one dot on the surface of the balloon, the nearby dots aren’t expanding away too quickly, maybe just a few centimeters. But the dots on the other side of the balloon are quite far away. It took the same amount of time for all the dots to change their positions, so the more distant dots appeared to be moving faster.

That’s how it works with the Universe. Because space itself is expanding, the more further a galaxy is, the faster it seems to be receding.

Thanks to Cassandra for the question.

Why are there Black Holes in the Middle of Galaxies?

Question: Why are Black Holes in the Middle of Galaxies?

Answer: The black holes you’re thinking of are known as supermassive black holes. Stellar mass black holes are created when a star at least 5 times larger than the Suns out of fuel and collapses in on itself forming a black hole. The supermassive black holes, on the other hand, can contain hundreds of millions of times the mass of a star like our Sun.

Astronomers are now fairly certain that these supermassive black holes are at the heart of almost every galaxy in the Universe. Furthermore, the mass of these black holes is somehow tied to the mass of the rest of the galaxy. They grown in tandem with each other.

When large quantities of material falls into the black hole, it chokes up, unable to get consumed all at once. This “accretion disk” begins to heat up and blaze brightly in many different wavelengths, including X-rays. When supermassive black holes are actively feeding, astronomers call these quasars.

So how do these black holes get there in the first place? Astronomers aren’t sure, but it could be that the dark matter halo that surrounds every galaxy serves to focus and concentrate material as the galaxy was first forming. Some of this material became the supermassive black hole, while the rest became the stars of the galaxy. It’s also possible that the black hole formed first, and collected the rest of the galaxy around it.

Astronomers just don’t know.

It’s a Bird! It’s Tinker Bell! It’sThree Galaxies!


There once was a galaxy known as ESO 593-IG 008. It was thought to be a relatively mild-mannered galaxy, even though scientists believed it was a collision of two different galaxies; one a barred spiral and the other an irregular galaxy. But now, an international team of astronomers has discovered that it actually is a stunning rare case of three interacting galaxies, with the third galaxy forming stars at a frantic rate.

Using adaptive optics on the European Southern Observatory’s (ESO) Very Large Telescope (VLT), astronomers were able to see through the all-pervasive dust clouds of the object that has been dubbed as “The Bird” because of its resemblance to a winged creature. With the adaptive optics of what’s called the NACO instrument, very fine details were able to be resolved.

“Examples of mergers of three galaxies of roughly similar sizes are rare,” says Petri Vaisanen, lead author of the paper which will appear in the journal of the Royal Astronomical Society. “Only the near-infrared VLT observations made it possible to identify the triple merger nature of the system in this case.”

NACO is the combination of NAOS – Nasmyth Adaptive Optics System that is equipped with both visible and infrared sensors, and CONICA, a Near-Infrared Imager and Spectrograph.

The Anatomy of a Bird.  Image Credit:  ESO
Looking like a bird or a cosmic Tinker Bell, the NACO images show two unmistakable galaxies that form the body and wings of “The Bird.” Astronomers were surprised with the new images that identify a third, clearly separate component that forms the head. This irregular, yet fairly massive galaxy is forming stars violently, at a rate of nearly 200 solar masses per year. It appears to be the major source of infrared luminosity in the system, even though it is the smallest of the three galaxies. The other two galaxies appear to be at a quieter stage of their interaction-induced star formation history. The object is 650 million light years distant but it is quite large with the “wings” alone extending more than 100,000 light-years, or the size of our own Milky Way.

Subsequent optical spectroscopy with the new Southern African Large Telescope, and archive mid-infrared data from the NASA Spitzer space observatory, confirmed the separate nature of the ‘head’, but also added further surprises. The ‘head’ and major parts of the ‘Bird’ are moving apart at more than 400 km/s (1.4 million km/h). Observing such high velocities is very rare in merging galaxies.

“The Bird” belongs to the prestigious family of luminous infrared galaxies, with an infrared luminosity nearly one thousand billion times that of the Sun. This family of galaxies has long been thought to signpost important events in galaxy evolution, such as mergers of galaxies, which in turn trigger bursts of star formation, and may eventually lead to the formation of a single elliptical galaxy.

The galaxy is also designated as IRAS 19115-2124. The ESO is more formally known as the European Organization for Astronomical Research in the Southern Hemisphere.

Original News Source: ESO Press Release: