Tag Archives: andromeda

The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.

Supermassive black hole pairs imaged in merging galactic cores

For the first time, astronomers have observed pairs of galaxies in the final stages of merging into a single, larger galactic body. The findings suggest that such events are more common than astronomers used to think.

The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.

The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.

Astronomers suspect that supermassive black holes lurk at the heart of every sizable galaxy, holding the galactic fiber together. For instance, at the galactic core of the Milky Way, there should be a supermassive black hole called Sagittarius A*, a staggering 4.5 million solar masses in size.

Black holes likely reach this sort of dizzying size through the merger of galaxies. However, evidence has been conflicting due to a lack of direct imaging of the process, which is obscured by swirling clouds of gas and dust. What’s more, simulations show that the more these galaxies progress in their merger, the greater the concealment effect.

To peer through all the matter that obscures supermassive black holes, scientists combed through a huge catalog of 10 years’ worth of X-ray measurements taken by the Burst Alert Telescope (BAT) aboard NASA’s Neil Gehrels Swift Observatory.

“The advantage to using Swift’s BAT is that it observes high-energy, ‘hard’ X-rays,” said study co-author Richard Mushotzky, a professor of astronomy at the University of Maryland. “These X-rays penetrate through the thick clouds of dust and gas that surround active galaxies, allowing the BAT to see things that are literally invisible in other wavelengths.”

Next, the research team analyzed another catalog of galaxies from NASA’s Hubble Space Telescope and the Keck Observatory in Hawaii whose X-ray signatures matched the Swift readings.

Various colliding galaxies along with closeup views of their coalescing nuclei in the bright cores. The images were taken in near-infrared light by the Keck Observatory in Hawaii. Credit: Keck images: W. M. Keck Observatory and M. Koss.

Various colliding galaxies along with closeup views of their coalescing nuclei in the bright cores. The images were taken in near-infrared light by the Keck Observatory in Hawaii. Credit: Keck images: W. M. Keck Observatory and M. Koss.

The breakthrough lied with the Keck Observatory’s adaptive optics technology whose deformable mirrors controlled by a computer enable a phenomenal increase in resolution. Using this tech, researchers were able to produce extremely sharp, near-infrared images of X-ray-producing black holes not found in the Hubble archive.

“People had conducted studies to look for these close interacting black holes before, but what really enabled this particular study were the X-rays that can break through the cocoon of dust,” explained Koss. “We also looked a bit farther in the universe so that we could survey a larger volume of space, giving us a greater chance of finding more luminous, rapidly-growing black holes.”

In total, 96 galaxies were observed with the Keck telescope and 385 galaxies from the Hubble archive. According to the results, 17 percent of these galaxies host a pair of black holes at their center, which are locked in the late stages of a galactic merger. This was surprising to learn since previously simulations suggested that black hole pairs spend very little time in this phase.

“Seeing the pairs of merging galaxy nuclei associated with these huge black holes so close together was pretty amazing,” said Michael Koss, co-author of the new study published in the journal Nature. “In our study, we see two galaxy nuclei right when the images were taken. You can’t argue with it; it’s a very ‘clean’ result, which doesn’t rely on interpretation.”

The findings suggest that galactic mergers may indeed be a key process by which black holes grow to stupendous masses. And the stages that the astronomers have mapped out in this study likely foretell the fate of our very own galaxy. In about 6 billion years, scientists estimate that the Milky Way will merge with the Andromeda galaxy into one big galaxy.

There is still much to learn about black hole mergers, though. Right now, hardware is a huge limitation, but, once the James Webb Space Telescope is deployed in 2021, scientists will be able to measure masses, growth rates, and other physical parameters of black hole pairs.

The Andromeda Galaxy. Credit: Wikimedia Commons.

The Milky Way once had a large sister galaxy — but Andromeda devoured it

The Andromeda Galaxy. Credit: Wikimedia Commons.

The Andromeda Galaxy. Credit: Wikimedia Commons.

Billions of years ago, the Milky Way once had a huge sister galaxy, according to a surprising new study. Astronomers found that this long-lost sibling was devoured by the Andromeda galaxy, the largest galaxy in the Local Group and our closest galactic neighbor — and we’re next!

“Astronomers have been studying the Local Group — the Milky Way, Andromeda and their companions — for so long,” study co-author Eric Bell, a professor of astronomy at the University of Michigan (UM), said in a statement. “It was shocking to realize that the Milky Way had a large sibling, and we never knew about it.”

Androdema, also known as M32, is considered to be the largest galaxy in a group of 54 galaxies, which astronomers refer to as the Local Group. However, a recent study published this year, which used a new tool to measure the galaxy, suggests that Andromeda is about the same size as the Milky Way, which is traditionally seen as the second largest in the galaxy group.

In any event, Andromeda is huge — and it didn’t reach its king-sized status by accident. Astronomers think that during its rich history, Andromeda has collided, shredded, and appropriated hundreds of smaller galaxies.

It’s almost impossible to discern what happened to each of these lost galaxies. However, Bell and colleagues were lucky — they found that a faint halo of stars in the outer reaches of Andromeda mostly got there as a result of the shredding of a single large galaxy. Simulations that backtrack the collision in time suggest that about two billion years ago Andromeda must have merged with the onetime third-biggest member of the Local Group. The timing seems right too — a team of French researchers independently determined earlier this year that Andromeda likely underwent an important merger between 1.8 billion and 3 billion years ago.

“It was a ‘eureka’ moment. We realized we could use this information of Andromeda’s outer stellar halo to infer the properties of the largest of these shredded galaxies,” said lead author Richard D’Souza, a postdoctoral researcher at UM.

This galaxy, called M32p, which was shredded by the Andromeda galaxy, was at least 20 times larger than any galaxy which merged with the Milky Way over the course of its lifetime. M32p must have been massive, possibly once being at some point the third largest galaxy in the Local Group after the Andromeda and the Milky Way galaxies.

The devoured galaxy, called M32p, isn’t entirely gone. Instead, the astronomers think that Andromeda’s enigmatic M32 satellite galaxy is actually the surviving center of the Milky Way’s long-lost sibling — the remnants of a galactic corpse. Previously, scientists had no clue how M32, with its many contradictory features, could have surfaced. M32 is the smallest galaxy in the Messier catalog: just 6,500 light years across, with ~3 billion solar masses of material.

“M32 is a weirdo,” Bell continued. “While it looks like a compact example of an old, elliptical galaxy, it actually has lots of young stars. It’s one of the most compact galaxies in the universe. There isn’t another galaxy like it.”

The findings will help scientists improve their basic understanding of how galaxies form and evolve. For instance, it was previously thought that huge, dramatic crashes destroy the disks of spiral galaxies, turning them into the boring elliptical variety. However, Andromeda still retains its spiral shape, showing that this assertion is no rule.

“The Andromeda Galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago,” Bell said. “When I was at graduate school, I was told that understanding how the Andromeda Galaxy and its satellite galaxy M32 formed would go a long way towards unraveling the mysteries of galaxy formation.”

Perhaps most importantly, the Andromeda-M32p collision will teach us what to expect from the galactic cannibal when it will have a taste of its largest meal yet — the Milky Way. The two galaxies will likely collide four billion years from now in an epic clash of the titans that will light the sky in other worlds that are far away enough from the mayhem.

A series of stills showing the Milky Way-Andromeda merger, and how the sky will appear different from Earth as it happens. Credit: NASA; Z. LEVAY AND R. VAN DER MAREL, STSCI; T. HALLAS; AND A. MELLINGER.

A series of stills showing the Milky Way-Andromeda merger, and how the sky will appear different from Earth as it happens. Credit: NASA; Z. LEVAY AND R. VAN DER MAREL, STSCI; T. HALLAS; AND A. MELLINGER.

The findings were reported in the journal Nature Astronomy.

Andromeda Galaxy.

Two billion years ago, Andromeda ‘ate’ a sister-galaxy of the Milky Way

Our closest galactic neighbor, Andromeda, seems to like the taste of its brethren.

Andromeda Galaxy.

The Andromeda Galaxy imaged through a hydrogen-alpha filter.
Image credits Adam Evans.

Researchers from the University of Michigan (UoM) report that the Andromeda galaxy smote and consumed one of its brethren some two billion years ago. Although its victim was shredded almost completely, the team pieced together evidence of the collision from the thin halo of stars that spans the gap between Andromeda and its enigmatic companion, Meiser 32 (M32).

The discovery helps further our understanding of how galaxies like the Milky Way evolve, and of their behavior during large mergers.

Family dinner

Our own galaxy, the Milky Way, and our closest neighbor, Andromeda, are the two largest members of a group known as the Local Group (of galaxies). The extended family includes some 54 different galaxies — most of them dwarf galaxies acting as satellites for their larger relatives — all orbiting around a point roughly between Andromeda and the Milky Way.

It may sound idyllic, but researchers have found that at least one member of this group found its demise at the hands of Andromeda. This once-galaxy, christened M32p, was the third-largest member of the Local Group — a distinction that now falls on the galaxy Triangulum.

The team started their research using data pertaining to the halo of stars around Andromeda. It’s not a unique feature; many galaxies harbor such wispy-thin groupings of stars around their bulk, the final remnants of smaller galaxies that they absorbed over time. Since Andromeda is so large and rich in matter (it has over double the diameter of the Milky Way and double its number of stars), the researchers expected it to have consumed hundreds of smaller galaxies — which they thought would make it impossible to study a single such meal.


Size comparison between M32p and today’s M32.
Image credits Richard D’Souza; for the image of M64: NOAO/AURA/NSF.

However, the team’s computer simulations revealed that although Andromeda did dine on many of its companion galaxies, most stars in the outer halo originate from a single, large galaxy. Piecing the evidence together to peer back in time, the team found that M32p would have been massive — likely the third-largest in the Local Group, after Andromeda and the Milky Way. The paper adds that M32p was at least 20 times larger than any galaxy the Milky Way ever merged with.

“The stars in Andromeda are very metal-rich and considerably young,” Richard D’Souza, lead author of the paper, explained in an e-mail. “In general, the larger the galaxy the more metal rich the stars are. We suspected that since the stars in the halo of Andromeda were so metal-rich, it must have come from a large metal-rich galaxy.”

One big bite

A metal-rich halo large enough to encompass a galaxy such as Andromeda could only be formed “through a single large merger,” he adds, noting that “there are not many smaller galaxies in the Universe to build up to the mass of the halo”.

“In terms of a business analogy, galaxies also grow through mergers and acquisitions. In order for a major company to grow at a very fast pace, it would need to acquire a similar large company into its business. Such was the case with Andromeda,” D’Souza adds.

The findings call into question our models of how mergers between two massive galaxies play out. Until now, astronomers believed that such an event would flatten the disk of a spiral galaxy into an elliptical one, but Andromeda’s disk evidently pulled through still very spiral-shaped. Some effects of this collision can still be seen, D’Souza told me. Among them are the thickness of Andromeda’s disk and the higher speeds its stars travel at (90 km/s compared to around 30 km/s in the Milky Way).

Collision path.

The process of shredding of the large galaxy M32p by the Andromeda (M31) galaxy which eventually resulted in M32 and a giant halo of stars.
Image credits Richard D’Souza; M31, courtesy of Wei-Hao Wang; Stellar halo of M31: AAS/IOP.

Still, he admits that it came as “a major surprise” that Andromeda could retain its spiral shape following this collision. One explanation could be that the particular angle of the collision between the two galaxies helped keep Andromeda spiral-like, “but we need to run more computer simulations to see which set of orbits helps preserve the disk”.

Beyond this, it helps us better understand Andromeda’s evolution over time. The timing of the merger coincides with a burst of intense star formation in Andromeda two billion years ago. All this star-forming activity also suggests that M32p must have been gas-rich in order to supply enough building blocks.

Finally, the findings point to Andromeda’s mysterious, compact, and very dense, satellite galaxy M32 (the one today) as the last sliver of the once-mighty galaxy — the naked core. This piece of data could help explain why we see so few galaxies similar to M32 zipping around in the universe.

“M32 is a weirdo,” co-author Eric Bell, UoM professor of astronomy, said in a press release. “While it looks like a compact example of an old, elliptical galaxy, it actually has lots of young stars. It’s one of the most compact galaxies in the universe. There isn’t another galaxy like it.”

“Galaxies like M32 are considerably rare in the Universe,” D’Souza adds. “The term used for them in the literature is called ‘compact ellipticals’, and they are one of the most rarest galaxies in the Universe. We do know a dozen or so compact ellipticals in the nearby Universe, and we have inferred that further out (where we cannot resolve them), the number is equally low.”

As part of the paper, the team also found that the merger scenario could help explain the scarcity of M32-like objects. It seems the secret is not just in the merging process itself, but also in the particular makeup of the galaxies involved. “What one really needs is a galaxy with a high central surface density of stars comparable to M32,” D’Souza explains. It seems to be quite a rare occurrence — the team only identified 8 potential progenitors for M32-like objects.

Their study may alter the traditional understanding of how galaxies evolve, the researchers say. The realization that Andromeda’s disk survived an impact with a massive galaxy flies in the face of our current models, which suggests that such large interactions would destroy disks and form an elliptical galaxy.

It went so fundamentally against the grain of our understanding of galaxy-formation that, previously, we didn’t even consider the possibility that this scenario could have ever occurred.

“Astronomers have been studying the Local Group–the Milky Way, Andromeda and their companions–for so long. It was shocking to realize that the Milky Way had a large sibling, and we never knew about it,” Bell concludes.

Such investigative methods can be applied to other galaxies as well, the team explains, to help us tease out the merger history of other galaxies besides Andromeda.

The paper “The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor” has been published in the journal Nature Astronomy.

Hubble Discovers Huge Halo Around Andromeda Galaxy

In an article published in the Astrophysical Journal last week, astronomers described a massive halo around the Andromeda Galaxy, extending up close to Earth. The team spotted the halo through NASA’s Hubble Space Telescope and consider it one of the galaxy’s most important features.

The Andromeda Galaxy seen in infrared by the Spitzer Space Telescope, one of NASA’s four Great Space Observatories. Image via Wikipedia.

“Halos are the gaseous atmospheres of galaxies,” Lehner said in a statement released by Notre Dame News. “The properties of these gaseous halos control the rate at which stars form in galaxies.”

The Andromeda Galaxy is located about 2.5 million light years from our own, and it is the nearest galaxy to the Milky Way. It’s the largest galaxy in the Local Group. The halo itself is huge, extending one million light years from the galaxy, containing about as much mass as half the stars in Andromeda.

So how is it that we didn’t discover something as huge as this until now? Well, as the team explains, the gas in the halo is invisible, so they had to rely on an indirect method to view it, using light from 18 different quasars. Quasars are a class of amazing objects – compact regions in the center of a massive galaxy surrounding a central supermassive black hole. They rotate at high speeds and emit electromagnetic radiation which can be detected when they are faced towards the Earth. Imagine a rotating lighthouse – you can see its light when it’s facing you.

“As the light from the quasars travels toward Hubble, the halo’s gas will absorb some of that light and make the quasar appear a little darker in just a very small wavelength range,” the study’s co-author J. Christopher Howk said.

Halos have been observed before, but never before has one so big been imaged. We don’t know if the Milky Way also has a halo; if it does, then the two halos might merge sooner than the two galaxies, and the consequences are still not yet understood.

If you’re worried about the collision of the two galaxies, then you should know that’s only going to happen about 4 billion years from now.

Milky Way spins faster

Milky Way

Well ladies and gentlemen, you’d better fasten your seat belts, because we’re in for quite a spin. The Milky Way is spinning a bit faster than scientists believed; with about 100.000 miles / hour that is. What does that mean, basically?? Well, we’re faster, heavier, and there’s a bigger chance that we’ll collide with… something. It won’t be good, anyway.

According to Mark Reid, of the Harvard-Smithsonian Center for Astrophysics, that means that the mass of our galaxy increases with about 50%, which is about as heavy as the Andromeda galaxy is (or as much as we believe it is).

“No longer will we think of the Milky Way as the little sister of the Andromeda Galaxy in our Local Group family.”

To explain this as simple as possible, at this kind of speeds which are relatively close to that of the light, mass increases significantly as the speed increases. As a consequence, the gravitational pull (the attraction force) increases too, meaning the probability of a collision with the Andromeda galaxy or other smaller galaxies increases as well.

The measurements seem to be more precise than anything done before.

“The new VLBA observations of the Milky Way are producing highly-accurate direct measurements of distances and motions,” said Karl Menten of the Max Planck Institute for Radio Astronomy in Germany, a member of the team. “These measurements use the traditional surveyor’s method of triangulation and do not depend on any assumptions based on other properties, such as brightness, unlike earlier studies.”
“These direct measurements are revising our understanding of the structure and motions of our Galaxy,” Menten said. “Because we’re inside it, it’s difficult for us to determine the Milky Way’s structure. For other galaxies, we can simply look at them and see their structure, but we can’t do this to get an overall image of the Milky Way. We have to deduce its structure by measuring and mapping,” he added.