Not All Black Holes are Ravenous Gluttons

This artist’s impression shows the record-breaking quasar J059-4351, the bright core of a distant galaxy that is powered by a supermassive black hole. The light comes from gas and dust that's heated up before it's drawn into the black hole. Credit: ESO/M. Kornmesser

Some Supermassive Black Holes (SMBHs) consume vast quantities of gas and dust, triggering brilliant light shows that can outshine an entire galaxy. But others are much more sedate, emitting faint but steady light from their home in the heart of their galaxy.

Observations from the now-retired Spitzer Space Telescope help show why that is.

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Are Andromeda and the Milky Way Already Exchanging Stars?

Artist's illustration of Andromeda/Milky Way Merger. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

I often drag out the amazing fact that the Andromeda Galaxy, that faint fuzzy blob just off the corner of the Square of Pegasus, is heading straight for us! Of course I continue to tell people it won’t happen for a few billion years yet but a recent study suggests that we are already seeing hypervelocity stars that have been ejected from Andromeda already. It is just possible that the two galaxies have already started to exchange stars long before they are expected to merge. 

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We Might Be Able to Measure Dark Energy Through the Milky Way's Collision With Andromeda

How the sky might look when the Andromeda galaxy approaches the Milky Way. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

The Universe is expanding, and it’s doing so at an ever-increasing pace. Whether due to a dark energy field throughout the cosmos or due to a fundamental of spacetime itself, the cosmos is stretching the space between distant galaxies. But nearby galaxies, those part of our local group, are moving closer together. And how they are falling toward each other could tell us about the nature of cosmic expansion.

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Astronomers Uncover Mass Migration of Stars into Andromeda

Astronomers at NSF’s NOIRLab found new evidence for a mass immigration of stars into the Andromeda Galaxy. This image shows individual stars from blue (moving toward us) to red (moving away from us). Image Credit: KPNO/NOIRLab/AURA/NSF/E. Slawik/D. de Martin/M. Zamani

Astronomers know that galaxies grow over time through mergers with other galaxies. We can see it happening in our galaxy. The Milky Way is slowly absorbing the Large and Small Magellanic Clouds and the Sagittarius Dwarf Spheroidal Galaxy.

For the first time, astronomers have found evidence of an ancient mass migration of stars into another galaxy. They spotted over 7,000 stars in Andromeda (M31), our nearest neighbour, that merged into the galaxy about two billion years ago.

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A Dark Matter map of our Local Cosmic Neighborhood

Simulation of dark matter and gas. Credit: Illustris Collaboration (CC BY-SA 4.0)

Since it was first theorized in the 1970s, astrophysicists and cosmologists have done their best to resolve the mystery that is Dark Matter. This invisible mass is believed to make up 85% of the matter in the Universe and accounts for 27% of its mass-energy density. But more than that, it also provides the large-scale skeletal structure of the Universe (the cosmic web), which dictates the motions of galaxies and material because of its gravitational influence.

Unfortunately, the mysterious nature of Dark Matter means that astronomers cannot study it directly, thus prevented them from measuring its distribution. However, it is possible to infer its distribution based on the observable influence its gravity has on local galaxies and other celestial objects. Using cutting-edge machine-learning techniques, a team of Korean-American astrophysicists was able to produce the most detailed map yet of the local Universe that shows what the “cosmic web” looks like.

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Hubble Shows the True Size of Andromeda

This illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo. These quasars—the very distant, brilliant cores of active galaxies powered by black holes—are scattered far behind the halo, allowing scientists to probe multiple regions. Looking through the immense halo at the quasars’ light, the team observed how this light is absorbed by the halo and how that absorption changes in different regions. By tracing the absorption of light coming from the background quasars, scientists are able to probe the halo’s material. Image Credit: NASA, ESA, and E. Wheatley (STScI)

It’s possible that you’ve seen the Andromeda galaxy (M31) without even realizing it. The massive spiral galaxy appears as a grey, spindle-shaped blob in the night sky, visible with the naked eye in the right conditions. It’s the nearest major galaxy to ours, and astronomers have studied it a lot.

Now astronomers have used the Hubble Space Telescope to map out Andromeda’s enormous halo of hot gas.

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Now We Know That Dark Matter Isn’t Primordial Black Holes

The early universe. Credit: Tom Abel & Ralf Kaehler (KIPACSLAC)/ AMNH/NASA

For over fifty years, scientists have theorized that roughly 85% of matter in the Universe’s is made up of a mysterious, invisible mass. Since then, multiple observation campaigns have indirectly witnessed the effects that this “Dark Matter” has on the Universe. Unfortunately, all attempts to detect it so far have failed, leading scientists to propose some very interesting theories about its nature.

One such theory was offered by the late and great Stephen Hawking, who proposed that the majority of dark matter may actually be primordial black holes (PBH) smaller than a tenth of a millimeter in diameter. But after putting this theory through its most rigorous test to date, an international team of scientists led from the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) has confirmed that it is not.

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This is What It’ll Look Like When the Milky Way and Andromeda Galaxies Collide Billions of Years from Now

Located in the constellation of Hercules, about 230 million light-years away, NGC 6052 is a pair of colliding galaxies. Image Credit: ESA/Hubble & NASA, A. Adamo et al.
Located in the constellation of Hercules, about 230 million light-years away, NGC 6052 is a pair of colliding galaxies. Image Credit: ESA/Hubble & NASA, A. Adamo et al.

What happens when two galaxies collide? The Milky Way and the Andromeda Galaxy are on a collision course, and in about 4.5 billion years, they will meet. Now astronomers using the Hubble have provided some visual insight into what that collision might look like.

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Has a New Dwarf Galaxy Been Found Hiding Behind Andromeda?

The Andromeda Galaxy will collide with the Milky Way in the future. Credit: Adam Evans
The Andromeda Galaxy. Credit: Adam Evans

One of the greatest challenges of astronomy is locating objects in space that are obscured by the light of nearby, brighter objects. In addition to making extra-solar planets very difficult to directly image, this problem also intrudes on surveys of the local Universe, where astronomers are unable to detect dwarf and isolated galaxies because of all the brighter ones surrounding them.

Because of this, astronomers are unable to do a full inventory of small galaxies in the volume of space surrounding the Milky Way (aka. the Local Volume). However, thanks to the efforts of an amateur astronomer and an international team of scientists, a dwarf spheroidal galaxy was recently discovered lurking behind the Andromeda Galaxy. The discovery of this object, named Donatiello I, could help astronomers learn more about the process of galaxy formation.

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Andromeda Shredded and Consumed a Massive Galaxy About Two Billion Years Ago

Andromeda Galaxy. Credit: Wikipedia Commons/Adam Evans

Scientists have long understood that in the course of cosmic evolution, galaxies become larger by consuming smaller galaxies. The evidence of this can be seen by observing galactic halos, where the stars of cannibalized galaxies still remain. This is certainly true of the Andromeda Galaxy (aka. M31, Earth’s closest neighbor) which has a massive and nearly-invisible halo of stars that is larger than the galaxy itself.

For some time, scientists believed that this halo was the result of hundreds of smaller mergers. But thanks to a new study by a team of researchers at the University of Michigan, it now appears that Andromeda’s halo is the result of it cannibalizing a massive galaxy some two billion years ago. Studying the remains of this galaxy will help astronomers understand how disk galaxies (like the Milky Way) evolve and survive large mergers.

The study, titled “The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor“, recently appeared in the scientific journal Nature. The study was conducted by Richard D’Souza, a postdoctoral researcher at the University of Michigan and the Vatican Observatory; and Eric F. Bell, the Arthur F. Thurnau Professor at the University of Michigan.

In this image, the Andromeda galaxy shreds the large galaxy M32p, which eventually resulted in M32 and a giant halo of stars. Credit: Richard D’Souza. Credit: AAS/IOP/Wei-Hao Wang

Using computer models, Richard D’Souza and Eric Bell were able to piece together how a once-massive galaxy (named M32p) disrupted and eventually came to merge with Andromeda. From their simulations, they determined that M32p was at least 20 times larger than any galaxy that has merged with the Milky Way over the course of its lifetime.

M32p would have therefore been the third-largest member of the Local Group of galaxies, after the Milky Way and Andromeda galaxies, and was therefore something of a “long-lost sibling”. However, their simulations also indicated that many smaller companion galaxies merged with Andromeda over time. But for the past, Andromeda’s halo is the result of a single massive merger. As D’Souza explained in a recent Michigan News press statement:

“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. 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.”

This study will not only help astronomers understand how galaxies like the Milky Way and Andromeda grew through mergers, it might also shed light on a long-standing mystery – which is how Andromeda’s satellite galaxy (M32) formed. According to their study, D’Souza and Bell believe that M32 is the surviving center of M32p, which is what remained after its spiral arms were stripped away.

Messier 31 (the Andromeda Galaxy), along with Messier 32 and Messier 110. Credit: Wikisky

“M32 is a weirdo,” said Bell. “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.” According to D’Souza and Bell, this study may also alter the traditional understanding of how galaxies evolve. In astronomy, conventional wisdom says that large interactions would destroy disk galaxies and form elliptical galaxies.

But if Andromeda did indeed survive an impact with a massive galaxy, it would indicate that this is not the case. The timing of the merger may also explain recent research findings which indicated that two billion years ago, the disk of the Andromeda galaxy thickened, leading to a burst in star formation. As Bell explained:

“The Andromeda Galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago. 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.”

In the end, this method could also be used to study other galaxies and determine which were the most massive mergers they underwent. This could allow scientists to better understand the complicated process that drives galaxy growth and how mergers affect galaxies. This knowledge will certainly come in handy when it comes to determining what will happen to our galaxy when it merges with Andromeda in a few billion years.

Further Reading: Michigan News