Gaia Spots an Enormous Ghost Galaxy Right Next Door that’s Being Dismantled by the Milky Way

From left to right: Large Magellanic Cloud, the Milky Way, and Antlia 2, our next door neighbor and so-called ghost galaxy. Credit: V. Belokurov based on the images by Marcus and Gail Davies and Robert Gendler

Astronomers combing through data from the ESA’s Gaia spacecraft have discovered what they’re calling a ghost galaxy. The galaxy, named Antlia 2 (Ant 2) is an extremely low-density dwarf galaxy that was formed in the early days of the universe. And it is being stripped of its mass by the tidal forces of the Milky Way.
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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

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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The Milky Way Could Be Spreading Life From Star to Star

For almost two centuries, scientists have theorized that life may be distributed throughout the Universe by meteoroids, asteroids, planetoids, and other astronomical objects. This theory, known as Panspermia, is based on the idea that microorganisms and the chemical precursors of life are able to survive being transported from one star system to the next.

Expanding on this theory, a team of researchers from the Harvard Smithsonian Center for Astrophysics (CfA) conducted a study that considered whether panspermia could be possible on a galactic scale. According to the model they created, they determined that the entire Milky Way (and even other galaxies) could be exchanging the components necessary for life.

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This Beautiful Photo of Galaxy NGC 3981 was Taken by the Most Powerful Telescope in the World for no Scientific Reason at all. Just Because it’s Pretty

The world’s most powerful telescopes have a lot of work to do. They’re tasked with helping us unravel the mysteries of the universe, like dark matter and dark energy. They’re burdened with helping us find other habitable worlds that might host life. And they’re busy with a multitude of other tasks, like documenting the end of a star’s life, or keeping an eye on meteors that get too close to Earth.

But sometimes, they have to take a break.
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We’re in the Milky Way’s Second Life. Star Formation was Shut Down for Billions of Years

Since the birth of modern astronomy, scientists have sought to determine the full extent of the Milky Way galaxy and learn more about its structure, formation and evolution. According to current theories, it is widely believed that the Milky Way formed shortly after the Big Bang (roughly 13.51 billion years ago). This was the result of the first stars and star clusters coming together, as well as the accretion of gas directly from the Galactic halo.

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

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

Astronomers See a Pileup of 14 Separate Galaxies in the Early Universe

Looking deep into the observable Universe – and hence, back to the earliest periods of time – is an immensely fascinating thing. In so doing, astronomers are able to see the earliest galaxies in the Universe and learn more about how they evolved over time. From this, they are not only able to see how large-scale structures (like galaxies and galaxy clusters) formed, but also the role played by dark matter.

Most recently, an international team of scientists used the Atacama Large Millimeter-submillimeter Array (ALMA) to observe the Universe when it was just 1.4 billion years old. What they observed was a “protocluster”, a series of 14 galaxies located 12.4 billion light-years away that were about to merge. This would result in the formation of a massive galaxy cluster, one of the largest objects in the known Universe.

The study which described their findings, titled “A massive core for a cluster of galaxies at a redshift of 4.3“, recently appeared in the journal Nature. The study was led by Tim Miller – an astronomer from Dalhousie University, Halifax, and Yale University – and included members from NASA’s Jet Propulsion Laboratory, the European Southern Observatory (ESO), Canada’s National Research Council, the Harvard-Smithsonian Center for Astrophysics, the National Radio Astronomy Observatory, and multiple universities and research institutions.

ALMA image of 14 galaxies forming a protocluster known as SPT2349-56. These galaxies are in the process of merging and will eventually form the core of a truly massive galaxy cluster. Credit: ALMA (ESO/NAOJ/NRAO); B. Saxton (NRAO/AUI/NSF)

As they indicate in their study, this protocluster (designated SPT2349-56) was first observed by the National Science Foundation’s South Pole Telescope. Using the Atacama Pathfinder Experiment (APEX), the team conducted follow-up observations that confirmed that it was an extremely distant galactic source, which was then observed with ALMA. Using ALMA’s superior resolution and sensitivity, they were able to distinguish the individual galaxies.

What they found was that these galaxies were forming stars at rate 1,000 times faster than our galaxy, and were crammed inside a region of space that was about three times the size of the Milky Way. Using the ALMA data, the team was also able to create sophisticated computer simulations that demonstrated how this current collection of galaxies will likely grow and evolve over billion of years.

These simulations indicated that once these galaxies merge, the resulting galaxy cluster will rival some of the most massive clusters we see in the Universe today. As Scott Chapman, and astrophysicist at Dalhousie University and a co-author on the study, explained:

“Having caught a massive galaxy cluster in throes of formation is spectacular in and of itself. But, the fact that this is happening so early in the history of the universe poses a formidable challenge to our present-day understanding of the way structures form in the universe.”

Zooming in to the galaxies discovered by ALMA that are evolving into a galaxy cluster. Credit: ALMA (ESO/NAOJ/NRAO), T. Miller & S. Chapman et al.; Herschel; South Pole Telescope; (NRAO/AUI/NSF) B. Saxton

The current scientific consensus among astrophysicists states that a few million years after the Big Bang, normal matter and dark matter began to form larger concentrations, eventually giving rise to galaxy clusters. These objects are the largest structures in the Universe, containing trillions of stars, thousands of galaxies, immense amounts of dark matter and massive black holes.

However, current theories and computer models have suggested that protoclusters – like the one observed by ALMA – should have taken much longer to evolve. Finding one that dates to just 1.4 billion years after the Big Bang was therefore quite the surprise. As Tim Miller, who is currently a doctoral candidate at Yale University, indicated:

“How this assembly of galaxies got so big so fast is a bit of a mystery, it wasn’t built up gradually over billions of years, as astronomers might expect. This discovery provides an incredible opportunity to study how galaxy clusters and their massive galaxies came together in these extreme environments.”

Looking to the future, Chapman and his colleagues hope to conduct further studies of SPT2349-56 to see how this protoclusters eventually became a galaxy cluster. “ALMA gave us, for the first time, a clear starting point to predict the evolution of a galaxy cluster,” he said. “Over time, the 14 galaxies we observed will stop forming stars and will collide and coalesce into a single gigantic galaxy.”

The study of this and other protoclusters will be made possible thanks to instruments like ALMA, but also next-generation observatories like the Square Kilometer Array (SKA). Equipped with more sensitive arrays and more advanced computer models, astronomers may be able to create a truly accurate timeline of how our Universe became what it is today.

Further Reading: NRAO, Nature

Hubble Finds a Galaxy with Almost no Dark Matter

Since the 1960s, astrophysicists have postulated that in addition to all the matter that we can see, the Universe is also filled with a mysterious, invisible mass. Known as “Dark Matter”, it’s existence was proposed to explain the “missing mass” of the Universe, and is now considered a fundamental part of it. Not only is it theorized to make up about 80% of the Universe’s mass, it is also believed to have played a vital role in the formation and evolution of galaxies.

However, a recent finding may throw this entire cosmological perspective sideways. Based on observations made using the NASA/ESA Hubble Space Telescope and other observatories around the world, astronomers have found a nearby galaxy (NGC 1052-DF2) that does not appear to have any dark matter. This object is unique among galaxies studied so far, and could force a reevaluation of our predominant cosmological models.

The study which details their findings, titled “A galaxy lacking dark matter“, recently appeared in the journal Nature. Led by Pieter van Dokkum of Yale University, the study also included members from the Max Planck Institute for Astronomy, San Jose State University, the University of California Observatories, the University of Toronto, and the Harvard-Smithsonian Center for Astrophysics

Image of the ultra diffuse galaxy NGC 1052-DF2, created from images forming part of the Digitized Sky Survey 2. Credit:ESA/Hubble, NASA, Digitized Sky Survey 2. Acknowledgement: Davide de Martin

For the sake of their study, the team consulted data from the Dragonfly Telephoto Array (DFA), which was used to identify NGC 1052-DF2. Based on data from Hubble, the team was able to determined its distance – 65 million light-years from the Solar System – as well as its size and brightness. In addition, the team discovered that NGC 1052-DF52 is larger than the Milky Way but contains about 250 times fewer stars, which makes it an ultra diffuse galaxy.

As van Dokkum explained, NGC 1052-DF2 is so diffuse that it’s essentially transparent. “I spent an hour just staring at this image,” he said. “This thing is astonishing: a gigantic blob so sparse that you see the galaxies behind it. It is literally a see-through galaxy.”

Using data from the Sloan Digital Sky Survey (SDSS), the Gemini Observatory, and the Keck Observatory, the team studied the galaxy in more detail. By measuring the dynamical properties of ten globular clusters orbiting the galaxy, the team was able to infer an independent value of the galaxy’s mass – which is comparable to the mass of the stars in the galaxy.

This led the team to conclude that either NGC 1052-DF2 contains at least 400 times less dark matter than is predicted for a galaxy of its mass, or none at all. Such a finding is unprecedented in the history of modern astronomy and defied all predictions. As Allison Merritt – an astronomer from Yale University, the Max Planck Institute for Astronomy and a co-author on the paper – explained:

“Dark matter is conventionally believed to be an integral part of all galaxies — the glue that holds them together and the underlying scaffolding upon which they are built… There is no theory that predicts these types of galaxies — how you actually go about forming one of these things is completely unknown.”

“This invisible, mysterious substance is by far the most dominant aspect of any galaxy. Finding a galaxy without any is completely unexpected; it challenges standard ideas of how galaxies work,” added van Dokkum.

However, it is important to note that the discovery of a galaxy without dark matter does not disprove the theory that dark matter exists. In truth, it merely demonstrates that dark matter and galaxies are capable of being separate, which could mean that dark matter is bound to ordinary matter through no force other than gravity. As such, it could actually help scientists refine their theories of dark matter and its role in galaxy formation and evolution.

In the meantime, the researchers already have some ideas as to why dark matter is missing from NGC 1052-DF2. On the one hand, it could have been the result of a cataclysmic event, where the birth of a multitude of massive stars swept out all the gas and dark matter. On the other hand, the growth of the nearby massive elliptical galaxy (NGC 1052) billions of years ago could have played a role in this deficiency.

However, these theories do not explain how the galaxy formed. To address this, the team is analyzing images that Hubble took of 23 other ultra-diffuse galaxies for more dark-matter deficient galaxies. Already, they have found three that appear to be similar to NGC 1052-DF2, which could indicate that dark-matter deficient galaxies could be a relatively common occurrence.

If these latest findings demonstrate anything, it is that the Universe is like an onion. Just when you think you have it figured out, you peal back an additional layer and find a whole new set of mysteries. They also demonstrate that after 28 years of faithful service, the Hubble Space Telescope is still capable of teaching us new things. Good thing too, seeing as the launch of its successor has been delayed until 2020!

Further Reading: Hubble Space Telescope

Maybe There’s no Connection Between Supermassive Black Holes and Their Host Galaxies?

For decades, astrophysicists have puzzled over the relationship between Supermassive Black Holes (SMBHs) and their respective galaxies. Since the 1970s, it has been understood the majority of massive galaxies have an SMBH at their center, and that these are surrounded by rotating tori of gas and dust. The presence of these black holes and tori are what cause massive galaxies to have an Active Galactic Nucleus (AGN).

However, a recent study conducted by an international team of researchers revealed a startling conclusion when studying this relationship. Using the Atacama Large Millimeter/submillimeter Array (ALMA) to observe an active galaxy with a strong ionized gas outflow from the galactic center, the team obtained results that could indicate that there is no relationship between a an SMBH and its host galaxy.

The study, titled “No sign of strong molecular gas outflow in an infrared-bright dust-obscured galaxy with strong ionized-gas outflow“, recently appeared in the Astrophysical Journal. The study was led by Yoshiki Toba of the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan and included members from Ehime University, Kogakuin University, and the National Astronomical Observatory of Japan, The Graduate University for Advanced Studies (SOKENDAI), and Johns Hopkins University.

Images from the Sloan Digital Sky Survey (SDSS) (left), and mid-infrared image from WISE (right), respectively. Credit: Sloan Digital Sky Survey/NASA/JPLCaltech

The question of how SMBHs have affected galactic evolution remains one of the greatest unresolved questions in modern astronomy. Among astrophysicists, it is something of a foregone conclusion that SMBHs have a significant impact on the formation and evolution of galaxies. According to this accepted notion, SMBHs significantly influence the molecular gas in galaxies, which has a profound effect on star formation.

Basically, this theory holds that larger galaxies accumulate more gas, thus resulting in more stars and a more massive central black hole. At the same time, there is a feedback mechanism, where growing black holes accrete more matter on themselves. This results in them sending out a tremendous amount of energy in the form of radiation and particle jets, which is believed to curtail star formation in their vicinity.

However, when observing an infrared (IR)-bright dust-obscured galaxy (DOG) – WISE1029+0501 – Yoshiki and his colleagues obtained results that contradicted this notion. After conducting a detailed analysis using ALMA, the team found that there were no signs of significant molecular gas outflow coming from WISE1029+0501. They also found that star-forming activity in the galaxy was neither more intense or suppressed.

This indicates that a strong ionized gas outflow coming from the SMBH in WISE1029+0501 did not significantly affect the surrounding molecular gas or star formation. As Dr. Yoshiki Toba explained, this result:

“[H]as made the co-evolution of galaxies and supermassive black holes more puzzling. The next step is looking into more data of this kind of galaxies. That is crucial for understanding the full picture of the formation and evolution of galaxies and supermassive black holes”.

Emission from Carbon Monoxide (Left) and Cold Dust (Right) in WISE1029 Observed by ALMA (image). Credit: ALMA (ESO/NAOJ/NRAO), Toba et al.

This not only flies in the face of conventional wisdom, but also in the face of recent studies that showed a tight correlation between the mass of central black holes and those of their host galaxies. This correlation suggests that supermassive black holes and their host galaxies evolved together over the course of the past 13.8 billion years and closely interacted as they grew.

In this respect, this latest study has only deepened the mystery of the relationship between SMBHs and their galaxies. As Tohru Nagao, a Professor at Ehime University and a co-author on the study, indicated:

“[W]e astronomers do not understand the real relation between the activity of supermassive black holes and star formation in galaxies. Therefore, many astronomers including us are eager to observe the real scene of the interaction between the nuclear outflow and the star-forming activities, for revealing the mystery of the co-evolution.”

The team selected WISE1029+0501 for their study because astronomers believe that DOGs harbor actively growing SMBHs in their nuclei. In particular, WISE1029+0501 is an extreme example of galaxies where outflowing gas is being ionized by the intense radiation from its SMBH. As such, researchers have been highly motivated to see what happens to this galaxy’s molecular gas.

Artist’s impression of the black hole wind at the center of a galaxy. Credit: ESA

The study was made possible thanks to ALMA’s sensitivity, which is excellent when it comes to investigating the properties of molecular gas and star-forming activity in galaxies. In fact, multiple studies have been conducted in recent years that have relied on ALMA to investigate the gas properties and SMBHs of distant galaxies.

And while the results of this study contradict widely-held theories about galactic evolution, Yoshiki and his colleagues are excited about what this study could reveal. In the end, it may be that radiation from a SMBH does not always affect the molecular gas and star formation of its host galaxy.

“[U]nderstanding such co-evolution is crucial for astronomy,” said Yoshiki. “By collecting statistical data of this kind of galaxies and continuing in more follow-up observations using ALMA, we hope to reveal the truth.”

Further Reading: ALMA Observatory, Astrophysical Journal