Finding explanation for Milky Way's warp

The Milky Way is often depicted as a flat, spinning disk of dust, gas, and stars. But if you could zoom out and take an edge-on photo, it actually has a distinctive warp -- as if you tried to twist and bend a vinyl LP.

Though scientists have long known through observational data that the Milky Way is warped and its edges are flared like a skirt, no one could explain why.

Now, Harvard astronomers at the Center for Astrophysics | Harvard and Smithsonian (CfA) have performed the first calculations that fully explain this phenomenon, with compelling evidence pointing to the Milky Way's envelopment in an off-kilter halo of dark matter. The work also bolsters current thinking about how the galaxy evolved and may offer clues into some of the mysteries of dark matter.

The new calculations were led by Jiwon Jesse Han, a Griffin Graduate School of Arts and Sciences student affiliated with the CfA. Published in Nature Astronomy, the work includes co-authors Charlie Conroy and Lars Hernquist, both faculty members at the CfA and in the Department of Astronomy.

Our galaxy is located inside a diffuse cloud called the stellar halo, which extends much farther out into the universe. In groundbreaking work published last year, the Harvard team deduced that the stellar halo is tilted and elliptical in shape, like a zeppelin or football.

Building on that, the team assumed the same shape for the dark matter halo, the larger entity that encompasses everything in and around the Milky Way. Dark matter makes up 80 percent of the galaxy's mass but is invisible because it doesn't interact with light, so the shape of that halo must be inferred. Using models to calculate the orbits of stars within a tilted, oblong dark matter halo, the team found a near-perfect match to existing observations of a warped, flared galaxy.

"A tilted dark halo is actually fairly common in simulations, but no one had explored its effect on the Milky Way," Conroy said. "It turns out that the tilt is an elegant way to explain both the magnitude and direction of our galaxy's wobbly disk."

Scientists had long surmised that the Milky Way formed due to a galactic collision; the astronomers' work further underscores that hypothesis.

"If the galaxy was just evolving on its own, it would have had this nice, spherical halo, this nice, flat disk," Han said. "So the fact that the halo is tilted and has a football-like shape suggests that our galaxy experienced a merger event, where two galaxies collide."

Their calculation of the dark matter halo's probable shape may also provide clues as to the properties and particle nature of dark matter itself, which remain unsolved mysteries in physics. "The fact that the galaxy is not spherical in our data implies that there is some limit to which dark matter can interact with itself," Han explained.

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Signs of disturbance in nearby dwarf galaxies indicate an alternative gravity theory

Dwarf galaxies are small, faint galaxies that can usually be found in galaxy clusters or near larger galaxies. Because of this, they might be affected by the gravitational effects of their larger companions. "We introduce an innovative way of testing the standard model based on how much dwarf galaxies are disturbed by gravitational ,tides' from nearby larger galaxies," said Elena Asencio, a PhD student at the University of Bonn and the lead author of the story. Tides arise when gravity from one body pulls differently on different parts of another body. These are similar to tides on Earth, which arise because the moon pulls more strongly on the side of Earth which faces the moon.

The Fornax Cluster has a rich population of dwarf galaxies. Recent observations show that some of these dwarfs appear distorted, as if they have been perturbed by the cluster environment. "Such perturbations in the Fornax dwarfs are not expected according to the Standard Model," said Pavel Kroupa, Professor at the University of Bonn and Charles University in Prague. "This is because, according to the standard model, the dark matter halos of these dwarfs should partly shield them from tides raised by the cluster."

The authors analyzed the expected level of disturbance of the dwarfs, which depends on their internal properties and their distance to the gravitationally powerful cluster center. Galaxies with large sizes but low stellar masses and galaxies close to the cluster centre are more easily disturbed or destroyed. They compared the results with their observed level of disturbance evident from photographs taken by the VLT Survey Telescope of the European Southern Observatory.

"The comparison showed that, if one wants to explain the observations in the standard model" - said Elena Asencio - "the Fornax dwarfs should already be destroyed by gravity from the cluster center even when the tides it raises on a dwarf are sixty-four times weaker than the dwarf's own self-gravity." Not only is this counter-intuitive, she said, it also contradicts previous studies, which found that the external force needed to disturb a dwarf galaxy is about the same as the dwarf's selfgravity.

Contradiction to the standard model

From this, the authors concluded that, in the standard model, it is not possible to explain the observed morphologies of the Fornax dwarfs in a self-consistent way. They repeated the analysis using Milgromian dynamics (MOND). Instead of assuming dark matter halos surrounding galaxies, the MOND theory proposes a correction to Newtonian dynamics by which gravity experiences a boost in the regime of low accelerations.

"We were not sure that the dwarf galaxies would be able to survive the extreme environment of a galaxy cluster in MOND, due to the absence of protective dark matter halos in this model - admitted Dr Indranil Banik from the University of St Andrews - "but our results show a remarkable agreement between observations and the MOND expectations for the level of disturbance of the Fornax dwarfs."

"It is exciting to see that the data we obtained with the VLT survey telescope allowed such a thorough test of cosmological models," said Aku Venhola from the University of Oulu (Finland) and Steffen Mieske from the European Southern Observatory, co-authors of the study.

This is not the first time that a study testing the effect of dark matter on the dynamics and evolution of galaxies concluded that observations are better explained when they are not surrounded by dark matter. "The number of publications showing incompatibilities between observations and the dark matter paradigm just keeps increasing every year. It is time to start investing more resources into more promising theories," said Pavel Kroupa, member of the Transdisciplinary Research Areas "Modelling" and "Matter" at the University of Bonn.

Dr. Hongsheng Zhao from the University of St Andrews added: "Our results have major implications for fundamental physics. We expect to find more disturbed dwarfs in other clusters, a prediction which other teams should verify."

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Astronomers detect extended dark matter halo around ancient dwarf galaxy

 The Milky Way is surrounded by dozens of dwarf galaxies that are thought to be relics of the very first galaxies in the universe. Among the most primitive of these galactic fossils is Tucana II -- an ultrafaint dwarf galaxy that is about 50 kiloparsecs, or 163,000 light years, from Earth.

Now MIT astrophysicists have detected stars at the edge of Tucana II, in a configuration that is surprisingly far from its center but nevertheless caught up in the tiny galaxy's gravitational pull. This is the first evidence that Tucana II hosts an extended dark matter halo -- a region of gravitationally bound matter that the researchers calculated to be three to five times more massive than scientists had estimated. This discovery of far-flung stars in an ancient dwarf galaxy implies that the very first galaxies in the universe were also likely extended and more massive than previously thought.

"Tucana II has a lot more mass than we thought, in order to bound these stars that are so far away," says MIT graduate student Anirudh Chiti. "This means that other relic first galaxies probably have these kinds of extended halos too."

The researchers also determined that the stars on the outskirts of Tucana II are more primitive than the stars at the galaxy's core. This is the first evidence of such a stellar imbalance in an ultrafaint dwarf galaxy.

The unique configuration suggests that the ancient galaxy may have been the product of one of the first mergers in the universe, between two infant galaxies -- one slightly less primitive than the other.

"We may be seeing the first signature of galactic cannibalism," says Anna Frebel, the Silverman Family Career Development Associate Professor of Physics at MIT. "One galaxy may have eaten one of its slightly smaller, more primitive neighbors, that then spilled all its stars into the outskirts."

Frebel, Chiti, and their colleagues have published their results today in Nature Astronomy.

Not-so-wimpy galaxies

Tucana II is one of the most primitive dwarf galaxies known, based on the metal content of its stars. Stars with low metal content likely formed very early on, when the universe was not yet producing heavy elements. In the case of Tucana II, astronomers had previously identified a handful of stars around the galaxy's core with such low metal content that the galaxy was deemed the most chemically primitive of the known ultrafaint dwarf galaxies.

Chiti and Frebel wondered whether the ancient galaxy might harbor other, even older stars, that might shed light on the formation of the universe's first galaxies. To test this idea, they obtained observations of Tucana II through the SkyMapper Telescope, an optical ground-based telescope in Australia that takes in wide views of the southern sky.

The team used an imaging filter on the telescope to spot primitive, metal-poor stars beyond the galaxy's core. The team ran an algorithm, developed by Chiti, through the filtered data to efficiently pick out stars with low metal content, including the previously identified stars at the center and nine new stars much further out from the galactic core.

"Ani's analysis shows a kinematic conection, that these far-out stars move in lockstep with the inner stars, like bathwater going down the drain," Frebel adds.

The results suggest that Tucana II must have an extended dark matter halo that is three to five times more massive than previously thought, in order for it to keep a gravitational hold on these far-off stars. Dark matter is a hypothetical type of matter that is thought to make up more than 85 percent of the universe. Every galaxy is thought to be held together by a local concentration, or halo, of dark matter.

"Without dark matter, galaxies would just fly apart," Chiti. says. "[Dark matter] is a crucial ingredient in making a galaxy and holding it together."

The team's results are the first evidence that an ultrafaint dwarf galaxy can harbor an extended dark matter halo.

"This probably also means that the earliest galaxies formed in much larger dark matter halos than previously thought," Frebel says. "We have thought that the first galaxies were the tiniest, wimpiest galaxies. But they actually may have been several times larger than we thought, and not so tiny after all."

"A cannibalistic history"

Chiti and Frebel followed up their initial results with observations of Tucana II taken by the Magellan Telescopes in Chile. With Magellan, the team focused in on the galaxy's metal-poor stars to derive their relative metallicities, and discovered the outer stars were three times more metal-poor, and therefore more primitive, than those at the center.

"This is the first time we've seen something that looks like a chemical difference beween the inner and outer stars in an ancient galaxy," Chiti says.

A likely explanation for the imbalance may be an early galactic merger, in which a small galaxy -- possibly among the first generation of galaxies to form in the universe -- swallowed another nearby galaxy. This galactic cannibalism occurs constantly throughout the universe today, but it was unclear whether early galaxies merged in a similar way.

"Tucana II will eventually be eaten by the Milky Way, no mercy," Frebel says. "And it turns out this ancient galaxy may have its own cannibalistic history."

The team plans to use their approach to observe other ultrafaint dwarf galaxies around the Milky Way, in hopes of discovering even older, farther-flung stars.

"There are likely many more systems, perhaps all of them, that have these stars blinking in their outskirts," Frebel says.

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The Milky Way galaxy has a clumpy halo

 The Milky Way galaxy is in the recycling business.

University of Iowa astronomers have determined our galaxy is surrounded by a clumpy halo of hot gases that is continually being supplied with material ejected by birthing or dying stars. This heated halo, called the circumgalactic medium (CGM), was the incubator for the Milky Way's formation some 10 billion years ago and could be where basic matter unaccounted for since the birth of the universe may reside.

The findings come from observations made by HaloSat, one of a class of minisatellites designed and built at Iowa -- this one primed to look at the X-rays emitted by the CGM. The researchers conclude the CGM has a disk-like geometry, based on the intensity of X-ray emissions coming from it. The HaloSat minisatellite was launched from the International Space Station in May 2018 and is the first minisatellite funded by NASA's Astrophysics Division.

"Where the Milky Way is forming stars more vigorously, there are more X-ray emissions from the circumgalactic medium," says Philip Kaaret, professor in the Iowa Department of Physics and Astronomy and corresponding author on the study, published online in the journal Nature Astronomy. "That suggests the circumgalactic medium is related to star formation, and it is likely we are seeing gas that previously fell into the Milky Way, helped make stars, and now is being recycled into the circumgalactic medium."

Each galaxy has a CGM, and these regions are crucial to understanding not only how galaxies formed and evolved but also how the universe progressed from a kernel of helium and hydrogen to a cosmological expanse teeming with stars, planets, comets, and all other sorts of celestial constituents.

HaloSat was launched into space in 2018 to search for atomic remnants called baryonic matter believed to be missing since the universe's birth nearly 14 billion years ago. The satellite has been observing the Milky Way's CGM for evidence the leftover baryonic matter may reside there.

To do that, Kaaret and his team wanted to get a better handle on the CGM's configuration.

More specifically, the researchers wanted to find out if the CGM is a huge, extended halo that is many times the size of our galaxy -- in which case, it could house the total number of atoms to solve the missing baryon question. But if the CGM is mostly comprised of recycled material, it would be a relatively thin, puffy layer of gas and an unlikely host of the missing baryonic matter.

"What we've done is definitely show that there's a high-density part of the CGM that's bright in X-rays, that makes lots of X-ray emissions," Kaaret says. "But there still could be a really big, extended halo that is just dim in X-rays. And it might be harder to see that dim, extended halo because there's this bright emission disc in the way.

"So it turns out with HaloSat alone, we really can't say whether or not there really is this extended halo."

Kaaret says he was surprised by the CGM's clumpiness, expecting its geometry to be more uniform. The denser areas are regions where stars are forming, and where material is being traded between the Milky Way and the CGM.

"It seems as if the Milky Way and other galaxies are not closed systems," Kaaret says. "They're actually interacting, throwing material out to the CGM and bringing back material as well."

The next step is to combine the HaloSat data with data from other X-ray observatories to determine whether there's an extended halo surrounding the Milky Way, and if it's there, to calculate its size. That, in turn, could solve the missing baryon puzzle.

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Hubble maps giant halo around Andromeda Galaxy

 In a landmark study, scientists using NASA's Hubble Space Telescope have mapped the immense envelope of gas, called a halo, surrounding the Andromeda galaxy, our nearest large galactic neighbor. Scientists were surprised to find that this tenuous, nearly invisible halo of diffuse plasma extends 1.3 million light-years from the galaxy -- about halfway to our Milky Way -- and as far as 2 million light-years in some directions. This means that Andromeda's halo is already bumping into the halo of our own galaxy.

They also found that the halo has a layered structure, with two main nested and distinct shells of gas. This is the most comprehensive study of a halo surrounding a galaxy.

"Understanding the huge halos of gas surrounding galaxies is immensely important," explained co-investigator Samantha Berek of Yale University in New Haven, Connecticut. "This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. It's full of clues regarding the past and future evolution of the galaxy, and we're finally able to study it in great detail in our closest galactic neighbor."

"We find the inner shell that extends to about a half million light-years is far more complex and dynamic," explained study leader Nicolas Lehner of the University of Notre Dame in Indiana. "The outer shell is smoother and hotter. This difference is a likely result from the impact of supernova activity in the galaxy's disk more directly affecting the inner halo."

A signature of this activity is the team's discovery of a large amount of heavy elements in the gaseous halo of Andromeda. Heavier elements are cooked up in the interiors of stars and then ejected into space -- sometimes violently as a star dies. The halo is then contaminated with this material from stellar explosions.

The Andromeda galaxy, also known as M31, is a majestic spiral of perhaps as many as 1 trillion stars and comparable in size to our Milky Way. At a distance of 2.5 million light-years, it is so close to us that the galaxy appears as a cigar-shaped smudge of light high in the autumn sky. If its gaseous halo could be viewed with the naked eye, it would be about three times the width of the Big Dipper. This would easily be the biggest feature on the nighttime sky.

Through a program called Project AMIGA (Absorption Map of Ionized Gas in Andromeda), the study examined the light from 43 quasars -- the very distant, brilliant cores of active galaxies powered by black holes -- located far beyond Andromeda. The quasars are scattered behind the halo, allowing scientists to probe multiple regions. Looking through the halo at the quasars' light, the team observed how this light is absorbed by the Andromeda halo and how that absorption changes in different regions. The immense Andromeda halo is made of very rarified and ionized gas that doesn't emit radiation that is easily detectable. Therefore, tracing the absorption of light coming from a background source is a better way to probe this material.

The researchers used the unique capability of Hubble's Cosmic Origins Spectrograph (COS) to study the ultraviolet light from the quasars. Ultraviolet light is absorbed by Earth's atmosphere, which makes it impossible to observe with ground-based telescopes. The team used COS to detect ionized gas from carbon, silicon, and oxygen. An atom becomes ionized when radiation strips one or more electrons from it.

Andromeda's halo has been probed before by Lehner's team. In 2015, they discovered that the Andromeda halo is large and massive. But there was little hint of its complexity; now, it's mapped out in more detail, leading to its size and mass being far more accurately determined.

"Previously, there was very little information -- only six quasars -- within 1 million light-years of the galaxy. This new program provides much more information on this inner region of Andromeda's halo," explained co-investigator J. Christopher Howk, also of Notre Dame. "Probing gas within this radius is important, as it represents something of a gravitational sphere of influence for Andromeda."

Because we live inside the Milky Way, scientists cannot easily interpret the signature of our own galaxy's halo. However, they believe the halos of Andromeda and the Milky Way must be very similar since these two galaxies are quite similar. The two galaxies are on a collision course, and will merge to form a giant elliptical galaxy beginning about 4 billion years from now.

Scientists have studied gaseous halos of more distant galaxies, but those galaxies are much smaller on the sky, meaning the number of bright enough background quasars to probe their halo is usually only one per galaxy. Spatial information is therefore essentially lost. With its close proximity to Earth, the gaseous halo of Andromeda looms large on the sky, allowing for a far more extensive sampling.

"This is truly a unique experiment because only with Andromeda do we have information on its halo along not only one or two sightlines, but over 40," explained Lehner. "This is groundbreaking for capturing the complexity of a galaxy halo beyond our own Milky Way."

In fact, Andromeda is the only galaxy in the universe for which this experiment can be done now, and only with Hubble. Only with an ultraviolet-sensitive future space telescope will scientists be able to routinely undertake this type of experiment beyond the approximately 30 galaxies comprising the Local Group.

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