The Milky Way. This image is constructed from data from the ESA's Gaia mission that's mapping over one billion of the galaxy's stars. Image Credit: ESA/Gaia/DPAC
Astronomers often use the Milky Way as a standard for studying how galaxies form and evolve. Since we’re inside it, astronomers can study it in detail with advanced telescopes. By examining it in different wavelengths, astronomers and astrophysicists can understand its stellar population, its gas dynamics, and its other characteristics in far more detail than distant galaxies.
However, new research that examines 101 of the Milky Way’s kin shows how it differs from them.
A mosaic of the images of 378 satellites across 101 Milky Way-like systems. Credit: Yao-Yuan Mao (Utah), with images from the DESI Legacy Surveys Sky Viewer
The Milky Way is special because it is our home. No matter where we are on Earth we can see its arc of light overhead if the night is dark enough. But how similar is our galaxy to others? Is it an unusual spiral galaxy, or is it rather typical in the cosmos?
The Large and Small Magellanic Clouds are well known satellite galaxies of the Milky Way but there are more. It is surrounded by at least 61 within 1.4 million light years (for context the Andromeda Galaxy is 2.5 million light years away) but there are likely to be more. A team of astronomers have been hunting for more companions using the Subaru telescope and so far, have searched just 3% of the sky. To everyone’s surprise they have found nine previously undiscovered satellite galaxies, far more than expected.
Hidden in this deep sky image (left) is Uma3/U1, an ultra faint galaxy. It contains fewer than 100 hundred stars, a tiny amount for a galaxy. Credit: CFHT/S. Gwyn (right) / S. Smith (left)
The Milky Way has many satellite galaxies, most notably the Large and Small Magellanic Clouds. They’re both visible to the naked eye from the southern hemisphere. Now astronomers have discovered another satellite that’s the smallest and dimmest one ever detected. It may also be one of the most dark matter-dominated galaxies ever found.
Modern astronomy holds that all major galaxies (with the Milky Way as no exception) are the accumulation of numerous small mergers. Thus, it should be expected that some of the globular clusters that are now part of our galaxy are likely inherited from other galaxies which have been cannibalized by the Milky Way, or even stolen from intact companion galaxies such as the Magellanic Clouds.
Associations between these clusters and the various progenitors began in the 1990’s, but recent research is beginning to paint a more comprehensive picture on exactly what percentage of our globular clusters were stolen, and precisely which ones.
Artist’s impression showing the increasing effect of ram-pressure stripping in removing gas from galaxies, sending them to an early death. Credit: ICRAR/NASA/ESA/Hubble Heritage Team (STScI/AURA)
It’s what you might call a case of galactic homicide (or “galacticide”). All over the known Universe, satellite galaxies are slowly being stripped of their lifeblood – i.e. their gases. This process is responsible for halting the formation of new stars, and therefore condemning these galaxies to a relatively quick death (by cosmological standards). And for some time, astronomers have been searching for the potential culprit.
But according to a new study by a team of international researchers from the International Center for Radio Astronomy Research (ICRAR) in Australia, the answer may have to do with the hot gas galactic clusters routinely pass through. According to their study, which appeared recently in The Monthly Notices of the Royal Astronomical Society, this mechanism may be responsible for the slow death we are seeing out there.
This process is known as “ram-pressure stripping“, which occurs when the force created by the passage of galaxies through the hot plasma that lies between them is strong enough that it is able to overcome the gravitational pull of those galaxies. At this point, they lose gas, much in the same way that a planet’s atmosphere can be slowly stripped away by the effects of Solar wind.
‘Radio color’ view of the sky above the Murchison Widefield Array radio telescope, part of the International Center for Radio Astronomy Research (ICRAC). Credit: Natasha Hurley-Walker (ICRAR/Curtin)/Dr John Goldsmith/Celestial Visions.
By measuring the amount of stripping that took place within each, they deduced that the extent to which a galaxy was stripped of its essential gases had much to do with the mass of its dark matter halo. For some time, astronomers have believed that galaxies are embedded in clouds of this invisible mass, which is believed to make up 27% of the known Universe.
“During their lifetimes, galaxies can inhabit halos of different sizes, ranging from masses typical of our own Milky Way to halos thousands of times more massive. As galaxies fall through these larger halos, the superheated intergalactic plasma between them removes their gas in a fast-acting process called ram-pressure stripping. You can think of it like a giant cosmic broom that comes through and physically sweeps the gas from the galaxies.”
The Arecibo Observatory in Puerto Rico, where the Arecibo Legacy Fast ALFA Survey is conducted. Credit: egg.astro.cornell.edu
This stripping is what deprives satellites galaxies of their ability to form new stars, which ensures that the stars they have enter their red giant phase. This process, which results in a galaxy populated by cooler stars, makes them that much harder to see in visible light (though still detectable in the infrared band). Quietly, but quickly, these galaxies become cold, dark, and fade away.
Already, astronomers were aware of the effects of ram-pressure stripping of galaxies in clusters, which boast the largest dark matter halos found in the Universe. But thanks to their study, they are now aware that it can affect satellite galaxies as well. Ultimately, this shows that the process of ram-pressure stripping is more prevalent than previously thought.
As Dr. Barbara Catinella, an ICRAR researcher and co-author on the study, put it:
“Most galaxies in the Universe live in these groups of between two and a hundred galaxies. We’ve found this removal of gas by stripping is potentially the dominant way galaxies are quenched by their surroundings, meaning their gas is removed and star formation shuts down.”
Another major way in which galaxies die is known as “strangulation”, which occurs when a galaxy’s gas is consumed faster than it can be replenished. However, compared to ram-pressure stripping, this process is very gradual, taking billions of years rather than just tens of millions – very fast on a cosmological time scale. Also, this process is more akin to a galaxy suffering from famine after outstripping its food source, rather than homicide.
Another cosmological mystery solved, and one that has crime-drama implications no less!
Arp 302 consists of a pair of very gas-rich spiral galaxies in their early stages of interaction. Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)
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A group of astronomers have discovered a vast structure of satellite galaxies and clusters of stars surrounding our Milky Way galaxy, stretching out across a million light years. The team says their findings may signal a “catastrophic failure of the standard cosmological model,” challenging the existence of dark matter. This joins another study released last week, where scientists said they found no evidence for dark matter.
PhD student Marcel Pawlowski and astronomy professor Pavel Kroupa from the University of Bonn in Germany are no strangers to the study – and skepticism — of dark matter. Together the two have a blog called The Dark Matter Crisis, and in a 2009 paper that also studied satellite galaxies, Kroupa declared that perhaps Isaac Newton was wrong. “Although his theory does, in fact, describe the everyday effects of gravity on Earth, things we can see and measure, it is conceivable that we have completely failed to comprehend the actual physics underlying the force of gravity,” he said.
While conventional cosmology models for the origin and evolution of the universe are based on the presence of dark matter, invisible material thought to make up about 23% of the content of the cosmos, this model is backed up by recent observations of the Cosmic Microwave Background that estimate the Universe is made of 4% regular baryonic matter, 73% dark energy and the remaining is dark matter.
But dark matter has never been detected directly, and in the currently accepted model – the Lambda-Cold Dark Matter model – the Milky Way is predicted to have far more satellite galaxies than are actually seen.
Pawlowski, Kroupa and their team say they have found a huge structure of galaxies and star clusters that extends as close as 33,000 light years to as far away as one million light years from the center of the galaxy, existing in right angles to the Millky Way, or in a polar structure both ‘north’ and ‘south’ of the plane of our galaxy.
This could be the ‘lost’ matter everyone has been searching for.
They used a range of sources to try and compile this new view of exactly what surrounds our galaxy, employing twentieth century photographic plates and images from the robotic telescope of the Sloan Deep Sky Survey. Using all these data they assembled a picture that includes bright ‘classical’ satellite galaxies, more recently detected fainter satellites and the younger globular clusters.
Altogether, it forms a huge structure.
“Once we had completed our analysis, a new picture of our cosmic neighbourhood emerged,” said Pawlowski.
The team said that various dark matter models struggle to explain what they have discovered. “In the standard theories, the satellite galaxies would have formed as individual objects before being captured by the Milky Way,” said Kroupa. “As they would have come from many directions, it is next to impossible for them to end up distributed in such a thin plane structure.”
Many astronomers, including astrophysicist Ethan Siegel in his Starts With a Bang blog, say the big picture of dark matter does a good job of explaining the structure of the Universe.
Siegel asks if any studies refuting dark matter “allow us to get away with a Universe without dark matter in explaining large-scale structure, the Lyman-alpha forest, the fluctuations in the cosmic microwave background, or the matter power spectrum of the Universe? The answers, at this point, are no, no, no, and no. Definitively. Which doesn’t mean that dark matter is a definite yes, and that modifying gravity is a definite no. It just means that I know exactly what the relative successes and remaining challenges are for each of these options.”
However, via Twitter today Pawlowski said, “Unfortunately the big picture of dark matter being reportedly fine only helps if looking from far away or with broken glasses.”
One explanation for how this structure formed is that the Milky Way collided with another galaxy in the distant past.
“The other galaxy lost part of its material, material that then formed our Galaxy’s satellite galaxies and the younger globular clusters and the bulge at the galactic centre.” said Pawlowski. “The companions we see today are the debris of this 11 billion year old collision.”
The team wrote in their paper: “If all the satellite galaxies and young halo clusters have been formed in an encounter between the young Milky Way and another gas-rich galaxy about 10-11 Gyr ago, then the Milky Way does not have any luminous dark-matter substructures and the missing satellites problem becomes a catastrophic failure of the standard cosmological model.”
“We were baffled by how well the distributions of the different types of objects agreed with each other,” said Kroupa. “Our model appears to rule out the presence of dark matter in the universe, threatening a central pillar of current cosmological theory. We see this as the beginning of a paradigm shift, one that will ultimately lead us to a new understanding of the universe we inhabit.”
About a decade ago, standard cosmological models encountered a slight problem when applied to the Milky Way… missing satellite galaxies. While the calculations predicted as many as 500, only 10 are documented and modern figures state as many as 20. So what happened to the other 480 that should be out there? Either they don’t exist – or we can’t see them for some reason. Thanks to research done by the LIDAU project and two researchers from Observatoire Astronomique de Strasbourg, we might just have an answer.
About 150 million years after the Big Bang, the Universe’s first stars began to appear out of the cold, electrically neutral hydrogen and helium gas which filled it. As their intense light cut through the hydrogen atoms, it returned them to their plasma state in a process called reionisation. Things really began to heat up from there… gas began escaping the gravity of low-mass galaxies and as a consequence, they lost their star-forming abilities. By computing the observable consequences of this process, Pierre Ocvirk and Dominique Aubert demonstrated that the Milky Way’s first stars had the power of reionisation and it “is indeed an essential process in the standard model of galaxy formation.” This photo-evaporation state neatly explains the sparsity and age of Milky Way companions and offers up the reason satellite galaxies are rare in this neighborhood.
“On the other hand, their sensitivity to UV radiation means satellite galaxies are good probes of the reionisation epoch. Moreover, they are relatively nearby, from 30000 to 900000 light-years, which allows us to study them in great details, especially with the forthcoming generation of telescopes.” says Ocvirk. “In particular, the study of their stellar content with respect to their position could give us precious insight into the structure of the local UV radiation field during the reionisation.”
Current theory states this photo-evaporation was simply caused by nearby galaxies, resulting in a uniform event – but the new model built by the two French researchers proves this assumption wrong. Their high resolution numerical simulation accounts for the dynamics of the dark matter haloes from beginning to end, as well as their resultant gas impacted star formation and UV radiation.
“It is the first time that a model accounts for the effect of the radiation emitted by the first stars formed at the center of the Milky way, on its satellite galaxies. Indeed, contrary to previous models, the radiation field produced in this configuration is not uniform, but decreases in intensity as one moves away from the source.” explains Ocvirk. “On one hand, the satellite galaxies close to the galactic center see their gas evaporate very quickly. They form so few stars that they can be undetectable with current telescopes. On the other hand, the more remote satellite galaxies experience on average a weaker irradiation. Therefore they manage to keep their gas longer, and form more stars. As a consequence they are easier to detect and appear more numerous.”
Where did initial assumptions fall short? In previous models reionisation was thought to occur over an evenly distributed UV background, but the MIlky Way’s first stars had already done its damage by consuming its satellites. As the study suggests, our own galaxy is responsible for the lack of smaller companions.
Says Ocvirk; “This new scenario has deep consequences on the formation of galaxies and the interpretation of the large astronomical surveys to come. Indeed, satellite galaxies are affected by our galaxy’s tidal field, and can be slowly digested into our galaxy’s stellar halo. They can also be stretched into filaments and form stellar streams.”
It’s a very interesting new concept and will be one of the main science goals of the Gaia space mission, scheduled for launch in 2013. Until then, the Observatoire Astronomique de Strasbourg team will continue in their efforts to further understand radiative processes during reionisation.
