# List of galaxies with their dark matter halo

We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Where can one find the list of galaxies with their dark halo masses, I've searched several sites but I found no such proper list where the masses of their dark matter halo is compiled?

Can anybody help?

This paper by Wechsler & Tinker (2018) should give you a better direction. Depending on the model, different estimates of DM halos come up. You just need to go through the sections on observational evidence and the underlying papers.

https://arxiv.org/pdf/1804.03097.pdf

Do you mean observed or simulated? I assume the former, but observing the mass $$M_mathrm{h}$$ of a dark matter (DM) halo is not easy (I mean, they're invisible), so you need a model to relate $$M_mathrm{h}$$ to some observable.

### "Observed" dark matter halos

Typically, such measurements are not made in large bulks, so I don't think you can find a "list". Rather, models are constructed based on theory and a few observations, together with previous observations. Popular ways to obtain $$M_mathrm{h}$$ is abundance matching, where a property of the DM halos (in this case mass) is assumed to correlate tightly with some observable such as stellar mass/luminosity (e.g. Reddick et al. 2013; Behroozi et al. 2013).

A more direct way is - as discussed in the last section of this answer - to measure the width of emission lines, but in order to convert this into $$M_mathrm{h}$$ requires measuring the radius as well, which can only be done for the luminous matter, and is particularly difficult for small, faint galaxies.

### Clusters and groups of galaxies

Inferring $$M_mathrm{h}$$ for clusters of galaxies is easier, I think, as one can be moderately confident that the gas is in hydrostatic equilibrium and hence relate the observed X-ray brightness to halo mass (e.g. Bhattacharya et al. 2013). See also Yang et al. (2009).

### Simulated dark matter halos

I don't know why you need such a list, but if you're okay with (hydrodynamically) simulated galaxies, then there are plenty of such catalogues, since in this case we know exactly the masses of their different components. Joanne Cohn at Berkeley has compiled a list of simulated galaxy catalogues (note that the list also contains catalogues of dark matter-only simulations).

## Dwarf galaxy’s “suburban” sprawl confirms ancient galaxies formed in dark matter halos

Pasadena, CA&mdashAn MIT-led team of astronomers that includes Carnegie&rsquos Joshua Simon, Lina Necib, and Alexander Ji has discovered an unexpected outer suburb of stars on the distant fringes of the dwarf galaxy Tucana II. Their detection, published by Nature Astronomy, confirms that the cosmos&rsquo oldest galaxies formed inside massive clumps of dark matter&mdashwhat astronomers refer to as a &ldquodark matter halo."

Our own Milky Way is surrounded by a cadre of orbiting dwarf galaxies&mdashrelics of the ancient universe. A new technique developed by lead author Anirudh Chiti of MIT extended the astronomers&rsquo reach and revealed never-before-seen stars on the outskirts of Tucana II. Their discovery opens up new questions about the origin of these galactic outskirts.

The stars of Tucana II were already among the most primitive known.

&ldquoStars manufacture elements throughout their lifetimes, which they spread into the surrounding gas when they explode as powerful supernovae,&rdquo explained Simon. &ldquoThese raw materials are then incorporated into new stars, making each successive stellar generation more chemically complex than its predecessors. As a result, we know that stars containing very small amounts of most elements are incredibly old.&rdquo

The newly discovered stars on Tucana II&rsquos fringes are even more ancient than the ones close to its center&mdasha phenomenon never previously observed in such a small dwarf galaxy. In larger galaxies, this type of distribution can be the remnant of a collision between two galaxies made up of stars of different ages. If a galactic merger is the source of this arrangement, these would be the smallest galaxies yet known to merge.

&ldquoWe may be seeing the first signature of galactic cannibalism,&rdquo said MIT&rsquos Anna Frebel in a statement. &ldquoOne galaxy may have eaten one of its slightly smaller, more primitive neighbors, that then spilled all its stars into the outskirts.&rdquo

The spatial distribution of Tucana II&rsquos stars is highly unusual for a dwarf galaxy, which are usually denser. The motions of the fringe stars show that all of them are gravitationally bound to the galaxy&rsquos center, allowing researchers to make a much better estimate of the galaxy&rsquos total mass than would usually be possible.

&ldquoThe mass of all the stars in Tucana II is considerably less than the total mass of the galaxy, which is predominantly supplied by dark matter,&rdquo Ji explained. &ldquoDark matter comprises about a quarter of the universe&rsquos mass and its gravity holds galaxies together.&rdquo

A galaxy&rsquos dark matter extends far beyond its center, where most of its stars are located. This means that ordinarily, astronomers measure the mass of a galaxy&rsquos center and extrapolate to estimate its total mass. Discovering these stars on the outer edges of Tucana II meant that less extrapolation was necessary.

The technique Chiti developed will hopefully be able to identify other dwarf galaxies with sprawling suburbs of stars on their fringes.

&ldquoThis means that other relic first galaxies probably have these kinds of extended halos too,&rdquo he concluded in the MIT statement.

Quotations from Chiti and Frebel were sourced from the MIT News article &ldquoAstronomers detect extended dark matter halo around ancient dwarf galaxy&rdquo written by Jennifer Chu.

## List of galaxies with their dark matter halo - Astronomy

In our modern understanding of galaxy formation, every galaxy forms within a dark matter halo. The formation and growth of galaxies over time is connected to the growth of the halos in which they form. The advent of large galaxy surveys as well as high-resolution cosmological simulations has provided a new window into the statistical relationship between galaxies and halos and its evolution. Here, we define this galaxy-halo connection as the multivariate distribution of galaxy and halo properties that can be derived from observations and simulations. This galaxy-halo connection provides a key test of physical galaxy-formation models it also plays an essential role in constraints of cosmological models using galaxy surveys and in elucidating the properties of dark matter using galaxies. We review techniques for inferring the galaxy-halo connection and the insights that have arisen from these approaches. Some things we have learned are that galaxy-formation efficiency is a strong function of halo mass at its peak in halos around a pivot halo mass of 10 12 M ☉ , less than 20% of the available baryons have turned into stars by the present day the intrinsic scatter in galaxy stellar mass is small, less than 0.2 dex at a given halo mass above this pivot mass below this pivot mass galaxy stellar mass is a strong function of halo mass the majority of stars over cosmic time were formed in a narrow region around this pivot mass. We also highlight key open questions about how galaxies and halos are connected, including understanding the correlations with secondary properties and the connection of these properties to galaxy clustering.

## National Science Foundation - Where Discoveries Begin

Results combine observations made by NSF-funded ALMA observatory in Chile and South Pole Telescope

The Aurora Australis, or Southern Lights, over the South Pole Telescope.

December 6, 2017

For b-roll of the South Pole Telescope and NSF's Amundsen-Scott South Pole Station, please contact Dena Headlee at (703) 292-7739 or [email protected]

This material is available primarily for archival purposes. Telephone numbers or other contact information may be out of date please see current contact information at media contacts.

Observations of two galaxies made with the National Science Foundation-funded Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope suggest that large galaxies formed faster than scientists had previously thought.

The two galaxies, first discovered by the South Pole Telescope at NSF's Amundsen-Scott South Pole Station in Antarctica, were massive and star-filled at a time when the cosmos was less than a billion years old.

The observation came as a surprise, considering astronomers had thought that the first galaxies, which formed just a few hundred million years after the Big Bang, were similar to today's dwarf galaxies -- collections of stars much smaller than the Milky Way. After a few billion years, these early, smaller galaxies became the building blocks of the larger galaxies that came to dominate the universe, scientists believed.

But the latest ALMA observations push this epoch of massive-galaxy formation back further into the past, as the two galaxies were giants when the universe was only 780 million years old. ALMA also revealed that these large galaxies were nestled inside an even-more-massive cosmic structure, a halo of dark matter several trillion times more massive than the sun. The discovery provides new details about the emergence of large galaxies and the role that dark matter plays in assembling the most massive structures in the universe.

The researchers report their findings in the journal Nature.

"With these exquisite ALMA observations, astronomers are seeing the most massive galaxy known in the first billion years of the Universe in the process of assembling itself," said Dan Marrone, an associate professor of astronomy at the University of Arizona in Tucson and lead author on the paper, whose research received NSF support, including an NSF CAREER grant.

Viewing distant galaxies means looking back through time, in a sense. The energy from those objects takes so long to reach Earth that researchers today view events that occurred billions of years ago. The astronomy team captured data from these two galaxies as they were during a period of cosmic history known as the Epoch of Reionization, when most of intergalactic space was suffused with an obscuring fog of cold hydrogen gas. As more stars and galaxies formed, their energy eventually ionized the hydrogen between the galaxies, revealing the universe as we see it today.

The observations showed the two galaxies in such close proximity -- less than the distance from the Earth to the center of our galaxy -- that they were certainly on course to merge and form the largest galaxy ever observed in the Epoch of Reionization.

"We usually view that as the time of little galaxies working hard to chew away at the neutral intergalactic medium," said Marrone. "Mounting observational evidence with ALMA, however, has helped to reshape that story and continues to push back the time at which truly massive galaxies first emerged in the universe."

The galaxies that Marrone and his team studied, collectively known as SPT0311-58, were originally identified as a single luminous source by the 10-meter South Pole Telescope (SPT) survey. SPT is supported by NSF's Office of Polar Programs, which manages the U.S. Antarctic Program.

"These discoveries are made possible by close cooperation between NSF's Division of Astronomical Sciences and Office of Polar Programs, both supporting the ALMA and SPT facilities such cooperation will be essential to achieving the goals of Windows on the Universe: The Era of Multi-messenger Astrophysics, one of the "10 Big Ideas for Future NSF Investments," said Vladimir Papitashvili, NSF program director for Antarctic Astrophysics and Geospace Sciences.

These first observations indicated an object was very distant and glowing brightly in infrared light, meaning that it was extremely dusty and likely going through a burst of star formation. Subsequent observations with ALMA revealed the distance and dual nature of the object, clearly resolving the pair of interacting galaxies.

To make this observation, ALMA had some help from a gravitational lens, which provided an observing boost to the telescope. Gravitational lenses form when an intervening massive object, like a galaxy or galaxy cluster, bends the light from more distant galaxies. They do, however, distort the appearance of the object being studied, requiring sophisticated computer models to reconstruct the image as it would appear in its unaltered state.

This "de-lensing" process provided intriguing details about the galaxies, showing that the larger of the two is forming stars at a rate of 2,900 solar masses per year. It also contains about 270 billion times the mass of our sun in gas and nearly 3 billion times the mass of our sun in dust.

"That's a whopping large quantity of dust, considering the young age of the system," noted ALMA team member Justin Spilker, a recent graduate of the University of Arizona and now a postdoctoral fellow at the University of Texas at Austin.

The astronomers determined that this galaxy's rapid star formation was likely triggered by a close encounter with its slightly smaller companion, which already hosts about 35 billion solar masses of stars and is increasing its rate of starburst at the breakneck pace of 540 solar masses per year.

The researchers note that galaxies of this earlier era are messier than the ones we see in the nearby universe. Their more jumbled shapes would be due to the vast stores of gas raining down on them and their ongoing interactions and mergers with their neighbors.

The new observations also allowed the researchers to infer the presence of a truly massive dark matter halo surrounding both galaxies. Dark matter provides the pull of gravity that causes the universe to collapse into structures (galaxies, groups and clusters of galaxies, etc.).

"If you want to see if a galaxy makes sense in our current understanding of cosmology, you want to look at the dark matter halo -- the collapsed dark matter structure -- in which it resides," said Chris Hayward, an associate research scientist at the Center for Computational Astrophysics at the Flatiron Institute in New York City who provides theoretical support for the ALMA follow-up of SPT-discovered galaxies. "Fortunately, we know very well the ratio between dark matter and normal matter in the universe, so we can estimate what the dark matter halo mass must be."

By comparing their calculations with current cosmological predictions, the researchers found that this halo is one of the most massive that should exist at that time.

"There are more galaxies discovered with the South Pole Telescope that we're following up, and there is a lot more survey data that we are just starting to analyze. Our hope is to find more objects like this, possibly even more distant ones, to better understand this population of extreme dusty galaxies and especially their relation to the bulk population of galaxies at this epoch," said Joaquin Vieira, an assistant professor of astronomy at the University of Illinois at Urbana-Campaign and member of the SPT team whose study of SPT-discovered galaxies is funded through NSF's Astronomy and Astrophysics Research Grants program.

"In any case, our next round of ALMA observations should help us understand how quickly these galaxies came together and improve our understanding of massive galaxy formation during reionization," Marrone said.

## Map of Milky Way halo reveals dark matter ocean

Astronomers observed distant stars in the faint halo surrounding our Milky Way galaxy and have now created a map of the halo, the first of its kind of these outermost parts of our galaxy. These new observations, the astronomers said in April 2021, show how the Large Magellanic Cloud – one of the Milky Way’s satellite galaxies – has created a wake, like a ship sailing through calm waters, as it travels through the Milky Way’s halo. The wake shows up as a distinct bright pathway of stars on the map, telling us that the Magellanic Clouds are still traveling in their very first orbit around the Milky Way galaxy. And the wake itself may be made up of dark matter, dragging the stars along with it!

The discovery was published in the peer-reviewed journal Nature on April 21.

There are many interesting things about galactic haloes. They are faint and hard to observe, extend to large distances out from their galaxies and are – apart from a few stars, gas and dust – thought to contain a large amount of dark matter. Dark matter is called “dark” not only because we know little about it, but because it does not reveal its existence to us through light, only through gravitational interaction with other matter. The video above illustrates how far out from our galaxy’s main disk the halo stretches, as well as the pathway – the wake – of the Large Magellanic Cloud traveling through it.

The new map of the Milky Way’s halo (blue elliptical region) with the disk of the Milky Way galaxy, seen from the side, and with the satellite galaxy the Large Magellanic Cloud in the bottom right (the small one is there too). What is interesting in this image are the bright areas in the halo: the lower one forms a distinct pathway – a wake – behind the Large Magellanic cloud. The top one is a region of more stars in the northern hemisphere of the halo. Both of these halo anomalies were first predicted by computer models and have now been observationally confirmed. Image via NASA/ ESA/ JPL-Caltech/ Conroy et al.

If dark matter indeed does make up most of the halo – and all different theories on the nature of dark matter agree on that – a galaxy traveling through the halo would also leave a wake in the dark matter, not only the stars. As a NASA/JPL statement described it:

The wake observed in the new star map is thought to be the outline of this dark matter wake the stars are like leaves on the surface of this invisible ocean, their position shifting with the dark matter.

The inner regions of the Milky Way halo have already been investigated in detail, but this is the first time astronomers have been able to similarly map the outer regions of the halo, including the wake, at a distance of 200,000 to 325,000 light-years from the center of our galaxy. (As a comparison, the visible Milky Way galaxy disk that we are more familiar with has a diameter of about 100,000 light years, so this is very far out indeed).

With the halo being so faint, how do you go about observing it? Although the stars are extremely sparse in the halo, there are still some there. The researchers measured 1,301 stars located at the vast distances of the halo, using data from the European Space Agency’s Gaia mission and NASA’s Near Earth Object Wide Field Infrared Survey Explorer (NEOWISE, which also gave name to the comet of 2020). But accurately pinpointing their distances was one of the major hurdles. So they picked only a specific kind of red giant stars, classified as K giants in the stellar classification scheme. NEOWISE could efficiently detect these stars in the infrared part of the electromagnetic spectrum, which helped the team find their precise distances in the halo and create the map.

Part of the team behind this research had predicted how the dark matter in the Milky Way halo should look like, using computer models. So when the observational data showed a wake behind the Large Magellanic Cloud and another higher density region of stars in the northern part of the halo, this was not entirely a surprise.

Team member Gurtina Besla at University of Arizona’s Steward Observatory said:

What has been a purely theoretical prediction has now been validated by observational data, providing a compelling argument for the existence of dark matter.

Lead author Charlie Conroy, professor at Harvard University, described how we can learn more about dark matter, such as what it consists of, through combining models and data:

You can imagine that the wake behind a boat will be different if the boat is sailing through water or through honey. In this case, the properties of the wake are determined by which dark matter theory we apply.

Charlie Conroy is a professor at Harvard University and the lead researcher of the discovery of a wake in the Milky Way’s halo, created by the Large Magellanic Cloud as it orbits around the Milky Way. Image via Harvard University.

Nicolás Garavito-Camargo, a co-author of the study at University of Arizona, explained how this research applies to other galaxies as well:

The Milky Way is the only galaxy in which we can resolve the stars and the halo to this level of detail, so it is our most important ‘natural laboratory’ in which we can study how galaxies work in general. We think that what we observe here likely applies to similar galaxies throughout the universe.

The Large Magellanic Cloud is a small galaxy rotating around the Milky Way, about 160,000 light years away from us. It and its smaller companion, the Small Magellanic Cloud (often abbreviated LMC and SMC, respectively), are clearly visible with the unaided eye from the Southern Hemisphere, where they look exactly like their namesake: like curious stationary clouds. LMC is predicted to collide with the Milky Way in the distant future, and in essence, this collision has already started if you take the halo into account as a part of our galaxy.

Bottom line: Astronomers have created a map of the halo of our Milky Way galaxy – its far outer regions – showing how the Large Magellanic Cloud has created a wake along its traveled path, evidence that the satellite dwarf galaxy is only on its very first orbit around the Milky Way. The map provides a way to learn more about the nature of dark matter, thought to compose a large part of the galactic halo.

## Astronomers offer possible explanation for elusive dark-matter-free galaxies

Laura Sales (seated, left) with her research group of former and current students, including Jessica Doppel (seated, right). Credit: Stan Lim, UC Riverside

A team led by astronomers at the University of California, Riverside, has found that some dwarf galaxies may today appear to be dark-matter free even though they formed as galaxies dominated by dark matter in the past.

Galaxies that appear to have little to no dark matter—nonluminous material thought to constitute 85% of matter in the universe—complicate astronomers' understanding of the universe's dark matter content. Such galaxies, which have recently been found in observations, challenge a cosmological model used by astronomers called Lambda Cold Dark Matter, or LCDM, where all galaxies are surrounded by a massive and extended dark matter halo.

Dark-matter-free galaxies are not well understood in the astronomical community. One way to study the possible formation mechanisms for these elusive galaxies—the ultradiffuse DF2 and DF4 galaxies are examples—is to find similar objects in numerical simulations and study their time evolution and the circumstances that lead to their dark matter loss.

Jessica Doppel, a graduate student in the UC Riverside Department of Physics and Astronomy and the first author of research paper published in the Monthly Notices of the Royal Astronomical Society, explained that in a LCDM universe all galaxies should be dark matter dominated.

"That's the challenge," she said. "Finding analogs in simulations of what observers see is significant and not guaranteed. Beginning to pin down the origins of these types of objects and their often-anomalous globular cluster populations allows us to further solidify our theoretical framework of dark matter and galaxy formation and confirms that no alternative forms of dark matter are needed. We found cold dark matter performs well."

For the study, the researchers used cosmological and hydrodynamical simulation called Illustris, which offers a galaxy formation model that includes stellar evolution, supernova feedback, black hole growth, and mergers. The researchers found a couple of "dwarf galaxies" in clusters had similar stellar content, globular cluster numbers, and dark matter mass as DF2 and DF4. As its name suggests, a dwarf galaxy is small, comprising up to several billion stars. In contrast, the Milky Way, which has more than 20 known dwarf galaxies orbiting it, has 200 to 400 billion stars. Globular clusters are often used to estimate the dark matter content of galaxies, especially dwarfs.

The researchers used the Illustris simulation to investigate the origin of odd dwarf galaxies such as DF2 and DF4. They found simulated analogs to dark-matter-free dwarfs in the form of objects that had evolved within the galaxy clusters for a long time and lost more than 90% of their dark matter via tidal stripping—the stripping away of material by galactic tidal forces.

"Interestingly, the same mechanism of tidal stripping is able to explain other properties of dwarfs like DF2 and DF4—for example, the fact that they are 'ultradiffuse' galaxies," said co-author Laura Sales, an associate professor of physics and astronomy at UCR and Doppel's graduate advisor. "Our simulations suggest a combined solution to both the structure of these dwarfs and their low dark matter content. Possibly, extreme tidal mass loss in otherwise normal dwarf galaxies is how ultradiffuse objects are formed."

In collaboration with researchers at the Max Planck Institute for Astrophysics in Germany, Sales' group is currently working with improved simulations that feature more detailed physics and a numerical resolution about 16 times better than the Illustris simulation.

"With these data, we will be able to extend our study to even lower-mass dwarfs, which are more abundant in the universe and expected to be more dark matter dominated at their centers, making them more challenging to explain," Doppel said. "We will explore if tidal stripping could provide a path to deplete dwarfs of their inner dark matter content. We plan to make predictions about the dwarfs' stellar, globular cluster, and dark matter content, which we will then compare to future observations."

## Astronomers find origins of "galactic cannibalism" with discovery of ancient dark matter halo

Astronomers have detected what they believe to be one of the earliest instances of "galactic cannibalism" &mdash when one galaxy consumes one of its smaller neighbors &mdash in an ultrafaint dwarf galaxy called Tucana II. The findings stem from the discovery of an ancient dark matter halo, located in a galaxy 163,000 light years from Earth.

Tucana II is just one of dozens of dwarf galaxies surrounding the Milky Way. They are thought to be artifacts left over from the first galaxies in the universe &mdash and Tucana II is among the most primitive of them.

In a new study, published Monday in the journal Nature Astronomy, astrophysicists report detecting nine previously unknown stars at the edge of Tucana II, using the SkyMapper Telescope in Australia and the Magellan Telescopes in Chile. The stars are shockingly far away from its center but remain in the small galaxy's gravitational pull.

The configuration of stars provides the first evidence that the galaxy contains an extended dark matter halo &mdash a region of matter three to five times larger than scientists originally believed &mdash in order to keep a gravitational hold on its distant stars. The findings suggest that the earliest galaxies in the universe were much more massive than previously believed.

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

Every galaxy is believed to be held together by a halo of dark matter, a type of hypothetical matter thought to make up over 85% of the universe, MIT News explains. But the new findings represent the first time one has been detected in an ultrafaint dwarf galaxy.

### Trending News

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

The vicinity of the Tucana II ultra-faint dwarf galaxy, as imaged with the SkyMapper Telescope. Anirudh Chiti, MIT

Scientists also found that these far-flung stars are older than the stars at Tucana II's core &mdash the first evidence of such an imbalance in this type of galaxy. Their discovery points to the possibility that the galaxy could be the product of one of the first mergers between two galaxies in the universe, which scientists refer to as "galactic cannibalism."

"We may be seeing the first signature of galactic cannibalism," said MIT Professor Anna Frebel. "One galaxy may have eaten one of its slightly smaller, more primitive neighbors, that then spilled all its stars into the outskirts."

Using a telescope's imaging filter, astronomers are able to study the metal content of a galaxy's stars to determine just how primitive it is. They had previously found stars at Tucana II's core with such low metal content that the galaxy was identified as the most chemically primitive of the known ultrafaint dwarf galaxies.

New research found the outer stars were three times more metal-poor than the ones at the center, making them even more primitive.

"This probably also means that the earliest galaxies formed in much larger dark matter halos than previously thought," Frebel said. "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."

An early galactic merger is one likely explanation for the imbalance. Galactic cannibalism occurs "constantly" across today's universe, according to MIT News, but mergers in the early universe are not so certain.

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

The team hopes to use their approach to discover even older, more distant stars in other ultrafaint dwarf galaxies.

First published on February 2, 2021 / 4:45 PM

Sophie Lewis is a social media producer and trending writer for CBS News, focusing on space and climate change.

## University of California, San Diego Center for Astrophysics & Space Sciences

Observations of clusters and their galaxies, have uncovered one of the major mysteries in astronomy today. Clusters appear to be very stable entities - they contain mature galaxies with old stars, and seem to have been formed billions of years ago. But, when we calculate the amount of mass in a cluster using the orbital motions of its member galaxies, the result is too low for the cluster to be gravitationally bound. If the cluster contains only the mass we can observe, the gravitational force is insufficient to prevent the galaxies from "escaping".There must be more mass in the cluster than what we see.

Rotation Curves for 3 Spiral Galaxies - Galaxy Image(left), Spectrum (center - photographic negative), & Plot (right).
The flatness of the rotation curve with no downward turn indicates that the mass distribution extends far beyond
the measured values, probably in the form of a massive halo of dark matter.

The same problem arises when we look at the galaxies themselves. The rotation curve of a galaxy shows how the orbital velocities of the stars change with distance from the center. If the galaxy rotated as a solid disk, the velocity would increase linearly with distance. If most of the mass were concentrated at the the center, as in our solar system, the velocities of the stars would decrease with the square root of the distance. But, that is not what is observed. Far past the point where no mass is visible, the rotation curves are flat! This means that the mass is still increasing as we move outward, even though we can't see anything! One again we have to call upon "dark matter". The galaxy must extend much farther out than the luminous matter indicates. In fact, the calculations require that there be at least 10 times more mass than we can see! Calculations suggest that this dark matter is likely to be in an extensive halo of dark matter.

The nature of this dark matter or "missing mass" is unknown. There are theories ranging from the bizzare to the mundane, none of which successfully answer all of the questions.

Possible forms of "Normal" (Baryonic) Matter
Planets/Brown Dwarfs/Stellar Remnants (Black Holes, Neutron Stars, & White Dwarfs)

• Planets - but the mass of planets is a small fraction of the mass of the Solar System. Are there free-roaming planets like Jupiter out there?
• Brown Dwarfs - down to 0.085M the number of stars increases dramatically as you go to stars of lower mass. Does this trend continue as one goes below the cutoff for the ignition of nuclear reactions? If so, failed stars, called Brown Dwarfs, might account for a significant fraction of the Dark Matter. Brown Dwarfs are hard to spot since they are cool and very low in luminosity. Recent infrared studies are finding Brown Dwarfs, but not in sufficient numbers to make up the dark matter needed in the Milky Way.
• Stellar Remnants - Dead stars, in the form of white dwarfs, neutron stars or black holes, could make up the Dark Matter, but our understanding of the history of the Milky Way makes it unlikely that stars could have formed and died sufficiently rapidly in the past to make up the necessary mass of 10 or more times the current mass of stars.

Perhaps galaxies have large amounts of gas that has not been accounted for. But, cool atomic hydrogen would emit 21-cm radio waves, and these are not seen. Molecular hydrogen should be visible by its ultraviolet emission, but this is not observed. Hot gas emits x-rays, and several galaxy clusters are strong x-ray sources. The mass of intergalactic gas is calculated to be a considerable amount, perhaps greater than the amount in galaxies and stars, but is still too little to account for the cluster stability.

According to the most commonly accepted theory, Big Bang Nucleosynthesis, ordinary atomic nuclei formed as the Universe expanded and cooled. The theory allows detailed calculations of the amount of helium ( 4 He) produced (also 2 H - deuterium, 3 He, 4 Li, 4 Be, 4 B) that should be present, and these have been confirmed by observations. But the theory only agrees with observation if the total amount of baryons (protons & neutrons) is restricted. There are enough baryons to account for some dark matter, but not enough to solve the problem.

• The Dark Matter Universe from the University of Oregon
• Really nice DM Tutorial from Jon [email protected] Univ. CA.
• Berkeley Center for Particle Astrophysics Dark Matter Tutorial
• Galaxy Rotation Curves
• Dark Matter Candidates

Gravitational Lenses   Clusters   Outreach & Education   CASS Home   Comments? Gene Smith
Conducted by:
Prof. H. E. (Gene) Smith
CASS 0424 UCSD
9500 Gilman Drive
La Jolla, CA 92093-0424

Last updated: 26 April 1999

## Dark matter halos could help scientists find the missing force in the universe

A virtual universe could help scientists find dark matter in the real universe, a new study suggests.

Dark matter accounts for some of the universe's greatest mysteries, but as a force, it too generates a seemingly never-ending list of unanswered questions.

Scientists believe dark matter makes up around 80 percent of the universe. Yet despite this abundance, it has never been successfully detected. Its presence can only be inferred from its gravitational pull, rather than any kind of visual evidence in the cosmos.

But by building a virtual universe, scientists were able to zoom in on clumps of dark matter, known as dark matter halos in unprecedented detail. By studying these virtual bodies, scientists may be inching closer to finding this mysterious force in the real universe.

The experiment is detailed in a study published Wednesday in the journal Nature.

Dark matter was first discovered in 1933, when astronomer Fritz Zwicky noticed there was something missing from the universe. Some form of mass had to account for the space in between cosmic bodies, and hold matter in place through gravitational force.

This missing massive force was dubbed dark matter.

In one of the universe's most fundamental processes, galaxies are formed from the cooled and condensed gas at the center of enormous clumps of this dark matter, dubbed dark matter halos. The galaxies, which are visible to us, enable astronomers to infer the existence of dark matter halos, and predict some of their properties.

But these halos can't be observed using our current technology. So to capture images of the dark matter halos, the team behind the new study used supercomputers based in Europe and China to recreate an average region of the universe in virtual reality. The area of the simulated universe simulation measured 2.4 billion light years on each side.

The simulation allowed them to zoom in on the universe, at a resolution equivalent to zooming in on a flea on the surface of the Moon, according to the researchers.

As a result, they were able to see hundreds of clumps of dark matter, ranging in size from incredibly large to extremely small.

The largest halos can contain massive galaxy clusters, or groups of hundreds of galaxies, each weighing around 1,000 trillion times more than the Sun. Because of that, it was relatively easier to figure out these halos' structures.

By contrast, not much was known about smaller halos, some of which scientists believe may be about the same mass as Earth. Their size means they are too small to contain any galaxies, and thus, harder to infer details on.

"By zooming in on these relatively tiny dark matter haloes we can calculate the amount of radiation expected to come from different sized haloes," Carlos Frenk, Ogden Professor of Fundamental Physics at the Institute for Computational Cosmology at Durham University and co-author of the new study, said in a statement.

Counterintuitively, the largest and smallest dark matter halos appear to have a very similar internal structure, and both are extremely dense in the center.

"We were really surprised by our results," Simon White, a researcher at the Max Planck Institute for Astrophysics, and co-author of the study, said in a statement.

"Everyone had guessed that the smallest clumps of dark matter would look quite different from the big ones we are more familiar with. But when we were finally able to calculate their properties, they looked just the same," he added.

By studying these smaller dark matter halos, scientists hope to apply their findings to detect real dark matter. One theory of dark matter halos is that particles of dark matter may collide with each other in the center of the halos, producing a burst of radiation. These emissions may be detectable in halos too small to contain any stars.

"Future gamma-ray observatories might be able to detect these emissions, making these small objects individually or collectively 'visible,'" Frenk said.

"This would confirm the hypothesized nature of the dark matter, which may not be entirely dark after all."

## The Smallest Galaxies In The Universe Have The Most Dark Matter

If you want to find dark matter, there's one simple rule: you follow the mass. Indeed, if you look at the largest structures in the Universe -- big galaxies, groups of galaxies, or even the most massive clusters -- they all show the same thing: their internal motions are all too fast to be explained by the gravitation of the matter we know is there.

Image credit: Wikimedia Commons user Stefania.deluca.

Inside individual spiral galaxies, their rotation speeds remain large, and in some cases even get larger and larger, as we move away from the galactic center. This cannot be explained by the sum total of all the different types of normal (atomic-based) matter we know exists: stars, gas, dust, plasma, even black holes.

Meanwhile, inside groups and clusters of galaxies, the speeds of the galaxies inside is also far too large to be explained by the normal matter. In all of these cases, if the only gravitation were due to the normal matter alone, these bound structures would fly apart, as their internal speeds are too great (greater than the escape velocity) for the mass due to all sources of protons, neutrons and electrons.

Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona.

The dark matter that we need to account for the internal motions, however, is remarkably similar for each any every one of these structures. If we add in dark matter that is:

• cold, or moving slowly compared to the speed of light,
• collisionless, or unable to interact electromagnetically or through the nuclear forces, and
• in about a 5-to-1 ratio to the normal matter everywhere we look,

we can then explain the motions of individual galaxies, small groups, large groups and even the largest clusters of galaxies. It all works out beautifully.

But what if we look at smaller objects? Not the grand, spiral galaxies containing tens of billions of stars or more, but rather the dwarfs of the cosmos?

Image credit: ESO/Digitized Sky Survey 2.

If we look at the smallest galaxies known, the ones with under a billion stars, or even just a few million, we find something counterintuitive: the motions of the stars are slower than in the large galaxies, but in order to keep these structures bound, there has to be more dark matter than the 5-to-1 ratio we find everywhere else!

In some cases, the ratio is more like 20-to-1, while in more extreme cases (at lower masses), the ratio rises into the hundreds-to-1. The smallest known galaxies in the Universe are actually tiny satellites of the Milky Way: objects like Segue 1 and Segue 3. They contain only a few hundred stars, orbiting their combined center of mass at less than the speed that Earth orbits the Sun: just 15 km/s.

Image credit: Marla Geha and Keck Observatories, of the stars making up the dwarf satellite Segue 1.

But if you were to ask how much total mass you need in this volume of space to keep these stars moving at that orbital speed, the answer is shocking: you need hundreds of thousands of solar masses worth of dark matter! Put that all together, and it means you need over a thousand times as much dark matter as normal matter in these extreme cases.

This should bother you! The Universe should have been born with the same amount of dark matter everywhere, and that dark matter should be essential to structure formation. This is what our greatest cosmic simulations indicate, and on the largest scales, they line up with observations fantastically well.

Images credit: Virgo consortium/A. Amblard/ESA (top and middle), of a simulation of dark matter and . [+] where the galaxies should be ESA / SPIRE Consortium / HerMES consortia (bottom), of the Lockman Hole, where each dot is a galaxy.

So where is the discrepancy? As it turns out, the answer comes from one of the properties necessary to dark matter: that it be collisionless! If the dark matter doesn't interact electromagnetically, that means that the presence of energetic photons -- particles of light -- can't affect it, or impart momentum and energy to it. This is very, very important when you think about what happens when the normal matter collapses to form stars.

Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration.

New stars are hot, emitting lots of radiation, and the most massive ones are volatile, resulting in catastrophic supernova explosions. The radiation from both of these sources streams out in all directions, giving a "kick" to everything it interacts with. All of the normal (atomic) matter that gets accelerated to speeds greater than the galaxy's escape velocity winds up getting expelled from the galaxy and sent into intergalactic space.

In small galaxies, the gravitational pull is insufficient to hang onto this normal matter, leaving behind only a small number of stars, but also all of the dark matter, which is unaffected by the radiation. But in larger galaxies, even the most catastrophic episodes of star formation are unable to expel the normal matter there's so much dark matter, and so much gravity, that practically nothing gets out!

Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA), of the Cigar Galaxy, Messier 82.

It's kind of amazing, when you think about it, that the only reason galaxies like our own Milky Way have hung onto so many hundreds of billions of stars is because of the dark matter present. Without it, the ejected material from ultra-massive stars would have been sent into intergalactic space, meaning that the building blocks of planets like Earth and organisms like us wouldn't have been present in the great abundances we needed!

But dark matter is real, and so instead, we're here, too. It's only in the smallest galaxies that gravitation can't do its job, and so we're left with a mediocre blob of dark matter and only a few leftover stars inside. The smallest galaxies in the Universe have the highest percentages of dark matter, but only because the normal matter didn't have what it takes to tag along for the entire ride!