dark matter halo

arvit  asked:

About the dark matter thing, I have heard about a few possible candidates that are being examined more closely at the moment. One are the neutralinos, the other are black holes between 20 and 100 solar masses, which are the only size possible for dark matter and which have been discovered more often with LIGO and the other detectors for gravitational waves. What do you think about those theories? (And about MOND, that hardly ever gets coverage for the general public)

Thanks for asking! To start out, we don’t know much about dark matter, but here’s what we do know: Stars are orbiting galaxies too fast for the visible matter to be the only thing holding the galaxy together; in order for the speeds of stars orbiting to be reconciled with the amount of mass we can see, we’ve concluded that there has to be about 5-10x as much matter that can’t be seen because it doesn’t interact with light, giving it the nickname “dark matter.” Based on rotational curves (which show the speed of stars rotating around the galactic center at a given distance from the core), we know that the dark matter extends in a “halo” significantly larger than the visible galaxy. (image source)

This is a rotational curve; as you can see, the rotational curves don’t follow the path expected at all. Astrophysicists did a bunch of math and figured out that the dark matter must be in a huge halo around the galaxy: (image source)

Now for the different theories you talked about! I’m not too familiar with neutralinos; from what I (just) read, they’re theoretical particles with a lot of mass, and one of the predicted ones are WIMPs (Weakly Interacting Massive Particles - don’t you love how astronomers name things), which is a good candidate for dark matter, but hasn’t yet been proven. A lot of articles I’ve read about dark matter searches spent some time talking about WIMPs so they’re probably a good candidate and could end up being right. Another particle had the acronym MACHOs and i believe it also has to do with dark matter.

Black holes are definitely not the source of dark matter, because there is 5-10x more dark matter than luminous matter in the universe, meaning that there would have to be at least 109 or more of these black holes (essentially one black hole for every 1-10 stars), so we can write this theory off. Also, this theory wouldn’t match the observed rotational curves - whatever dark matter there is, it has to be whizzing around in the dark matter halo. 

MOND, or modified-newtonian dynamics, is almost definitely not true. The theory of gravity (general relativity) is pretty pretty solid and extremely unlikely to be wrong, so redoing our entire theory of gravity to fit dark matter is probably not the answer; this is why it gets essentially no coverage.

If you want my personal opinion, I think the most likely candidate would be a massive but very weakly (if at all) interacting subatomic particle that we haven’t discovered yet. From what I understand, we’re trying to figure out how to detect them, but it’s very very hard since these particles don’t interact with light (and only really interact gravitationally). 

If you want more information about dark matter, you should check out this post by @quantanaut about what we know (or more accurately don’t know), along with the dark matter tag on my blog. ps im sorry I literally typed you an essay but I wanted to be thorough. If i did a poor job explaining anything or you want to talk more about it or want clarification or something, feel free to shoot me another question!!

NGC 660 is featured in this cosmic snapshot. Over 40 million light-years away and swimming within the boundaries of the constellation Pisces, NGC 660’s peculiar appearance marks it as a polar ring galaxy. A rare galaxy type, polar ring galaxies have a substantial population of stars, gas, and dust orbiting in rings strongly tilted from the plane of the galactic disk. The bizarre-looking configuration could have been caused by the chance capture of material from a passing galaxy by a disk galaxy, with the captured debris eventually strung out in a rotating ring. The violent gravitational interaction would account for the myriad pinkish star forming regions scattered along NGC 660’s ring. The polar ring component can also be used to explore the shape of the galaxy’s otherwise unseen dark matter halo by calculating the dark matter's gravitational influence on the rotation of the ring and disk. Broader than the disk, NGC 660’s ring spans over 50,000 light-years.

Image Credit & Copyright: CHART32 Team,Processing - Johannes Schedler

Time And Space

New insight into dark matter halos

Research from the University of Pennsylvania could shed light on the distribution of one of the most mysterious substances in the universe.

In the 1970s, scientists noticed something strange about the motion of galaxies. All the matter at the edge of spiral galaxies was rotating just as fast as material in the inner part of the galaxy. But according to the laws of gravity, objects on the outskirts should be moving slower.

The explanation: A form of matter called dark matter that does not directly interact with light.

Keep reading


Dark matter faces its biggest challenge of all

“Yet in all of them, a very interesting and unexpected property shows itself: there’s a relationship between the observed gravitational acceleration and the distribution of the normal (baryonic, or protons, neutrons and electrons) matter alone. In other words, if you measure how quickly the galaxies rotate, it seems to depend — within a reasonable set of errors — only on the presence of the normal matter.”

Dark matter is a hugely successful theory for explaining a whole slew of observations about the Universe. Just by adding this one ingredient to the mix, we can successfully simulate and reproduce the large-scale structure, CMB fluctuations, galaxy clustering and cluster collision properties observed in our Universe. Without dark matter, there’s no other way known to make the Universe work in line with what we see. And yet, if you go down to the small scales of individual galaxies, dark matter predicts a dark matter halo of a specific profile with specific rotation properties. When we look at the actual galaxies, those rotation properties don’t match! Even worse, they appear to be correlated solely with the normal matter content of the galaxies, and have no dependence on whether the galaxy is rich-or-poor in dark matter.

Could this observation be the demise of dark matter? No matter what, it’s a challenge that even the most robust theory must face!


Dark Matter: Giver of Life

“It’s only the presence of these massive dark matter halos, surrounding our galaxies, that allow the carbon-based life that took hold on Earth — or a planet like Earth, for that matter — to even be a possibility within our Universe. As we’ve come to understand what makes up our Universe and how it came to be the way it is, we’re left with one inescapable conclusion: dark matter is absolutely necessary for the origin of life.”

Without dark matter — a substance that doesn’t interact in any (yet) measurable, non-gravitational way with anything else (or even itself) in the Universe — life as we know it would be unable to exist. The gravitation from dark matter is the only thing keeping supernova ejecta from escaping from our galaxy, and enabling heavy elements to participate in later generations of stars, planets, and biochemical reactions.


Dark matter is necessary for the origin of life

“Without the additional gravitation of a massive dark matter halo surrounding a galaxy, the overwhelming amount of material ejected from a supernova would escape from galaxies and wind up floating freely in the intergalactic medium, never to become incorporated into future generations of star systems. In a Universe without dark matter, we’d still have stars and galaxies, but the only planets would be gas giant worlds, with no rocky ones, no liquid water, and insufficient ingredients for life as we know it.”

Dark matter is necessary to explain the motions of stars, galaxies and the formation of structure in the Universe, but most surprisingly is how its presence and abundance is essential to the existence of life in the Universe.

My latest for Forbes!


Is Dark Matter Required For Life To Exist?

“Dark matter is the most mysterious, non-interacting substance in the Universe. Its gravitational effects are necessary to explain the rotation of galaxies, the motions of clusters, and the largest scale-structure in the entire Universe. But on smaller scales, it’s too sparse and diffuse to impact the motion of the Solar System, the matter here on Earth, or the origin and evolution of humans in any meaningful way. Yet the gravity that dark matter provides is an absolute necessity for allowing our galaxy to hold onto the raw ingredients that made life like us and planets like Earth possible at all. Without dark matter, the Universe would likely have no signs of life at all.”

Making up some 85% of the mass in our Universe, dark matter is necessary to explain the motions of individual galaxies, the grouping and clustering of assemblies of galaxies, the large-scale structure of the Universe and more. But on a much closer-to-home level, dark matter may be absolutely essential to the origin of life, too! Without dark matter, supernova explosions and starburst events would still create copious amounts of heavy elements, driven outwards by winds and the force of the explosions. But it’s the extra gravity of the dark matter that prevents most of this material from escaping, and allows it to take part in the formation of future generations of stars, to participate in rocky planet formation, and to deliver the ingredients necessary for life.

Go get the whole, detailed story today, and find out why life needs dark matter to exist after all!


The Stars Beyond

“Each galaxy has a story. Some are small but growing rapidly. Others look bland but betray a complex, vibrant past. What’s more, most large galaxies — again like some cities — appear to be built upon the ruins of smaller, more ancient ones. Our home galaxy, the Milky Way, is not unlike Rome in this respect. Ancient stellar remains show up viscerally in the the faint, extended outer reaches of galaxies — regions of light so diffuse that they’ve been difficult to study until recently.”

You’ve no doubt heard of dark matter halos around galaxies: vast, extended, spherical collection of mass that reach for hundreds of thousands of light-years beyond what we typically think of as a spiral or elliptical galaxy. But did you know that galaxies contain vast, extended stellar halos as well? Moreover, they look nothing like you’d expect! They’re not spherical or even ellipsoidal, but highly irregular, and have an awful lot to teach us about how galaxies came to be the way they are today. Galaxy evolution expert James Bullock has the story.

Polar Ring Galaxy NGC 660 : NGC 660 is featured in this cosmic snapshot, a sharp composite of broad and narrow band filter image data from the Gemini North telescope on Mauna Kea. Over 20 million light-years away and swimming within the boundaries of the constellation Pisces, NGC 660’s peculiar appearance marks it as a polar ring galaxy. A rare galaxy type, polar ring galaxies have a substantial population of stars, gas, and dust orbiting in rings nearly perpendicular to the plane of the galactic disk. The bizarre-looking configuration could have been caused by the chance capture of material from a passing galaxy by a disk galaxy, with the captured debris eventually strung out in a rotating ring. The violent gravitational interaction would account for the myriad pinkish star forming regions scattered along NGC 660’s ring. The polar ring component can also be used to explore the shape of the galaxy’s otherwise unseen dark matter halo by calculating the dark matter’s gravitational influence on the rotation of the ring and disk. Broader than the disk, NGC 660’s ring spans over 50,000 light-years. via NASA



– The discovery of two massive holes punched through a stream of stars could help answer questions about the nature of dark matter, the mysterious substance holding galaxies together.–

Researchers have detected two massive holes which have been ‘punched’ through a stream of stars just outside the Milky Way, and found that they were likely caused by clumps of dark matter, the invisible substance which holds galaxies together and makes up a quarter of all matter and energy in the universe.

The scientists, from the University of Cambridge, found the holes by studying the distribution of stars in the Milky Way. While the clumps of dark matter that likely made the holes are gigantic in comparison to our Solar System – with a mass between one million and 100 million times that of the Sun – they are actually the tiniest clumps of dark matter detected to date.

The results, which have been submitted to the Monthly Notices of the Royal Astronomical Society, could help researchers understand the properties of dark matter, by inferring what type of particle this mysterious substance could be made of. According to their calculations and simulations, dark matter is likely made up of particles more massive and more sluggish than previously thought, although such a particle has yet to be discovered.

“While we do not yet understand what dark matter is formed of, we know that it is everywhere,” said Dr Denis Erkal from Cambridge’s Institute of Astronomy, the paper’s lead author. “It permeates the universe and acts as scaffolding around which astrophysical objects made of ordinary matter – such as galaxies – are assembled.”

Current theory on how the universe was formed predicts that many of these dark matter building blocks have been left unused, and there are possibly tens of thousands of small clumps of dark matter swarming in and around the Milky Way. These small clumps, known as dark matter sub-haloes, are completely dark, and don’t contain any stars, gas or dust.

Dark matter cannot be directly measured, and so its existence is usually inferred by the gravitational pull it exerts on other objects, such as by observing the movement of stars in a galaxy. But since sub-haloes don’t contain any ordinary matter, researchers need to develop alternative techniques in order to observe them.

The technique the Cambridge researchers developed was to essentially look for giant holes punched through a stream of stars. These streams are the remnants of small satellites, either dwarf galaxies or globular clusters, which were once in orbit around our own galaxy, but the strong tidal forces of the Milky Way have torn them apart. The remnants of these former satellites are often stretched out into long and narrow tails of stars, known as stellar streams.

“Stellar streams are actually simple and fragile structures,” said co-author Dr Sergey Koposov. “The stars in a stellar stream closely follow one another since their orbits all started from the same place. But they don’t actually feel each other’s presence, and so the apparent coherence of the stream can be fractured if a massive body passes nearby. If a dark matter sub-halo passes through a stellar stream, the result will be a gap in the stream which is proportional to the mass of the body that created it.”

The researchers used data from the stellar streams in the Palomar 5 globular cluster to look for evidence of a sub-halo fly-by. Using a new modelling technique, they were able to observe the stream with greater precision than ever before. What they found was a pair of wrinkled tidal tails, with two gaps of different widths.

By running thousands of computer simulations, the researchers determined that the gaps were consistent with a fly-by of a dark matter sub-halo. If confirmed, these would be the smallest dark matter clumps detected to date.

“If dark matter can exist in clumps smaller than the smallest dwarf galaxy, then it also tells us something about the nature of the particles which dark matter is made of – namely that it must be made of very massive particles,” said co-author Dr Vasily Belokurov. “This would be a breakthrough in our understanding of dark matter.”

The reason that researchers can make this connection is that the mass of the smallest clump of dark matter is closely linked to the mass of the yet unknown particle that dark matter is composed of. More precisely, the smaller the clumps of dark matter, the higher the mass of the particle.

Since we do not yet know what dark matter is made of, the simplest way to characterise the particles is to assign them a particular energy or mass. If the particles are very light, then they can move and disperse into very large clumps. But if the particles are very massive, then they can’t move very fast, causing them to condense – in the first instance – into very small clumps.

“Mass is related to how fast these particles can move, and how fast they can move tells you about their size,” said Belokurov. “So that’s why it’s so interesting to detect very small clumps of dark matter, because it tells you that the dark matter particle itself must be very massive.”

“If our technique works as predicted, in the near future we will be able to use it to discover even smaller clumps of dark matter,” said Erkal. “It’s like putting dark matter goggles on and seeing thousands of dark clumps each more massive than a million suns whizzing around.”

TOP IMAGE….artist’s impression of dark matter clumps around a Milky Way-like galaxy. These clumps are invisible and can only be detected through their gravitational effect on visible matter. The clumps, also known as subhaloes, come in a range of sizes with the smallest one set by the mass of the yet to be discovered dark matter particle. The more massive the dark matter particle, the slower the dark matter moves, and the easier it is for regions in the early universe to collapse and form small subhaloes. In this work, a tidal stream from a disrupting globular cluster is used to probe their presence. Credit: V. Belokurov, D. Erkal, S.E. Koposov (IoA, Cambridge). Photo: Color image of M31 from Adam Evans. Dark matter clumps from Aquarius, Volker Springel (HITS)

LOWER IMAGE….Comparison between the observed stream and two simulated streams. The blue points show the observed stream which has been colored blue to distinguish it from the other streams. In reality, the color of its stars look more like the previous figure. Note the underdense regions on the left and right. The green points show a simulated stream evolved in a smooth potential without dark matter clumps. In contrast to the observed stream, this stream appears smooth and does not have any gaps. The red points show a simulated stream which has been struck by two clumps of dark matter with masses of one million Suns (left) and fifty million Suns (right). These perturbations produce the same gaps as what is seen in the data. Although the dark matter clumps themsevlves are invisible, they create gaps in the stream which can be detected. If confirmed, these two dark subhaloes would represent the lowest mass clumps detected to date. Credit: V. Belokurov, D. Erkal, S.E. Koposov (IoA, Cambridge)

Gas cloud survives collision with Milky Way

A high-velocity cloud hurtling toward the Milky Way should have disintegrated long ago when it first collided with and passed through our Galaxy. The fact that it’s still intact suggests it’s encased in a shell of dark matter, like a Hobbit wrapped in a mithril coat.

Mapping dark matter — the unseen stuff that makes up more than 80 percent of cosmic matter — near our Galaxy is crucial to fully understanding how the Milky Way assembled over cosmic time.

This firstly requires detailed observations of nearby dwarf galaxies — galaxies each totaling a mass less than 10% of the Milky Way’s 200 to 400 billion stars — because they’re enshrouded in dark matter. More recently, it has been suggested that nearby high velocity clouds of hydrogen gas are encased in dark matter as well. But the effects of their dark matter halos remain unknown.

So Matthew Nichols from the Sauverny Observatory in Switzerland and colleagues set out to observe the Smith Cloud — a high-velocity cloud of hydrogen gas located 8,000 lightyears away in the constellation Aquila — in order to better constrain its dark matter halo. They used the Green Bank Telescope (GBT) in west Virginia in order to detect the faint radio emission of neutral hydrogen.

“The Smith Cloud is really one of a kind. It’s fast, quite extensive, and close enough to study in detail,” said Nichols in a press release.  At its distance the cloud (9,800 lightyears long and 3,300 lightyears wide) covers almost as much sky as the constellation Orion.

“It’s also a bit of a mystery; an object like this simply shouldn’t survive a trip through the Milky Way, but all the evidence points to the fact that it did,” said Nichols. Previous studies of the Smith Cloud revealed that it first passed through our Galaxy many millions of years ago. By reexamining and carefully modeling the cloud, Nichols’ team now believes that it’s actually wrapped in a substantial halo of dark matter.

“Based on the currently predicted orbit, we show that a dark matter free cloud would be unlikely to survive this disk crossing,” said coauthor Jay Lockman from the National Radio Astronomy Observatory. “While a cloud with dark matter easily survives the passage and produces an object that looks like the Smith Cloud today.”

Image credit: NRAO / AUI / NSF