Astronomy

The contribution of components like dark matter, stars and gas to the mass of a galaxy

The contribution of components like dark matter, stars and gas to the mass of a galaxy


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I am doing an introductory course in astrophysics and astronomy undergrad level. In it we were taught that the main components of matter in a galaxy are dark matter halo ($M_h$), gas ($M_{gas}$) and stars ($M_*$). Thus the mass of the galaxy can be given as the sum of these three components. I want to read more about this but when I search this on net I don't get any appropriate results or they are very advanced. I guess this is because the actual terminology and nomenclature of these masses is different. Thus if anyone can give me accurate terminology of these masses it would be helpful.

Additionally we were told that given the mass of dark matter halo the other two masses can have only a certain range of values. I want to know how we can determine these range of values possible for a galaxy. Also how do these values change as the galaxy ages? Any links to resources regarding the above topics are also welcome.

Edit 1: Regarding my question above about the change in mass of galaxies over time I am giving my thinking. Please correct me if I am wrong somewhere.

Initially galaxy consists of only dark matter and gas. This initially contracts but later undergoes segmentation to form stars. Thus initially there should be decrease in $M_{gas}$. But slowly some gas is returned as stars die and also gathered from surroundings, hence the gas mass should remain constant or decrease at a very slow rate. Now for the stars initially $M_*$ should increase due to creation of new stars. But after some time as some of the early stars start dying out there will be simultaneous birth and death of stars, hence the $M_*$ should fairly remain constant during this period. Finally as the star formation rate decreases due to depletion of $M_{gas}$ the birth of new stars will be reduced. Slowly more and more stars will start dying and $M_*$ will be depleted leaving behind only heavier elements. Hence $M_*$ should decrease during this period. Finally coming to halo mass, I am still not sure about it. I think it should not change much over the lifetime of the galaxy because dark matter does not seem to interact with the baryonic mass. I know that these changes will be slow but can anyone give me a rough number on the slowness of these changes?


This is a rather complex question, for several reasons.

  • First, galaxies come in many variations, regarding such diverse properties as mass, morphology, and environment.
  • Second, different observational techniques, and different models, yield different observables - you can observe the same field of the sky with two different instruments and deduce a different distribution of galaxies and their properties.
  • Third, as you mention, galaxies evolve, and you will not necessarily obtain the same ratio between, say, gas mass and stellar mass, even for a given galaxy type, at different redshifts.

Nevertheless, some things can be said to be true "in general":

Stellar mass - gas mass relation

The more massive a galaxy is (in terms of stellar mass, $M_*$), the more efficient it is at forming stars. Hence, the gas fraction $f_mathrm{gas} equiv M_mathrm{gas}/(M_mathrm{gas}+M_*)$ decreases with $M_*$. Moreover, although part of the gas in stars is returned to the interstellar medium (ISM), as time goes, and a galaxy forms stars, it will "deplete" the ISM, further reducing $f_mathrm{gas}$.

This can be seen in this plot from Magdis et al. (2012), showing the gas fraction as a function of stellar mass today (open circles) and ~10 billion years ago (closed circles):

The galaxies used in this survey are "main sequence" galaxies, and other selection criteria apply as well.

Stellar mass - halo mass relation

The dark matter (DM) component of a galaxy is much more extended and diffuse than the baryons (because DM is collisionless), rather lying in a large "halo" around the gas and stars. Of course, we cannot see DM, making measurements of its mass difficult. Only in numerical simulations, we know exactly its mass.

The larger the DM halo mass ($M_mathrm{h}$), the more stars the galaxy has. But the relation is not straightforward. In general, $M_*$ increases with $M_mathrm{h}$ more rapidly for low-mass galaxies, while for $M_mathrm{h} gtrsim 10^{12},M_odot$ (roughly Milky Way-sized galaxies) the relation flattens out:

This is seen in the left panel of this plot from Behroozi et al. (2013):

The different colors correspond to different epochs in the Universe. The data is from a cosmological simulation, but the simulation was calibrated to match various observations.

Another way to show this relation is seen in the right panel, where the stellar fraction $M_*/M_mathrm{h}$ is seen to rise until around $M_mathrm{h} sim 10^{12},M_odot$, after which it decreases again.

Why is this? In general it is thought that star formation is suppressed at low masses because gas is more easily expelled from a shallow gravitational potential, while a high masses, active galactic nuclei become very efficient at blowing out gas, thus quenching star formation.

How are masses measured?

There are several techniques to measuring these masses.

Stellar masses are measured using known relations between the amount of stars and the amount of light from some physical process - either a single emission line, or a broader band of light. For highly star-forming galaxies, where there are still many hot O and B stars that ionize the surrounding gas, nebular lines such as H$alpha$ or Ly$alpha$ can be used, while for non-star-forming galaxies you can use e.g. the continuum radiation from heated dust.

The conversion depends on the assumed initial mass function of the stellar population.

Likewise, gas masses and molecular masses may be measured knowing how much light a given amount of gas emits (at a given temperature, pressure,… ).

Measurements of halo masses are typically done looking at the width of various spectral lines, thus deducing the velocity dispersion $sigma_V$ of the gas and stars. Then, the total mass $M$ can be calculated from $$ sigma^2 = frac{GM}{CR}, $$ where $G$ is the gravitational constant, $R$ is the radius, and $C$ is a geometrical factor (see this answer for an explanation).


A comprehensive analysis using 9 dark matter halo models on the spiral galaxy NGC 4321

This paper addressed the dark matter analysis on the spiral galaxy NGC 4321 (M100) by considering the nine different dark matter profiles, so far lacking in the scientific literature, i.e., pseudoisothermal, Burkert, NFW, Moore, Einasto, core-modified, DC14, coreNFW and Lucky13 profiles. In this paper, we analyzed the rotation curve analysis on the galaxy NGC 4321 by using nonlinear fitting of star, gaseous and dark matter halo equations with selected VLA HI observation data. Among the nine dark matter profiles, four dark matter profiles (DC14, Lucky13, Burkert and Moore profiles) showed declining features and hence not suitable for this galaxy. This is concluded to be mainly due to the characteristics of those dark matter profiles and also the varying levels of problems within the inner region fittings. For the remaining five accepted dark matter profiles, we further conducted the analysis by using reduced Chi-square test. Four out of the five accepted dark matter profiles lie within the range of 0.40 < (chi _>^2) < 1.70, except for the core-modified profile. In addition, pseudoisothermal profile achieved the best fitting with (chi _>^2) nearest to 1, mainly due to its linearity in the inner region and flatness at large radii.

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Dark Matter

In the universe, what you see is certainly not what you get.

All of the stars, galaxies, and nebulae seen by telescopes make up only four percent of the contents of the universe. Scientists are unsure about the nature of the rest.

A large part of our universe is made up of so-called "dark matter," which emits no detectable energy, such as visible light, X-rays, or radio waves. However, it reveals itself by its gravity, just like a magnet underneath a table betrays its presence by attracting paperclips and pins.

The mystery of dark matter is more than 70 years old. In 1933, Fritz Zwicky studied the motions of galaxies in the Coma cluster and found that the galaxies were moving around much too fast. The cluster is flying apart, unless it's much more massive than it appears. One year earlier, Jan Oort studied the motions of stars in the Milky Way, and used similar arguments to conclude that our galaxy contains more mass than meets the eye.

In the late 1970s, Vera Rubin and Kent Ford announced the results of their pioneering studies of distant spiral galaxies. The outer regions of every galaxy they observed rotated so fast that there was one inescapable conclusion: Galaxies are embedded in extended "halos" of dark matter.

Why does fast rotation imply the presence of dark matter? Think about the solar system. Since most of the mass is in the Sun, which sits in the center of the system, Mercury's orbital speed is much higher than Pluto's.

Likewise, if most of a galaxy's mass is concentrated in its core (where most of the light comes from), you would expect stars and gas clouds to orbit slower with increasing distance from the core. This happens out to a certain distance. But past that point, orbital speeds of stars and gas clouds remain almost constant at increasing distances from the core. This can be explained only if there's a lot of invisible mass outside their orbits. Rubin and Ford concluded that the universe must contain about 10 times more dark matter than ordinary "luminous" matter.

Some of the dark matter consists of ordinary, or "baryonic" matter (matter consisting of protons and neutrons) that simply does not give off energy. Candidates include tenuous gas clouds, remnants of dead stars, or primordial black holes. But this is just the tip of the dark-matter iceberg: the amount of strange dark matter -- new types of elementary particles -- may be almost 10 times as large.

There's a simple reason why astronomers are so interested in dark matter. The mass of the universe determines its fate. The universe began expanding at the Big Bang, and is still expanding today. If the visible mass were all the mass in the universe, the universe would expand forever. However, the gravity of large amounts of dark matter might stop the expansion and cause the universe to contract, causing it to end in a "Big Crunch."

Recent observations of distant stellar explosions seem to indicate that the Big Crunch is not in our future. In fact, the expansion of the universe is speeding up, goaded by a mysterious "dark energy."

MACHOs and WIMPs

Most dark-matter searches have focused on our galaxy's halo -- a sphere around the flat, main disk, like the halo seen in blue around the edge-on spiral galaxy NGC 4631. Suppose it harbors a large amount of dark matter that consists of normal atoms (called "baryonic" dark matter). It may be locked up in small, cold bodies, like dead stars, cool brown dwarfs, rogue planets, or maybe even small black holes. These hypothetical bodies are called MACHOs, for MAssive Compact Halo Objects.

The nature of the "non-baryonic" dark matter -- dark matter not made of normal atoms -- is more mysterious. It may consist of particles that rarely or never interact with normal matter, except through gravity. Astronomers call this non-baryonic dark matter WIMPs -- Weakly Interacting Massive Particles.


Dark matter and Dark energy (Part 1) – Discovering the main components of the Universe

Gravity is a force that makes objects move, bringing things together. Everything with mass has gravity. We can feel the force of gravity as we jump up and get pulled down to the ground. Planets, stars, moons and other objects in the Universe also have gravity. That’s why they orbit around each other, like for example the Earth orbits the Sun, or the Moon orbiting around the Earth, instead of flying randomly in space. That’s why we see the Moon and the Sun every day. Groups of stars are held together forming galaxies and even galaxies are held together by gravity forming galaxy clusters. Thus, gravity can be seen as the universal glue.


The more mass something has, the stronger the gravity it produces. The Earth’s gravity is stronger than the Moon’s because it is more massive. So our bodies are pulled down on Earth more than if we were on the Moon. That’s why astronauts can jump higher and more easily on the Moon than on Earth. Our bodies also exert gravitational forces on other objects, but because our mass is so low, the gravity from our bodies does not affect objects any way we can see. The strength of gravity also changes with the distance to an object. The pull between the Earth and the Moon is stronger than that between the Earth and Jupiter. This is because the Earth is closer to the Moon than to the Jupiter.

Gravity was first described by Newton as a force. Described more than 300 years ago, Newton’s theory of gravity is still applied today and it was used when scientists plotted the course to land man on the moon. Although Newton’s theory describes the strength of gravity fairly accurately, he didn’t know what caused gravity or how it worked. These concepts were left unknown for nearly 250 years, until Albert Einstein described gravity as the curvature of space. Space has 3 dimensions: up-down, left-right and forward-backward and it can be visualized as a fabric, like a stretchy sheet. Any object with mass deforms space, just like a marble creating a dimple on the surface of the stretchy sheet. This curvature of space causes objects to interact and move towards one another, which is seen as gravity, a natural consequence of a mass’s influence on space. The more mass something has, the more the space is curved, and, therefore, the more gravity there is.

The Universe

The Earth, the planet on which we live is only a relatively small planet in the Solar system and in the vast Universe. In our Solar System, there are 8 planets, including Earth, orbiting a big star called the Sun in the center. Our Solar System also includes other smaller objects, including dwarf planets, moons orbiting planets, comets, asteroids etc. Outside the Solar System, there are similar systems, called planetary systems which also consist of planets orbiting a star. Gravity holds all these stars and planets together in something we call galaxy. We live in a galaxy called the Milky Way. It is estimated that there are about 200 billion stars in it so it is a very big place. Outside our enormous Milky Way, there are about 100 to 200 billion more galaxies like our Milky Way. These galaxies are at enormous distances from us, in fact even with the most powerful telescope we cannot see individual stars from these distant galaxies. All the stars that we can see in our night sky with the naked eye are only within our Milky Way. All galaxies, stars, planets, from matter to energy to time itself, make up what we call the Universe. All the visible things we can observe in our Universe, we call them normal matter.

The normal matter that we know of only makes up less than 5% of the Universe. While this seems shocking, it is not that impossible. Think about what elements comprise the Earth. Everything that we encounter every day here on Earth is made of elements like nitrogen, hydrogen, oxygen, carbon and heavy elements like iron and silicon. Because we see them in such abundance, we might easily conclude that the rest of the Universe must be the same as Earth, a lot of heavy elements, a bit of hydrogen and no helium. However, these elements make up only 1% of the Sun. The Sun is composed mainly of hydrogen, some helium and not much else. Many stars, gas and other objects in space that are observable also consist of the same elements as the Sun. Just as some materials commonly found on Earth are a fraction of the Sun composition, the materials that comprise the Sun and similar space objects are not necessarily representative of all the matter in the Universe. Isn’t it possible that everything on Earth and in space that we know might be as well just a tiny fraction of what really exist in the Universe? Astronomers have found evidence for the existence of other components, the main components, of the Universe they are called dark matter and dark energy.

Dark matter

Gravity allowed scientists to discover dark matter. In 1933, a Swiss astronomer, Fritz Zwicky tried to measure the total mass of a galaxy cluster by summing the mass of each individual visible galaxy in the cluster. He found out that their total mass was not enough to create the observed gravity that holds the galaxies together to form a cluster. With just the gravity created by all of their visible matter, the galaxies would not cluster easily, if at all. Thus, Zwicky concluded that there must be something invisible, inside and around the galaxies. This matter adds the extra mass to create that gravity, strong enough to form a galaxy cluster. Zwicky called the unseen mass dark matter.

More evidence for dark matter has emerged over time. Photographs of galaxies showed that most of their light, i.e. most of their stars, were concentrated near the center. So most of the mass of a galaxy is concentrated in its center, meaning that gravity is stronger at the center of a galaxy than in the outskirt. Therefore, it is expected that the stars near the center of the galaxy would move faster than those farther away. However, the measurements indicated that the orbital speed of stars was the same everywhere, regardless of their distance from the center. The conclusion is that there must be invisible matter that spreads throughout a galaxy, such that stars far away from the center will feel the gravitational pull of not only the central material, but all the other matter between them. The extra force of gravity from dark matter can cause them to speed up roughly to the same speed of the stars near the center.

Although it’s been discovered for some time, it’s still unknown what dark matter really is because it is invisible and does not interact with other normal matter that we know e.g. light, magnets, electricity. Scientists have proposed various ideas about the particles that make dark matter and designed experiments to test those ideas. However, the quest to define dark matter has been a process of elimination. Experiments have only ruled out possible candidates and left a few leading hypotheses, but yet to find the exact dark matter particles. The only thing we know about dark matter is that it is what makes it possible for galaxies to exist. Dark matter makes up 25% of the entire Universe, 5 times more than the normal matter that we know.

Dark energy

Something even more mysterious, called dark energy, makes up 70% of the Universe, besides normal matter and dark matter. Normal and dark matter generate gravity which holds things together. Dark energy is completely opposite to gravity it causes things to fly apart. To understand dark energy, think about what happens when you toss a ball up into the air. It goes up and gradually slows down due to the pull of gravity. Eventually, the ball stops in mid-air and falls back to the ground. Now imagine a ball, once tossed up, keeps flying up further and further, instead of being attracted back down to the ground. This event seems impossible to happen, but this is the property of dark energy.
In the 1990s, dark energy was discovered as astronomers observed such peculiar effect in the Universe. Scientists know that Universe has continually expanded i.e. galaxies moving away from each other, since its formation, the Big Bang. They also observed that the speed of expansion has increased, which is unexpected, because like the tossed ball, the expansion should slow down as gravity pulled on all of the galaxies. Or, the Universe would stop expanding and finally collapse, if gravity won and halted the expansion. Scientists concluded that the accelerated expansion cannot be caused by dark matter and normal matter, which generate gravity for the Universe, but must be by some form of mysterious energy. As it is invisible, they called it dark energy. Remember that the Universe has always been expanding dark energy only speeds up this expansion. By measuring how fast the Universe is expanding, which is a combined effect of dark matter pulling galaxies together and dark energy pushing them apart, astronomers can determine the proportion between dark energy and dark matter. Like dark matter, scientists have many possible explanations, but no concrete answer for the nature of dark energy. It remains one of the greatest mysteries of the Universe. If one day we understand dark energy, it will change what we already know and the way we think about the Universe.

Introduction: (11 min)

  1. Start with the video “The known Universe” in the accompanied slideshow.
  2. Together with students, organize the structures in the Universe in a flow chart from the Earth out to Solar system, other stars and planets outside solar system, our Milky Way galaxy, other galaxies that may form galaxy clusters. Altogether, these structures exist in a vast Universe. Use background information to aid this warm-up activity.
    If not possible to use slideshow in step 1, use background information (with provided Universe image) to guide students through step 2.
  3. Elicit a discussion if it is possible that these structures are only a tiny fraction of the Universe. Start by using the background information to impart the mindset that what we commonly see on Earth and know of in space might not be representative for everything.
  4. Tell students that in this activity they will play the role of astronomers to find evidence for whether or not the Universe is only comprised of everything we can see, or something else exists. Previously available knowledge about the subject and observation of something unexpected usually help solving a mystery.
  5. Discuss with the students that they know stars and planets are beautifully organized into galaxies. And they are held in place because of gravity.

After the introduction activity, the class can be divided in small groups to do the activity. For part 1 and 2, students can explore on their own using the activity guide sheet. Between each part, give the students a quick introduction and explanation about the activity. For part 3, it is best that the teacher instructs, explains and guides the students through the steps.

Part 1: What is gravity? (7min)

  1. Use the provided background information to explain the concept of gravity as an attractive force and that this attraction can be explained as a result of space being bent.
  2. Cover a a large round bowl with a stretchy sheet . Introduce the surface of the sheet as a small portion of space and point out that this is only space in 2 dimensions but in reality space is 3 dimensions.
  3. Place a heavy marble on the sheet. Ask students to observe that there is a curvature in space (the sheet) due to the mass of the marble. Then roll a lighter marble on the sheet so that the light marble moves toward the heavier one and circles around it.

Figure 1: Gravity – stretchy sheet + marbles

Part 2: The discovery of dark matter (15 min)

  1. Use the washbowl/stretchy sheet set-up from activity 1 and place a number of marbles on the sheet. Label this as set A. Prepare, in advance, another setup (labeled set B) which is a washbowl and a stretchy sheet that has an extra heavy weight tied by a string at the middle of the sheet. Fix the sheet on the washbowl so that the extra weight is hidden and hanged underneath. Cover the dip created by the hanging extra weight by placing marbles on the sheet (total weight and number of marbles are same as set A).
    Note: Prepare in advance separate marble bags for the two setups to bring into class.

    Figure 2: Dark matter experiment Set A



Figure 3: Dark matter experiment Set B


Figure 4: Dark matter set B


Figure 4: Dark matter experiment Set A (left) vs Set B (right)

Part 3: Dark energy (15min)

    Use the accompanying materials (slideshow or chart) to point out scientists calculated that all the visible matter (seen in the warm-up video) contributes only 5% of the Universe. Dark matter is only 25% of the Universe. The remaining 70% is something else completely different from normal matter and dark matter. It’s called dark energy and, like dark matter, also invisible.

Tell students that dark energy was discovered from an unexpected observation, different from the facts that we already knew and what we expected. Present students
Fact 1: The Universe has continually expanded since its formation. It’s like an expanding balloon.

Slightly inflate a balloon with some dots drawn on it. Tell students that this is our Universe at birth, everything is compact (the dots are close together). Continue to blow the balloon and the dots on the balloon move far away from each other. This is like our Universe, it has been expanding and the dots are like the galaxies moving apart in our expanding Universe.

Figure 5: Dark energy experiment balloons

Part 6: Wrap up (3 min)

  1. Conclude that the normal matter we know is only a tiny fraction the main components of our Universe are invisible. However, it’s still not known what dark matter and dark energy really are. Scientists are still working and debating on different theories they have.
  2. Go through the entire activity with the students to point out how they have worked like a scientist. Working in the field of science means there will always be unexpected results, which must be carefully recorded as they may signal for something new to be discovered. To solve the mysteries, it requires observation, deduction, make hypothesis based on observation and available knowledge, and finding proof to support one’s idea.
  3. Point out the fluid nature of science as our view and understanding continuously evolve with changes or updates in current knowledge, made by observations, measurements and theories.

The use of stretchy sheet and marbles for demonstration of gravity is inspired by previous Astroedu activity ‘Model of a black hole’ (http://astroedu.iau.org/en/activities/1304/model-of-a-black-hole/).
For more activities about finding proofs for dark matter and to understand how much scientists currently know about what make dark matter and dark energy, continue with the second session using the Activity Part 2 - Understanding the nature of dark matter and dark energy.

In this activity, students play role of scientists to explore the possible existence of other components of the Universe besides the normal matter. As they understand how dark matter and dark energy are discovered, they also realize that science understanding can change over time as new discoveries are made, adding to or modifying our prior knowledge. They also realize that what they can see is not everything in the Universe, there are still a lot of mysteries. This activity helps students develop scientific thinking and method of scientific investigation.


It's dark out there.

"Dark matter" refers to matter of an unknown nature that many astronomers and cosmologists think must make up the majority of the mass in the universe. Its presence is revealed by the gravitational effects on objects that we can see. According to the current understanding of how gravity works, the way the visible matter behaves indicates that there should be much more matter than we can detect &mdash and therefore, much more mass exerting a gravitational influence &mdash in objects in space, like stars in galaxies, or galaxies in clusters. The clusters move at speeds that are too high to be attributed just to the visible galaxies.

If we were to apply the rules of gravity to the matter that we can see, galaxies (and galaxy clusters) would fly apart, losing the swiftly moving outer components, because there isn't enough mass (and therefore gravity) present to hold them in place. So, if more mass is added to the visible matter, the equations work, objects remain in their paths, and everything makes sense, mathematically. Results from the Wilkinson Microwave Anisotropy Probe (WMAP) show that roughy 1/4 of the mass of the universe is composed of dark matter. But what is it, really?

There are many theories as to what comprises dark matter. It is unlikely to be any one substance, but rather a variety of substances that contribute to the total needed to hold things together. Some aspects are fairly certain: the objects would not give off much, if any, visible light, and so may include black holes, brown dwarfs, neutron stars, red dwarfs, and planets they may also be very small individually, distributed somewhat equally in a very large volume, like a cloud. That could include particles such as axions, neutrinos and neutralinos, as well as other particles, both exotic and commonplace. However, there are still many missing pieces to this puzzle, and scientists continue to search for more pieces.

While the individual components are difficult to find, the influence of dark matter is easily detected, and comparatively simple to measure. Astronomers measure high temperature gas in these galaxy clusters. This gas is at too high a temperature to remain bound to the cluster without some additional mass, hidden from view. For galaxies and groups, the X-ray data have often indicated very extended dark matter halos far beyond the radius at which one sees starlight or galaxies. The total inferred dark matter mass is often several times that in the "visible" galaxies alone. Additionally, a phenomenon called gravitational lensing acts as a more visually obvious way to demonstrate how a galaxy's mass (and therefore gravity) can bend the rays of light traveling from a distant object to Earth.


Dark star (dark matter)

A dark star is a type of star that may have existed early in the universe before conventional stars were able to form and thrive. The stars would be composed mostly of normal matter, like modern stars, but a high concentration of neutralino dark matter present within them would generate heat via annihilation reactions between the dark-matter particles. This heat would prevent such stars from collapsing into the relatively compact and dense sizes of modern stars and therefore prevent nuclear fusion among the 'normal' matter atoms from being initiated. [1]

Under this model, a dark star is predicted to be an enormous cloud of molecular hydrogen and helium ranging between 4 and 2,000 astronomical units in diameter and with a surface temperature and luminosity low enough that the emitted radiation would be invisible to the naked eye. [2]

In the unlikely event that dark stars have endured to the modern era, they could be detectable by their emissions of gamma rays, neutrinos, and antimatter and would be associated with clouds of cold molecular hydrogen gas that normally would not harbor such energetic, extreme, and rare particles. [3] [2]


National Aeronautics and Space Administration

The chief property of dark matter is that it is "dark", i.e. that it emits no light. Not visible, not x-ray, not infrared. So it is not large clouds of hydrogen gas, since we can usually detect such clouds in the infrared or radio. In addition, dark matter must interact with visible matter gravitationally. So the dark matter must be massive enough to cause the gravitational effects that we see in galaxies and clusters of galaxies. Large clouds of hydrogen gas don't have enough mass to do what the dark matter does.

The two main categories of objects that scientists consider as possibilities for dark matter include MACHOs, and WIMPs. These are acronyms which help us to remember what they represent. Listed below are some of the pros and cons for the likelihood that they might be a component of dark matter.

MACHOs (MAssive Compact Halo Objects): MACHOs are objects ranging in size from small stars to super massive black holes. MACHOS are made of ordinary matter (like protons, neutrons and electrons). They may be black holes, neutron stars, or brown dwarfs.

Neutron Stars and Black Holes are the final result of a supernova of a massive star. They are both compact objects resulting from the supernovae of very massive stars. Neutron stars are 1.4 to 3 times the mass of the sun. Black holes are greater than 3 times the mass of the sun. Because a supernova usually leaves behind a remnant cloud of gas, these objects must travel far from the remnant to be "hidden."

Pros:Neutron stars are very massive, and if they are isolated, they both can be dark.
Cons:Because they result from supernovae, they are not necessarily common objects. As a result of a supernova, a release of a massive amount of energy and heavy elements should occur. However, there is no such evidence that they occur in sufficient numbers in the halo of galaxies.

Brown Dwarfs have a mass that is less than eight percent of the mass of the Sun, resulting in a mass too small to produce the nuclear reactions that make stars shine.

Astronomers have been detecting MACHOs using their gravitational effects on the light from distant objects. In formulating his theory of gravity, Einstein discovered that the gravitational attraction of a massive object can bend the path of a light ray, much like a lens does. So when a massive object passes in front of a distant object (e.g. a star or another galaxy), the light from the distant object is "focused" and the object appears brighter for a short time. Astronomers search for MACHOs (usually brown dwarfs) in the halo of our galaxy by monitoring the brightness of stars near the center of our galaxy and of stars in the Large Magellanic Cloud.

The MACHO Project, one of the groups using this "gravitational lens" technique, observed about 15 lensing events toward the LMC over a span of 6 years of observations. They set a limit of 20% as the contribution to the dark matter in our Galaxy due to objects with mass less than 0.5 that of the sun.

Pros:Astronomers have observed objects that are either brown dwarfs or large planets around other stars using the properties of gravitational lenses.
Cons:While they have been observed, astronomers have found no evidence of a large enough population of brown dwarfs that would account for all the dark matter in our Galaxy.

WIMPs (Weakly Interacting Massive Particles): WIMPs are the subatomic particles which are not made up of ordinary matter. They are "weakly interacting" because they can pass through ordinary matter without any effects. They are "massive" in the sense of having mass (whether they are light or heavy depends on the particle). The prime candidates include neutrinos, axions, and neutralinos.

Neutrinos were first "invented" by physicists in the early 20th century to help make particle physics interactions work properly. They were later discovered, and physicists and astronomers had a good idea how many neutrinos there are in the universe. But they were thought to be without mass. However, in 1998 one type of neutrino was discovered to have a mass, albeit very small. This mass is too small for the neutrino to contribute significantly to the dark matter.

Axions are particles which have been proposed to explain the absence of an electrical dipole moment for the neutron. They thus serve a purpose for both particle physics and for astronomy. Although axions may not have much mass, they would have been produced abundantly in the Big Bang. Current searches for axions include laboratory experiments, and searches in the halo of our Galaxy and in the Sun.

Neutralinos are members of another set of particles which has been proposed as part of a physics theory known as supersymmetry. This theory is one that attempts to unify all the known forces in physics. Neutralinos are massive particles (they may be 30x to 5000x the mass of the proton), but they are the lightest of the electrically neutral supersymmetric particles. Astronomers and physicists are developing ways of detecting the neutralino either underground or searching the universe for signs of their interactions.

Pros:Theoretically, there is the possibility that very massive subatomic particles, created in the right amounts, and with the right properties in the first moments of time after the Big Bang, are the dark matter of the universe. These particles are also important to physicist who seek to understand the nature of sub-atomic physics.
Cons:The neutrino does not have enough mass to be a major component of Dark Matter. Observations have so far not detected axions or neutralinos.

There are other factors which help scientists determine the mix between MACHOs and WIMPs as components of the dark matter. Recent results by the WMAP satellite show that our universe is made up of only 4% ordinary matter. This seems to exclude a large component of MACHOs. About 23% of our universe is dark matter. This favors the dark matter being made up mostly of some type of WIMP. However, the evolution of structure in the universe indicates that the dark matter must not be fast moving, since fast moving particles prevent the clumping of matter in the universe. So while neutrinos may make up part of the dark matter, they are not a major component. Particles such as the axion and neutralino appear to have the appropriate properties to be dark matter. However, they have yet to be detected.


Milky Way’s Dark Matter Halo is Slowing Down Galactic Bar’s Spin, Astronomers Say

According to new research by astronomers from the University of Oxford and University College London, the spin of the bar of our Milky Way Galaxy — made up of billions of stars and trillions of solar masses — has slowed by more than 24% since its formation galaxy models have long predicted such a slowdown by the postulated dark halo, but this is the first time it has been measured.

An artist’s conception of the Milky Way Galaxy. Image credit: Pablo Carlos Budassi / CC BY-SA 4.0.

“Astrophysicists have long suspected that the spinning bar at the center of our Galaxy is slowing down, but we have found the first evidence of this happening,” said Dr. Ralph Schoenrich, an astronomer in the Mullard Space Science Laboratory at University College London.

Dr. Schoenrich and his colleague, University of Oxford Ph.D. student Rimpei Chiba, analyzed data from ESA’s Gaia satellite on a large group of stars, the Hercules stream, which are in resonance with and gravitationally trapped by the Milky Way’s bar.

If the bar’s spin slows down, the stars in this stream would be expected to move further out in the Galaxy, keeping their orbital period matched to that of the bar’s spin.

The astronomers found that these stars carry a chemical fingerprint — they are richer in heavier elements (called metals in astronomy), proving that they have traveled away from the Galactic center, where stars and star-forming gas are about 10 times as rich in metals compared to the outer Galaxy.

Using these data, they inferred that the bar had slowed down its spin by at least 24% since it first formed.

“The counterweight slowing this spin must be dark matter,” Dr. Schoenrich said.

“Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter.”

“Our research provides a new type of measurement of dark matter — not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar’s spin.”

“Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar,” Chiba added.

“The finding also poses a major problem for alternative gravity theories — as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar.”

The results were published in the Monthly Notices of the Royal Astronomical Society.

Rimpei Chiba & Ralph Schönrich. 2021. Tree-ring structure of Galactic bar resonance. MNRAS 505 (2): 2412-2426 doi: 10.1093/mnras/stab1094


Dark matter is slowing the spin of the Milky Way’s galactic bar

The spin of the Milky Way’s galactic bar, which is made up of billions of clustered stars, has slowed by about a quarter since its formation, according to a new study by UCL and University of Oxford researchers.

For 30 years, astrophysicists have predicted such a slowdown, but this is the first time it has been measured.

The researchers say it gives a new type of insight into the nature of dark matter, which acts like a counterweight slowing the spin.

In the study, published in the Monthly Notices of the Royal Astronomical Society, researchers analysed Gaia space telescope observations of a large group of stars, the Hercules stream, which are in resonance with the bar – that is, they revolve around the galaxy at the same rate as the bar’s spin.

These stars are gravitationally trapped by the spinning bar. The same phenomenon occurs with Jupiter's Trojan and Greek asteroids, which orbit Jupiter's Lagrange points (ahead and behind Jupiter) . If the bar’s spin slows down, these stars would be expected to move further out in the galaxy, keeping their orbit al period matched to that of the bar’s spin.

The researchers found that the stars in the stream carry a chemical fingerprint – they are richer in heavier elements (called metals in astronomy), proving that they have travelled away from the galactic centre, where stars and star-forming gas are about 10 times as rich in metals compared to the outer galaxy .

Using this data, the team inferred that the bar – made up of billions of stars and trillions of solar masses – had slowed down its spin by at least 24% since it first formed.

Co-author Dr Ralph Schoenrich (UCL Mullard Space Science Laboratory) said: “Astrophysicists have long suspected that the spinning bar at the centre of our galaxy is slowing down, but we have found the first evidence of this happening.

“The counterweight slowing this spin must be dark matter. Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter.

“Our research provides a new type of measurement of dark matter – not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar’s spin.”

Co-author and PhD student Rimpei Chiba, of the University of Oxford, said: "Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar.

“Our finding also poses a major problem for alternative gravity theories – as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar.”

The Milky Way, like other galaxies, is thought to be embedded in a ‘halo’ of dark matter that extends well beyond its visible edge.

Dark matter is invisible and its nature is unknown, but its existence is inferred from galaxies behaving as if they were shrouded in significantly more mass than we can see . There is thought to be about five times as much dark matter in the Universe as ordinary, visible matter.

Alternative gravity theories such as modified Newtonian dynamics reject the idea of dark matter, instead seeking to explain discrepancies by tweaking Einstein’s theory of general relativity.

The Milky Way is a barred spiral galaxy, with a thick bar of stars in the middle and spiral arms extending through the disc outside the bar . The bar rotates in the same direction as the galaxy.

The research received support from the Royal Society, the Takenaka Scholarship Foundation, and the DiRAC supercomputing facility of the Science and Technology Facilities Council (STFC) .


Where Was Dark Matter in the Early Universe?

In the modern universe, dark matter dominates the motions of stars in the outskirts of the Milky Way and other disk-shaped galaxies. But new research suggests that wasn’t the case 10 billion years ago. Instead, galaxies were dominated by the “normal” matter that makes up gas, dust, and stars—everything we can see and touch.

Dark matter produces no detectable energy, but reveals its presence by exerting a gravitational pull on visible matter. Leading theories say it consists of subatomic particles created in the Big Bang, although efforts to find the particles using detectors in underground laboratories so far have been unsuccessful.

Astronomers discovered dark matter in part by measuring its influence on the motions of stars in spiral galaxies. If a galaxy consists only of the matter that we can see, then stars at the edge of its disk should move more slowly than those in the densely packed center, just as planets farther from the Sun move slower than those closer to the Sun.

Instead, stars on the rims of galaxies in the present-day universe move just as fast as those near the center. That means that some unseen matter is tugging at the stars to make them move faster: dark matter. Astronomers calculate that dark matter outweighs the normal matter in the universe by about five to one.

But a new study has found that when the universe was only about four billion years old, when the rate of galaxy formation reached its peak, galaxies were dominated by normal matter. The study was led by Reinhard Genzel of the University of California, Berkeley, and the Max Planck Institute for Extraterrestrial Physics in Germany and published in the March 16 issue of Nature.

Researchers measured the rotation of six disk-shaped galaxies with the Very Large Telescope in Chile. The galaxies are all of similar mass to the Milky Way. The measurements showed that stars in the inner regions of the galaxies moved much faster than those at the edges, suggesting that the role of dark matter was “modest to negligible,” according to the Nature paper.

Several other recent studies have found similar results. One averaged the observations of about 100 galaxies, while another modeled the motions of 240 galaxies. A third, led by team member Hannah Übler but not yet published, “also shows that the contribution of dark matter to the dynamical mass on the galaxy scale is larger for galaxies that are [closer],” Übler said in an email.

The findings don’t mean that dark matter didn’t exist in that earlier epoch. Normal matter was just more densely packed in the younger, smaller early universe, the researchers say.

“Our early, star-forming galaxies are very gas rich and compact,” Genzel and Übler said in the email. “Gas. can migrate to the centers of galaxies through loss of angular momentum, creating dense galactic cores and disks.” With the normal matter squeezed together more tightly, it exerted a stronger gravitational influence on stars at a galaxy’s edge than it does today.

To more fully understand the role of dark matter in the earlier universe, the team plans to study less massive galaxies, “which are the progenitors of galaxies like our Milky Way,” Genzel and Übler said. “Will they have a different contribution from dark matter to their dynamics, as it seems to be the case for the Milky Way today? This is what we want to find out next.”