Tag Archives: #darkmatter

How To Detect And Confirm The Presence Of Antistars? (Cosmology)

In the recent paper, Postnov and colleagues proposed an idea to detect electromagnetic signal from annihilation in outer layers of antistars. Their idea is to search for antistars in the Galaxy through X-rays in the ∼ (1–10) keV energy band. The reason is, prior to annihilation, protons and antiprotons could form atomic-type excited bound states (‘protonium’, Pn), similar to 𝑒+𝑒¯-positronium (Ps) atoms, and in the process of de-excitation of protonium, an antistar could emit not only ∼ 100-MeV gamma-rays but a noticeable flux of X-rays with energies in the keV range. Their study recently appeared in Arxiv.

Antistars are objects that could have form from smaller high baryonic number (HBB). They were created in the very early universe after the QCD phase transition at 𝑇 ∼ 100 MeV & should also populate the galactic halo. Such stars are not only too old, but also they are moving very fast, and have a highly unusual chemical content. Present observations also favored the possibility of their existence.

Now, Postnov and colleagues explored the possibility that, when antistars interact with interstellar medium (ISM) gas it can give rise to excited protonium atoms. Formation of these atoms takes place most effectively during interaction of protons with neutral (or molecular) antimatter. This can happen if an antistar has a noticeable wind mass-loss.

“These (protonium) atoms rapidly cascade down to low levels prior to annihilation giving rise to a series of narrow lines which can be associated with the hadronic annihilation gamma-ray emission.”

— wrote authors of the study.

They have also shown that these protonium atoms cascade to the 2p-state producing mostly L (Balmer) 3d-2p X-rays around ∼ 1.7 keV line before the 𝑝𝑝¯ hadronic annihilation.

While, antistars formed in higher HBBs should have an enhanced helium abundance. Therefore, the 4.86 keV M (4-3) and 11.13 L (3-2) narrow X-ray lines from cascade transitions in ⁴He𝑝¯ atoms can also be associated with gamma-rays from hadronic annihilations.

“These lines are interesting from the observational point of view because the protonium 3d-2p transition line energy 1.73 keV is close to the Si K-shell complex lines, which could hamper its disentangling from the background.”

— wrote authors of the study.

Finally, it has been suggested, these lines can be probed in dedicated observations by forthcoming sensitive X-ray spectroscopic missions XRISM and Athena and in wide-field X-ray surveys like SRG/eROSITA all-sky survey.


Reference: Bondar et al., “X-ray signature of antistars in the Galaxy”, pp. 1-10, 2021.
arXiv:2109.12699


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Can Direct Inflaton Couplings Give Rise To Dark Matter? (Quantum)

Oleg Lebedev and colleagues studied scalar dark matter production and reheating via direct inflaton/renormalizable couplings. They found that dark matter (DM) can be abundantly produced even at very small couplings. Their study recently appeared in Arxiv.

The unique renormalizable gauge–invariant interaction between the Standard Model and the inflaton φ has the form:

It is thus natural to expect that these couplings play a leading role in producing the Standard Model (SM) particles after inflation, i.e. reheating. If dark matter is a scalar field s, analogous renormalizable terms can be written down for the interaction between inflation and scalar field. Although a similar statement applies to the Higgs–DM interaction, in the present work Lebedev and colleagues focused on non–thermal dark matter and assumed that such couplings are negligible. The above interactions are sufficient to fully describe both dark matter production and reheating.

Figure 1: Energy density of the inflaton and the Higgs normalized to the total energy density for the φ² preheating potential with λφh = 10¯4, mφ = 5 × 10¯6 in Planck units. Left: no Higgs self–interaction. Right: λh = 10¯2. Produced with LATTICEEASY. © Lebedev et al.

After inflation ends, the inflaton oscillation epoch sets in. During this epoch, particle production can be very efficient. Indeed, renormalizable couplings of the above type can lead to parametric or tachyonic resonance signifying explosive particle production. However, Lebedev and colleagues by using LATTICEEASY tool, found that the collective effects such as resonances, back scattering and rescattering of the produced particles make a crucial impact on the dynamics of the system and the resulting dark matter abundance. In particular, the produced DM quanta can efficiently scatter against the inflaton background thereby destroying it and possibly bringing the system to quasi–equilibrium state. In this case, the dark matter abundance becomes independent of the inflaton–dark matter coupling and is described by a universal formula.

“The renormalizable inflaton couplings to dark matter and the Higgs field are sufficient to fully describe the reheating and DM production, leading to a realistic picture of the Early Universe.”, they concluded.


Reference: Oleg Lebedev, Fedor Smirnov, Timofey Solomko, Jong-Hyun Yoon, “Dark matter production and reheating via direct inflaton couplings: collective effects”, pp. 1-19, 2021.
arXiv:2107.06292


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Which Profile Is More Appropriate To Represent Dark Matter Haloes & Estimating Galaxy Total Mass? (Cosmology)

Rotation curves are major tools for determining the dynamical mass distribution in the Milky Way and spiral galaxies. Discoveries of extended rotation curves have suggested the presence of dark matter in spiral galaxy haloes. It has led to many studies that estimated the galaxy total mass, mostly by using the Navarro Frenk and White (NFW) density profile.

Now, Jiao and colleagues aimed at verifying how the choice of the dark-matter profile may affect the predicted values of extrapolated total masses.

They have considered the recent Milky Way rotation curve (MW RC) for two reasons, firstly because of its unprecedented accuracy, and secondly because the Galactic disk is amongst the least affected by past major mergers having fully reshaped the initial disk.

They found that, for calculating the dark-matter contribution to the Milky Way rotation curve, the use of NFW profile (or its generalized form, gNFW) generates apparently inconsistent results, e.g., an increase of the baryonic mass leads to increase of the dark matter mass.

In addition, it has been found that NFW and gNFW profile narrow the total mass range, leading to a possible methodological bias particularly against small milky way (MW) masses.

Fig. 1. Top: Contribution to the rotation curve of different baryonic models and model components. Red points indicate the rotation curve of the Milky Way. The error-bars are estimated via bootstrapping and include the systematic uncertainties from the neglected term (see text). Bottom: Fit of the rotation curve by the best-fit model (solid blue curve, total mass of 2.6 × 10¹¹ M), and with the most massive MW model for which the χ² probability reaches P=0.05 (orange dash-dotted line, total mass of 18 × 10¹¹ M), both associated to the baryonic distribution from model I of Pouliasis et al. © Jiao et al.

Finally, they suggested, the use of Einasto profile is more appropriate to represent cold dark matter haloes and found that the Milky Way slightly decreasing rotation curve favors total mass that can be as small as 2.6 ×10¹¹ M, disregarding any other dynamical tracers further out in the Milky Way. It is inconsistent with values larger than 18 ×10¹¹ M for any kind of cold dark matter (CDM) halo profiles, under the assumption that stars and gas do not influence the predicted dark matter distribution in the Milky Way.

“Our study encourages the use of the Einasto profile for characterizing rotation curves with the aim of evaluating their total masses.”

— concluded authors of the study

Reference: Yongjun JIAO, Francois HAMMER, Jianling WANG, Yanbin YANG, “Which Milky Way masses are consistent with the slightly declining 5-25 kpc rotation curve?”, Arxiv, pp. 1-10, 2021. https://arxiv.org/abs/2107.00014


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What Are The Effects Of Dark Matter On Stars Present In Globular Clusters? (Cosmology)

If you would ever study rotational curves of spiral galaxies you will realize that 80% mass of galaxies is in the form of dark matter (DM). By studying DM density profile like NFW you will came to know that, the DM is distributed non-uniformly throughout the galaxies. It means, in central regions of galaxies, DM density is the highest and when you move towards the edges of galaxies, the DM density reduces. From this, you can say that all the objects inside galaxies like stars, stellar clusters etc. are immersed in the DM halo. Thus, you can say that DM will affect everything inside galaxies.

Stars which are present inside galaxies can absorb DM particle from their surrounding and this can affect their stellar properties in two ways: First, DM particles can transfer their energy between different layers of stars. This can affect chemical composition, temperature, pressure and many more properties of stars. Secondly, DM particles can act a new source of energy inside stars if they annihilate. This can also alter luminosity and temperature of stars. So, if you consider the two stars having same masses and initial chemical compositions but different DM densities, you will find that both the stars are following different evolutionary path in the Hertzsprung-Russell diagram (H-R) diagram.

According to classical view of globular clusters (GC’s), stars inside GC’s have similar chemical compositions as they evolved from the same gaint molecular cloud. But recent photometric and spectroscopic studies of globular clusters reveal the presence of more-than-one or multiple stellar populations (MSPP) inside globular clusters. This finding challenges our classical view of globular clusters. Now, Hassani and Mousavi investigated the possibility of solving MSPP in globular cluster using DM assumptions.

The results of their simulations, showed that stars in different locations of globular clusters (corresponding to different dark matter densities) follow different evolutionary paths (e.g. on Hertzsprung-Russell diagram). It means, stars in high dark matter density environments like the central region of globular clusters, are more affected by the presence of dark matter than the stars present in outer regions of same GC’s.

Depending on their mass, stars convert Hydrogen to Helium through PP or CNO energy production cycles. Thus, it has been shown that, the presence of dark matter can alter these energy production cycles inside stars and can affect their chemical compositions.

But, these two cycles are also strong function of temperature. Thus, if the DM annihilate inside stars, it can alter the core temperature too, which causes stars to consume Hydrogen atoms at different rates in comparison to the models without DM. Thus, dark matter presence can affect the elemental abundances of star too. Overall, they found that, if the presence of DM can alter the temperature of stars, then it can alter their age, luminosity, and many other physical parameters of stars too.

Finally, they concluded that, if the presence of DM alters the luminosity, temperature, chemical composition, age, etc. of stars, then its presence can be considered as a possible solution to the multiple stellar populations problem in GCs.

All images credit (except featured): Hassani and Mousavi


Reference: Ebrahim Hassani, Seyyed Milad Ghaffarpour Mousavi, “Dark Matter Effects on Stellar Populations in Globular Clusters”, Arxiv, pp. 1-11, 2021.
arXiv:2106.03398


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The Mystery Of The Missing Dark Matter (Cosmology)

New distance measurements of the diffuse spheroid galaxy Ngc 1052-Df2 place this galaxy at a distance of 72 million light years and confirm that the galaxy is practically devoid of dark matter, a very rare case in the galaxy landscape. This absence of dark matter compared to other galaxies suggests that dark matter exists as a real physical entity and not as a result of a different law of gravitation on a galactic scale.

According to the current paradigm, dark matter makes up about 86% of all matter in our Universe. Its peculiarity is that it does not interact electromagnetically like ordinary matter, but only by gravitational way . For this reason it is difficult to study it, in fact it can only be detected on a large scale by observing the gravitational effects it causes on ordinary matter: unfortunately there is no experimental detection.of dark matter particles. The presence of this matter has been deduced thanks to studies on the velocity curves of spiral galaxies: as we move away from the nucleus of a galaxy, the stars do not decrease their speed as one might expect, but continue to move. quickly. If Newton’s law of gravity holds, this excess of speed indicates that most of the mass of galaxies is made up of invisible matter capable of holding the stars of which they are composed bound together with its own force of gravity: unlike galaxies. they would fall apart. Dark matter in the evolution of the Universe is very important because it is thanks to its intense gravitational effects that, within immense haloes of dark mattergalaxies were formed . Otherwise, after the Big Bang, ordinary matter would never have undergone any process of gravitational collapse and galaxies would not have formed. From this theoretical framework it is expected that each galaxy contains a consistent amount of dark matter: for example the value of the average ratio between dark matter and ordinary , measured for galaxies such as our Milky Way, is of the order of 30 times and increases both for more massive galaxies, and for less massive galaxies.

However, things seem more complex than that, at least as far as the galaxy Ngc 1052-Df2 is concerned . It is an ultra-diffuse galaxy with low surface brightness that is prospectively located in the constellation of the Whale, identified thanks to a large-field survey of the group of galaxies of Ngc 1052. The galaxy contains so little ordinary matter that it is practically transparent, so much is it It is true that in the images that portray it you can see the background galaxies much further away. Morphologically, this galaxy has a spheroidal appearance and does not appear to have a core, spiral arms or a disk of stars. The geometric dimensions are similar to those of the Milky Way.

In a March 2018 article published in Nature, the results of the radial velocity measurements of 10 luminous globular clusters belonging to this evanescent galaxy were published for the estimation of the total mass of the system. The result was that the ratio of dark to bright matter in Ngc 1052-Df2 was about 1, a value about 400 times lower than expected and in stark contrast to what is observed in other galaxies. Put simply, the case of NGC1052-DF2 showed that dark matter is not always coupled with baryon matter , at least on a galactic scale. To confirm this incredible result, the discovery team, led by Pieter van Dokkum of Yale University, focused on precise distance measurementby Ngc 1052-Df2, publishing a new paper in The Astrophysical Journal Letters . In the work of 2018, the distance of the galaxy was assumed to be similar to that of the group of galaxies to which it seemed to belong, namely that of Ngc 1052 at about 65 million light years from us. How does distance fit into estimating the relationship between dark and ordinary matter? To understand this, just think of the fact that the estimation of the mass of a star can be done by measuring its intrinsic brightness and this is obtained by measuring both the apparent brightness and the distance at which the star is located. By scaling this reasoning on a galactic scale we understand that if Df2 were closer to Earth than the 65 million light years adopted, thenits stars would be intrinsically weaker and less massive , so the luminous matter would make a minor contribution to the total mass (which is measured with the radial velocity of globular clusters) and the ratio between dark and luminous matter would increase accordingly. Distance measurement thus becomes a crucial parameter for determining the amount of luminous matter in the galaxy.

To measure the distance of a galaxy you need ” standard candles “, ie stars whose intrinsic brightness is known a priori . The team of astronomers, using the “Hubble” space telescope, focused on measuring the apparent brightness of the red giants located on the periphery of Ngc 1052-Df2 and which, during their evolution, all reach the same brightness peak. In this way, the difference between intrinsic and apparent brightness can be used to measure large intergalactic distances. The new distance estimate tells us that Df2 is 72 million light years awaythat is, the galaxy is further away than the original estimate of 65 million light years. From here it follows that Df2 is really devoid of dark matter, it is not an observational bias .

Moreover, Df2 is not the only galaxy without dark matter, another galaxy, Ngc 1052-Df4 , is also devoid of dark matter. In this case, however, some scientists suggest that dark matter may have been removed from the galaxy due to tidal forces exerted by another passing galaxy.

The discovery of these galaxies devoid of dark matter, paradoxically, confirms that dark matter really exists. In fact, if dark matter were only an effect of a gravitational law different from the Newtonian one, all galaxies should show its presence. The fact that there are galaxies without dark matter means that something is really missing in their structure. Understanding why Df2 is devoid of dark matter will require further observation, the mystery continues.

Featured image: The galaxy poor in dark matter Ngc 1052-Df2 taken with the Hubble Advanced Camera for Surveys between December 2020 and March 2021. The galaxy is so poor in matter that, through it, you can see the background galaxies (Credits: Nasa , Esa, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (Ias), Alyssa Pagan (STScI))


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Is Dark Matter Real, Or Have We Misunderstood Gravity? (Cosmology)

For many years now, astronomers and physicists have been in a conflict. Is the mysterious dark matter that we observe deep in the Universe real, or is what we see the result of subtle deviations from the laws of gravity as we know them? In 2016, Dutch physicist Erik Verlinde proposed a theory of the second kind: emergent gravity. New research, published in Astronomy & Astrophysics this week, pushes the limits of dark matter observations to the unknown outer regions of galaxies, and in doing so re-evaluates several dark matter models and alternative theories of gravity. Measurements of the gravity of 259,000 isolated galaxies show a very close relation between the contributions of dark matter and those of ordinary matter, as predicted in Verlinde’s theory of emergent gravity and an alternative model called Modified Newtonian Dynamics. However, the results also appear to agree with a computer simulation of the Universe that assumes that dark matter is ‘real stuff’.

The new research was carried out by an international team of astronomers, led by Margot Brouwer (RUG and UvA).  Further important roles were played by Kyle Oman (RUG and Durham University) and Edwin Valentijn (RUG). In 2016, Brouwer also performed a first test of Verlinde’s ideas; this time, Verlinde himself also joined the research team.

Matter or gravity?

So far, dark matter has never been observed directly – hence the name. What astronomers observe in the night sky are the consequences of matter that is potentially present: bending of starlight, stars that move faster than expected, and even effects on the motion of entire galaxies. Without a doubt all of these effects are caused by gravity, but the question is: are we truly observing additional gravity, caused by invisible matter, or are the laws of gravity themselves the thing that we haven’t fully understood yet?

To answer this question, the new research uses a similar method to the one used in the original test in 2016. Brouwer and her colleagues make use of an ongoing series of photographic measurements that started ten years ago: the KiloDegree Survey (KiDS), performed using ESO’s VLT Survey Telescope in Chili. In these observations one measures how starlight from far away galaxies is bent by gravity on its way to our telescopes. Whereas in 2016 the measurements of such ‘lens effects’ only covered an area of about 180 square degrees on the night sky, in the mean time this has been extended to about 1000 square degrees – allowing the researchers to measure the distribution of gravity in around a million different galaxies.

Comparative testing

Brouwer and her colleagues selected over 259,000 isolated galaxies, for which they were able to measure the so-called ‘Radial Acceleration Relation’ (RAR). This RAR compares the amount of gravity expected based on the visible matter in the galaxy, to the amount of gravity that is actually present – in other words: the result shows how much ‘extra’ gravity there is, in addition to that due to normal matter. Until now, the amount of extra gravity had only been determined in the outer regions of galaxies by observing the motions of stars, and in a region about five times larger by measuring the rotational velocity of cold gas. Using the lensing effects of gravity, the researchers were now able to determine the RAR at gravitational strengths which were one hundred times smaller, allowing them to penetrate much deeper into the regions far outside the individual galaxies.

This made it possible to measure the extra gravity extremely precisely – but is this gravity the result of invisible dark matter, or do we need to improve our understanding of gravity itself? Author Kyle Oman indicates that the assumption of ‘real stuff’ at least partially appears to work: “In our research, we compare the measurements to four different theoretical models: two that assume the existence of dark matter and form the base of computer simulations of our universe, and two that modify the laws of gravity – Erik Verlinde’s model of emergent gravity and the so-called ‘Modified Newtonian Dynamics’ or MOND. One of the two dark matter simulations, MICE, makes predictions that match our measurements very nicely. It came as a surprise to us that the other simulation, BAHAMAS, led to very different predictions. That the predictions of the two models differed at all was already surprising, since the models are so similar. But moreover, we would have expected that if a difference would show up, BAHAMAS was going to perform best. BAHAMAS is a much more detailed model than MICE, approaching our current understanding of how galaxies form in a universe with dark matter much closer. Still, MICE performs better if we compare its predictions to our measurements. In the future, based on our findings, we want to further investigate what causes the differences between the simulations.”

Young and old galaxies

Thus it seems that, at least one dark matter model does appear to work. However, the alternative models of gravity also predict the measured RAR. A standoff, it seems – so how do we find out which model is correct? Margot Brouwer, who led the research team, continues: “Based on our tests, our original conclusion was that the two alternative gravity models and MICE matched the observations reasonably well. However, the most exciting part was yet to come: because we had access to over 259,000 galaxies, we could divide them into several types – relatively young, blue spiral galaxies versus relatively old, red elliptical galaxies.” Those two types of galaxies come about in very different ways: red elliptical galaxies form when different galaxies interact, for example when two blue spiral galaxies pass by each other closely, or even collide. As a result, the expectation within the particle theory of dark matter is that the ratio between regular and dark matter in the different types of galaxies can vary. Models such as Verlinde’s theory and MOND on the other hand do not make use of dark matter particles, and therefore predict a fixed ratio between the expected and measured gravity in the two types of galaxies – that is, independent of their type. Brouwer: “We discovered that the RARs for the two types of galaxies differed significantly. That would be a strong hint towards the existence of dark matter as a particle.”

RAR
A plot showing the Radial Acceleration Relation (RAR). The background is an image of the elliptical galaxy M87, showing the distance to the centre of the galaxy. The plot shows how the measurements range from high gravitational acceleration in the centre of the galaxy, to low gravitational acceleration in the far outer regions. Image: Chris Mihos (Case Western Reserve University) / ESO.

However, there is a caveat: gas. Many galaxies are probably surrounded by a diffuse cloud of hot gas, which is very difficult to observe. If it were the case that there is hardly any gas around young blue spiral galaxies, but that old red elliptical galaxies live in a large cloud of gas – of roughly the same mass as the stars themselves – then that could explain the difference in the RAR between the two types. To reach a final judgement on the measured difference, one would therefore also need to measure the amounts of diffuse gas – and this is exactly what is not possible using the KiDS telescopes. Other measurements have been done for a small group of around one hundred galaxies, and these measurements indeed found more gas around elliptical galaxies, but it is still unclear how representative those measurements are for the 259,000 galaxies that were studied in the current research.

Dark matter for the win?

If it turns out that extra gas cannot explain the difference between the two types of galaxies, then the results of the measurements are easier to understand in terms of dark matter particles than in terms of alternative models of gravity. But even then, the matter is not settled yet. While the measured differences are hard to explain using MOND, Erik Verlinde still sees a way out for his own model. Verlinde: “My current model only applies to static, isolated, spherical galaxies, so it cannot be expected to distinguish the different types of galaxies. I view these results as a challenge and inspiration to develop an asymmetric, dynamical version of my theory, in which galaxies with a different shape and history can have a different amount of ‘apparent dark matter’.”

Therefore, even after the new measurements, the dispute between dark matter and alternative gravity theories is not settled yet. Still, the new results are a major step forward: if the measured difference in gravity between the two types of galaxies is correct, then the ultimate model, whichever one that is, will have to be precise enough to explain this difference. This means in particular that many existing models can be discarded, which considerably thins out the landscape of possible explanations. On top of that, the new research shows that systematic measurements of the hot gas around galaxies are necessary. Edwin Valentijn formulates is as follows: “As observational astronomers, we have reached the point where we are able to measure the extra gravity around galaxies more precisely than we can measure the amount of visible matter. The counterintuitive conclusion is that we must first measure the presence of ordinary matter in the form of hot gas around galaxies, before future telescopes such as Euclid can finally solve the mystery of dark matter.”

Featured image: In the centre of the image the elliptical galaxy NGC5982, and to the right the spiral galaxy NGC5985. These two types of galaxies turn out to behave very differently when it comes to the extra gravity – and therefore possibly the dark matter – in their outer regions. Images: Bart Delsaert (www.delsaert.com).


Publication

The weak lensing radial acceleration relation: Constraining modified gravity and cold dark matter theories with KiDS-1000”, M. Brouwer et al., Astronomy & Astrophysics 2021.


Provided by University of Amsterdam

What Would Happen If Dark Asteroid Travels Through A Star? (Cosmology)

Anirban Das and colleagues in their recent paper showed that, when dark asteroids travels through a star, it produces shock waves, which quickly propagate to the stellar surface, where it is released in the form of a transient optical, UV or X-ray emission. They also suggested how we can search and detect such signature. Their study recently appeared in Arxiv.

If you read our articles everyday, you may came across several studies which demonstrated that light dark matter (DM) particles can capture or produce inside stars or compact objects and can change their properties like mass, orbital period, luminosity etc. However, DM could also be in the form of objects of macroscopic mass and size. Such objects are hard to detect because of their rarity. According to several studies heavy DM asteroids can pass through earth but we haven’t detected one yet since the advent of human civilization. Now, Anirban Das and colleagues suggested that dark asteroids in the mass range of 10¯20–10¯11 can pass through stars. Thus, we must look them in the stars.

Figure 1. Depiction of the phases of shock propagation (left). A cylindrical blast wave solution is matched to an N-wave (right) when the shock become weak. The N-wave is propagated along acoustic rays, and becomes strong and deposits its energy near the surface. © Anirban Das et al.

They point out that, because dark asteroids move supersonically in stars, dissipation through any non-gravitational interaction will generate shock waves. This allows the dissipated energy to quickly propagate to the stellar surface, where it is released in the form of a transient, thermal ultraviolet (UV) emission.

“Crucially, such events are correlated with the local DM density, but uncorrelated with the underlying activity of the star.”

We can detect such events without requiring a dedicated search with the help of next-generation survey telescopes. While, in a dense globular cluster, such events occur far more often than flare backgrounds, so, an existing UV telescopes could find them by monitoring regions of high DM density.

“At the opposite end of the mass range, impacts on the Sun are expected to occur annually for mass of dark matter, MDM ≲ 10¯19M, and would be energetic enough to be easily detected by solar observatories.”

“It would be interesting to see if the resolution of these instruments permits such impacts to be distinguished from solar flares. In many of these cases, it may be possible to find impact events in a reanalysis of archival data.”, concluded authors of the study.


Reference: Anirban Das, Sebastian A. R. Ellis, Philip C. Schuster, Kevin Zhou, “Stellar Shocks From Dark Matter”, Arxiv, pp. 1-13, 2021. https://arxiv.org/abs/2106.09033


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How A Supermassive Black Hole Originates? (Cosmology)

UC Riverside-led study points to a seed black hole produced by a dark matter halo collapse

Supermassive black holes, or SMBHs, are black holes with masses that are several million to billion times the mass of our sun. The Milky Way hosts an SMBH with mass a few million times the solar mass. Surprisingly, astrophysical observations show that SMBHs already existed when the universe was very young. For example, a billion solar mass black holes are found when the universe was just 6% of its current age, 13.7 billion years. How do these SMBHs in the early universe originate?

A team led by a theoretical physicist at the University of California, Riverside, has come up with an explanation: a massive seed black hole that the collapse of a dark matter halo could produce.

Dark matter halo is the halo of invisible matter surrounding a galaxy or a cluster of galaxies. Although dark matter has never been detected in laboratories, physicists remain confident this mysterious matter that makes up 85% of the universe’s matter exists. Were the visible matter of a galaxy not embedded in a dark matter halo, this matter would fly apart.

“Physicists are puzzled why SMBHs in the early universe, which are located in the central regions of dark matter halos, grow so massively in a short time,” said Hai-Bo Yu, an associate professor of physics and astronomy at UC Riverside, who led the study that appears in Astrophysical Journal Letters. “It’s like a 5-year-old child that weighs, say, 200 pounds. Such a child would astonish us all because we know the typical weight of a newborn baby and how fast this baby can grow. Where it comes to black holes, physicists have general expectations about the mass of a seed black hole and its growth rate. The presence of SMBHs suggests these general expectations have been violated, requiring new knowledge. And that’s exciting.”

A seed black hole is a black hole at its initial stage — akin to the baby stage in the life of a human.

“We can think of two reasons,” Yu added. “The seed — or ‘baby’ — black hole is either much more massive or it grows much faster than we thought, or both. The question that then arises is what are the physical mechanisms for producing a massive enough seed black hole or achieving a fast enough growth rate?”

“It takes time for black holes to grow massive by accreting surrounding matter,” said co-author Yi-Ming Zhong, a postdoctoral researcher at the Kavli Institute for Cosmological Physics at the University of Chicago. “Our paper shows that if dark matter has self-interactions then the gravothermal collapse of a halo can lead to a massive enough seed black hole. Its growth rate would be more consistent with general expectations.”

In astrophysics, a popular mechanism used to explain SMBHs is the collapse of pristine gas in protogalaxies in the early universe.

“This mechanism, however, cannot produce a massive enough seed black hole to accommodate newly observed SMBHs — unless the seed black hole experienced an extremely fast growth rate,” Yu said. “Our work provides an alternative explanation: a self-interacting dark matter halo experiences gravothermal instability and its central region collapses into a seed black hole.”

The explanation Yu and his colleagues propose works in the following way:

Dark matter particles first cluster together under the influence of gravity and form a dark matter halo. During the evolution of the halo, two competing forces — gravity and pressure — operate. While gravity pulls dark matter particles inward, pressure pushes them outward. If dark matter particles have no self-interactions, then, as gravity pulls them toward the central halo, they become hotter, that is, they move faster, the pressure increases effectively, and they bounce back. However, in the case of self-interacting dark matter, dark matter self-interactions can transport the heat from those “hotter” particles to nearby colder ones. This makes it difficult for the dark matter particles to bounce back.

Yu explained that the central halo, which would collapse into a black hole, has angular momentum, meaning, it rotates. The self-interactions can induce viscosity, or “friction,” that dissipates the angular momentum. During the collapse process, the central halo, which has a fixed mass, shrinks in radius and slows down in rotation due to viscosity. As the evolution continues, the central halo eventually collapses into a singular state: a seed black hole. This seed can grow more massive by accreting surrounding baryonic — or visible — matter such as gas and stars.

“The advantage of our scenario is that the mass of the seed black hole can be high since it is produced by the collapse of a dark matter halo,” Yu said. “Thus, it can grow into a supermassive black hole in a relatively short timescale.”

The new work is novel in that the researchers identify the importance of baryons–ordinary atomic and molecular particles — for this idea to work.

“First, we show the presence of baryons, such as gas and stars, can significantly speed up the onset of the gravothermal collapse of a halo and a seed black hole could be created early enough,” said Wei-Xiang Feng, Yu’s graduate student and a co-author on the paper. “Second, we show the self-interactions can induce viscosity that dissipates the angular momentum remnant of the central halo. Third, we develop a method to examine the condition for triggering general relativistic instability of the collapsed halo, which ensures a seed black hole could form if the condition is satisfied.”

Over the past decade, Yu has explored novel predictions of dark matter self-interactions and their observational consequences. His work has shown that self-interacting dark matter can provide a good explanation for the observed motion of stars and gas in galaxies.

“In many galaxies, stars and gas dominate their central regions,” he said. “Thus, it’s natural to ask how the presence of this baryonic matter affects the collapse process. We show it will speed up the onset of the collapse. This feature is exactly what we need to explain the origin of supermassive black holes in the early universe. The self-interactions also lead to viscosity that can dissipate angular momentum of the central halo and further help the collapse process.”

The study was funded by the U.S. Department of Energy; NASA; the Kavli Institute for Cosmological Physics; and the John Templeton Foundation.

The research paper is titled “Seeding Supermassive Black Holes with Self-Interacting Dark Matter: A Unified Scenario with Baryons.”

Featured image: Hai-Bo Yu is a theoretical physicist at UC Riverside with expertise in the particle properties of dark matter. © Samantha Tieu.


Provided by University of California Riverside

Dark Matter is Slowing the Spin of the Milky Way’s Galactic Bar (Cosmology)

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 orbital 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 Physics & Astronomy) 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 Science and Technology Facilities Council (STFC).

Image

  • An artist’s conception of the Milky Way. Source: Wikimedia Commons. Credit: Pablo Carlos Budassi.

Link


Reference: Rimpei Chiba, Ralph Schönrich, Tree-ring structure of Galactic bar resonance, Monthly Notices of the Royal Astronomical Society, Volume 505, Issue 2, August 2021, Pages 2412–2426, https://doi.org/10.1093/mnras/stab1094


Provided by UCL