Dark Sector Experiments

A dark energy experiment was recently searching for a so-called scalar “chameleon field”. Chameleon particles could be an explanation for dark energy. They would have to make the field strength vanishingly small when they are in regions of significant matter density, coupling to matter more weakly than does gravity. But in low-density regions, say between the galaxies, the chameleon particle would exert a long range force.

Chameleons can decay to photons, so that provides a way to detect them, if they actually exist.

Chameleon particles were originally suggested by Justin Khoury of the University of Pennsylvania and another physicist around 2003. Now Khoury and Holger Muller and collaborators at UC Berkeley have performed an experiment which pushed millions of cesium atoms toward an aluminum sphere in a vacuum chamber. By changing the orientation in which the experiment is performed, the researchers can correct for the effects of gravity and compare the putative chameleon field strength to gravity.

If there were a chameleon field, then the cesium atoms should accelerate at different rates depending on the orientation, but no difference was found. The level of precision of this experiment is such that only chameleons that interact very strongly with matter have been ruled out. The team is looking to increase the precision of the experiment by additional orders of magnitude.

For now the simplest explanation for dark energy is the cosmological constant (or energy of the vacuum) as Einstein proposed almost 100 years ago.

Large_Underground_Xenon_detector_inside_watertank

The Large Underground Xenon experiment to detect dark matter (CC BY 3.0)

Dark matter search broadens

“Dark radiation” has been hypothesized for some time by some physicists. In this scenario there would be a “dark electromagnetic” force and dark matter particles could annihilate into dark photons or other dark sector particles when two dark matter particles collide with one another. This would happen infrequently, since dark matter is much more diffusely distributed than ordinary matter.

Ordinary matter clumps since it undergoes frictional and ordinary radiation processes, emitting photons. This allows it to cool it off and to become more dense under mutual gravitational forces. Dark matter rarely decays or interacts, and does not interact electromagnetically, thus no friction or ordinary radiation occurs. Essentially dark matter helps ordinary matter clump together initially since it dominates on the large scales, but on small scales ordinary matter will be dominant in certain regions. Thus the density of dark matter in the solar system is very low.

Earthbound dark matter detectors have focused on direct interaction of dark matter with atomic nuclei for the signal. John Cherry and co-authors have suggested that dark matter may not interact directly, but rather it first annihilates to light particles, which then scatter on the atomic nuclei used as targets in the direct detection experiments.

So in this scenario dark matter particles annihilate when they encounter each other, producing dark radiation, and then the dark radiation can be detected by currently existing direct detection experiments. If this is the main channel for detection, then much lower mass dark matter particles can be observed, down to of order 10 MeV (million electron-Volts), whereas current direct detection is focused on masses of several GeV (billion electron-Volts) to 100 GeV or more. (The proton rest mass is about 1 GeV)

A Nobel Prize awaits, most likely, the first unambiguous direct detection of either dark matter, or dark energy, if it is even possible.

References

https://en.wikipedia.org/wiki/Chameleon_particle – Chameleon particle

http://news.sciencemag.org/physics/2015/08/tiny-fountain-atoms-sparks-big-insights-dark-energy?rss=1 – dark energy experiment

http://www.preposterousuniverse.com/blog/2008/10/29/dark-photons/ – dark photons

http://scitechdaily.com/physicists-work-on-new-approach-to-detect-dark-matter/ – article on detecting dark matter generated dark radiation

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.231303 – Cherry et al. paper in Physical Review Letters


Most Distant Galaxy Known: over 95% of the way back to the origin

Recently, a team of astronomers from the U.S., U.K. and The Netherlands have confirmed the most distant galaxy known. This galaxy had previously been estimated to have a redshift of z = 8.57, from photometric methods, that is, from the general shape of the spectrum.

EGSY8p7-a

Image: Hubble Space Telescope, NASA/STScI

More accurate redshifts are obtained by measuring particular emission or absorption lines, which have precisely known laboratory (z = 0) wavelengths.

The team measured Lyman alpha line emission, and have determined the redshift to be z = 8.68, in good agreement with the photometric redshift. The Lyman alpha line is a main transition line in neutral hydrogen that occurs at 1216 Angstroms (.1216 microns) in the rest frame. The authors observed the line in the infrared and centered at 11,776 Angstroms (1.1776 microns) on 2 separate observing nights, detecting the Lyman alpha line each night. The redshift is given by 1 + z = 11,776/1216 = 9.68, thus z for this galaxy is 8.68.

The galaxy image is thought to be somewhat magnified by intervening dark matter gravitational lensing, but less than a factor of 2, and perhaps only around 20%.

The significance here is in the detection of Lyman alpha at such a high redshift, corresponding to a time when the universe was only 600 million years old, less than 5% of its current age. Not only does this result determine the age of this earliest known galaxy, but it also provides insight into the nature of the intergalactic medium.

The cosmic microwave background radiation is the most distant source we can see. It comes from all directions, filling the universe and reflects a time when the universe was only 380,000 years old and transitioned from ionized plasma to neutral hydrogen and helium.

Later on in the universe’s evolution, as the first galaxies and stars form, hot blue stars produce ionizing ultraviolet radiation, and the neutral gas is reionized – electrons are stripped from their atoms. This process has generally thought to have completed by redshift ~ 6, at a time when the universe was around 1 billion years old.

Lyman alpha emission is not expected in a region which is still neutral, that has not yet undergone the reionization process. So the implication here is that the surrounding intergalactic medium in the neighborhood of EGSY8p7 has already been reionized at a significantly higher redshift.

The universe does not become reionized in a uniform way, rather the process would be expected to happen in “bubbles” or regions surrounding energetic galaxies with hot blue stellar populations. Eventually all the ionized regions overlap and the intergalactic medium becomes fully ionized.

This detection helps astronomers to better understand how reionization occurred.

The team’s paper is submitted to the Astrophysical Journal Letters and can be found here:

http://arxiv.org/pdf/1507.02679v2.pdf


Galaxy Formation in an Expanding Universe: Dark Matter Halos and Supermassive Black Holes

This blog is based on a recent talk on the Horizon supercomputer simulation for galaxy formation. The talk (in English) was given at the Ecole Normale Superieure by Julien Devrient, of the University of Oxford, available on YouTube here:

The background for the simulation of galaxy formation on supercomputers is the standard Lambda-Cold Dark Matter cosmology with 4.8% ordinary matter, 26.8% dark matter and 68.4% dark energy, which are the measured values from the Planck satellite and other observations. These are the proportions at present, but until the last few billion years, dark matter was dominant over dark energy. The ratio of dark matter to ordinary matter has stayed essentially fixed since the universe was 1 second old, with about 5 times or so as much dark matter as ordinary matter.

The collisionless components to consider are cold dark matter (CDM) and stars, as the stars form inside the simulation.

Then there is a collisional fluid composed of gas, in both atomic (neutral and ionized) and molecular forms and consisting primarily of hydrogen, helium and a small amount, up to around 1% by mass, of heavy elements including carbon, nitrogen, oxygen, silicon, iron and so forth. This fraction increases during the history of the universe as star formation and evolution proceeds. This ‘primordial’ gas is heated by falling into the gravitational potential determined primarily by the CDM (but also by the ordinary matter) and it cools via various radiative processes that depend on density, temperature and composition.

There are many complicating factors and feedback processes. This is an extremely messy problem to address. Dust, supernovae, turbulent gas dynamics, magnetic fields, and black holes that merge and grow into supermassive black holes (SMBH) are all things to consider. The SMBH are surrounded by accretion disks and also may emit jets and these components are visible as highly luminous AGN (active galactic nuclei). Not all of these can be included in simulations at present, or they are treated empirically.

Although the physics is well understood for the collisionless component behavior and for the atomic and molecular gas, including the cooling (radiative) functions, the modeling must occur over many, many orders of magnitude, since scales range from less than 1 parsec to 100s of Megaparsecs (a million parsecs, where 1 parsec = 3.26 light-years). This huge range in scale, plus complex physics, makes the calculation extremely computationally expensive.

The Horizon simulation had 7 billion grid cells and 1 billion dark matter particles. The highest resolution is down to 1 kiloparsec. Gas cooling, star formation, stellar winds, two types of supernovae are included and the abundances of C, N, O, Si, Mg, and Fe tracked. Black hole formation was included. Two million CPU core hours were required for the simulation.

MultiScaleProblemFigure 1. The multi-scale problem

Many scales are involved in simulating galaxy formation – 11 or 12 orders of magnitude. Each tick mark in the above Figure 1 is 3 orders of magnitude (a factor of 1000) in linear scale. From the largest to the smallest objects (moving from right to left) we have LSS = large-scale structure: the universe has evolved into a web-like structure with filaments and sheets of galaxies and high-density and low-density regions. The scale is 100s of Megaparsecs to more than a Gigaparsec. Below this are the galaxy clusters, which are the largest gravitationally bound structures, at around 1 Megaparsec, and then galaxies which are found primarily in the 1 kiloparsec to 100 kiloparsec range.

Then within galaxies, star formation happens within molecular clouds and the scales are parsecs to 100s of parsecs. At the smallest scale, we have highly energetic active galactic nuclei (AGN), that are powered by SMBH (supermassive black holes), with millions to billions of solar masses, and have surrounding accretion disks, confined within a very small region of order 1/1000 of a parsec, reaching down towards the scale of our solar system.

multiscaleproblem.part2Figure 2. The Dark Matter Halo mass function and the galaxy mass function

It is impossible with current supercomputers and techniques to directly model across all these scales, but the Horizon-AGN Simulation, one of the largest galaxy formation simulations today, spans around 5 orders of magnitude by using adaptive mesh refinement strategies. When and where the density of matter is high and the physics is interesting, an increasingly finer mesh is employed for the calculations. Without this method, it would be impossible to make progress.

Galaxies are formed within the gravitational potentials of dark matter halos (DMH). There is about 5 times as much mass in dark matter as in ordinary matter (baryons, e.g. protons and neutrons). So the ordinary matter falls into the gravitational potentials of DMH, is heated up, and cools by radiation which allows for further collapse, and so on until galaxies are formed.

The interesting scales for DMH are from about 100 billion to 1000 trillion solar masses. The size distribution for the density perturbations that self-collapse under their own gravity follows a power law (with an index of close to -1 in the inverse linear scale). This comes from the cosmic microwave background measurements and inflationary Big Bang theory. How these density perturbations evolve and collapse to DMH is now a well-studied problem in cosmology.

One might assume that each DMH results in a single galaxy, and in the mid-range, this matches observations fairly well. But at the low-end and the high-end, this simple model breaks down, when comparison is made to the observed galaxy mass function (which is simply a measurement of how many galaxies we see per unit volume with a given mass).

At the low end we see fewer galaxies than expected. These are very faint however more and more dwarf galaxies with low luminosity yet with significant mass dominated by dark matter are being detected, and this is helping to resolve this issue. An important factor is most likely feedback from supernovae. As supernovae explode they produce blast waves which drive gas out and prevent molecular cloud formation and star formation.

Supernova physics is tricky as it can result in gas compression which enhances the star formation rate but also can drive gas out of a galaxy, partcularly if it is smaller and has a lower gravitational field, and this suppresses star formation.

In the left panel of Figure 2 above, the first black line is the DMH mass function, and the second black line is just shifted to the left by the baryon to dark matter ratio. What is being plotted is the frequency of galaxies expected for a given mass.  The actual observed curve for galaxy stellar masses is in red, and one sees fewer galaxies at the low end and especially at the very high end. The right panel shows the observational data which is replotted as the red line in the left panel.

At the high end of the mass function there are fewer galaxies with a rapid cutoff around 1 to 10 trillion solar masses for baryon content, which is about an order of magnitude lower than the DMH  mass function would suggest. At the high end it is believed that feedback from AGN (SMBH) is the cause of inhibited star formation, placing a limit on the maximum size of an individual galaxy. Of course multiple galaxies may form out of a single halo as well.

horizonagnFigure 3. The Horizon simulation without and with Active Galactic Nuclei included

The upper panel on the right in Figure 3 is the simulation without AGN, the lower one with AGN. The simulation including AGN is a better fit to observed galaxy properties.

The simulation had 7 billion grid cells and 1 billion dark matter particles. The highest resolution is down to 1 kiloparsec. Gas cooling, star formation, stellar winds, two types of supernovae are included and the abundances of C, N, O, Si, Mg, and Fe tracked. Black hole formation was included. Two million CPU core hours were required for the simulation.

Including modeling of AGN, the larger galaxies in the simulation are less massive and dimmer, and are more likely to be ellipticals than spiral galaxies. The high mass galaxies in the center of clusters are generally observed to be ellipticals, so this is a desired result.

There is much room for refining and improving galaxy simulation work, including adding additional physics and more small-scale resolution to the models. I encourage you to look at the YouTube video, there are many other interesting results discussed by Prof. Devrient from the Horizon-AGN simulation work.

References:

https://www.youtube.com/watch?v=ZRDITkkqqUg – Prof. Devrient’s talk

http://www.horizon-simulation.org/about.html – Horizon simulation home page


Black Holes Destroy Dark Matter (and Emit Gamma Rays)

Black holes can cause dark matter to annihilate in their vicinity by concentrating the dark matter and enhancing the collision rate between dark matter particles. The best observational candidates are supermassive black holes, such as the 4 million solar mass black hole found at the center of our Milky Way galaxy. Some galaxies have much larger supermassive black holes, reaching as high as several billion or even tens of billions of solar masses. Most massive galaxies appear to have supermassive black holes in their centers.

Artist's conception of a supermassive black hole (public domain; courtesy NASA JPL)

Artist’s conception of a supermassive black hole (public domain; courtesy NASA JPL)

We infer the existence of supermassive black holes through their effect on nearby stellar or molecular cloud orbits. And we more directly detect supermassive black holes (SMBHs) by the radiation emitted from ordinary matter that is near the black hole (BH), but has not yet fallen into the BH’s event horizon (from which nothing, not even light, can escape). Such matter will often form a hot accretion disk around the SMBH. The disk or other infalling matter can be heated to millions of degrees by the strong gravitational potential of the BH as the kinetic energy of infall is converted to thermal energy by frictional processes. Ordinary matter (OM) heated to such high temperatures will give off X-rays.

Now if OM is being pulled into a SMBH, so is dark matter, which pervades every galaxy. Dark matter (DM) responds to the same gravitational potential from the SMBH. The difference is that OM is collisional since it easily interacts with other OM via the electromagnetic force, whereas DM is generally collisionless, since it does not interact via electromagnetism.

Nevertheless DM – DM collisions can occur, rarely, via a ‘direct hit’ (as if two bullets hit each other in mid-air) and this leads to annihilation. Two DM particles meet directly and their entire energy content, from their rest mass as well as their kinetic energy of motion, is converted into other particles. The cross-section strength is not known, but it must be small due to observational limits, yet is expected to be non-zero. The most likely candidates for decay products are expected to be photons, neutrinos, and electrons.

The leading candidate for DM is some sort of weakly interacting massive particle with a mass of perhaps 5 to 300 GeV; this is the range where DM searches from satellites and on Earth are focused. (The proton mass is a little less than 1 GeV = billion electron Volts.) So if two DM particles mutually annihilate, there is of order 10 GeV to 600 GeV of available rest mass energy to produce highly energetic gamma rays.

The likelihood of a direct hit is proportional to the square of the density of the DM. A SMBH’s gravitational potential acts as a concentrator for DM, allowing the density to be high enough that there could be a significant number of annihilation events, resulting in a detectable flux of escaping photons reaching Earth. Relativistic effects work to further increase the annihilation rate. And it is possible that the annihilation signal could scale as M³ (mass of the SMBH cubed), and thus the most massive SMBHs would be very strong gamma ray emitters. These would be highly energetic gamma rays with well over 1 GeV of energy.

Movie from NASA Goddard showing Dr. Jeremy Schnittman’s simulation

Dr. Jeremy Schnittman of the NASA Goddard Space Flight Center has investigated possible annihilation rates and the nature of the observable gamma ray spectrum for some simple dark matter models. He used a compute cluster to simulate hundreds of millions of DM particles moving in the general direction of a SMBH. One of his remarkable findings is that much higher gamma ray energies can be produced than previously believed, in the case of SMBHs which are rapidly spinning.

This is a result of something known as the Penrose process, which allows energy to be extracted from a rotating BH. There is a region called the ergosphere outside of the event horizon and when two DM particles annihilate in this region and produce two gamma rays, one gamma ray photon would fall into the event horizon (into the BH), and the other photon would escape to infinity, possibly in the direction of Earth. Dr. Schnittman’s simulation indicates that the energy boost can be as high as 6 times or more. The faster the SMBH is spinning, the greater the potential energy boost.

He also has looked at DM particles on bound orbits, which are likely to form into a (donut-shaped) corotating torus around the SMBH, aligned with its spin vector. The bound DM particle annihilations lead to lower energy gamma ray production, as compared to the unbound particles.

One of the important considerations is that the influence radius of the BH is very large. The size of the BH itself (event horizon or Schwarzschild radius) is small, even for SMBHs. The radius is proportional to the mass, via the relation 2GM/R = c² (G is the gravitational constant, c the speed of light and M and R are the BH mass and radius, respectively). A SMBH with a mass of 10 million solar  masses will have a radius of only around 30 million kilometers, or about 1/5 of the Earth-Sun distance (an AU, or astronomical unit).

But the gravitational influence is much greater, since DM particles are typically expected to be moving at around only a couple of hundred kilometers per second far away from the SMBH. Thus DM particles that are 1 million times further away than the SMBH will have their orbits in their galaxy perturbed by the SMBH. And the scale of influence is thus parsecs (1 parsec = 3.26 light-years) or tens of parsecs or even hundreds of parsecs, depending on the SMBH mass.

The most energetic gamma rays can be produced by unbound DM particles. These are on orbits which can approach near to the SMBH after falling from far away (a “swan dive” toward the SMBH) and these DM particles would then typically head out away from the SMBH in the opposite direction. But before they are able to, they have a direct hit with another DM particle and annihilate into gamma rays or some other decay products.

The search for gamma rays from annihilating DM around SMBHs is already underway. There is in fact a possible detection by the Fermi telescope at 130 GeV in our Milky Way galaxy, from the direction of the Sagittarius A* SMBH. Future more sensitive gamma ray surveys may lead to many detections, helping us to better understand both dark matter and black holes.

References

J.D. Schnittman, 2015. “The Distribution and Annihilattion of Dark Matter around Black Holes”, http://arxiv.org/abs/1506.06728

J.D. Schnittman, 2014. Phys. Rev. Letters 113, 261102,  “Revised Upper Limit to Energy Extraction from a Kerr Black Hole”


Dark Lenses Magnify Star Formation in Dusty Galaxies

Dusty star-forming galaxies (DSFGs) are found in abundance in the early universe. They are especially bright because they are experiencing a large burst of high-rate star formation. Since they are mainly at higher redshifts, we are seeing them well in the past; the high star formation rates occur typically during the early life of a galaxy.

The optical light from new and existing stars in such galaxies is heavily absorbed by interstellar dust interior to the galaxy. The dust is quite cold, normally well below 100 Kelvins. It reradiates the absorbed energy thermally at low temperatures. As a result the galaxy becomes bright in the infrared and far infrared portions of the spectrum.

Dark matter has two roles here. First of all, each dusty star-forming galaxy would have formed from a “halo” dominated by dark matter. Secondly, dark matter lenses magnify the DSFGs significantly, allowing us to observe them and get decent measurements in the first place.

An international team of 27 astronomers has observed half a dozen DSFGs at 3.6 micron and 4.5 micron infrared wavelengths with the space-borne Spitzer telescope. These objects were originally identified at far infrared wavelengths with the Herschel telescope. Combining the infrared and far infrared measurements allows the researchers to determine the galaxy stellar masses and the star formation rates.

The six DSFGs observed by the team have redshifts ranging from 1.0 to 3.3 (corresponding to  look back times of roughly 8 to 12 billion years). Each of the 6 DSFGs has been magnified by “Einstein” lenses. The lensing effect is due to intervening foreground galaxies, which are also dominated by dark matter, and thus possessing sufficient gravitational fields that are able to significantly deflect and magnify the DSFG images. Each of the 6 DSFGs is therefore magnified by a lens that is mostly dark.

The lenses can result in the images of the DSFGs appearing as ring-shaped or arc-shaped. Multiple images are also possible. The magnification factors are quite large, ranging from a factor of 4 to a factor of more than 16 times. (Without dark matter’s contribution the magnification would be very much less).

It is a delicate process to subtract out the foreground galaxy, which is much brighter. The authors build a model for the foreground galaxy light profile and gravitational lensing effect in each case. They remove the light from the foreground galaxy computationally in order to reveal the residual light from the background DSFG. And they calculate the magnification factors so that they can determine the intrinsic luminosity of the DSFGs.

The stellar masses for these 6 DSFGs are found to be in the range of 80 to 400 billion solar masses, and their star formation rates are in the range of 100 to 500 solar masses per year.

One of the 6 galaxies, nicknamed HLock12, is shown in the Spitzer infrared image below, along with the foreground galaxy. The model of the foreground galaxy is subtracted out, such that in the rightmost panes, the DSFG image is more apparent. There are two rows of images, the top row shows measurements at 3.6 microns, and the bottom row is for observations at 4.5 microns.

This particular DSFG among the six was found to have a stellar mass of 300 billion solar masses and a total mass in dust of 3 billion solar masses. So the dust component is just about 1% of the stellar component. The estimated star formation rate is 500 solar masses per year, which is hundreds of times larger than the current star formation rate in our own Milky Way galaxy.

It is only because of the significant magnification through gravitational lensing (“dark lenses”) that researchers are able to obtain good measurements of these DSFGs. This lensing due to intervening dark matter allows astronomers to advance our understanding of galaxy formation and early evolution, much more quickly than would otherwise be possible.

HLock12

The figure 6 is from the paper referenced below. The top row shows (a) a Hubble telescope image of the field in the near infrared at 1.1 microns, and (b) the field at 3.6 microns from the Spitzer telescope. The arc is quite visible in the Hubble image in the upper right quadrant just adjacent to the foreground galaxy in the center. The model for the foreground galaxy is in column (c) and after subtraction the background galaxy image is in column (d), along with several other faint objects. The corresponding images in the bottom row are from Spitzer observations at 4.5 microns.

Reference

B. Ma et al. 2015, “Spitzer Imaging of Strongly-lensed Herschel-selected Dusty Star Forming Galaxies” http://arxiv.org/pdf/1504.05254v3.pdf


Dusty Star-Forming Galaxies Brightened by Dark Matter

The first galaxies were formed within the first billion years of the Universe’s history. Our Milky Way galaxy contains very old stars with ages indicating formation around 500 or 600 million years after the Big Bang.

Astronomers are very eager to study galaxies in the early universe, in order to understand galaxy formation and evolution. They can do this by looking at the most distant galaxies. With the expanding universe of the Big Bang, the farther away a galaxy is, the farther back in time we are looking. Astronomers often use redshift to measure the distance, and hence age, of a galaxy. The larger the redshift, z, the farther back in time, and the closer to a galaxy’s birth and the universe’s birth.

The interstellar medium of a galaxy consists of gas and dust. The gas can be hot or cold, and in atomic or molecular form. Atomic gas may be ionized by ultraviolet starlight, or X-radiation from neutron stars or black holes (not the black holes themselves, but hot matter near the black hole), from cosmic rays or from other astrophysical mechanisms. Our Milky Way galaxy is rich in gas and dust, and contains thousands of molecular clouds. These are very cold clouds composed mainly of molecular hydrogen but also many other molecular species. Molecular clouds are the primary sites of new star formation. The Horsehead Nebula is an example of a molecular cloud in the constellation of Orion.

"Hubble Sees a Horsehead of a Different Color" by ESA/Hubble. Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Hubble_Sees_a_Horsehead_of_a_Different_Color.jpg#/media/File:Hubble_Sees_a_Horsehead_of_a_Different_Color.jpg

“Hubble Sees a Horsehead of a Different Color” by ESA/Hubble. Licensed under CC BY 3.0 via Wikimedia Commons 

During their most active phase of star formation, a large galaxy might give birth to over 1000 solar masses worth of stars per year. By comparison, in the Milky Way galaxy, the new star formation rate is only of order 1 solar mass per year, the equivalent of 1 Sun, or, say, 2 stars with half the mass of our Sun, per annum. Over its entire 13 billion year life the Milky Way has formed many hundreds of billions of stars, so clearly the star formation rate was higher in the past.

Before the first stars and galaxies form, the universe contains only hydrogen and helium, and no heavier elements. Those are produced by thermonuclear reactions in stellar interiors. This is a wonderful thing, because carbon, oxygen and other heavy elements are essential to life.

After a galaxy produces its first generation of massive stars, its interstellar medium will begin to contain carbon, nitrogen, oxygen and other heavy elements (heavy means anything above helium, in this context). Massive stars (above a few solar masses) evolve rapidly, with timescales in the millions of years, rather than billions, and explode as supernovae at the end of their lives. A large portion of their material, now containing heavy elements as well as hydrogen and helium, is expelled at high velocity and mixed into the interstellar medium. The carbon, nitrogen and oxygen which is then in the respective galaxy’s interstellar medium can be detected in atomic (including ionized) or molecular forms. The relative abundance of heavy elements grows with time as more stars are formed, evolve, and recycle matter into the interstellar medium.

High-redshift (z > 2) galaxies with active star formation are best observed in the infrared. The gas and dust in molecular clouds is quite cold, usually less than 100 K (100 degrees above absolute zero). And their radiation is shifted further toward the far infrared and sub-millimeter portions of the spectrum by the redshift factor of 1+z. So radiation emitted at 100 microns is detected at the Earth at 400 microns for a source at z = 3.

These are difficult measurements to make, because if the galaxy is very distant, it is also very faint. However the possibility of getting good measurements is helped by two things. One is that galaxies with very active star formation are intrinsically brighter.

And the other reason is that intervening clusters of galaxies are massive and contain mostly dark matter. As we look far back through the universe toward an early galaxy, there is a good chance that the line of sight passes through a cluster of galaxies. Clusters of galaxies contain hundreds or even thousands of galaxies, and are dominated by dark matter. Most of the infrared radiation can pass through the intracluster medium – the space between galaxies – without being absorbed; it does not interact with dark matter. The clusters are sufficiently massive to bend the light, however, according to general relativity. As the background galaxy’s light passes through the cluster during its multi-billion year journey to the Earth and our telescopes, the cluster’s gravitational potential modifies the light ray’s path. Actually the intervening cluster of galaxies does more than displace the light, it acts as a lens, causing the image to brighten by as much as 10 times or more. This makes it much easier to gather enough photons from the target galaxy to obtain good quality results.

An international research team with participants from Germany, the U.S., Chile, the U.K. and Canada has identified 20 high redshift “dusty star forming galaxies” at very high redshift (DSFG is a technical term for galaxies with high star formation rates and lots of dust) from the South Pole Telescope infrared galaxy survey. They have been able to further elucidate the nature of 17 of these early galaxies by measuring C II emission from singly ionized atomic carbon, and CO emission from carbon monoxide molecules for 11 of those. They have also determined the total far infrared luminosity for these target galaxies. Their results allow them to place constraints on the nature of the interstellar medium and the properties of molecular clouds.

The galaxies’ high redshifts, ranging from z = 2.1  to 5.7, actually makes it possible to make Earth-bound measurements in most cases. At lower redshifts the observations would not be possible from Earth because the Earth’s atmosphere is highly opaque at the observation frequencies. But it is much more transparent at longer wavelengths, so as the redshift exceeds z = 3, Earth-based observations are possible from favorable locations, in this case the Chilean desert. For three sources with redshifts around 2 the atmosphere prohibits ground-based observations and the team therefore made observations from the orbiting Herschel Space Telescope, designed for infrared work.

The figure labelled Figure 3 below is taken from their paper. It indicates the redshift z on the x-axis (logarithmically) and the far infrared luminosity of the galaxy on the y-axis (as the log) as well. The 17 galaxies studied by the authors are indicated with red dots and labelled “SPT DSFGs”. Their very high luminosities are in the range of 10 to 100 trillion times the Sun’s luminosity. Note that the luminosities must be very high for detection at such a high redshift (distance from Earth). Also these luminosities are uncorrected for the lensing magnification, so the true luminosities are around an order of magnitude lower.

CIIEmissionDSFGfig3

The redshift range covered in this research corresponds to ages for the universe of around 1 billion years old (z = 5.7) to a little over 3 billion years old (z = 2.1). So the lookback time is roughly 11 to 13 billion years.

For those of us interested in dark matter, their findings regarding the degree of magnification by dark matter are also interesting. They find “strong lensing” or magnification in the range of 5 to 21 times for 4 sources that allowed for lens modeling. The other sources do not have magnifications measured, but they are presumed to be of the same order of magnitude of around 10 times or so, to within a factor of 2 either way.

It is only because the lensing is so substantial that they are able to measure these galaxies with sufficient fidelity to arrive at their results. So not only is dark matter key to galaxy formation and evolution, it is key to allowing us to study galaxies in the early universe. Dark matter forms galaxies and then helps us understand how they form!

Reference

B. Gullberg et al. 2015, ”The nature of the [CII] emission in dusty star-forming galaxies from the SPT survey” to be published, Monthly Notices of the Royal Astronomical Society, http://arxiv.org/pdf/1501.06909v2.pdf

C.M. Casey, D. Narayanan, A. Cooray 2015, “Dusty Star-Forming Galaxies at High Redshift”, http://arxiv.org/abs/1402.1456


Dark Globular Clusters

Globular clusters are highly compact star clusters containing hundreds of thousands or even millions of old stars. The stars found in a globular cluster can be up to almost 13 billion years old, and thus act as good tracers of the early history of our Milky Way Galaxy, or of other galaxies in which they are found.

Our Galaxy has about 150 or so globular clusters, and our neighbor the Andromeda Galaxy has over 500. Because they are so compact, they are tightly gravitationally bound, and tend to be very spherical, hence the description as globular. They also orbit our Galaxy in the halo and in various directions; they are not confined to the disk where most of the other stars of the Milky Way are found. This suggests a history for globulars (as they are also called) that predates the formation of the Milky Way’s disk and spiral arms. Many globulars may be remnants of dwarf galaxies that have been pulled into the Milky Way during its long lifetime, or have been captured by the Milky Way as it consumed another, smaller galaxy in its vicinity.

M13

M13 globular cluster

The large majority of globular clusters are thought to contain little dark matter. They contain no dust or gas, their matter appears to be all in stars. Astronomers are able to measure the mass of globulars by determining how fast stars are moving around in the core, and by measuring the size of the globulars. Assuming the globulars are gravitationally relaxed, which most appear to be, then the mass within a certain radius is proportional to the radius and the square of the velocities of stars relative to the globular’s center and within that radius. Some assumptions need to be made about the falloff of matter density, and projection effects corrected, but these can be calculated and compared with observations to provide a self-consistent model.

Going through these steps, and also measuring the absolute brightness of the globular (which requires a distance measurement) allows something called a mass-to-light ratio for the globular to be determined. This is stated in terms of solar units, i.e., using the mass in relation to the Sun’s mass and the absolute brightness in relation to the Sun’s luminosity.

Typical mass-to-light ratios for most globulars are of order unity, say 1 or 2 or 3. This indicates little dark matter is present (we know the dark matter content of ordinary stars such as the Sun is low).

But now some possible dark matter-containing globular clusters have been found. To be more precise, some globular clusters with quite high mass-to-light ratios have been found around Centaurus A.

eso1519a

Centaurus A galaxy and globular clusters observed by the Very Large Telescope (ESO)

Centaurus A is a peculiar galaxy about 10 million light-years or so away, and is the fifth brightest galaxy in the sky by apparent magnitude. It is classified as a giant elliptical, or lenticular, galaxy with peculiar characteristics. It has a very prominent dust lane through the center as seen in the photograph above, and also possesses a large relativistic jet visible at radio and X-ray wavelengths. There is a black hole at the center of Centaurus A with a mass of over 50 million solar masses. Centaurus A may have as many as 2000 globular clusters.

A study of 125 of the brighter globular clusters in the Centaurus A galaxy, by Matthew Taylor and co-authors, was made at the European Southern Observatory’s Very Large Telescope in Chile. The locations of the globulars are indicated in the green, blue and red circles superimposed on Centaurus A’s image. A certain fraction of the globulars with masses above a million solar masses showed the characteristic that the mass-to-light ratio was abnormally high, and becomes higher nearly in proportion to the mass of the globular. This set of globulars is denoted in red circles in the image. The most massive of the red circle globulars have the highest mass-to-light ratios.

The authors find, quoting from their paper, “a distinct group of objects which require significant dark gravitating components such as central massive black holes and/or exotically concentrated dark matter distributions.” These objects have mass-to-light ratios above 6, and half a dozen have mass-to-light ratios over 15, including one object with a very high ratio of 67.

Several explanations are proposed. One is rotation of the globulars in question, but the authors are able to rule this explanation out since the stability of the cluster would be destroyed. Another is massive black holes in the centers of these globulars. This would require black holes with masses in the range 40,000 to over 1 million solar masses, and would be quite an exciting finding in its own right. There is evidence for intermediate mass black holes in a few other globular clusters found in the Milky Way and other galaxies. An additional possibility is the accumulation of many smaller stellar-sized black holes and/or neutron stars in the center of the cluster that would modify the cluster’s dynamical properties.

Yet another possibility is the presence of significant dark matter. If verified there would be important implications for globular cluster formation histories. This study should lead to a rush for other observations to ferret out high mass-to-light ratio globular clusters and to allow astronomers to distinguish between the black hole, dark stellar remnant, and dark matter possible scenarios.

Already there is a Wikipedia entry for “dark globular clusters”.

References

http://www.eso.org/public/news/eso1519/

http://www.eso.org/public/archives/releases/sciencepapers/eso1519/eso1519a.pdf, submitted to the Astrophysical Journal


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