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Tag Archives: galaxy formation

Supernovae Destroy Dwarf Galaxies: Dark Matter is Safe

The existence of dark matter has not exactly been under threat – the ratio of dark matter to ordinary matter in the universe is well established, at about 5:1 in favor of dark matter. Consistent results are found between observations of the cosmic microwave background, observations of clusters of galaxies, and observations of the rotation curves of galaxies. (The MOND theory as an alternative to dark matter does not do well at scales greater than that of individual galaxy rotation curves.)

But there has been an issue around galaxy formation. It has been expected that many more dwarf galaxies should be seen in our Local Group, which is dominated by the Andromeda Galaxy (#1) and our Milky Way Galaxy (#2, sorry folks), along with the aptly named Triangulum Galaxy (#3).

Where are the Dwarfs?

Our Milky Way has only around 30 dwarf galaxies as companions, the best known of which are the Large and Small Magellanic Clouds. While a few more have been discovered only recently, simulations of galaxy formation have previously suggested this number ought to be more than 1000! This posed a problem for both our understanding of dark matter and our understanding of galaxy formation.

Now, from CalTech comes a much more detailed simulation of how galaxies similar to the Milky Way are formed. The researchers used over 700,000 CPU hours of supercomputer time to create the most detailed simulation ever of the galaxy formation and evolution processes.

“In a galaxy, you have 100 billion stars, all pulling on each other, not to mention other components we don’t see like dark matter. To simulate this, we give a supercomputer equations describing those interactions and then let it crank through those equations repeatedly and see what comes out at the end.”  – Caltech’s Phil Hopkins, associate professor of theoretical astrophysics.

Death by Supernova

Postdoc Andrew Wetzel and Prof. Hopkins paid special attention to the effects of supernovae. When supernovae explode they release tremendous amounts of kinetic energy. They generate powerful winds that reach speeds of over a thousand kilometers per second.

In a dwarf galaxy an individual supernova can have substantial effect. The researchers’ simulations indicate that dwarf galaxies can actually be destroyed by the effect of even a single supernova during their early history. Stars and gas that would form future stars can both be blown out of the dwarf galaxies. In addition, many dwarf galaxies in the Milky Way’s neighborhood would have been destroyed by the gravitational tidal forces of the Milky Way, the simulations show.

These advanced galaxy evolution simulations appear to solve the dark matter and dwarf galaxy problem. The authors plan to refine their results and develop even greater understanding of galaxy formation with simulations of even greater power in the future.

simgalaxy-face-on-hr

Simulated View of Milky Way Galaxy
The formation and evolution of the galaxy were done on a supercomputer. Credit: Hopkins Research Group/Caltech

References:

https://www.caltech.edu/news/recreating-our-galaxy-supercomputer-51995

https://youtu.be/e7KuwjGGxBw

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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


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


Forming the First Galaxies: Blue Tides on Blue Waters

The largest high-redshift cosmological simulation of galaxy formation ever has been recently completed by a group of astrophysicists from the U.S. and the U.K. This tour-de-force simulation was performed on the Blue Waters Cray XE/XK system at NCSA and employed 648,000 CPU cores. They utilized approximately 700 billion particles (!) to represent dark matter and ordinary matter and to create virtual galaxies inside the supercomputer. The authors, who represent Carnegie Mellon University, UC Berkeley, Princeton University, and the University of Sussex, have given their simulation the moniker BlueTides.

The astrophysicists simulated galaxy formation in a “box” 2 billion light-years on a side. At redshifts of  z = 8 to 10,  which is the maximum for which we have observed real galaxies, their simulation matches the data from the Hubble Space Telescope. And yes, dark matter plays a key role – it is the main gravitational sink that pulls in ordinary matter that forms the stars, gas, and dust that are the primary visible components of galaxies. Without dark matter, our Milky Way would be much smaller, and you probably wouldn’t be here. You can learn more about the BlueTides simulation results and methodology at insidehpc.com.

  Hubble Ultra Deep Field; STScI


Herschel infrared telescope studies Dark Matter

The space-borne Herschel infrared telescope, launched by the European Space Agency with NASA participation, is studying galaxy formation in the early universe. It allows astronomers to look back to the first 3 billion years of the universe’s approximately 14 billion year history.

http://www.jpl.nasa.gov/news/news.cfm?release=2011-057

The degree of clustering of these early galaxies is a reflection of the amount of dark matter relative to ordinary matter. The researchers also note that to form large galaxies (such as our Milky Way) that are efficient sites for robust star formation that one needs enough dark matter relative to the ordinary matter in stars and interstellar gas, but not so much that one ends up with many smaller galaxies.