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Tag Archives: Hubble Space Telescope

Blue Tides and the Milky Way

I recently wrote about the  largest high-redshift cosmological simulation of galaxy formation ever, which has been recently completed by a group of astrophysicists in the U.S. and the U.K. This tour-de-force simulation, named BlueTides, was performed on the Blue Waters Cray XE supercomputer at NCSA and employed 648,000 cores. The researchers utilized approximately 700 billion particles (!) to represent dark matter and ordinary matter and to create galaxies inside the supercomputer.

You can find the full article describing the simulation at insidehpc.com.

Galaxies are the fundamental building blocks of the large scale structure of the universe. Very early on, before the first galaxies formed, the universe was a highly uniform mix of dark matter and ordinary matter, but with about 5 times as much dark matter by mass relative to the ordinary matter (protons, neutrons, electrons) that makes up the visible parts of galaxies, including stars, gas and dust. Ares of higher dark matter density play a key role in gravitationally attracting ordinary matter that forms galaxies and stars.

When we think of the word galaxy we typically think of beautiful modern day spiral galaxies, such as the Andromeda Galaxy. Spiral galaxies are flattened, rotating disks; the spiral arms represent concentrations of matter and of star-forming regions. The most distant disk-shaped galaxies that have been detected are at redshifts of 2 to 3, so we are seeing them as they were when the universe was around 2 to 3 billion years old. (The higher the redshift the more distant the galaxy and also the farther back in time we are looking, toward the universe’s origin some 13.8 billion years ago).

ngc2207_hubble_960

Two spiral galaxies starting to collide. Image Credit: Debra Meloy Elmegreen (Vassar College) et al.,
& the Hubble Heritage Team (AURA/STScI/NASA)

The BlueTides simulation provides insight into what was going on when the universe was only around 1/2 a billion years old, with galaxy redshifts around 8 to 10. It does a good job of matching the limited observational data we have at such highredshifts, in particular the rest frame (before redshift) ultraviolet luminosities of the earliest detected galaxies from the Hubble Space Telescope surveys.

Their simulation finds that, among the most massive galaxies in their simulation, “a significant fraction are visually disk-like, and surprisingly regular in shape”. In other words, they appear to be the progenitors of present-day spiral galaxies. They find that, at a redshift of 8, a full 70% of their virtual galaxies with masses above 10 billion solar masses are classifiable as disks, since a majority of stellar orbits lie in the plane of the disk.

Fig1.Fengetal.milkyway

Simulated high-redshift galaxies from BlueTides – Figure 1 from Feng et al.

They also find that mergers are not of major significance in the build-up of these early massive galaxies. Rather it appears that they grew primarily by cold gas arriving from preferred directions, namely along filaments in the density distribution of the background gas. It is well known that the universe has a web-like or filamentary structure of high density regions interspersed with voids (relatively empty regions). This filamentary structure is believed on the basis of many simulations of the large-scale universe to have begun at an early date.

A future infrared satellite known as WFIRST will have a field of view 200 times larger than the Wide Field Camera on the Hubble. Also, its design for infrared radiation detection makes it appropriate for studying the light from high-redshift galaxies. The authors predict that a survey of 2000 square degrees with WFIRST should find roughly 8000 massive disk-type galaxies at redshifts above 8. Future very large ground-based telescopes will be able to make follow-up observations of galaxies discovered by WFIRST. Such observations will provide further insight into the nature of galaxy formation, including accretion of material from the background and the details of dark matter’s role in the process.

Reference

Feng et al. 2015, “The Formation of Milky Way-Mass Disk Galaxies in the First 500 Million Years of a Cold Dark Matter Universe” http://arxiv.org/abs/1504.06618

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Super Colliders in Space: Dark Matter not Colliding

What’s bigger and more powerful than the Large Hadron Collider at CERN? Why colliding galaxy clusters of course.

A cluster of galaxies consists of hundreds or even thousands of galaxies bound together by their mutual gravitation. Both dark matter and ordinary matter in and between galaxies is responsible for the gravitational field of a cluster. And typically there is about 5 times as much dark matter as ordinary matter. The main component of ordinary matter is hot intracluster gas; only a small percentage of the mass is locked up in stars.

One stunning example of dark matter detection is the Bullet Cluster. This is the canonical example found revealing dark matter separation from ordinary matter in a pair of clusters colliding and merging. The dark matter just passes right through, apparently unaffected by the collision. The hot gas (ordinary matter) is seen through its X-ray emission, since the gas is heated by collisions to of order 100 million degrees. The Chandra X-ray Observatory (satellite) provided these measurements.

Image courtesy of Chandra X-ray Observatory

Bullet Cluster. The blue color shows the distribution of dark matter, which passed through the collision without slowing down. The purple color shows the hot X-ray emitting gas. Image courtesy of Chandra X-ray Observatory

The distribution of matter overall in the Bullet Cluster or other clusters is traced by gravitational lensing effects; general relativity tells us that  background galaxies will have their images displaced, distorted, and magnified as their light passes through a cluster on its way to Earth. The magnitude of these effects can be used to “weigh” the dark matter. These measurements are made with the Hubble Space Telescope.

In the Bullet Cluster the dark matter is displaced from the ordinary matter. The interpretation is that the ordinary matter from the two clusters, principally in the form of hot gas, is slowed by frictional, collisional processes as the clusters interact and form a larger single cluster of galaxies. Another six or so examples of galaxy clusters showing the displacement between the dark matter and the ordinary matter in gas and stars have been found to date.

Now, a team of astrophysicists based in the U.K. and Switzerland have examined 30 additional galaxy clusters with data from both Chandra and Hubble, and with redshifts typically 0.2 to 0.6. In aggregate there are 72 collisions in the 30 systems, since some have more than two subclusters. The offsets between the gas and dark matter are quite substantial, and in aggregate indicate the existence of dark matter in these clusters with over 7 standard deviations of statistical significance (probability of the null hypothesis of no dark matter is 1 in 30 trillion).

They then look at the possible drag force on the dark matter due to dark matter particles colliding with other dark matter particles. There are already much more severe constraints on ordinary matter – dark matter interactions from Earth-based laboratory measurements. But the dark matter mutual collision cross section could potentially be large enough to result in a drag. They measure the relative positions of hot gas, galaxies, and dark matter for all of the 72 subclusters.

From paper "The non-gravitational interactions of dark matter in colliding galaxy clusters"

From paper “The non-gravitational interactions of dark matter in colliding galaxy clusters” D. Harvey et al. 2015

The gas should and does lag the most, relative to the direction of the galaxies in a collision. If there is a dark matter drag, then dark matter should lag behind the positions of the stars. They find no lag of the dark matter average position, which allows them to place a new, tighter constraint on the mutual interaction cross-section for dark matter.

Their constraint is σ(DM)/m < 0.47 cm^2/g at 95% confidence level, where σ (sigma) is the cross-section and m is the mass of a single dark matter particle. This limit is over twice as tight as that previously obtained from the Bullet Cluster. And some dark matter models predict a cross section per unit mass of 0.6 cm^2/g, so these models are potentially ruled out by these new measurements.

In summary, using Nature’s massive particle colliders, the authors have found further highly significant evidence for the existence of dark matter in clusters of galaxies, and they have placed useful constraints on the dark matter self-interaction cross-section. Dark matter continues to be highly elusive.

Reference:

D. Harvey et al. 2015 “The non-gravitational interactions of dark matter in colliding galaxy clusters” http://arxiv.org/pdf/1503.07675v1.pdf