Lots of Dark Matter Near the Sun


Astronomers at the University of Zürich and the ETH Zürich, together with other international researchers, have found large amounts of invisible “dark matter” near the Sun. Their results are consistent with the theory that the Milky Way Galaxy is surrounded by a massive “halo” of dark matter, but this is the first study of its kind to use a method rigorously tested against mock data from high quality simulations. The authors also find tantalising hints of a new dark matter component in our Galaxy.

The high resolution simulation of the Milky Way used to test the mass-measuring technique. Image Credit: Dr A. Hobbs/UZH

Dark matter was first proposed by the Swiss astronomer Fritz Zwicky in the 1930s. He found that clusters of galaxies were filled with a mysterious dark matter that kept them from flying apart. At nearly the same time, Jan Oort in the Netherlands discovered that the density of matter near the Sun was nearly twice what could be explained by the presence of stars and gas alone. In the intervening decades, astronomers developed a theory of dark matter and structure formation that explains the properties of clusters and galaxies in the Universe, but the amount of dark matter in the solar neighbourhood has remained more mysterious. For decades after Oort’s measurement, studies found 3-6 times more dark matter than expected. Then last year new data and a new method claimed far less than expected. The community was left puzzled, generally believing that the observations and analyses simply weren’t sensitive enough to perform a reliable measurement.

Testing the method on a simulated Milky Way

Now an international team, lead by researchers of the University of Zürich with the participation of the ETH Zürich, have developed a new technique. The researchers used a state-of-the-art simulation of the Milky Way to test their mass-measuring method before applying it to real data. This threw up a number of surprises: they noticed that standard techniques used over the past twenty years were biased, always tending to underestimate the amount of dark matter. The researchers then developed a new unbiased technique that recovered the correct answer from the simulated data. Applying their technique to the positions and velocities of thousands of orange K dwarf stars near the Sun, they obtained a new measure of the local dark matter density.

If the dark matter should be a new fundamental particle the accurate measure of the local dark matter is vital. Image Credit: Dr A. Hobbs/UZH

Evidence for dark matter near the Sun

“We are 99% confident that there is dark matter near the Sun,” says the lead author Silvia Garbari. In fact, if anything, the authors’ favoured dark matter density is a little high: they find more dark matter than expected at 90% confidence. There is a 10% chance that this is merely a statistical fluke, but if future data confirms this high value the implications are exciting as Silvia explains: “This could be the first evidence for a “disk” of dark matter in our Galaxy, as recently predicted by theory and numerical simulations of galaxy formation, or it could mean that the dark matter halo of our galaxy is squashed, boosting the local dark matter density.”

Many physicists are placing their bets on dark matter being a new fundamental particle that interacts only very weakly with normal matter, but strongly enough to be detected in experiments deep underground. An accurate measure of the local dark matter density is vital for such experiments as co-author Prof. George Lake explains: “If dark matter is a fundamental particle, billions of these particles will have passed through your body by the time your finish reading this article. Experimental physicists hope to capture just a few of these particles each year in experiments like XENON and CDMS currently in operation. Knowing the local properties of dark matter is the key to revealing just what kind of particle it consists of.”

Source: The University of Zürich

Galaxies Reveal Surprises


A team of astronomers has found unexpected relationships between star formation, stellar mass, and dark matter halo mass in central galaxies of groups.

This view shows a section of the widest deep view of the sky ever taken using infrared light, with a total effective exposure time of 55 hours. It was created by combining more than 6000 individual images from the VISTA survey telescope at ESO’s Paranal Observatory in Chile. This picture shows a region of the sky known as the COSMOS field in the constellation of Sextans (The Sextant). More than 200 000 galaxies have been identified in this picture. Credit: ESO/UltraVISTA team. 

Galaxy formation and evolution is one of the most intriguing topics in cosmology. We know that most of the matter in the Universe is in the form of dark matter, with its vast web of filaments and halos of increased density where structures of ordinary matter form. Most of the ordinary, non dark matter is in the form of swarms of gas between the galaxies in agglomerations ranging from giant clusters to somewhat smaller groups. The galaxies themselves, representing only a tiny fraction of the matter but most of what the matter does that is interesting, form amongst the dark matter halos where the gravity can gather enough gas together to collapse into stars.

But how did galaxies come about, how do they change over time, why are there so many different shapes and sizes and colors, and what do all of the properties have to do with each other? This is a literally astronomical problem that can only be tackled through a combination of observation and simulation. One question that has challenged astronomers is the form and evolution of the relationship between galaxy stellar mass, galaxy color, and dark matter halo mass. Color itself is indicative of star formation, as galaxies full of large, young stars glow blue while those full of older, smaller stars are red. There is likely some connection between the mass of stars in the galaxy and and the mass of the dark matter halo in which it sits, and between those and the color which indicates how much gas is collapsing to form stars, but what are these connections?

Galaxies Reveal Surprises Statistically

A measure of the amount of clustering on different distance scales in galaxy groups, for central galaxies with a lot (green) and little star formation (red). The star forming galaxies show enhanced clustering, implying a larger halo mass.  (Click to enlarge.)

A team led by Jeremy Tinker of New York University and including KIPAC professor Risa Wechsler and colleagues from several institutions has examined the relation between the star formation rate, stellar mass, and halo mass of galaxies in groups and found a surprising twist. The team used observational data from a small patch of sky known as the COSMOS field, which has been extensively studied and observed by telescopes in many different kinds of light from radio to optical to X-ray.

The results indicate that there is a lot of correlation between star formation rate and stellar mass, and that it changes completely over time. The farther away galaxies at the centers of groups – which, because of the time it takes light to travel we are seeing earlier in time – show an inverse relationship between stellar mass and star formation rate, meaning that star formation rate is higher in central galaxies with a lower mass. Unexpectedly, this relationship completely reverses for the nearer galaxies studied, which we are seeing as they are closer to the present time.

The data also show that central galaxies with a lot of star formation at the higher redshifts considered in the sample live in groups that are much more tightly clustered, on average, than those without a lot of star formation. Tighter clustering indicates a larger halo mass. This presents an intriguing contradiction with previous results that suggested that the opposite is true for very nearby groups, again indicating an evolution in behavior over time. It seems that central galaxies in groups have evolved in an unexpected way, a finding which will challenge models of galaxy formation and evolution that have been derived from large simulation efforts.

This work is described in a paper submitted to the Astrophysical Journal Letters and available from astro-ph at arXiv:1205.4245.

Source: Kavli Institute for Particle Astrophysics and Cosmology (KIPAC)