jun 042012


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)

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