COSMOS Survey Reveals Galaxies' Split Personalities


A team of astronomers led by Niv Drory of the Max Planck Institute for
Extraterrestrial Physics, Garching, Germany), Kevin Bundy (University of California, Berkeley), and Alexie Leauthaud (Lawrence Berkeley National Lab and Berkeley Center for Cosmological Physics, Berkeley) used data from the COSMOS project to explore the connections between the growth of dark matter halos and the growth of galaxies within these dark matter halos. They found previously unseen features in the mass distribution of galaxies and its evolution with time (looking back to half the current age of the universe) and interpreted these features as corresponding to the known sub-populations of galaxies in the local universe.

The Cosmic Evolution Survey project (COSMOS), an international collaboration of more than 100 scientists led by Nick Scoville at the California Institute of Technology, combines observations from major space and ground-based observatories to explore the evolution of galaxies over 75% of the age of the universe. It is based around the largest survey ever conducted by the Hubble Space Telescope and includes observations by the Spitzer Space Telescope, the XMM-Newton spacecraft, the Chandra X-ray Observatory, the GALEX spacecraft, the ESO Very Large Telescope, the Subaru Telescope, the Canada-France-Hawaii Telescope, and others.

The team has used the extensive imaging data available in COSMOS to measure the mass distribution of galaxies (how many galaxies there are as a function of their mass) over a time spanning about half of the present-day age of the universe reaching more than 10 times smaller masses than before. This exceptional dataset has allowed the team to see features in the mass distribution that point towards the presence of multiple galaxy populations that together make up the more complicated shape of the distribution.

In particular, the mass distribution was found to steepen significantly towards its faint end (the number of faint galaxies increases very quickly with decreasing mass). The team was able to detect both a faint blue population and a corresponding faint red population showing the same steep faint end behavior. They associate the faint red population with faint blue galaxies that have become satellite galaxies (galaxies that have fallen into the halo of another, more massive galaxy, but have not merged with the more massive galaxy yet; if the host galaxy is massive enough, this will shut off star formation by stripping the gas from the infalling satellite). This effect was known from observations of nearby galaxies and their satellites, but the detection of the same populations in the mass distributions (and therefore the quantitative treatment) is new.

Additionally, there is an increase in numbers of brighter galaxies before the mass distribution cuts off at very high masses. It was previously thought that this bump is made up of high mass elliptical galaxies, but it was now found that the mass distribution of regular disk galaxies does show the same feature at high redshift, i.e., at earlier times. This bump must be intricately linked to galaxy formation. It is a combination of the central-satellite distinction (there is a "pile-up" of central galaxies caused by the assembly of their halos and stellar components in mergers) and the increase in star formation efficiency as the halo mass increases: a galaxy in a more massive halo is able to retain more of its gas as supernova explosions of dying massive stars drive winds that are able to expel gas from the galaxy. These two effects provide a link between the assembly of dark matter halos and the visible components of galaxies.

These observations provide new and exciting statistical measures of the properties of the galaxy population as a whole and also of its sub-components.

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