Did neighborhood harassment or strangulation stop galaxies from forming stars?

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Clusters of galaxies are rare regions of the Universe consisting of hundreds of galaxies containing trillions of stars, as well as hot gas and mysterious dark matter. It has long been known that when a galaxy falls into a cluster, interactions with other cluster galaxies and hot gas rapidly shut off its star formation in a process known as “quenching”, causing galaxies found in clusters to appear very similarly red in color. The UCR-led SpARCS survey has pioneered utilizing this uniformity of red color of quenched galaxies to discover hundreds of new clusters in the distant Universe.foltz2015b

While observationally indisputable, the details of how a galaxy becomes quenched within the neighborhood of a cluster are poorly understood. 

In SpARCS clusters which contain lots of galaxies, the main possibility is that quenching occurs due to interactions with the other galaxies in the cluster: they can be harassed (frequent, high speed, gravitationally-disturbing encounters), tidally stripped (material pulled from a smaller galaxy to a larger one) or merged (two or more galaxies joining into a single entity); in X-ray luminous clusters which contain lots of hot gas, the main possibility is that quenching occurs when this hot gas strips cold gas (which is needed to form stars) out of the galaxies before that gas is able to cool even further to condense and form new stars (ram pressure stripping). Because one set of quenching processes depend on there being many other galaxies nearby and the other set of processes depend on there being lots of hot gas present, one might have expected to observe older quenched galaxies in one sample of clusters relative to the other depending on whether other galaxies or hot gas were doing most of the quenching.

A team led by UCR graduate student Ryan Foltz compared the properties of quenched galaxies in a sample of ten galaxy-rich SpARCS clusters (so distant their light has traveled for 7- 8 billion years) to a previously published sample of gas-rich X-ray selected clusters at the same distance. To obtain the necessary observations, the SpARCS team carried out a large 25-night survey, named “GCLASS”,  on the twin Gemini telescopes (8 m in diameter) in Hawaii and Chile. Remarkably, the team learned that the galaxies in both samples stopped forming stars at approximately the same time. This suggests that neither interactions with other galaxies nor interactions with hot gas are primarily responsible for turning star formation off.

Instead, the Foltz et al analysis suggest that another, yet to be determined, quenching mechanism which does not depend on the other galaxies or the hot gas in the cluster may be the culprit. One possibility is that galaxies interact with each other and become “pre-processed” from star-forming to quenched, before they even fall into the cluster. Another is that they are “strangled” i.e., they slowly stop forming stars because their reservoirs cannot continue to replenish with more cold gas once they fall inside the cluster.

Clearly, more information is required. The team has recently been awarded 50 nights of Gemini time and a >$1M dollar grant from the National Science Foundation to continue investigating the complex and fascinating topic of how the neighborhood in which a galaxy lives determines how  it will stop forming stars.

The paper can be found here: http://adsabs.harvard.edu/abs/2015ApJ…812..138F

Figures: Red-sequence selected galaxy clusters from the SpARCS survey which were targeted for the follow-up Gemini GCLASS survey observations  which were used in this analysis.