CHANGES (Complete H-Alpha imaging of Nearby Group EnvironmentS)

The Complete H-Alpha imaging of Nearby Group EnvironmentS (CHANGES) survey (PI: C. Haines) is a new project obtaining wide-field Hα and optical imaging of galaxies from CLoGS, a complete volume-limited sample of galaxy groups within 80 Mpc, using Steward Observatory's 2.3m Bok telescope and the WIYN 0.9m telescope at the Kitt Peak National Observatory in Arizona.

The properties of galaxies are known to be strongly correlated with their local environment. Isolated galaxies in the field are typically spirals which form stars continually within their gas-rich spiral arms, while in dense groups and clusters the dominant populations are featureless elliptical and lenticular galaxies for which star formation has permanently shut down. These trends have been known for many years, and have been quantified by the star-formation (SF)-density and morphology-density relations. However, the physical mechanisms behind these trends remain poorly understood.

Hα imaging is a powerful technique which allows astronomers to resolve exactly where within a galaxy stars are forming. As environmental processes by their nature act from outside the galaxy, they will affect some parts of a galaxy much more than others. Hence by looking for localized regions within a galaxy in which star formation is being quenched, or alternatively being triggered, we can identify how star formation in that galaxy is being affected by the interaction of the galaxy with its environment, and by which physical process.

  Optical image of the nearby galaxies M81 and M82 taken with the WIYN 0.9m telescope by the CHANGES team in April 2012, combining broadband g-band and r-band images with a narrow-band H-alpha image (red colors) tracing the emission from hydrogen gas ionized by star formation. In M82 (on the left) a massive starburst has produced an enormous galactic wind of ionized hydrogen gas perpendicular to the disk. This starburst is believed to have been triggered by a recent interaction with its more massive neighbor spiral galaxy M81 (on the right) which shows many star forming regions embedded in its spiral arms. The degree field of view of the Mosaic-1.1 imager on the 0.9m enables us to view both galaxies in the same image.  

Hα imaging surveys of galaxies in the nearby Virgo cluster showed that gas removal processes such as ram-pressure stripping due to the passage of the galaxy through the dense intra-cluster medium (ICM) and the gentler starvation mechanism are very effective in quenching star formation in spiral galaxies falling into the cluster for the first time. However, clusters are rare and extreme over-densities which contain only a few percent of all galaxies in the local Universe. The majority of galaxies instead reside in small groups containing a few to a few dozen member galaxies. While it is believed that galaxy groups are the primary environment for transforming galaxies, it remains far from clear how this is achieved, as the maximal ram pressures witnessed by group galaxies are a factor 10-100x lower than those seen in clusters.

While there have been Hα imaging surveys of galaxies in nearby rich clusters and also isolated field regions, a comparable survey of galaxies in a statistical sample of galaxy groups is lacking. For this reason, we have recently embarked on CHANGES, a survey designed to chart the impact of group environments on star formation within their member galaxies, by covering a complete volume-limited sample of 53 galaxy groups within 80 Mpc (CLoGS) with panoramic Hα imaging.

We are at present in the process of observing these galaxy groups using wide-field optical imagers (1 degree fields of view) on 1-2m class telescopes, having completed a total of 39 nights of observations to date during 2012. Once we have completed the observations, we aim on involving the public to help us identify galaxies whose star formation is being affected by their interaction with their local environment.



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