Group Sample


The galaxy groups we are observing within the CHANGES survey are all taken from the Complete Local-Volume Groups Sample (CLoGS). CLoGS consists of 53 groups in the local Universe (<80 Mpc) selected from the shallow, all-sky LGG catalog of Garcia (1993) which identified groups by a combination of friends-of-friends and hierarchical clustering algorithms from an early version of the all-sky LEDA galaxy catalog which is complete to B=14 and vrec=5500 km/s (equivalent to D<80Mpc).

The CLoGS team then selected groups within the LGG sample which had 4 or more member galaxies, including at least one early-type galaxy with LB>3x1010 Lsun. The requirement for a bright early-type galaxy member came from the observation that such groups are typically more X-ray luminous (Mulchaey et al. 2003) and are more likely to be collapsed systems able to drive galaxy evolution through mergers. Only galaxy groups with declination >-30 degrees were kept, to ensure visibility from Northern hemisphere observatories such as the GMRT and the VLA. Group membership was then refined and expanded using the latest HyperLEDA catalogue.

The key motivations for using the CLoGS group sample as a basis for our CHANGES H-alpha imaging survey are:

  • The CLoGS groups are all sufficiently nearby (<80Mpc) that most of the member galaxies are 10-100" across, allowing us to resolve star forming regions on 100-300 pc scales within each galaxy, and easily distinguish between truncated H-alpha disks indicative of ram-pressure stripping, anemic spirals or nuclear starbursts. The advantage of H-alpha imaging over fiber-fed spectrographs is greatest for the nearest galaxies which show the largest extents on the sky. The CLoGS galaxy groups all have recession velocities within 5500 km/s, sufficiently low that they can be covered by one of the standard offset H-alpha filters available on the WIYN 0.9m or Bok 2.3m telescopes at Kitt Peak. While larger group samples based on the SDSS or 2dFGRS are available (e.g. 2PIGG, Yang et al. 2005), these are typically much more distant such that the member galaxies are that much smaller thus reducing the resolving capability of H-alpha imaging, while their recession velocities are too high to be covered by the standard H-alpha filtersets available on most telescopes.

  • The CLoGS team are working to obtain X-ray data from Chandra and/or XMM-Newton for all of these 53 groups, vital to measure the density profiles of the Intra-Group Medium providing a basis for estimating the ram pressures that are being felt by their member galaxies. This is fundamental to our science goal of understanding the importance of ram-pressure stripping for quenching star formation in galaxy groups.

  • As the galaxy groups are so nearby, we can take full advantage of a wealth of multi-wavelength all-sky surveys such as GALEX (UV), 2MASS (NIR), WISE (NIR/MIR), AKARI (FIR) to characterise the panchromatic properties of each galaxy, deriving stellar masses and infrared-based star formation rates. The CLoGS team are also obtaining low-frequency radio observations (235 and 610MHz) for all of these groups with the Giant Metrewave Radio Telescope (GMRT) in India, which will allow both AGN and star formation to be detected.

A full list of the 53 groups can be found here on the CLoGS website.

The spatial locations of the groups among the filamentary structures of the local volume as seen in the plane of the sky is shown below. Blue circles mark member galaxies, red points are dominant ellipticals. All other galaxies in the local volume are marked in black.


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