Journal Article

Measures of galaxy environment – I. What is ‘environment’?

Stuart I. Muldrew, Darren J. Croton, Ramin A. Skibba, Frazer R. Pearce, Hong Bae Ann, Ivan K. Baldry, Sarah Brough, Yun-Young Choi, Christopher J. Conselice, Nicolas B. Cowan, Anna Gallazzi, Meghan E. Gray, Ruth Grützbauch, I-Hui Li, Changbom Park, Sergey V. Pilipenko, Bret J. Podgorzec, Aaron S. G. Robotham, David J. Wilman, Xiaohu Yang, Youcai Zhang and Stefano Zibetti

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 419, issue 3, pages 2670-2682
Published in print January 2012 | ISSN: 0035-8711
Published online January 2012 | e-ISSN: 1365-2966 | DOI:
Measures of galaxy environment – I. What is ‘environment’?

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The influence of a galaxy’s environment on its evolution has been studied and compared extensively in the literature, although differing techniques are often used to define environment. Most methods fall into two broad groups: those that use nearest neighbours to probe the underlying density field and those that use fixed apertures. The differences between the two inhibit a clean comparison between analyses and leave open the possibility that, even with the same data, different properties are actually being measured. In this work, we apply 20 published environment definitions to a common mock galaxy catalogue constrained to look like the local Universe. We find that nearest-neighbour-based measures best probe the internal densities of high-mass haloes, while at low masses the interhalo separation dominates and acts to smooth out local density variations. The resulting correlation also shows that nearest-neighbour galaxy environment is largely independent of dark matter halo mass. Conversely, aperture-based methods that probe superhalo scales accurately identify high-density regions corresponding to high-mass haloes. Both methods show how galaxies in dense environments tend to be redder, with the exception of the largest apertures, but these are the strongest at recovering the background dark matter environment. We also warn against using photometric redshifts to define environment in all but the densest regions. When considering environment, there are two regimes: the ‘local environment’ internal to a halo best measured with nearest neighbour and ‘large-scale environment’ external to a halo best measured with apertures. This leads to the conclusion that there is no universal environment measure and the most suitable method depends on the scale being probed.

Keywords: methods: numerical; methods: statistical; galaxies: evolution; galaxies: haloes; dark matter; large-scale structure of Universe

Journal Article.  9452 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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