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Title

Measures of galaxy environment I: what is 'environment'?

Author(s)

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

Abstract

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.

Publication type

Journal article

Research centre

Swinburne University of Technology. Faculty of Information and Communication Technologies. Centre for Astrophysics and Supercomputing

Source

Monthly Notices of the Royal Astronomical Society, Vol. 419, no. 3 (Jan 2012), pp. 2670-2682

Publication year

2012

FOR Code(s)

0201 Astronomical and Space Sciences

Keyword(s)

Dark matter;  Galaxy evolution;  Galaxy haloes;  Large-scale structure of Universe

Publisher

Wiley-Blackwell

ISSN

0035-8711

Publisher URL

http://dx.doi.org/10.1111/j.1365-2966.2011.19922.x

Copyright

Copyright © 2011 The Authors. Journal compilation copyright © 2011 Royal Astronomical Society. The accepted manuscript is reproduced in accordance with the copyright policy of the publisher. The definitive publication is available at www.interscience.wiley.com.

Additional information

The authors acknowledge support from a QEII Fellowship by the Australian Research Council, the NRF for a research grant 2010-0023319, a CIERA Postdoctoral Fellowship and a STFC Advanced Fellowship.

Full text

Peer reviewed