A Robust Quantification of Galaxy Cluster Morphology Using Asymmetry and Central Concentration
Abstract
We present a novel quantitative scheme of cluster classification based on the morphological properties that are manifested in X-ray images. We use a conventional radial surface brightness concentration parameter (c SB) as defined previously by others and a new asymmetry parameter, which we define in this paper. Our asymmetry parameter, which we refer to as photon asymmetry (A phot), was developed as a robust substructure statistic for cluster observations with only a few thousand counts. To demonstrate that photon asymmetry exhibits better stability than currently popular power ratios and centroid shifts, we artificially degrade the X-ray image quality by (1) adding extra background counts, (2) eliminating a fraction of the counts, (3) increasing the width of the smoothing kernel, and (4) simulating cluster observations at higher redshift. The asymmetry statistic presented here has a smaller statistical uncertainty than competing substructure parameters, allowing for low levels of substructure to be measured with confidence. A phot is less sensitive to the total number of counts than competing substructure statistics, making it an ideal candidate for quantifying substructure in samples of distant clusters covering a wide range of observational signal-to-noise ratios. Additionally, we show that the asymmetry-concentration classification separates relaxed, cool-core clusters from morphologically disturbed mergers, in agreement with by-eye classifications. Our algorithms, freely available as Python scripts (https://github.com/ndaniyar/aphot), are completely automatic and can be used to rapidly classify galaxy cluster morphology for large numbers of clusters without human intervention.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2013
- DOI:
- 10.1088/0004-637X/779/2/112
- arXiv:
- arXiv:1309.7044
- Bibcode:
- 2013ApJ...779..112N
- Keywords:
-
- galaxies: clusters: intracluster medium;
- methods: statistical;
- X-rays: galaxies: clusters;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- doi:10.1088/0004-637X/779/2/112