X-ray Luminosity Function of Optically Selected Galaxy Groups

Student thesis: Master's ThesisMaster of Science by Research (MScR)


The Galaxy and Mass Assembly (GAMA) spectroscopic survey consists of roughly 300,000 galaxies covering 286 deg2 down to an r-band magnitude of 19.8. A 16.6 deg2 area of the GAMA survey overlaps with the XXL survey, which is the largest homogeneous survey carried out with XMM-Newton to date. The overlap between the GAMA optical survey and the XXL X-ray survey creates an exciting opportunity to explore galaxy groups across multiple wavelengths with independent selection criteria. This overlap region contains 234 galaxy groups with five or more members detected by GAMA. The XXL X-ray mosaic created by combining multiple XMM-Newton observations is used to determine the X-ray luminosity of GAMA’s optically selected sample of galaxy groups. Forced X-ray aperture photometry was used, with emission from AGN and other non-group Xray sources modelled out or removed, and then luminosity posteriors were sampled to deal with non-detections. This sample of galaxy groups are then presented as an X-ray luminosity function (XLF): the number density of groups as a function of X-ray luminosity. Due to the optical selection criteria used the XLF is able to explore lower X-ray luminosities than X-ray selected samples could. However, incompleteness from the optical selection function only including groups with five or more members results in a lower luminosity limit of 1.9×1040 erg s-1 being implemented in the XLF fitting. The XLF demonstrates that the Schechter function remains a good description down to lower-luminosities and for an optically selected sample which has different biases than an X-ray selected sample. The Schechter function best fit parameters observed are consistent with values determined using X-ray selected samples. The issue of incompleteness due to the optical selection function needs to be further explored, and by combining the observed XLF with a theoretical mass function this work will ultimately lead to new constraints on the scaling relation between X-ray luminosity and mass in the galaxy group regime.
Date of Award23 Jan 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorBen J Maughan (Supervisor)

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