The XXL Survey

XLV. Linking the ages of optically selected groups to their X-ray emission^⋆

¹ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Valparaíso, Chile
e-mail: jacob.crossett@uv.cl
² School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
³ Max-Planck Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching bei München, Germany
⁴ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
⁵ H. H. Wills Physics Laboratory, University of Bristol, Tyndall Ave, Bristol BS8 1TL, UK
⁶ ICRAR, University of Western Australia, Crawley, WA 6009, Australia
⁷ Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, V8P 5C2 BC, Canada
⁸ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
⁹ School of Physics, University of New South Wales, NSW 2052, Australia
¹⁰ Department of Physics and Astronomy, 102 Natural Science Building, University of Louisville, Louisville, KY 40292, USA
¹¹ Australian Astronomical Optics, Macquarie University, 105 Delhi Rd, North Ryde 2113, Australia
¹² Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK
¹³ Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia
¹⁴ Astronomy, Astrophysics and Astrophotonics Research Centre, Macquarie University, Sydney, NSW 2109, Australia
¹⁵ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
¹⁶ E.A. Milne Centre for Astrophysics, University of Hull, Cottingham Road, Kingston-upon-Hull HU6 7RX, UK

Received: 20 August 2021
Accepted: 21 February 2022

Abstract

Why are some galaxy groups pervaded by a hot X-ray emitting intracluster medium, whilst others have no detectable X-ray emission? Is the presence of hot gas a reliable indicator of dynamical maturity, and can some virialised groups contain little or none of it? What are the main differences between samples of groups selected in the X-ray and optical bands? We address these questions by studying 232 optical spectroscopically selected groups from the Galaxy And Mass Assembly (GAMA) survey that overlap the XXL X-ray cluster survey. X-ray aperture flux measurements combined with GAMA group data provides the largest available sample of optical groups with detailed galaxy membership information and consistently measured X-ray fluxes and upper limits. A sample of 142 of these groups is divided into three subsets based on the relative strength of X-ray and optical emission, and we see a trend in galaxy properties between these subsets: X-ray overluminous groups contain a lower fraction of both blue and star forming galaxies compared with X-ray underluminous systems. X-ray overluminous groups also have a more dominant central galaxy, with a magnitude gap between first and second ranked galaxies on average 0.22 mag larger than in underluminous groups. Moreover, the central galaxy in overluminous groups lies closer to the luminosity-weighted centre of the group. We examine a number of other structural properties of our groups, such as axis ratio, velocity dispersion, and group crossing time, and find evidence of trends with X-ray emission in some of these properties despite the high stochastic noise arising from the limited number of group galaxies. We attribute the trends we see primarily to the evolutionary state of groups, with X-ray overluminous systems being more dynamically evolved than underluminous groups. The X-ray overluminous groups have had more time to develop a luminous intragroup medium, quench member galaxies, and build the mass of the central galaxy through mergers compared to underluminous groups. However, an interesting minority of X-ray underluminous groups have properties that suggest them to be dynamically mature. We find that the lack of hot gas in these systems cannot be accounted for by high star formation efficiency, suggesting that high gas entropy resulting from feedback is the likely cause of their weak X-ray emission.