Using Strong Gravitational Lensing to Identify Fossil Group Progenitors

Abstract

Fossil galaxy systems are classically thought to be the end result of galaxy group/cluster evolution, as galaxies experiencing dynamical friction sink to the center of the group potential and merge into a single, giant elliptical that dominates the rest of the members in both mass and luminosity. Most fossil systems discovered lie within z < 0.2, which leads to the question: what were these systems' progenitors? Such progenitors are expected to have imminent or ongoing major merging near the brightest group galaxy (BGG) that, when concluded, will meet the fossil criteria within the look back time. Since strong gravitational lensing preferentially selects groups merging along the line of sight, or systems with a high mass concentration like fossil systems, we searched the CASSOWARY survey of strong lensing events with the goal of determining if lensing systems have any predisposition to being fossil systems or progenitors. We find that ∼13% of lensing groups are identified as traditional fossils while only ∼3% of non-lensing control groups are. We also find that ∼23% of lensing systems are traditional fossil progenitors compared to ∼17% for the control sample. Our findings show that strong lensing systems are more likely to be fossil/pre-fossil systems than comparable non-lensing systems. Cumulative galaxy luminosity functions of the lensing and non-lensing groups also indicate a possible, fundamental difference between strong lensing and non-lensing systems' galaxy populations with lensing systems housing a greater number of bright galaxies even in the outskirts of groups.

Properties of Fossil Groups of Galaxies

“…A novel technique to search for FG's progenitors is the use of strong gravitational lensing in galaxy groups. Johnson et al (2018b) investigated the fraction of FGs in lensed and non-lensed galaxy groups, finding that the fraction of FGs is larger in lensed systems (13% versus 3%). They also identified 12 possible FG progenitors, that were later investigated in details in Johnson et al (2018a) using Chandra and the Hubble Space Telescope.…”

Chandra and HST Snapshots of Fossil System Progenitors

“…In our previous work (Johnson et al 2018), we identified 12 Jones fossil progenitors in the CASSOWARY catalog at varying stages of BGG formation. Sloan Digital Sky Survey (SDSS) images allowed us to observe how the galaxy population of a system evolves as it makes the transition to fossil status on average, with intermediate mass galaxies being cannibalized by the BGG.…”

“…Finding the Cheshire Cat also demonstrated that the progenitors to today's fossil systems can be found; to that point little concerted effort had gone into locating these outside simulations (Kanagusuku et al 2016). After the Cheshire Cat's discovery, we attempted to locate more fossil progenitors in the CAmbridge Sloan Survey of Wide ARcs in the SkY (CASSOWARY) catalog of which the Cheshire Cat (CSWA 2) was a member (Johnson et al 2018).…”

The lens SW05 J143454.4+522850: a fossil group at redshift 0.6?

“…have not yet been translated to constraints on galaxy formation scenarios. A recent study by Johnson et al (2018) estimates that a substantial percentage of lensing systems within the group mass range are fossil or pre-fossil groups. If this is indeed the case, models of such lenses could provide a new and independent diagnosticbesides the common criteria to identify fossil groups through Xray observations -to assess whether a lens in question is in fact a fossil system, in contrast to a more standard group.…”