Fossil group origins

Abstract

Context. In nature we observe galaxy aggregations that span a wide range of magnitude gaps between the two first-ranked galaxies of a system (Δm 12 ). Thus, there are systems with gaps close to zero (e.g., the Coma cluster), and at the other extreme of the distribution, the largest gaps are found among the so-called fossil systems. The observed distribution of magnitude gaps is thought to be a consequence of the orbital decay of M * galaxies in massive halos and the associated growth of the central object. As a result, to first order the amplitude of this gap is a good statistical proxy for the dynamical age of a system of galaxies. Fossil and non-fossil systems could therefore have different galaxy populations that should be reflected in their luminosity functions. Aims. In this work we study, for the first time, the dependence of the luminosity function parameters on Δm 12 using data obtained by the fossil group origins (FOGO) project. Methods. We constructed a hybrid luminosity function for 102 groups and clusters at z ≤ 0.25 using both photometric data from the SDSS-DR7 and redshifts from the DR7 and the FOGO surveys. The latter consists of ∼1200 new redshifts in 34 fossil system candidates. We stacked all the individual luminosity functions, dividing them into bins of Δm 12 , and studied their best-fit Schechter parameters. We additionally computed a "relative" luminosity function, expressed as a function of the central galaxy luminosity, which boosts our capacity to detect differences -especially at the bright end. Results. We find trends as a function of Δm 12 at both the bright and faint ends of the luminosity function. In particular, at the bright end, the larger the magnitude gap, the fainter the characteristic magnitude M * . The characteristic luminosity in systems with negligible gaps is more than a factor three brighter than in fossil-like ones. Remarkably, we also find differences at the faint end. In this region, the larger the gap, the flatter the faint-end slope α. Conclusions. The differences found at the bright end support a dissipationless, dynamical friction-driven merging model for the growth of the central galaxy in group-and cluster-sized halos. The differences in the faint end cannot be explained by this mechanism. Other processes -such as enhanced tidal disruption due to early infall and/or prevalence of eccentric orbits -may play a role. However, a larger sample of systems with Δm 12 > 1.5 is needed to establish the differences at the faint end.

Fossil groups in the Millennium simulation

“…Grey region show the results for galaxies in groups in the SDSS DR7 obtained by Zandivarez & Martínez (2011). Upper and lower arrows represent the faint end slope of the luminosity function obtained by Popesso et al (2005) and Zarattini et al (2015), while middle arrow correspond to the value obtained in this work.…”

“…The bright end of the luminosity function of the semi-analytical galaxies in groups agrees with the observations. The behaviour of the faint-end slope of the luminosity function was compared with the observational results obtained by Popesso et al (2005) and Zarattini et al (2015;arrows in Fig. 2).…”

“…A special mention should be given to a recent effort to collect observational evidence to study fossil systems, which it is known as the "Fossil Group Origins" project. This is a collaboration to study galaxy systems previously identified as fossil groups by Santos et al (2007), which has attempted to address several questions such as studying high-redshift massive systems and their fossil-like behaviour (Aguerri et al 2011), the intrinsic difference between the brightest central galaxies in fossils and normal galaxy systems (Méndez-Abreu et al 2012), the correlation between their optical and X-ray luminosity , confirming the fossil nature of part of the original group sample (Zarattini et al 2014), and analysing the dependence of the luminosity function on the magnitude gap (Zarattini et al 2015).…”

Fossil group origins

“…In particular, FSs show a lack of L * galaxies (e.g., Khosroshahi et al 2006;Mendes de Oliveira et al 2006;Aguerri et al 2011;Adami et al 2012;Lieder et al 2013;Khosroshahi et al 2014;Zarattini et al 2014). Additionally, there is also evidence for differences in the number of low-mass galaxies -FSs show LFs with shallower faint-end than non-FSs (Zarattini et al 2015). This lack of substructure in fossil halos has been interpreted as a challenge for the hierarchical formation (D'Onghia & Lake 2004); nevertheless, this is still a puzzle and a matter of debate (Zibetti et al 2009).…”

“…This project aims at a multiwavelength characterization of a large sample of FSs (see Aguerri et al 2011). We have analyzed so far the properties of their (i) dark matter halos Kundert et al 2015), (ii) the central BGGs (Méndez-Abreu et al 2012;Zarattini et al 2014), and (iii) the galaxy populations (Zarattini et al 2015(Zarattini et al , 2016. These observational properties have been compared with state-of-the-art cosmological simulations (Kundert et al 2017).…”