Fossil group origins

IV. Characterization of the sample and observational properties of fossil systems

¹ Instituto de Astrofísica de Canarias, calle vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: stefano@iac.es
² Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Dipartimento di Fisica e Astronomia “G. Galilei”, Università degli Studi di Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁴ Dipartimento di Fisica-Sezione Astronomia, Università degli Studi di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁵ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁶ Fundación Galileo Galilei – INAF, Rambla José Ana Fernández Pérez 7, 38712 Breña Baja, La Palma, Spain
⁷ NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria V9E 2E7, Canada
⁸ Departamento de Astrofísica y CC. de la Atmósfera, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁹ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁰ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Sta. Ma. Tonantzintla, Puebla, México
¹¹ Astronomy Department, University of Wisconsin, 475 Charter St., Madison WI 53706, USA
¹² Alfred P. Sloan Fellow  
¹³ Astronomisches Institut der Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany
¹⁴ Instituto de Astrofísica de Andalucía – C.S.I.C., 18008 Granada, Spain
¹⁵ Centro Astronómico Hispano Alemán, C/ Jesús Durbán Remón 2-2. 04004 Almería, Spain
¹⁶ Instituto de Astronomía Apdo. 70-264, Cd. Universitaria, México DF 04510 México
¹⁷ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁸ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK

Received: 28 December 2013
Accepted: 26 February 2014

Abstract

Context. Virialized halos grow by the accretion of smaller ones in the cold dark matter scenario. The rate of accretion depends on the different properties of the host halo. Those halos for which this accretion rate was very fast and efficient resulted in systems dominated by a central galaxy surrounded by smaller galaxies that were at least two magnitudes fainter. These galaxy systems are called fossil systems, and they can be the fossil relics of ancient galaxy structures.

Aims. We started an extensive observational program to characterize a sample of 34 fossil group candidates spanning a broad range of physical properties.

Methods. Deep r-band images were obtained with the 2.5-m Isaac Newton Telescope and Nordic Optic Telescope. Optical spectroscopic observations were performed at the 3.5-m Telescopio Nazionale Galileo for ~1200 galaxies. This new dataset was completed with Sloan Digital Sky Survey Data Release 7 archival data to obtain robust cluster membership and global properties of each fossil group candidate. For each system, we recomputed the magnitude gaps between the two brightest galaxies (Δm₁₂) and the first and fourth ranked galaxies (Δm₁₄) within 0.5 R₂₀₀. We consider fossil systems to be those with Δm₁₂ ≥ 2 mag or Δm₁₄ ≥ 2.5 mag within the errors.

Results. We find that 15 candidates turned out to be fossil systems. Their observational properties agree with those of non-fossil systems. Both follow the same correlations, but the fossil systems are always extreme cases. In particular, they host the brightest central galaxies, and the fraction of total galaxy light enclosed in the brightest group galaxy is larger in fossil than in non-fossil systems. Finally, we confirm the existence of genuine fossil clusters.

Conclusions. Combining our results with others in the literature, we favor the merging scenario in which fossil systems formed from mergers of L^∗ galaxies. The large magnitude gap is a consequence of the extreme merger ratio within fossil systems and therefore it is an evolutionary effect. Moreover, we suggest that at least one fossil group candidate in our sample could represent a transitional fossil stage. This system could have been a fossil in the past, but not now owing to the recent accretion of another group of galaxies.