Fossil group origins

III. The relation between optical and X-ray luminosities^⋆

¹ Dipartimento di Fisica dell’Università degli Studi di Trieste − Sezione di Astronomia, via Tiepolo 11, 34143 Trieste, Italy
e-mail: girardi@oats.inaf.it
² INAF − Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
³ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna, Tenerife, Canary Islands, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, Av. del Astrofísico Franciso Sánchez s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain
⁵ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁶ Astronomy Department, University of Wisconsin, 475 Charter St., Madison, WI 53706, USA
⁷ Alfred P. Sloan Fellow  
⁸ Fundación Galileo Galilei – INAF, Rambla José Ana Fernández Perez 7, 38712 Breña Baja, La Palma, Canary Islands, Spain
⁹ School of Physics and Astronomy, University of St Andrews (SUPA), North Haugh, St Andrews, KY16 9SS, UK
¹⁰ NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
¹¹ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹² INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹³ Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Aptdo. Postal 51 y 216, 72000 Puebla, Pue., Mexico
¹⁴ Instituto de Astrofísica de Andalucia – C.S.I.C., 18008 Granada, Spain
¹⁵ Centro Astronómico Hispano Alemán, C/ Jesús Durbán Remón 2-2, 04004 Almería, Spain

Received: 20 December 2013
Accepted: 28 February 2014

Abstract

Aims. This study is part of the Fossil group origins (FOGO) project which aims to carry out a systematic and multiwavelength study of a large sample of fossil systems. Here we focus on the relation between the optical luminosity (L_opt) and X-ray luminosity (L_X).

Methods. Out of a total sample of 28 candidate fossil systems, we consider a sample of 12 systems whose fossil classification has been confirmed by a companion study. They are compared with the complementary sample of 16 systems whose fossil nature has not been confirmed and with a subsample of 102 galaxy systems from the RASS-SDSS galaxy cluster survey. Fossil and normal systems span the same redshift range 0 <z< 0.5 and have the same L_X distribution. For each fossil system, the L_X in the 0.1−2.4 keV band is computed using data from the ROSAT All Sky Survey to be comparable to the estimates of the comparison sample. For each fossil and normal system we homogeneously compute L_opt in the r-band within the characteristic cluster radius, using data from the Sloan Digital Sky Survey Data Release 7.

Results. We sample the L_X-L_opt relation over two orders of magnitude in L_X. Our analysis shows that fossil systems are not statistically distinguishable from the normal systems through the 2D Kolmogorov-Smirnov test nor the fit of the L_X-L_opt relation. Thus, the optical luminosity of the galaxy system does strongly correlate with the X-ray luminosity of the hot gas component, independently of whether the system is fossil or not. We discuss our results in comparison with previous literature.

Conclusions. We conclude that our results are consistent with the classical merging scenario of the brightest galaxy formed via merger/cannibalism of other group galaxies with conservation of the optical light. We find no evidence for a peculiar state of the hot intracluster medium.