Fossil group origins
IX. Probing the formation of fossil galaxy groups with stellar population gradients of their central galaxies⋆,⋆⋆
1 Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy
2 INAF–Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 2, 35122 Padova, Italy
3 Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama, Copayapu 485, Copiapó, Chile
4 Dipartimento di Fisica–Sezione Astronomia, Università di Trieste, via Tiepolo 11, 34143 Trieste, Italy
5 INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
6 Institut de Recherche sur les Lois Fondamentales de l’Univers, Commissariat à l’Énergie Atomique, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
7 Instituto de Astrofísica de Canarias, calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
8 Departamento de Astrofísica, Universidad de La Laguna, Avenida Astrofísico Francisco Sánchez s/n, 38206 La Laguna, Tenerife, Spain
9 Astronomy Department, University of Wisconsin, 475 Charter St., Madison, WI 53706, USA
10 Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
11 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
Accepted: 10 August 2018
Context. Fossil groups (FGs) are galaxy aggregates with an extended and luminous X-ray halo, which are dominated by a very massive early-type galaxy and lack of L∗ objects. FGs are indeed characterized by a large magnitude gap between their central and surrounding galaxies. This is explained by either speculating that FGs are failed groups that formed without bright satellite galaxies and did not suffer any major merger, or by suggesting that FGs are very old systems that had enough time to exhaust their bright satellite galaxies through multiple major mergers.
Aims. Since major mergers leave signatures in the stellar populations of the resulting galaxy, we study the stellar population parameters of the brightest central galaxies (BCGs) of FGs as a benchmark against which the formation and evolution scenarios of FGs can be compared.
Methods. We present long-slit spectroscopic observations along the major, minor, and diagonal axes of NGC 6482 and NGC 7556, which are the BCGs of two nearby FGs. The measurements include spatially resolved stellar kinematics and radial profiles of line-strength indices, which we converted into stellar population parameters using single stellar-population models.
Results. NGC 6482 and NGC 7556 are very massive (M∗ ≃ 1011.5 M⊙) and large (D25 ≃ 50 kpc) galaxies. They host a centrally concentrated stellar population, which is significantly younger and more metal rich than the rest of the galaxy. The age gradients of both galaxies are somewhat larger than those of the other FG BCGs studied so far, whereas their metallicity gradients are similarly negative and shallow. Moreover, they have negligible gradients of α-element abundance ratio.
Conclusions. The measured metallicity gradients are less steep than those predicted for massive galaxies that formed monolithically and evolved without experiencing any major merger. We conclude that the observed FGs formed through major mergers rather than being failed groups that lacked bright satellite galaxies from the beginning.
Key words: galaxies: elliptical and lenticular, cD / galaxies: formation / galaxies: individual: NGC 6482 / galaxies: individual: NGC 7556 / galaxies: kinematics and dynamics / galaxies: stellar content
Based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC).
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© ESO 2018