Accretion processes in the galaxy cluster Hydra A/Abell 780^⋆

¹ Dipartimento di Fisica dell’Università degli Studi di Trieste – Sezione di Astronomia, Via Tiepolo 11, 34143 Trieste, Italy
² INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
e-mail: marisa.girardi@inaf.it
³ Fundación Galileo Galilei – INAF (Telescopio Nazionale Galileo), Rambla José Ana Fernández Perez 7, 38712 Breña Baja (La Palma), Canary Islands, Spain
⁴ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain
⁵ Departamento de Astrofísica, Univ. de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, 38205 La Laguna, Tenerife, Spain
⁶ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy

Received: 13 September 2021
Accepted: 8 December 2021

Abstract

Context. Clusters of galaxies evolve and accrete mass, mostly from small galaxy systems.

Aims. Our aim is to study the velocity field of the galaxy cluster Abell 780, which is known for the powerful radio source Hydra A at its center and where a spectacular X-ray tail associated with the galaxy LEDA 87445 has been discovered.

Methods. Our analysis is based on the new spectroscopic data for hundreds of galaxies obtained with the Italian Telescopio Nazionale Galileo and the Very Large Telescope. We have constructed a redshift catalog of 623 galaxies and selected a sample of 126 cluster members. We analyzed the internal structure of the cluster using a number of techniques.

Results. We estimate the mean redshift z = 0.0545, the line-of-sight velocity dispersion σ_V ∼ 800 km s⁻¹, and the dynamical mass M₂₀₀ ∼ 5.4 × 10¹⁴ M_⊙. The global properties of Abell 780 are typical of relaxed clusters. On a smaller scale, we can detect the presence of a galaxy group associated with LEDA 87445 in projected phase space. The mean velocity and position of the center of the group agree well with the velocity and position of LEDA 87445. We estimate the following parameters of the collision. The group is characterized by a higher velocity relative to the main system. It is infalling at a rest frame velocity of V_rf ∼ +870 km s⁻¹ and lies at a projected distance of D ∼ 1.1 Mpc to the south, slightly southeast of the cluster center. The mass ratio of the group to the cluster is ∼1:5. We also find evidence of an asymmetry in the velocity distribution of galaxies in the inner cluster region, which might be related to a small low-velocity group detected as a substructure at V_rf ∼ −750 km s⁻¹.

Conclusions. We conclude that A780, although dynamically relaxed at first sight, contains small substructures that may have some impact on the energetics of the core region.