65 kpc of ionized gas trailing behind NGC 4848 during its first crossing of the Coma cluster^⋆

¹ Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
e-mail: matteo.fossati@mib.infn.it; giuseppe.gavazzi@mib.infn.it
² Laboratoire d’Astrophysique de Marseille, UMR 6110 CNRS, 38 rue F. Joliot-Curie, 13388 Marseille, France
e-mail: alessandro.boselli@oamp.fr
³ Department of Astronomy and Astrophysics, University of California, 1156 High Street, Santa Cruz, CA 95064, USA
e-mail: mfumagalli@ucolick.org

Received: 2 July 2012
Accepted: 18 July 2012

Abstract

In a five hour Hα exposure of the northwest region of the Coma cluster with the 2.1 m telescope at San Pedro Martir (Mx), we discovered a 65 kpc cometary emission of ionized gas trailing behind the SBab galaxy NGC 4848. The tail points in the opposite direction of the cluster center, in the same direction where stripped HI had been detected in previous observations. The galaxy shows bright HII regions in an inner ring-like pattern, where the star formation takes place at the prodigious rate of  ~8.9   M_⊙ yr^-1. From the morphologies of the galaxy and the trailing material, we infer that the galaxy is suffering from ram pressure due to its high velocity motion through the intergalactic medium. We estimate that  ~4 × 10⁹   M_⊙ of gas is swept out from the galaxy forming the tail. Given the ambient conditions in the Coma cluster (ρ₀ = 6.3 × 10^-27   g   cm^-3; σ_vel = 940   km   s^-1), simulations predict that the ram pressure mechanism is able to remove such an amount of gas in less than 200 Myr. This, combined with the geometry of the interaction, is indicative of radial infall into the cluster, leading to the conclusion that NGC 4848 has been caught during its first passage through the dense cluster environment.