H₂ formation and excitation in the Stephan's Quintet galaxy-wide collision^*

¹ Institut d'Astrophysique Spatiale (IAS), UMR 8617, CNRS, Université Paris-Sud 11, Bâtiment 121, 91405 Orsay Cedex, France e-mail: pierre.guillard@ias.u-psud.fr
² LERMA, UMR 8112, CNRS, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France
³ NASA Science Center (NHSC), California Institute of Technology, Mail code 100-22, Pasadena, CA 91125, USA

Received: 30 October 2008
Accepted: 28 April 2009

Abstract

Context. The Spitzer Space Telescope has detected a powerful ( erg s^-1) mid-infrared H₂ emission towards the galaxy-wide collision in the Stephan's Quintet (henceforth SQ) galaxy group. This discovery was followed by the detection of more distant H₂-luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations place molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution.

Aims. The SQ postshock medium is observed to be multiphase, with H₂ gas coexisting with a hot (  10⁶ K), X-ray emitting plasma. The surface brightness of H₂ lines exceeds that of the X-rays and the 0-0 S(1)  linewidth is  km s^-1, of the order of the collision velocity. These observations raise three questions we propose to answer: (i) why is H₂ present in the postshock gas? (ii) How can we account for the  excitation? (iii) Why is H₂ a dominant coolant?

Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H₂ formation and excitation in the postshock medium.

Results. (i) The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume-filling intercloud gas. This produces a   K, dust-free, X-ray emitting plasma. The shock velocity is lower in clouds. We show that gas heated to temperatures of less than K cools, keeps its dust content and becomes H₂ within the SQ collision age (  years). (ii) Since the bulk kinetic energy of the H₂ gas is the dominant energy reservoir, we consider that the H₂ emission is powered by the dissipation of kinetic turbulent energy. We model this dissipation with non-dissociative MHD shocks and show that the H₂ excitation can be reproduced by a combination of low velocities shocks ( km s^-1) within dense () H₂ gas. (iii) An efficient transfer of the bulk kinetic energy to turbulent motion of much lower velocities within molecular gas is required to make H₂ a dominant coolant of the postshock gas. We argue that this transfer is mediated by the dynamic interaction between gas phases and the thermal instability of the cooling gas. We quantify the mass and energy cycling between gas phases required to balance the dissipation of energy through the H₂ emission lines.

Conclusions. This study provides a physical framework to interpret H₂ emission from H₂-luminous galaxies. It highlights the role that H₂ formation and cooling play in dissipating mechanical energy released in galaxy collisions. This physical framework is of general relevance for the interpretation of observational signatures, in particular H₂ emission, of mechanical energy dissipation in multiphase gas.