Integral field spectroscopy of Hα emission in cooling flow cluster cores: disturbing the molecular gas reservoir

Abstract

We present optical integral field spectroscopy of the Hα-luminous (>10⁴²ergs^-1) central cluster galaxies in the cores of the cooling flow clusters A1664, A1835, A2204 and Zw8193. From the [NII]+Hα complex in these moderate resolution (70-150kms^-1) spectra we derive 2D views of the distribution and kinematics of the emission-line gas, and further diagnostics from the [SII] and [OI] lines.

The Hα emission shows a variety of disturbed morphologies, ranging from smooth but distorted to clumpy and filamentary, with velocity gradients and splittings of several hundred kms^-1 on spatial scales of 20kpc or more. Despite the small sample size, there are some generic features. The most disturbed Hα emission appears to be associated with secondary galaxies within 10-20kpc (projected) of the central galaxy and close in velocity to the Hα. The global Hα kinematics match those of the CO(1-0) emission in single-dish data. The [NII]/Hα, [SII]/Hα and [OI]/Hα ratios vary little with position, local Hα surface brightness or between clusters.

We propose that the Hα and CO emission arise in molecular clouds heated by a starburst, and that the latter has been triggered by interaction with a secondary galaxy. Such CO emission is known to trace massive (>10¹⁰M_solar) compact (<20kpc) reservoirs of cool molecular gas, and it is plausible that an infalling galaxy would disturb this gas, distorting the Hα morphology and initiating widespread star formation. We also examine the role of cloud-cloud collisions in the undisturbed molecular gas reservoir, and suggest that they might be an important source of excitation for the emission-line gas in the cores of lower Hα luminosity clusters with less intense star formation.

Based on observations performed at the European Southern Observatory, Chile (Programme ID: 71.A-3014(A) and at the William Herschel Telescope, La Palma (Programme ID: SW2005b01).

E-mail: r.j.wilman@durham.ac.uk