Cold gas in the Perseus cluster core: excitation of molecular gas in filaments

¹ Institut of Radio Astronomy (IRAM), Domaine Universitaire, 300 rue de la piscine, 38400 St Martin d'Hères, France e-mail: salome@iram.fr
² Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
³ Laboratoire d'Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland
⁴ Department of Physics, University of Durham, South Road, Durham DH1 3LEi, UK
⁵ Leiden Observatory, 2300 RA Leiden, The Netherlands
⁶ Institute of Astronomy, Madingley Road, Cambridge CB3 OHA, UK

Received: 1 February 2008
Accepted: 13 March 2008

Abstract

We have recently detected CO lines in the well-known filaments around NGC 1275, the galaxy at the centre of the Perseus cluster of galaxies. These previous observations, with the HERA multi-beam array at the IRAM 30 m telescope enabled us to make a large map of the CO(2–1) line and to see hints of molecular gas far away from the cluster centre. To confirm the presence of CO emission lines in the outer filaments and to study the CO(2–1)/CO(1–0) line ratio, we observed seven regions of interest again with the 30 m telescope in both CO(1–0) and CO(2–1). The regions we observed were: the eastern filament, the horseshoe, the northern filament and a southern extension, all selected from Hα emission line mapping. Molecular gas is detected in all the observed regions. This result confirms the large extent of the cold molecular gas filaments. We discuss the CO(2–1)/CO(1–0) ratios in the filaments. The eastern filament has optically thick gas, whereas further away, the line ratio increases close to values expected for a warmer optically thin medium. We also show CO(1–0) and CO(2–1) lines in 9 regions closer to the centre. The kinematics of the CO is studied here in more detail and confirms that it follows the motions of the warm H₂ gas found in the near-infrared. Finally, we searched for dense gas tracers around 3C 84 and claim here the first detection of HCN(3-2).