Letter to the Editor

Interstellar atoms, molecules and diffuse bands toward SN2006X in M 100^*

¹ Herschel Science Centre, European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain e-mail: nick.cox@sciops.esa.int
² European Southern Observatory, K. Schwarzschild Str. 2, 85748, Garching b. München, Germany

Received: 14 March 2008
Accepted: 6 May 2008

Abstract

Aims. Supernovae offer the unique possibility to probe diffuse extra-galactic sightlines via observation of the optical transitions of atoms, molecules and the diffuse interstellar bands (DIBs). Through optical spectroscopy the presence of (complex) molecules in distant galaxies can be established and used to derive local physical conditions of the interstellar medium (ISM).

Methods. High resolution optical (3300–6800 Å) spectra of SN2006X at different phase obtained with UVES on the VLT were reduced and analysed.

Results. In addition to previously detected atomic (Na I and Ca II) and molecular (CN) transitions we present detections of DIBs (6196, 6283), diatomic molecules (CH, CH⁺) and neutral atoms (Ca I) in the spectra of SN2006X taken at different phases (at 2 days before and 14 and 61 days after the brightness maximum). An analysis of the absorption profiles shows no variation between phases in the abundance, nor the central velocities (within 3σ error bars) of the (dense) gas tracers (CH, CH⁺ and Ca I) and the DIBs. This is consistent with the conclusion in the literature that SN2006X exploded behind a dense interstellar cloud (inferred from strong atomic sodium and calcium lines and CN transitions) which caused strong photometric reddening but whose material was not directly affected by the supernova explosion. The CH and CN column densities correspond to a reddening of one magnitude following the Galactic correlation derived previously. The 6196 and 6283 lines detected in the M 100 ISM are under-abundant by factor of 2.5 to 3.5 (assuming a visual extinction of ~2 mag) compared to the average Galactic ISM relationship. Upper limits for 6379 and 6613 show that these are at least a factor of seven weaker. Therefore, the Galactic DIB-reddening relation does not seem to hold in M 100, although the lower gas-to-dust ratio may further reduce this discrepancy.