Letter to the Editor

Multiple CO lines in SMM J16359+6612 – further evidence for a merger


¹ IRAM, Avenida Divina Pastora 7, 18012 Granada, Spain e-mail: aweiss@iram.es
² MPIfR, Auf dem Hügel 69, 53121 Bonn, Germany
³ IRAM, 300 rue de la Piscine, 38406 St.-Martin-d'Hères, France
⁴ MPIA, Königstuhl 17, 69117 Heidelberg, Germany

Received: 8 June 2005
Accepted: 29 July 2005

Abstract

Using the IRAM 30 m telescope, we report the detection of the CO(3–2), CO(4–3), CO(5–4) and CO(6–5) lines in the gravitational lensed submm galaxy SMM J16359+6612 at . The CO lines have a double peak profile in all transitions. From a Gaussian decomposition of the spectra we show that the CO line ratios, and therefore the underlying physical conditions of the gas, are similar for the blue and the redshifted component. The CO line Spectral Energy Distribution (SED; i.e. flux density vs. rotational quantum number) turns over already at the CO 5–4 transition which shows that the molecular gas is less excited than in nearby starburst galaxies and high-z QSOs. This difference mainly arises from a lower average H₂ density, which indicates that the gas is less centrally concentrated than in nuclear starburst regions in local galaxies. We suggest that the bulk of the molecular gas in SMM J16359+6612 may arise from an overlap region of two merging galaxies. The low gas density and clear velocity separation may reflect an evolutionary stage of the merger event that is in between those seen in the Antennae and in the more evolved ultraluminous infrared galaxies (ULIRGs) like e.g. Mrk 231.