Molecular gas and dust in the highly magnified z  ~  2.8 galaxy behind the Bullet Cluster

¹ Onsala Space Observatory, Department of Earth and Space Sciences, Chalmers University of Technology, 439 92 Onsala, Sweden
e-mail: daniel.p.johansson@chalmers.se
² Instituto Nacional de Astrofísica, Optica y Electrónica (INAOE), Tonantzintla, 72840 Puebla, Mexico
³ HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
⁴ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 19 August 2011
Accepted: 14 May 2012

Abstract

Context. The gravitational magnification provided by massive galaxy clusters makes it possible to probe the physical conditions in distant galaxies that are of lower luminosity than those in blank fields and likely more representative of the bulk of the high-redshift galaxy population.

Aims. We aim to constrain the basic properties of molecular gas in a strongly magnified submm galaxy located behind the massive Bullet Cluster (1E 0657-56). This galaxy (SMM J0658) is split into three images, with a total magnification factor of almost 100.

Methods. We used the Australia Telescope Compact Array (ATCA) to search for ¹²CO(1–0) and ¹²CO(3–2) line emission from SMM J0658. We also used the SABOCA bolometer camera on the Atacama Pathfinder EXperiment (APEX) telescope to measure the continuum emission at 350 μm.

Results. CO(1–0) and CO(3–2) are detected at 6.8σ and 7.5σ significance when the spectra toward the two brightest images of the galaxy are combined. From the CO(1–0) luminosity we derive a mass of cold molecular gas of (1.8 ± 0.3) × 10⁹   M_⊙, using the CO to H₂ conversion factor commonly used for luminous infrared galaxies. This is 45 ± 25% of the stellar mass. From the width of the CO lines we derive a dynamical mass within the CO-emitting region L of (1.3 ± 0.4) × 10¹⁰(L/1   kpc)   M_⊙. We refine the redshift determination of SMM J0658  to z = 2.7793 ± 0.0003. The CO(3–2) to CO(1–0) brightness temperature ratio is 0.56_-0.15^+0.21, which is similar to the values found in other star-forming galaxies. Continuum emission at 350 μm from SMM J0658 was detected with SABOCA at a signal-to-noise ratio of 3.6. The flux density is consistent with previous measurements at the same wavelength by the Herschel satellite and BLAST balloon-borne telescope. We study the spectral energy distribution of SMM J0658 and derive a dust temperature of 33 ± 5 K and a dust mass of 1.1_-0.3^+0.8 × 10⁷ M_⊙.

Conclusions. SMM J0658 is one of the least massive submm galaxies discovered so far. As a likely representative of the bulk of the submm galaxy population, it is a prime target for future observations.