The properties of the interstellar medium within a star-forming galaxy at z=2.3

We present an analysis of the molecular and atomic gas emission in the rest-frame far-infrared and submillimetre from the lensed z = 2.3 submillimetre galaxy SMM J2135-0102. We obtain very high signal-to-noise ratio detections of 11 transitions from three species and limits on a further 20 transitions from nine species. We use the 12CO, [C i] and HCN line strengths to investigate the gas mass, kinematic structure and interstellar medium (ISM) chemistry and find strong evidence for a two-phase medium within this high-redshift starburst galaxy, comprising a hot, dense, luminous component and an underlying extended cool, low-excitation massive component. Employing a suite of photodissociation region models, we show that on average the molecular gas is exposed to an ultraviolet (UV) radiation field that is similar to 1000 times more intense than the Milky Way, with star-forming regions having a characteristic density of n similar to 104 cm-3. Thus, the average ISM density and far-UV radiation field intensity are similar to those found in local ultraluminous infrared galaxies (ULIRGs) and to those found in the central regions of typical starburst galaxies, even though the star formation rate is far higher in this system. The 12CO spectral line energy distribution and line profiles give strong evidence that the system comprises multiple kinematic components with different conditions, including temperature, and line ratios suggestive of high cosmic-ray flux within clouds, likely as a result of high star formation density. We find tentative evidence of a factor of similar to 4 temperature range within the system. We expect that such internal structures are common in high-redshift ULIRGs but are missed due to the poor signal-to-noise ratio of typical observations. We show that, when integrated over the galaxy, the gas and star formation surface densities appear to follow the Kennicutt-Schmidt relation, although by comparing our data to high-resolution submillimetre imaging, our data suggest that this relation breaks down on scales of < 100 pc. By virtue of the lens amplification, these observations uncover a wealth of information on the star formation and ISM at z similar to 2.3 at a level of detail that has only recently become possible at z < 0.1 and show the potential physical properties that will be studied in unlensed galaxies when the Atacama Large Millimeter Array is in full operation.