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We analyse the SLEDs of 13CO and C18O for the J=1-0 up to J=7-6 transitionsin the gravitationally lensed ultraluminous infrared galaxy SMMJ2135-0102 atz=2.3. This is the first detection of 13CO and C18O in a high-redshiftstar-forming galaxy. These data comprise observations of six transitions takenwith PdBI and we combine these with 33GHz JVLA data and our previous 12CO andcontinuum emission information to better constrain the properties of the ISMwithin this system. We study both the velocity-integrated and kinematicallydecomposed properties of the galaxy and coupled with an LVG model we find thatthe star-forming regions in the system vary in their cold gas properties. Wefind strong C18O emission both in the velocity-integrated emission and in thetwo kinematic components at the periphery of the system, where the C18O lineflux is equivalent to or higher than the 13CO. We derive an averagevelocity-integrated flux ratio of 13CO/C18O~1 suggesting a [13CO]/[C18O]abundance ratio at least 7x lower than that in the Milky Way. This may suggestenhanced C18O abundance, perhaps indicating star formation preferentiallybiased to high-mass stars. We estimate the relative contribution to the ISMheating from cosmic rays and UV of (30-3300)x10^(-25)erg/s and 45x10^(-25)erg/sper H2 molecule respectively and both are comparable to the total cooling rateof (0.8-20)x10^(-25)erg/s from the CO. However, our LVG models indicate high(>100K) temperatures and densities (>10^(3))cm^(-3) in the ISM which maysuggest that cosmic rays play a more important role than UV heating in thissystem. If cosmic rays dominate the heating of the ISM, the increasedtemperature in the star forming regions may favour the formation of massivestars and so explain the enhanced C18O abundance. This is a potentiallyimportant result for a system which may evolve into a local elliptical galaxy.

13CO and C18O emission from a dense gas disk at z=2.3: abundance variations, cosmic rays and the initial conditions for star formation