We analyse the spectral line energy distributions ( SLEDs ) of ^ { 13 } CO and C ^ { 18 } O for the J = 1 \rightarrow 0 up to J = 7 \rightarrow 6 transitions in the gravitationally lensed ultraluminous infrared galaxy SMM J2135-0102 at z = 2.3 .
This is the first detection of ^ { 13 } CO and C ^ { 18 } O in a high-redshift star-forming galaxy .
These data comprise observations of six transitions taken with PdBI and we combine these with \sim 33 GHz JVLA data and our previous spatially resolved ^ { 12 } CO and continuum emission information to better constrain the properties of the inter-stellar medium ( ISM ) within this system .
We study both the velocity-integrated and kinematically decomposed properties of the galaxy and coupled with a large velocity gradient model we find that the star-forming regions in the system vary in their cold gas properties , in particular in their chemical abundance ratios .
We find strong C ^ { 18 } O emission both in the velocity-integrated emission and in the two kinematic components at the periphery of the system , where the C ^ { 18 } O line flux is equivalent to or higher than the ^ { 13 } CO. We derive an average velocity-integrated flux ratio of ^ { 13 } CO/C ^ { 18 } O \sim 1 which suggests an abundance ratio of [ ^ { 13 } CO ] / [ C ^ { 18 } O ] which is at least 7 \times lower than that in the Milky Way .
This is suggestive of enhanced C ^ { 18 } O abundance , perhaps indicating star formation preferentially biased to high-mass stars .
We estimate the relative contribution to the ISM heating from cosmic rays and UV of ( 30–3300 ) \times 10 ^ { -25 } erg s ^ { -1 } and 45 \times 10 ^ { -25 } erg s ^ { -1 } per H _ { 2 } molecule respectively and find them to be comparable to the total cooling rate of ( 0.8–20 ) \times 10 ^ { -25 } erg s ^ { -1 } from the CO .
However , our LVG models indicate high ( > 100 K ) temperatures and densities ( > 10 ^ { 3 } ) cm ^ { -3 } in the ISM which may suggest that cosmic rays play a more important role than UV heating in this system .
If cosmic rays dominate the heating of the ISM , the increased temperature in the star forming regions may favour the formation of massive stars and so explain the enhanced C ^ { 18 } O abundance .
This is a potentially important result for a system which may evolve into a local elliptical galaxy .