We report on a measurement of the temperature of the cosmic microwave background radiation field, T CMB, at z = 0.88582 by imaging HC3N(3 N(3 N(3 rot = 5.6 2.5 - 0.9 K for the rotational temperature, T rot, which is consistent with the temperature of the cosmic microwave background at the absorber's redshift of 2.73(1 z) K. However, our high-resolution imaging reveals that the absorption peak position of the foreground gas is offset from the continuum peak position of the synchrotron radiation from PKS 1830-211SW, which indicates that the absorbing cloud is covering only part of the emission from PKS 1830-211, rather than the entire core-jet region. This changes the line-to-continuum ratios, and we find T rot between 1.1 and 2.5 K, which is lower than the expected value. This shows that previous T rot measurements could be biased due to unresolved structure.
