On Measuring the CMB Temperature at Redshift 0.89

We report on a measurement of the temperature of the cosmic microwave background radiation field, T_CMB, at z = 0.88582 by imaging HC3N (3-2) and (5-4) absorption in the foreground galaxy of the gravitationally lens magnified radio source PKS 1830-211 using the Very Long Baseline Array and the phased Very Large Array. Low-resolution imaging of the data yields a value of Trot = 5.6+2.5-0.9 K, for the rotational temperature, Trot, 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 1830-211 SW, 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 Trot between 1.1 and 2.5 K, which is lower than the expected value. This shows that previous, Trot, measurements could be biased due to unresolved structure.