On measuring the cosmic microwave background temperature at redshift 0.89

Abstract

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 T 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. © 2013. The American Astronomical Society. All rights reserved.