Setting the Stage for Cosmic Chronometers. I. Assessing the Impact of Young Stellar Populations on Hubble Parameter Measurements

The expansion history of the universe can be constrained in a cosmology-independent way by measuring the differential age evolution of cosmic chronometers. This yields a measurement of the Hubble parameter H(z) as a function of redshift. The most reliable cosmic chronometers known so far are extremely massive and passively evolving galaxies. Age-dating these galaxies is, however, a difficult task, and even a small contribution of an underlying young stellar population could, in principle, affect the age estimate and its cosmological interpretation. We present several spectral indicators to detect, quantify, and constrain such contamination in old galaxies and study how their combination can be used to maximize the purity of cosmic chronometers selection. In particular, we analyze the Ca ii H/K ratio, the presence (or absence) of H and [O ii] emission lines, higher-order Balmer absorption lines, and UV flux; each indicator is especially sensitive to a particular age range, allowing us to detect young components ranging between 10 Myr and 1 Gyr. The combination of these indicators minimizes the contamination to a level below 1% in the case of ideal data. More importantly, it offers a way to control the systematic error on H(z) as a function of the contamination by young stellar populations. We show that for our previous measurements of the Hubble parameter, the possible bias induced by the presence of a younger component is well below the current errors. We envision that these indicators will be instrumental in paving the road for a robust and reliable dating of the old population and its cosmological interpretation.