Measuring D(4000) n z~1.6 galaxies using WFC3/G102L observations with HST: a feasibility study

Abstract

This is a feasibility study for the analysis of Hubble Space Telescope observations of galaxy cluster IRC 0218A at a redshift of z = 1.62. The main goals were as follows: to investigate any systematic biases in D(4000) and Dn(4000), commonly used indices for probing a galaxy's average stellar age, for different star formation histories, ages, metallicities, and dust extinctions; to determine the precision with which D(4000) and Dn(4000) can be measured for galaxies at a range of magnitudes; and to constrain the conditions under which a measurement of D(4000) or Dn(4000) is a reliable probe of a galaxy's average stellar age. A grid of template galaxies was constructed using the GALAXEV package described by Bruzual & Charlot (2003), and the Hubble observations were simulated using the Space Telescope Science Institute's aXeSIM software package. No obvious systematic biases in D(4000) or Dn(4000) were observed. D(4000) and Dn(4000) could be accurately measured within a 10% uncertainty up to the study's J-band magnitude limit of 22.8 and within a 5% uncertainty up to a J-band magnitude of 21.6. However, even these small errors in D(4000) correspond to large errors in calculated stellar age. A measurement of D(4000) or Dn(4000) can only roughly constrain the age of a galaxy's stellar population for J-band magnitudes brighter than ∼ 21.6.