Evolution of massive galaxy structural properties and sizes via star formation in the GOODS NICMOS Survey

Abstract

We present a study of the resolved star-forming properties of a sample of distant massive (M > 1011M⊙) galaxies in the GOODS NICMOS Survey (GNS), based on deep Hubble Space Telescope imaging from the GOODS North and South fields. We derive dust corrected ultraviolet star formation rates (SFRs) as a function of radius for 45 massive galaxies within the redshift range of 1.5 < z < 3 in order to measure the spatial location of ongoing star formation in massive galaxies. We find that the SFRs present in different regions of a galaxy reflect the already existent stellar mass density, i.e. high-density regions have higher SFRs than lower density regions, on average. This observed star formation is extrapolated in several ways to the present day, and we measure the amount of new stellar mass that is created in individual portions of each galaxy to determine how the stellar mass added via star formation changes the observed stellar mass profile, the Sérsic index and effective radius over time. We find that these massive galaxies fall into three broad classifications of star formation distribution: (1) total stellar mass added via star formation is insignificant compared to the stellar mass that is already in place at high redshift. (2) Stellar mass added via star formation is only significant in the outer regions (R > 1kpc) of the galaxy. (3) Stellar mass added via star formation is significant in both the inner (R < 1kpc) and outer regions of the galaxy. These different star formation distributions increase the effective radii over time, which are on average a factor of ∼16 ± 5per cent larger, with little change in the Sérsic index (average Δn = -0.9 ± 0.9) after evolution. We also implement a range of simple stellar migration models into the simulated evolutionary path of these galaxies in order to gauge its effect on the properties of our sample. This yields a larger increase in the evolved effective radii than the pure static star formation model, with a maximum average increase of ΔRe ∼ 54 ± 19per cent, but with little change in the Sérsic index, Δn ∼ -1.1 ± 1.3. These results are not in agreement with the observed change in the effective radius and Sérsic index between z ∼ 2.5 and z ∼ 0 obtained via various observational studies. We conclude that star formation and stellar migration alone cannot account for the observed change in structural parameters for this galaxy population, implying that other mechanisms must additionally be at work to produce the evolution, such as merging. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.