Final Report Summary - HIGHZ (HIGHZ: Elucidating galaxy formation and evolution from very deep Near-IR imaging)
The project focused on the study of the formation and evolution of galaxies. The new Wide Field Camera 3 was installed on the Hubble Space Telescope in 2009, and caused a revolution in the study of distant galaxies. We were able to study the distant galaxies as part of the HUDF09 observing program, in which we made very sensitive images of the Hubble Ultra Deep Field. This study led to the discovery of a new “record” redshift galaxy candidate by the team members: the galaxy was at a redshift of about 10, which implies that the universe was a factor of 11 times smaller when the light was emitted. We also used these data to study the properties of galaxies at lower redshifts. Overall, the population of galaxies evolves strongly between a redshift of 6 (when the universe was about 1 billion years old) and a redshift of 10 (when the universe was half that age). It is clear that we are witnessing the growth of the first population of galaxies at that epoch. The low number of redshift 10 galaxies was a surprise, as many more would have been detected if the properties of galaxies at a redshift of 6 were similar to that at a redshift of 10. We also found that the colors of these galaxies evolve weakly, and we found direct evidence that optical emission lines are very strong for high redshift galaxies. This complicates the modelling of the galaxies, and the derivation of their stellar masses. The second part of the program focused on the study of the evolution of galaxies between a redshift of about 3 and 1, corresponding to an age of the universe of 2 billions years and 6 billion years. Due to Wide Field Camera 3 capabilities, we could study the restframe optical morphologies and spectra of these galaxies. We used publicly available imaging, and data from the “3D-HST” program. We were part of the 3D-HST team and were able to analyse the spectra and images of the galaxies in that program. We found several remarkable results : 1) we confirmed that a population of galaxies exists out to redshift 3 which form only few stars. These quiescent galaxies could be studied in great detail thanks to the deep imaging taken with the Hubble Space Telescope. These data ruled out that we had missed a substantial portion of the light, and confirmed that these galaxies are indeed very small, much smaller than comparable non-star forming galaxies in the nearby universe. We found that star forming galaxies were also substantially smaller at higher redshift, compared to nearby galaxies. 2) We used a novel technique to relate galaxies at different redshifts to each other as likely “progenitors” and “descendants”. This allowed us to study the structural evolution of galaxies across time. We found that the massive, very small galaxies at high redshift typically grow “inside-out”, which implies that they remain more-or-less constant in the inner parts, but a substantial amount of stars is added in the outer parts. This is most easily caused by the merging of small galaxies onto the more massive galaxies. We also found that lower mass galaxies like form in a different way: they also show the accumulation of stars in the outer parts, but in addition, they also “grow” in the central parts: they undergo a doubling of the mass in the central parts since a redshift of 1. Finally, we analysed the spectra of the galaxies. One of the striking results is the increase in the emission lines in the spectra of these galaxies. We measured this increase, and we showed that it agrees with a simple model. In addition, this model predicts even stronger emission lines at higher redshifts. This description gives a brief list of high-lights from the project, in total 43 publications were produced, which cannot all be described in this brief space.