GeminiFocus October 2018 | Page 6

Figure 2. Half-light radius versus stellar mass for galaxies with photometric redshifts 2 half-light radius versus stellar mass for galaxies with photometric redshifts 2 < z < 4. Red symbols indicate objects best fit with de Vaucouleurs profiles and blue symbols objects best fit by exponential disks. Objects from the CANDELS survey by HST are shown as faint dots. Stars indicate the host galaxies of starbursts detected by Herschel. The dotted cyan line indicates the resolution of HST, and the dot-dashed black line shows that of GeMS/GSAOI. Note that the estimates from the Gemini data tend towards smaller sizes at a given stellar mass than those from HST. Determining the physical mechanisms by which massive galaxies evolve into the ob- jects we see today requires imaging high- z galaxies on scales less than 1 kiloparsec (kpc). Imaging in the rest-frame optical/ near-infrared — longward of the Balmer break at 3646 Ångstroms, where the stellar population is dominated by the older stars that contribute most of the stellar mass in a galaxy — is particularly valuable. Such imaging can be used to measure both the changing distribution of galaxy sizes as a function of redshift and the frequency of interactions and mergers. Furthermore, by combining near-infrared imaging of the stel- lar light with high-resolution radio contin- uum imaging (which pinpoints the regions of star formation or nuclear activity in these systems) we can build up a much more com- plete picture of the nature of the galaxies. This is particularly useful in dusty star-form- ing systems, where the peak of star forma- tion activity may be offset from the peak of the visible stellar light. The GeMS/MCAO Advantage High-resolution imaging over a field of more than a few tens of arcseconds in extent has, until recently, been the exclusive domain of space-based telescopes such as the Hubble Space Telescope (HST). In the near-infrared, HST’s resolution is limited to between 0.1- 0.15 arcsecond. Conventional adaptive optics from the ground can deliver higher resolution, but only over a patch of sky ~30 arcseconds in extent. Multi-conjugate adaptive optics (MCAO) allows larger fields to be imaged by correcting multiple lay- ers of the atmosphere, probed by multiple guide stars. This overcomes two limitations of conventional AO: 1) the limitation of the ~30-arcsecond-radius isoplanatic patch over which correction from a single guide star is effective, and 2) the “cone effect” from laser guide stars, whereby the atmospheric turbu- lence probed by a single laser guide star is not the same as that from an arriving wave- front from a distant star. The Gemini Multi- conjugate adaptive optics System (GeMS) on the Gemini South 8-meter telescope (Rigaut et al., 2014; Neichel et al., 2014) uses a five-laser guide star and a natu- ral guide star constellation of between one and three stars to achieve a consis- tent point spread function (PSF) over a 1.5 arcminute field of view. The current use of GeMS is restricted to asterisms having stars brighter than R ≈ 15 (depending on observing condi- tions), ideally consisting of three stars in an approximate equilateral triangle, and within an ≈ 2 arcminute field of view. Such asterisms are rare (only about one per square degree outside of the Galactic Plane) and are even less commonly found in well-studied, small-area deep extragalactic fields, which are typically picked to avoid bright stars. 4 GeminiFocus October 2018