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