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 adap-
tive optics (MCAO) allows larger fields to be
imaged by correcting multiple layers of the
atmosphere, probed by multiple guide stars.
This overcomes two limitations of conven-
tional AO: 1) the limitation of the ~30-arc-
second-radius isoplanatic patch over which
correction from a single guide star is effec-
tive, and 2) the “cone effect” from laser guide
stars, whereby the atmospheric turbulence
probed by a single laser guide star is not
the same as that from an arriving wavefront
from a distant star. The Gemini Multiconju-
gate 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 natural guide star constel-
lation of between one and three stars to
achieve a consistent point spread function
(PSF) over a 1.5 arcminute field of view.
January 2019 / 2018 Year in Review
The current use of GeMS is restricted to as-
terisms having stars brighter than R ≈ 15
(depending on observing conditions), ide-
ally consisting of three stars in an approxi-
mate equilateral triangle, and within an ≈ 2
arcminute field of view. Such asterisms are
rare (only about one per square degree out-
side 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.
Fortunately, the new generation of deep,
wide-area (> 1 square degree) extragalactic
surveys — designed to study the evolution
of galaxies over a wide range in environ-
ment — can complement MCAO facilities
by both containing suitable asterisms and
having the multi-wavelength coverage
needed to obtain photometric redshifts and
star formation rates for the galaxies in the
field. The Spitzer Extragalactic Representa-
tive Volume Survey (SERVS; Mauduit et al.,
2012) and associated VISTA Deep Extraga-
lactic Observations (VIDEO) survey (Jarvis et
al., 2013) provide 12 square degrees of deep
near-infrared observations in seven bands
from 0.9-4.5 microns (μm), enough area to
find several such asterisms.
GeminiFocus
Figure 1.
The GSAOI image of the
ES1C field. Objects of
interest are shown as
insets, each measuring 6
arcseconds on a side. The
red circles indicate the
stars used to determine
the PSF in the field, and
the blue circles show
those used as natural
guide stars for the
adaptive optics system
(one is off the image).
9