CATS is a long-term survey aimed for an enhanced near-IR study of the structural,
chemical, star formation, and kinematic evolution of distant field galaxy subcomponents
on sub-kpc scales. This is possible by using adaptive optics (AO) on an 8-10m class telescope
that has diffraction-limits in the near-IR of 0.05 arcsec, which is 3x-4x better than
is possible with HSTs 2.4m mirror at a similar wavelength. This resolution corresponds to
the sizes of bulges, disks, bars, spiral arms, and merger/interaction or lensing signatures
at redshifts z > 0.5 and improves the study of unresolved star forming knots, supernovae,
and AGN’s. It also matches the optical diffraction-limit of HST, so the CATS strategy
is to work in well-studied HST survey fields with ACS images taken in 2 or more filters,
including GOODS, GEMS, & EGS. These regions are highly leveraged by deep Chandra,
XMM-Newton, GALEX, and Spitzer data from space and optical, near-IR, submm, and
radio data from the ground.
Keck’s AO is presently limited to the near-IR, which complements the optical by
providing superior penetration of dust-obscured regions and having higher sensitivity
to old stars. For high redshift objects, the near-IR measures light that was emitted as
restframe optical, including the important Hα and [NII] lines for spectroscopy. Keck II
now routinely provides laser guide star (LGS) AO (Wizinowich et al. 2006), which has
opened new doors for many areas of astronomy (see Liu 2006 for an overview of the
advantages, disadvantages, and potential of AO). With LGS AO supporting tip-tilt stars
120 Koo et al.
as faint as 18th mag within ∼ 60 arcsec (van Dam et al. 2006) and good characterization
of the off-axis point spread function (Steinbring et al. 2005), CATS now has, for the
first time, access to 20% or more of the area of the special HST galaxy survey fields,
and potentially over 1500 galaxies to be studied with AO. Prior to LGS, only about 1%
was reachable on Keck since much brighter natural guide stars of 12th mag were needed
(Wizinowich et al. 2000). Thus much of the early distant galaxy research with Keck
AO by Larkin et al. (2000) and Glassman, Larkin, & Lafreniere (2002) were undertaken
without having the benefit of adding HST data (c.f., Steinbring et al. 2004).
To date, CATS has obtained AO data with Keck II for about 10 pointings or 5 square
arcmin with the NIRC2 camera in GOODS, GEMS, and EGS. One Chandra X-ray
galaxy in GOODS-S was found to have two red nuclei for which the 4 filter HST ACS
photometry in the optical indicated either old stars or dusty, younger stars. The AO Kband
photometry provided the critical discrimination and suggested that both were old.
Thus we were witnessing a ”dry merger” (Melbourne et al. 2005). In another study, M.
Barczys (PhD 2006) exploited the higher sensitivity to stellar mass of near-IR photometry
to discover that many of the apparent major mergers with roughly equal luminosities seen
in the HST optical images are in fact minor mergers with unequal stellar masses. Finally,
CATS has recently achieved an important milestone by measuring the very faint (∼ 24
mag Vega) H band flux (and thus the restframe R luminosity) of a high redshift (z = 1.32)
supernova that was lying atop its much brighter host galaxy (Melbourne et al. 2007).
CATS plans to acquire more LGS AO images, mainly to study the host galaxies of
AGNs and the colors of bulges and disks. With the advent of OSIRIS (Larkin et al. 2006),
an AO compatible near-IR spectrograph with an integral field unit, CATS is poised to
gather high spatial resolution 2-D spectroscopic data. A major goal of CATS is to provide
the community with good quality AO data with which to explore its scientific potential.
To this end, CATS intends to release some Keck AO data by early 2007.
We thank the Keck Observatory staff for years of help and for making LGS AO a reality.
We thank S. Perlmutter and the Supernovae Cosmology Project team for providing CATS
with the supernova. This work was supported in part by the NSF Science and Technology
CfAO, managed by UC Santa Cruz under cooperative agreement No. AST-9876783. We
close with thanks to the Hawaiian people for use of their sacred mountain.