Galaxy Science with ORCAS: Faint Star-Forming Clumps to AB$\leq$31 mag
  and r$_e$$\geq$ 0.01"
  The NASA concept mission ORCAS (Orbiting Configurable Artificial Star) aims
to provide near diffraction-limited angular resolution at visible and
near-infrared wavelengths using laser signals from space-based cubesats as
Adaptive Optics beacons for ground-based 8-30 meter telescopes, in particular
the 10 meter Keck Telescopes. When built as designed, ORCAS+Keck would deliver
images of ~0.01-0.02" FWHM at 0.5-1.2 micron wavelength that reach AB<31 mag
for point sources in a few hours over a 5x5" FOV that includes IFU
capabilities. We summarize the potential of high-resolution faint galaxy
science with ORCAS. We show that the ability to detect optical-near-IR point
sources with r_e>0.01" FWHM to AB<31 mag will yield about 5.0x10^6 faint
star-forming (SF) clumps per square degree, or ~0.4 per arcsec^2. From recent
HST lensing data, the typical intrinsic (unlensed) sizes of SF clumps at z~1-7
will be r_e ~1-80 m.a.s. to AB<31 mag, with intrinsic (unmagnified) fluxes as
faint as AB<35-36 mag when searching with ORCAS around the critical curves of
lensing clusters imaged with HST and JWST. About half of these SF clumps will
have sizes below the ORCAS diffraction limit, and the other half will be
slightly resolved, but still mostly above the ORCAS surface brightness (SB)
limits. ORCAS will address how galaxies assemble from smaller clumps to stable
disks by measuring ages, metallicities, and gradients of clumps within
galaxies. ORCAS can monitor caustic transits of individual stars in SF clumps
at z>1-2 that have been detected with HST, and those that may be detected with
JWST at z>6 at extreme magnifications (mu>10^3-10^5) for the first stars and
their stellar mass black hole accretion disks. ORCAS provides a unique
opportunity to obtain a statistical census of individual stars at cosmological
distances, leveraging the largest telescopes only available on the ground.
