Quantifying the Faint Structure of Galaxies: The Late-type Spiral NGC
  2403
  Ground-based surveys have mapped the stellar outskirts of Local Group
galaxies in unprecedented detail, but extending this work to other galaxies is
necessary to overcome stochastic variations in evolutionary history and provide
more stringent constraints on cosmological galaxy formation models. As part of
our continuing program of ultra-deep imagery of galaxies beyond the Local
Group, we present a wide-field analysis of the isolated late-type spiral
NGC2403 using data obtained with Suprime-Cam on Subaru. The survey reaches a
maximum projected radius of 30 kpc or deprojected radius of R_dp~60 kpc. The
colour-magnitude diagram reaches 1.5 mag below the tip of the metal-poor red
giant branch (RGB) at a completeness rate > 50% for R_dp >12 kpc. Using the
combination of diffuse light photometry and resolved star counts, we are able
to trace the radial surface brightness (SB) profile over a much larger range of
radii and surface brightness than is possible with either technique alone. The
exponential disc as traced by RGB stars dominates the SB profile out to >8 disc
scale-lengths, or R_dp~18 kpc, and reaches a V-band SB of 29 mag per sq.
arcsec. Beyond this radius, we find evidence for an extended structural
component with a significantly flatter SB profile than the inner disc and which
we trace to R_dp~40 kpc and ~32 mag per sq. arcsec. This component can be fit
with a power-law index of ~3, has an axial ratio consistent with that of the
inner disc and has a V-band luminosity of 1-7% that of the whole galaxy. At
R_dp~20 - 30 kpc, we estimate a peak metallicity [M/H]= -1.0+/-0.3. Although
the extant data are unable to discriminate between stellar halo or thick disc
interpretations of this component, our results support the notion that faint,
extended stellar structures are a common feature of all disc galaxies, even
isolated, low-mass systems.
