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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.