2012MNRAS.419.1489B

Ground-based surveys have mapped the stellar outskirts of Local Group disc galaxies in unprecedented detail, but extending this work to other galaxies is necessary in order to overcome stochastic variations in evolutionary history and provide more stringent constraints on cosmological galaxy formation models. As part of our continuing programme of ultra-deep imagery of galaxies beyond the Local Group, we present a wide-field analysis of the isolated late-type spiral NGC 2403 using data obtained with Suprime-Cam on the Subaru telescope. The surveyed area reaches a maximum projected radius of 30 kpc or a 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 per cent for R_dp> rsim 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 SB than is possible with either technique alone. The exponential disc as traced by RGB stars dominates the SB profile out to ≳ 8 disc scalelengths, or R_dp ∼ 18 kpc, and reaches a V-band SB of µ_V ∼ 29 mag/arcsec2. 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 µ_V ∼ 32 mag/arcsec2. This component can be fit with a power-law index of γ ∼ 3, has an axial ratio consistent with that of the inner disc and has a V-band luminosity integrated over all radii of 1-7 per cent that of the whole galaxy. At R_dp ∼ 20-30 kpc, we estimate a peak metallicity [M/H] =-1.0 ±0.3 assuming an age of 10 Gyr and zero α-element enhancement. 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.