NGC 300: An extremely faint, outer stellar disk observed to 10 scale lengths

We have used the Gemini Multi-Object Spectrograph (GMOS) on the Gemini South 8 m telescope in exceptional conditions (0.″6 FWHM seeing) to observe the outer stellar disk of the Sculptor Group galaxy NGC 300 at two locations. At our point-source detection threshold of r′ = 27.0 (3 σ) mag, we trace the stellar disk out to a radius of 24′, or 2.2R₂₅, where R₂₅ is the 25 mag arcsec^-2 isophotal radius. This corresponds to about 10 scale lengths in this low-luminosity spiral galaxy (M_B = -18.6), or about 14.4 kpc at a Cepheid distance of 2.0 ± 0.07 Mpc. The background galaxy counts are derived in the outermost field, and these are within 10% of the mean survey counts from both Hubble Deep Fields. The luminosity profile is well described by a nucleus plus a simple exponential profile out to 10 optical scale lengths. We reach an effective surface brightness of μ_r′ = 30.5 mag arcsec^-2 (2 σ) at 55% completeness, which doubles the known radial extent of the optical disk. These levels are exceedingly faint in the sense that the equivalent surface brightness in B or V is about 32 mag arcsec^-2. We find no evidence for truncation of the stellar disk. Only star counts can be used to reliably trace the disk to such faint levels, since surface photometry is ultimately limited by nonstellar sources of radiation. In the Appendix, we derive the expected surface brightness of one such source: dust scattering of starlight in the outer disk.