Spatially correlated cluster populations in the outer disk of ngc 3184

We use deep (27.5 mag V-band point-source limiting magnitude) V- and U-band Large Binocular Telescope imaging to study the outer disk (beyond the optical radius R ₂₅) of the non-interacting, face-on spiral galaxy NGC 3184 (D = 11.1 Mpc; R ₂₅ = 11.1 kpc) and find that this outer disk contains >1000 objects (or marginally resolved "knots") resembling star clusters with masses 10²-10⁴ M and ages up to 1Gyr. We find statistically significant numbers of these cluster-like knots extending to 1.4 R ₂₅, with the redder knots outnumbering bluer at the largest radii. We measure clustering among knots and find significant correlation to galactocentric radii of 1.5 R ₂₅ for knot separations <1 kpc. The effective integrated surface brightness of this outer disk cluster population ranges from 30-32 mag arcsec^-2 in V. We compare the H I extent to that of the correlated knots and find that the clusters extend at least to the damped Lyman-α threshold of H I column density (2 × 10²⁰ cm^-2; 1.62 R ₂₅). The blue knots are correlated with H I spiral structure to 1.5 R ₂₅, while the red knots may be correlated with the outer fringes of the H I disk to 1.7 R ₂₅. These results suggest that outer disks are well populated, common, and long-lasting features of many nearby disk galaxies.