2009ApJ...700.1977H

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 R₂₅) of the non-interacting, face-on spiral galaxy NGC 3184 (D = 11.1 Mpc; R₂₅= 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 ∼1 Gyr. We find statistically significant numbers of these cluster-like knots extending to ∼1.4 R₂₅, 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 R₂₅ for knot separations <1 kpc. The effective integrated surface brightness of this outer disk cluster population ranges from 30-32 mag/arcsec2 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 (2x10²⁰/cm2; ∼1.62 R₂₅). The blue knots are correlated with H I spiral structure to ∼1.5 R₂₅, while the red knots may be correlated with the outer fringes of the H I disk to ∼1.7 R₂₅. These results suggest that outer disks are well populated, common, and long-lasting features of many nearby disk galaxies.