We use deep ( \sim 27.5 mag V -band point-source limiting magnitude ) V - and U -band LBT 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 \sim 10 ^ { 2 } -10 ^ { 4 } M _ { \sun } and ages up to \sim 1 Gyr .
We find statistically significant numbers of these cluster-like knots extending to \sim 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- \alpha threshold of H i column density ( 2 { \times 10 ^ { 20 } } { 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 .