Spatially Correlated Cluster Populations in the Outer Disk of NGC 3184

TL;DR

This study reveals that the outer disk of galaxy NGC 3184 hosts a large, spatially correlated population of star cluster-like objects extending beyond the optical radius, with implications for galaxy evolution.

Contribution

It provides the first detailed analysis of the spatial distribution and properties of star cluster populations in the outer disk of NGC 3184, extending beyond the optical radius.

Findings

Over 1000 cluster-like objects found in the outer disk.

Clusters extend beyond the optical radius up to 1.4 R_25.

Clusters are correlated with HI spiral structure and extend to the HI disk fringe.

Abstract

We use deep (~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 ~10^2 - 10^4 M_sun and ages up to ~1 Gyr. 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 HI extent to that of the correlated knots and find that the clusters extend at least to the damped Lyman-alpha threshold of HI column density (2e20 cm^-2; 1.62 R_25). The blue knots are correlated with HI spiral structure to 1.5 R_25, while the red knots may be correlated with the outer fringes of the HI 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.