Clues to nuclear star cluster formation from edge-on spirals

We find nine nuclear cluster candidates in a sample of 14 edge-on, late-type galaxies observed with the Hubble Space Telescope Advanced Camera for Surveys. These clusters have magnitudes (M-I similar to -11) and sizes (r(eff) similar to 3 pc) similar to those found in previous studies of face-on, late-type spirals and dE galaxies. However, three of the nuclear clusters are significantly flattened and show evidence for multiple, coincident structural components. The elongations of these three clusters are aligned to within similar to 10 degrees of the galaxies' major axes. Structurally, the flattened clusters are well fit by a combination of a spheroid and a disk or ring, with the disk preferred in two of three cases. The nuclear cluster disks/rings have F606W - F814W (similar to V - I) colors 0.3-0.6 mag bluer than the spheroid components, suggesting that the stars in these components have ages < 1 Gyr. In NGC 4244, the nearest of the nuclear clusters, we further constrain the stellar populations via spectroscopy and multiband photometry. This nuclear cluster is equally well fit by single stellar populations with ages of either similar to 70 Myr or similar to 0.8 Gyr and with masses of similar to 3 x 10(6) M-circle dot. However, significantly better fits to the spectroscopy and photometry are obtained by combining two or more stellar populations. Exploiting emission lines that appear to originate similar to 1 '' from the NGC 4244 nucleus, we determine a lower limit on the dynamical mass of 2.5(-1.2)(+1.7) x 10(6) M-circle dot within 19 pc, typical of values found for other nuclear clusters. We also present tentative evidence that another of the nuclear clusters ( in NGC 4206) may also host a supermassive black hole. Based on our observational results we propose an in situ formation mechanism for nuclear clusters in which stars form episodically in compact nuclear disks and then lose angular momentum or heat vertically to form an older spheroidal structure. We estimate the period between star formation episodes to be similar to 0.5 Gyr and discuss possible mechanisms for transforming the disklike components into spheroids. We also note the connection between our objects and massive globular clusters (e.g., w Cen), ultracompact dwarfs, and supermassive black holes.