NGC 3621 is a late-type ( Sd ) spiral galaxy with an active nucleus , previously detected through mid-infrared [ Ne v ] line emission .
Archival Hubble Space Telescope ( HST ) images reveal that the galaxy contains a bright and compact nuclear star cluster .
We present a new high-resolution optical spectrum of this nuclear cluster , obtained with the ESI Spectrograph at the Keck Observatory .
The nucleus has a Seyfert 2 emission-line spectrum at optical wavelengths , supporting the hypothesis that a black hole is present .
The line-of-sight stellar velocity dispersion of the cluster is \sigma _ { \star } = 43 \pm 3 km s ^ { -1 } , one of the largest dispersions measured for any nuclear cluster in a late-type spiral galaxy .
Combining this measurement with structural parameters measured from archival HST images , we carry out dynamical modeling based on the Jeans equation for a spherical star cluster containing a central point mass .
The maximum black hole mass consistent with the measured stellar velocity dispersion is 3 \times 10 ^ { 6 } M _ { \odot } .
If the black hole mass is small compared with the cluster ’ s stellar mass , then the dynamical models imply a total stellar mass of \sim 1 \times 10 ^ { 7 } M _ { \odot } , which is consistent with rough estimates of the stellar mass based on photometric measurements from HST images .
From structural decomposition of 2MASS images , we find no clear evidence for a bulge in NGC 3621 ; the galaxy contains at most a very faint and inconspicuous pseudobulge component ( M _ { K } \gtrsim - 17.6 mag ) .
NGC 3621 provides one of the best demonstrations that very late-type spirals can host both active nuclei and nuclear star clusters , and that low-mass black holes can occur in disk galaxies even in the absence of a substantial bulge .