The Centers of Early- to Intermediate-Type Spiral Galaxies: A Structural Analysis^*

A recent Hubble Space Telescope (HST)/WFPC2 visual survey of early- and intermediate-type spiral galaxies has unveiled a great complexity in the inner regions of these systems, which include a high fraction of photometrically distinct compact sources sitting at the galactic centers ("nuclei"). The faint nuclei (M_V ≳ -12) are typically hosted by rather amorphous, quiescent, bulgelike structures with an exponential (rather than the classical R^1/4) light profile. These "exponential bulges" are commonly found inside the intermediate-type disks, consistent with previous studies. Brighter nuclei (M_V ≲ -12) are typically found instead in the centers of galaxies with circumnuclear rings/arms of star formation or dust and an active, i.e., H II- or AGN-type, central spectrum at ground-based resolution. On the structural plane of half-light radius (R_e) versus mean surface brightness within the half-light radius (μ_e), faint and bright nuclei overlap with, and fill the region of parameter space between, the old Milky Way globular clusters and the young star clusters, respectively, with typical R_e of about a few up to ≈20 pc. On the same plane, the exponential bulges have significantly fainter μ_e than R^1/4 bulges for any given radius and follow a μ_e-R_e relation typical of disks, which strengthens the suggestion that the exponential bulges grow inside the disks as a result of the secular evolution of the latter. Under the likely assumption that the visual light from the faint nuclei embedded in the quiescent exponential bulges is of stellar origin and of a similar (≳1 Gyr) age for the central star clusters and their host bulges, the masses inferred for the former agree with those required to disrupt bars comparable in size to the latter. This offers support to scenarios in which the exponential bulges grow inside the disks owing to the orbital disruption of progenitor bars caused by the growth of a central concentration of mass and suggests that this specific mode of bulge formation is (still) active in the present-day universe. On the other hand, the presence of the massive clusters at the very center of the low-density exponential bulges should prevent any other "nuclear" bar from forming, thereby preventing further infall of dissipative fuel to the nuclear regions. This may argue against the possibility of evolving the exponential bulges into denser, R^1/4 bulges by a simple looping for several cycles of the bar formation/disruption mechanism.