Circumnuclear star-forming activities along the Hubble sequence

In order to study circumnuclear star-forming activity along the Hubble sequence, we cross-correlated the Sloan Digital Sky Survey Data Release 2 (SDSS DR2) with the Third Reference Catalog of Bright Galaxies (RC3) to derive a large sample of 1015 galaxies with both morphological and spectral information. Among these, 385 sources are classified as star-forming galaxies, and the SDSS fibre covered the circumnuclear regions (0.2-2.0 kpc). By using the spectral synthesis method to remove the contribution from the underlying old stellar population, we measured the emission lines fluxes accurately, which are then used to estimate the star-formation rates(SFRs). Our main findings are: (1) early-type spirals show much higher Hα luminosities, and hence higher SFRs, and also suffer more extinctions than late-type ones. The equivalent widths (EWs) of Hα emission lines show a similar trend; however, the very late types (Sdm ~ Irr) do have large fractions of high EWs; (2) we confirm that $D_n(4000)$ shows a strong correlation with the strengths of metallic absorption lines (such as CN band, G band, and Mg Ib). Both these lines and the Balmer absorption lines show interesting variations between Sbc and Sd type galaxies; (3) the bar structure tightly relates to the enhanced star formation activity, an effect that is even more significant in the early-type spirals. But we should note that the bar structure is not a necessary or sufficient condition for galaxies to harbor circumnuclear star formations.