Issue A&A Volume 450, Number 1, April IV 2006 Page(s) 15 - 23 Section Extragalactic astronomy DOI 10.1051/0004-6361:20054045

A&A 450, 15-23 (2006)
DOI: 10.1051/0004-6361:20054045

Circumnuclear star-forming activities along the Hubble sequence

L. Shi, Q. S. Gu and Z. X. Peng

Department of Astronomy, Nanjing University, Nanjing 210093, PR China
    e-mail: shileiastro@yahoo.com; qsgu@nju.edu.cn

(Received 13 August 2005 / Accepted 5 December 2005)

Abstract
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 H luminosities, and hence higher SFRs, and also suffer more extinctions than late-type ones. The equivalent widths (EWs) of H 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.

Key words: Galaxy: general -- galaxies: stellar content -- galaxies: statistics

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