The relationship between past and present star formation in galactic disks from CCD surface photometry

Abstract

We present some results of a major new multiband imaging survey of 34 nearby southern spiral galaxies. Images in V, I, Hα, and the adjacent red continuum have been obtained using a CCD and focal reducer on the 1.0 and 2.3 m telescopes at Siding Spring Observatory. Surface photometry using the GASP software package is used first to derive the disk orientation parameters, then to provide deprojected radial surface brightness profiles for each galaxy in V and I, as well as the continuum- subtracted Hα, which traces the present-day rate of massive star formation. In the outer disk, the Hα profile can be reasonably well fitted by an exponential disk, but with a scale length much longer than the V scale length, which itself tends to be slightly longer than the I scale length. An almost universal relationship is observed in the disk between the Hα surface brightness and the I-band surface brightness at a given radius, with any residual offset from the mean trend being a weak function of the morphological type. Thus the rate of massive star formation per unit area in the disk is closely related to the old stellar mass surface density at each radius, and to the mean H I surface density in the disk as a whole. This forms the basis for a law of (or rather, a constraint on) massive star formation in the disks of spiral galaxies, one that has a surprising degree of independence from both galactic dynamics and molecular gas content.