The stellar populations of spiral disks .1. A new observational approach: Description of the technique and spectral gradients for the inter-arm regions of NGC 4321 (M100)

We describe an imaging method that makes use of interference filters to provide integrated stellar spectral indices for spiral disks to faint surface brightness limits. We use filters with bandpasses similar to 60 Angstrom FWHM, centered on the Mg and Fe features (lambda lambda 5176 Angstrom and 5270 Angstrom, respectively) allowing the determination of the spatial distribution of the Lick indices Mg-2 and Fe5270. These two indices have been extensively modeled by different groups and used in the past mainly for the study of elliptical galaxies, bulges, and globular clusters. Azimuthal integration of the underlying smooth stellar signal, after removal of the signature of the spiral arms and associated extreme Pop. I structures, provides measurements of these spectral indices useful to radial distances where the surface brightness of the galaxy reaches similar to 24 mu(V). As a first example of this technique and its possibilities we conduct a preliminary study of the SABbc galaxy NGC 4321 (M100). We present spectral gradients for the inter-arm stellar population to about 4 exponential scale lengths. There is some evidence for a discontinuity in the run of the Mg-2 index near corotation, which we interpret as evidence for bar-driven secular evolution. There is also evidence that Mg is overabundant with respect to Fe in the inner regions of the projected image. (C) 1997 American Astronomical Society.