The Hα galaxy survey^*

VII. The spatial distribution of star formation within disks and bulges

¹ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK e-mail: [paj;cft]@astro.livjm.ac.uk
² Royal Observatory, Greenwich, London, SE10 9LF, UK
³ Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain e-mail: jhk@.iac.es

Received: 30 July 2008
Accepted: 24 April 2009

Abstract

Aims. We analyse the current build-up of stellar mass within the disks and bulges of nearby galaxies through a comparison of the spatial distributions of forming and old stellar populations.

Methods. Hα and R-band imaging are used to determine the distributions of young and old stellar populations in 313 S0a-Im field galaxies out to ~40 Mpc. Concentration indices and mean normalised light profiles are calculated as a function of galaxy type and bar classification.

Results. The mean profiles and concentration indices show a strong and smooth dependence on galaxy type. Apart from a central deficit due to bulge/bar light in some galaxy types, mean Hα and R-band profiles are very similar. Mean profiles within a given type are remarkably constant even given wide ranges in galaxy luminosity and size. SBc, SBbc, and particularly SBb galaxies have profiles that are markedly different from those of unbarred galaxies. The Hα emission from individual SBb galaxies is studied in detail; virtually all show resolved central components and concentrations of star formation at or just outside the bar-end radius.

Conclusions. Galaxy type is an excellent predictor of R-band light profile. In field galaxies, star formation has the same radial distribution as R-band light, i.e. stellar mass is building at approximately constant morphology, with no strong evidence for outer truncation or inside-out disk formation. Bars have a strong impact on the radial distribution of star formation, particularly in SBb galaxies.