Distribution of star-forming complexes in dwarf irregular galaxies

Astronomisches Institut der Universität Basel, Venusstrasse 7, 4102 Binningen, Switzerland e-mail: parodi@astro.unibas.ch;binggeli@astro.unibas.ch

Corresponding author: B. R. Parodi, parodi@astro.unibas.ch

Received: 21 August 2002
Accepted: 25 October 2002

Abstract

We study the distribution of bright star-forming complexes in a homogeneous sample of 72 late-type (“irregular”) dwarf galaxies located within the 10 Mpc volume. Star-forming complexes are identified as bright lumps in B-band galaxy images and isolated by means of the unsharp-masking method. For the sample as a whole the radial number distribution of bright lumps largely traces the underlying exponential-disk light profiles, but peaks at a 10 percent smaller scale length. Moreover, the presence of a tail of star forming regions out to at least six optical scale lengths provides evidence against a systematic star formation truncation within that galaxy extension. Considering these findings, we apply a scale length-independent concentration index, taking into account the implied non-uniform random spread of star formation regions throughout the disk. The number profiles frequently manifest a second, minor peak at about two scale lengths. Relying on a two-dimensional stochastic self-propagating star formation model, we show these secondary peaks to be consistent with triggered star formation; for a few of the brighter galaxies a peculiar peak distribution is observed that is conceivably due to the onset of shear provided by differential rotation. On scales between 100 and 1000 pc, and by taking into account exponential-disk structure, bright lumps reveal cluster dimensions between 1.3 and 2, with a weak trend to higher dimensions for brighter galaxies. Cluster dimension weakly anticorrelates with the lumpiness index (the fraction of the total galaxy light due to the light contributed by the lumps), the latter index showing no dependence on luminosity. Lump spreading within the disk, as measured by the concentration index, and lump clustering, as given by the cluster dimension, are not linked to each other. Interpreting cluster dimension in terms of porosity of a self-similar intragalactic medium, we derive a relation between current star formation rate, scale length, and porosity.