Star formation properties of a large sample of irregular galaxies

Abstract

We present the results of Ha imaging of a large sample of irregular galaxies. Our sample includes 94 galaxies with morphological classifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems. The sample spans a large range in galactic parameters, including integrated absolute magnitude (M V of -9 to -19), average surface brightness (20-27 mag arcsec -2), current star formation activity (0-1.3 M ⊙ yr -1 kpc -2), and relative gas content (0.02 - 5 M ⊙/L B). The Ha images were used to measure the integrated star formation rates, determine the extents of star formation in the disks, and compare azimuthally averaged radial profiles of current star formation to older starlight. The integrated star formation rates of Im galaxies normalized to the physical size of the galaxy span a range of a factor of 10 4 with 10% Im galaxies and one Sm system having no measurable star formation at the present time. The BCDs fall, on average, at the high star formation rate end of the range. We find no correlation between star formation activity and proximity to other cataloged galaxies. Two galaxies located in voids are similar in properties to the Sm group in our sample. The H II regions in these galaxies are most often found within the Holmberg radius R H, although in a few systems H II regions are traced as far as 1.7R H. Similarly, most of the star formation is found within three disk scale lengths R D, but in some galaxies H II regions are traced as far as 6R D. A comparison of Hα surface photometry with V-band surface photometry shows that the two approximately follow each other with radius in Sm galaxies, but in most BCDs there is an excess of Hα emission in the centers that drops with radius. In approximately half of the Im galaxies Hα and V correspond well, and in the rest there are small to large differences in the relative rate of falloff with radius. The cases with strong gradients in the L Hα/L V ratios and with high central star formation rate densities, which include most of the BCDs, require a significant fraction of their gas to migrate to the center in the last gigayear. We discuss possible torques that could have caused this without leaving an obvious signature, including dark matter bars and past interactions or mergers with small galaxies or H I clouds. There is now a substantial amount of evidence for these processes among many surveys of BCDs. We note that such gas migration will also increase the local pressure and possibly enhance the formation of massive dense clusters but conclude that the star formation process itself does not appear to differ much among BCD, Im, and Sm types. In particular, there is evidence in the distribution function for Hα surface brightness that the turbulent Mach numbers are all about the same in these systems. This follows from the Hα distribution functions corrected for exponential disk gradients, which are log-normal with a nearly constant dispersion. Thus, the influence of shock-triggered star formation is apparently no greater in BCDs than in Im and Sm types.