Star formation rates in interacting starburst galaxies

By narrowband imaging in Hα and in the adjacent red stellar continuum we have studied the rate and distribution of star formation in 43 systems of luminous and ultraluminous IR galaxies currently undergoing interaction and merging. These galaxies are amongst the most luminous at 60 μm and range in distance from ∼50 up to 100 Mpc. Here we present the Hα and the adjacent red-continuum narrowband images, and we compare the star formation rates derived from Hαa with those estimated from the IR luminosity. We find clear evidence for substantial extinction and obscuration of star-forming regions in the optical. Without correction for reddening in the host galaxy or correction for [N II] contamination, the star formation rates derived for Hα are typically 0.5-1.0 dex lower than those estimated from the IR flux, and the scatter in the correlation is very large. However, an unexpected result is that when spectroscopic data are used to eliminate objects dominated by an active nucleus, to determine the galaxian extinction, and to correct the Hα flux for both reddening and for the contamination by the [N II] emission, a remarkably good correlation emerges between the star formation rates estimated from the Hα flux and those derived from the FIR continuum. In addition, a strong correlation is found between the extinction in the line-emitting region, A_Hα and the rate of star formation. Our results invalidate the use of Hα imaging as a reliable indicator of star formation in starburst galaxies unless spectroscopic data are also available. This has important implications for the determination of star formation rates in high-redshift galaxies. Finally, we find no correlation between the measured star formation rates, and the interaction class, suggesting that the enhanced star formation rates triggered by the interaction continue throughout the whole of the merging sequence.