Spaxel Analysis: Probing the Physics of Star Formation in Ultraluminous Infrared Galaxies

This paper presents a detailed spectral pixel (spaxel) analysis of the ten Luminous Infrared Galaxies (LIRGs) previously observed with the Wide Field Spectrograph (WiFeS), an integral field spectrograph mounted on the ANU 2.3m telescope, and for which an abundance gradient analysis has already been presented by Rich et al. (2012). Here we use the strong emission line analysis techniques developed by Dopita et al. (2013) to measure the ionisation parameter and the oxygen abundance in each spaxel. In addition, we use the observed H$\alpha$ flux to determine the surface rate of star formation ($M_{\odot}$ yr$^{-1}$ kpc$^{-2}$) and use the [\ion{S}{2}] $\lambda\lambda6717/6731$ ratio to estimate the local pressure in the ionised plasma. We discuss the correlations discovered between these physical quantities, and use them to infer aspects of the physics of star formation in these extreme star forming environments. In particular, we find a correlation between the star formation rate and the inferred ionisation parameter. We examine the possible reasons for this correlation, and determine that the most likely explanation is that the more active star forming regions have a different distribution of molecular gas which favour higher ionisation parameters in the ionised plasma.