Effect of diffuse background on Schmidt relation in nearby star-forming galaxies

The Kennicutt-Schmidt relation relating the surface densities of the star-formation rate (SFR) and gas (atomic and molecular) is a widely-accepted star-formation relation. However, it is a disk-averaged law smoothing over local variations, and hence may not provide an explanation for the local relationship between SFR and gas density at the sub-galactic scale. To further probe this, various groups have carried out spatially-resolved studies of star-formation in nearby spiral galaxies using different methods. However, most of these studies do not take into account the effect of the spatially varying diffuse background which is potentially present in all star-forming galaxies and affects all the usual SFR tracers (optical, far-ultraviolet and mid-infrared). In this contribution, I present the results from an analysis (Kumari et al. 2020) of nearby spiral galaxies using aperture photometry where the effect of the diffuse background is taken into account. Making use of a novel split of the overall light distribution as a function of spatial scale allows us to subtract the diffuse background in the SFR tracers and determine the current localised SFR density. This is then combined with the gas density estimates (molecular gas from CO(2-1) and atomic gas from HI) to study the relation between SFR and gas density. An analysis is also done for the effect of diffuse background in the atomic gas, should it be present. This work indicates that removal of a diffuse background in SFR and atomic gas tracers results in a slope ~1.4 ± 0.1, which agrees with dynamical models of star formation accounting for flaring effects in the outer regions of galaxies.