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The H$\alpha$ and H$\beta$ emission line luminosities measured in a singleintegrated spectrum are affected in non-trivial ways by point-to-pointvariations in dust attenuation in a galaxy. This work investigates the impactof this variation when estimating global H$\alpha$ luminosities corrected forthe presence of dust by a global Balmer decrement. Analytical arguments showthat the dust-corrected H$\alpha$ luminosity is always underestimated whenusing the global H$\alpha$/H$\beta$ flux ratio to correct for dust attenuation.We measure this effect on 156 face-on star-forming galaxies from the MappingNearby Galaxies at APO (MaNGA) survey. At 1-2 kpc spatial resolution, theeffect is small but systematic, with the integrated dust-corrected H$\alpha$luminosity underestimated by $2$-$4$ per cent (and typically not more than by$10$ per cent), and depends on the specific star formation rate of the galaxy.Given the spatial resolution of MaNGA, these are lower limits for the effect.From Multi Unit Spectroscopic Explorer (MUSE) observations of NGC 628 with aresolution of 36 pc we find the discrepancy between the globally and thepoint-by-point dust-corrected H$\alpha$ luminosity to be $14 \pm 1$ per cent,which may still underestimate the true effect. We use toy models andsimulations to show that the true difference depends strongly on the spatialvariance of the H$\alpha$/H$\beta$ flux ratio, and on the slope of the relationbetween H$\alpha$ luminosity and dust attenuation within a galaxy. Largersamples of higher spatial resolution observations are required to quantify thedependence of this effect as a function of galaxy properties.

monthly notices of the royal astronomical society

Less than the sum of its parts: the dust-corrected H$\alpha$ luminosity  of star-forming galaxies explored at different spatial resolutions with MaNGA  and MUSE