Resolved stellar mass maps of galaxies. I: method and implications for global mass estimates

We introduce a novel technique to construct spatially resolved maps of stellar mass surface density in galaxies based on optical and near IR imaging. We use optical/NIR colour(s) to infer effective stellar mass-to-light ratios (M/L) at each pixel, which are then multiplied by the surface brightness to obtain the local stellar surface mass density. We build look-up tables to express M/L as a function of colour(s) by marginalizing over a Monte Carlo library of 50,000 stellar population synthesis (SPS) models by Charlot & Bruzual (2007), which include a revised prescription for the TP-AGB stellar evolutionary phase. Moreover, we incorporate a wide range of possible dust extinction parameters. In order to extract reliable flux and colour information at any position in the galaxy, we perform a median adaptive smoothing of the images that preserves the highest possible spatial resolution. As the most practical and robust, and hence fiducial method, we express the M/L in the H band as a function of (g− i) and (i−H). Stellar mass maps computed in this way have a typical accuracy of 30 per cent or less at any given pixel, determined from the scatter in the models. We compare maps obtained with our fiducial method with those derived using other combinations of band-passes: i) mass maps based on the M/L in NIR bands require one optical and one optical-NIR colour to avoid significant biases as a function of the local physical properties of a galaxy; ii) maps based on M/L in i band as a function of (g − i) only are generally in excellent agreement with our best optical-NIR set, except for extremely star forming and dust extincted regions. We further compute stellar mass maps using a model library identical to the previous one except for being based on older SPS models, which assume shorter-lived TP-AGB stars. The M/L in the NIR inferred using these old models may be up to 2.5 times larger than the new ones, but this varies strongly as a function of colours and is maximal for the bluest colours. Finally, we compare total stellar mass estimates obtained by integrating resolved mass maps with those obtained with unresolved photometry. In galaxies with evident dust lanes, unresolved estimates may miss up to 40 per cent of the total stellar mass because dusty regions are strongly under-represented in the luminous fluxes.