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We measure the star formation histories (SFH) of a sample of low-massgalaxies with $M_\ast<10^9M_\odot$ from the SDSS-IV MaNGA survey. The largenumber of IFU spectra for each galaxy are either combined to reach a highsignal to noise ratio or used to investigate spatial variations. We useBayesian inferences based on full spectrum fitting. Our analysis based onBayesian evidence ratio indicates a strong preferencefor a model that allowsthe presence of an old stellar population, and that an improper model for theSFH can significantly underestimate the old population in these galaxies. Theaddition of NIR photometry to the constraining data can further distinguishbetween different SFH model families and significantly tighten the constraintson the mass fraction in the old population. On average more than half of thestellar mass in present-day low-mass galaxies formed at least 8 Gyrs ago, whileabout 30\% within the past 4 Gyrs. Satellite galaxies on average have formedtheir stellar mass earlier than central galaxies. The radial dependence of theSFH is quite weak. Our results suggest that most of the low-mass galaxies havean early episode of active star formation that produces a large fraction oftheir present stellar mass.

SDSS-IV MaNGA: Bayesian analysis of the star formation history of low-mass galaxies in the local Universe