We have obtained the time and space-resolved star formation history ( SFH ) of M51a ( NGC 5194 ) by fitting GALEX , SDSS , and near infrared pixel-by-pixel photometry to a comprehensive library of stellar population synthesis models drawn from the Synthetic Spectral Atlas of Galaxies ( SSAG ) .
We fit for each space-resolved element ( pixel ) an independent model where the SFH is averaged in 137 age bins , each one 100 Myr wide .
We used the Bayesian Successive Priors ( BSP ) algorithm to mitigate the bias in the present-day spatial mass distribution .
We test BSP with different prior probability distribution functions ( PDFs ) ; this exercise suggests that the best prior PDF is the one concordant with the spatial distribution of the stellar mass as inferred from the near infrared images .
We also demonstrate that varying the implicit prior PDF of the SFH in SSAG does not affects the results .
By summing the contributions to the global star formation rate of each pixel , at each age bin , we have assembled the resolved star formation history of the whole galaxy .
According to these results , the star formation rate of M51a was exponentially increasing for the first 10 Gyr after the Big Bang , and then turned into an exponentially decreasing function until the present day .
Superimposed , we find a main burst of star formation at t \approx 11.9 Gyr after the Big Bang .