Recovering stellar population parameters via two full-spectrum fitting algorithms in the absence of model uncertainties

Abstract

Using mock spectra based on Vazdekis/MILES library fitted within the\nwavelength region 3600-7350\\AA, we analyze the bias and scatter on the\nresulting physical parameters induced by the choice of fitting algorithms and\nobservational uncertainties, but avoid effects of those model uncertainties. We\nconsider two full-spectrum fitting codes: pPXF and STARLIGHT, in fitting for\nstellar population age, metallicity, mass-to-light ratio, and dust extinction.\nWith pPXF we find that both the bias in the population parameters and the\nscatter in the recovered logarithmic values follows the expected trend. The\nbias increases for younger ages and systematically makes recovered ages older,\n$M_*/L_r$ larger and metallicities lower than the true values. For reference,\nat S/N=30, and for the worst case ($t=10^8$yr), the bias is 0.06 dex in\n$M_*/L_r$, 0.03 dex in both age and [M/H]. There is no significant dependence\non either E(B-V) or the shape of the error spectrum. Moreover, the results are\nconsistent for both our 1-SSP and 2-SSP tests. With the STARLIGHT algorithm, we\nfind trends similar to pPXF, when the input E(B-V)<0.2 mag. However, with\nlarger input E(B-V), the biases of the output parameter do not converge to zero\neven at the highest S/N and are strongly affected by the shape of the error\nspectra. This effect is particularly dramatic for youngest age, for which all\npopulation parameters can be strongly different from the input values, with\nsignificantly underestimated dust extinction and [M/H], and larger ages and\n$M_*/L_r$. Results degrade when moving from our 1-SSP to the 2-SSP tests. The\nSTARLIGHT convergence to the true values can be improved by increasing Markov\nChains and annealing loops to the "slow mode". For the same input spectrum,\npPXF is about two order of magnitudes faster than STARLIGHT's "default mode"\nand about three order of magnitude faster than STARLIGHT's "slow mode".\n