Measuring Resolved Star Formation Histories from High-precision Color–Magnitude Diagrams with <tt>StarFormationHistories.jl</tt>

Abstract Understanding how and when galaxies formed stars over the history of the Universe is fundamental to the study of galaxy evolution. The star formation histories (SFHs) of galaxies in the local Universe can be measured with high precision using deep imaging with space telescopes. Such resolved SFHs are based on modeling the observed color–magnitude diagram (CMD) with stellar evolution models and rely on age-sensitive features like the main-sequence turnoff to measure a galaxy’s star formation rate as a function of time. There are many other population-level parameters that factor into these measurements, such as the stellar initial mass function (IMF), binary fraction, and metallicity, to name a few. We present and release StarFormationHistories.jl, a modular, open-source Julia package for measuring resolved SFHs with a focus on model flexibility for these types of population parameters. The code can model unresolved photometric binaries and supports arbitrary IMFs. Random uncertainties in the SFH measurements can be quantified with Monte Carlo posterior sampling methods. We illustrate the performance of the package on JWST/NIRCAM data of the Local Group dwarf irregular galaxy Wolf–Lundmark–Melotte <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close=")" open="("> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mo>−</mml:mo> <mml:mn>14.2</mml:mn> <mml:mspace width="0.25em"/> <mml:mspace width="0.1em"/> <mml:mtext>mag</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> </mml:mfenced> </mml:math> , which exhibits a complex, well-sampled CMD, and Hubble Space Telescope/Advanced Camera for Surveys data of the ultrafaint Milky Way satellite dwarf galaxy Horologium I <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close=")" open="("> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mo>−</mml:mo> <mml:mn>3.7</mml:mn> <mml:mspace width="0.25em"/> <mml:mspace width="0.1em"/> <mml:mtext>mag</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> </mml:mfenced> </mml:math> , which has a much simpler but sparser CMD.