SFHs

Star Formation Histories (SFHs) describe how the rate at which a galaxy or region forms stars has changed over time. An SFH is typically expressed as the star formation rate, SFR(t), as a function of lookback time, or as the cumulative stellar mass formed up to a given time. SFHs capture the impact of gas accretion, feedback, mergers, and quenching on a system’s stellar content and structure.

Observationally, SFHs can be reconstructed in two main ways. For nearby galaxies with resolved stellar populations, SFHs are inferred from color-magnitude diagrams by comparing observed star counts to synthetic populations. For distant or unresolved systems, SFHs are derived from integrated light using population synthesis models to fit spectral energy distributions, colors, and spectral features. Common approaches include forward modeling of resolved star formation, and inverse methods that decompose observed spectra into combinations of simple stellar populations. Parameterizations often used to describe SFHs include constant or slowly varying SFRs, exponentially declining (tau) models, delayed-tau forms, rising histories, bursty histories, and lognormal shapes.

SFHs provide insights into the growth of stellar mass, quenching timescales, burst events, and the relative importance of different epochs in building a galaxy’s current stellar population. In a cosmological framework, the collective SFHs of many galaxies underpin the cosmic star formation history, which rises to a peak around redshift ~2 and declines toward the present.

Limitations and uncertainties arise from degeneracies among age, metallicity, and dust extinction, as well as model dependencies and incomplete knowledge of stellar evolution. Nonetheless, SFHs remain fundamental for interpreting galaxy formation and evolution in both observations and simulations.