sSFR

Specific star formation rate (sSFR) is a measure used in galaxy evolution to quantify how quickly a galaxy is forming stars relative to its existing stellar mass. It is defined as sSFR = SFR / M*, where SFR is the star formation rate (in solar masses per year) and M* is the stellar mass (in solar masses). The resulting units are inverse time (typically yr^-1 or Gyr^-1).

Conceptually, sSFR indicates the growth rate of a galaxy’s stellar mass if current star formation continues unchanged. Higher sSFR means the galaxy is building up mass rapidly compared with its current size, often associated with actively star-forming systems. Lower sSFR implies a more quiescent state, where star formation is comparatively slow.

Measuring sSFR requires estimating both SFR and M*. SFR is inferred from indicators such as ultraviolet light, hydrogen recombination lines (e.g., H-alpha), and infrared emission that traces dust-reprocessed starlight, with corrections for dust attenuation. Stellar mass is derived from modeling the galaxy’s spectral energy distribution to estimate the accumulated stellar content. Systematic uncertainties in the initial mass function, dust corrections, and stellar population models affect sSFR values.

sSFR evolves with cosmic time, generally increasing with redshift for star-forming galaxies. This evolution, along with the relationship between SFR and stellar mass (the star-forming main sequence), provides insight into how galaxies grow and quench over billions of years. sSFR is a widely used diagnostic but depends on model assumptions and observational calibrations.