Kennicutt–Schmidt law
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| Kennicutt–Schmidt law | |
|---|---|
| Name | Kennicutt–Schmidt law |
| Field | Astrophysics |
| Discovered | 1959; reformulated 1989–1998 |
| Key people | Maarten Schmidt; Robert C. Kennicutt Jr.; Lyman Spitzer; Edwin Hubble; Vera Rubin |
| Relates to | Star formation; Interstellar medium; Galaxy evolution |
Kennicutt–Schmidt law The Kennicutt–Schmidt law describes an empirical relation between surface density of gas and surface density of star formation in galaxies. It connects observations across scales from local star-forming regions to integrated measurements of Milky Way, Andromeda Galaxy, and high-redshift systems, and underpins models used by observatories such as Hubble Space Telescope, Spitzer Space Telescope, and Atacama Large Millimeter/submillimeter Array. The relation informs theoretical work by groups at institutions like Harvard–Smithsonian Center for Astrophysics, Max Planck Institute for Astronomy, and California Institute of Technology.
Introduction
The Kennicutt–Schmidt law links gas surface density to star formation surface density through a power law first proposed by Maarten Schmidt and later extended by Robert C. Kennicutt Jr., with antecedents in work by Lyman Spitzer and context from galaxy surveys by Edwin Hubble and kinematic studies by Vera Rubin. Observational programs using facilities such as Very Large Array, James Clerk Maxwell Telescope, European Southern Observatory, and Keck Observatory measured emission lines like Hα and CO to calibrate the relation. The law is central to interpretations by theorists from Princeton University, University of Cambridge, and Yale University working on star formation, feedback, and galactic dynamics.
Historical development and formulation
The original power-law proposal by Maarten Schmidt in 1959 related volumetric gas density to star formation rate density, inspired by studies of the Milky Way and early spectroscopic surveys by teams at Mount Wilson Observatory and Palomar Observatory. In the 1980s and 1990s, surveys led by Robert C. Kennicutt Jr. synthesized integrated galaxy measurements from programs at Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, producing an empirical surface-density form often expressed as Σ_SFR ∝ Σ_gas^N. The often-cited exponent N≈1.4 arose from comparisons including data from IRAS, ROSAT, and ground-based Hα imaging programs associated with National Optical Astronomy Observatory and Anglo-Australian Observatory. Subsequent refinements involved contributions from researchers at Carnegie Institution for Science, National Radio Astronomy Observatory, and the Royal Astronomical Society.
Observational evidence and measurements
Evidence comes from resolved studies of nearby disks like Andromeda Galaxy and Triangulum Galaxy using CO mapping by IRAM and ALMA, and Hα and ultraviolet imaging by Galaxy Evolution Explorer and Hubble Space Telescope. Integrated galaxy studies spanning starbursts such as M82 and ultraluminous systems cataloged by IRAS reinforced the power-law form, while surveys targeting dwarf galaxies cataloged by Sloan Digital Sky Survey and low-surface-brightness systems observed by the Anglo-Australian Telescope showed scatter and deviations. Measurements rely on tracers including CO (linked to studies at Institut de Radioastronomie Millimétrique), HI (mapped by Arecibo Observatory and Westerbork Synthesis Radio Telescope), far-infrared emission (characterized by Spitzer Space Telescope and Herschel Space Observatory), and recombination lines exploited in projects led by European Southern Observatory teams.
Theoretical interpretations and models
Interpretations draw on gravitational collapse theories developed by authors at Institute for Advanced Study and turbulence-regulated models from groups at Max Planck Institute for Astrophysics and University of California, Berkeley. Models invoking Toomre instability reference work by Alar Toomre and connect to simulations performed with codes from FLASH Center, Gadget (simulation code), and projects at Los Alamos National Laboratory. Feedback-regulated frameworks involve supernova-driven winds studied by researchers at Lawrence Berkeley National Laboratory and radiation-pressure models advanced by teams at Stanford University and Princeton University. Analytic approaches reference scaling relations from Kenneth C. Freeman-type disk models and aspects of dark matter halo assembly from Lambda Cold Dark Matter context developed by groups including Sloan Digital Sky Survey collaborators.
Variations, limitations, and extensions
Observed deviations occur in circumnuclear starbursts like NGC 253, low-metallicity dwarfs such as Small Magellanic Cloud, and high-redshift objects observed in surveys like COSMOS and CANDELS, prompting variants that separate molecular and atomic gas components or introduce thresholds tied to shielding and metallicity studied by teams at University of Michigan and University of Illinois Urbana-Champaign. Limitations arise from uncertainties in CO-to-H2 conversion factors debated by groups at Max Planck Institute for Extraterrestrial Physics and effects of cosmic rays investigated by collaborators at Jet Propulsion Laboratory. Extensions include volumetric formulations, multi-freefall models proposed by researchers at University of Bonn and non-linear prescriptions used in cosmological simulations run by Illustris and EAGLE consortia.
Applications in galaxy evolution and simulations
The Kennicutt–Schmidt relation is implemented in subgrid recipes in cosmological simulations such as IllustrisTNG, EAGLE, and Millennium Simulation and informs semi-analytic models developed by groups at Institute of Astronomy, Cambridge and Max Planck Institute for Astrophysics. It guides interpretations of star-formation histories in surveys like Sloan Digital Sky Survey, CANDELS, and DEEP2, and influences modeling of feedback processes studied by collaborations at Lawrence Livermore National Laboratory, NASA, and European Southern Observatory. Observational programs with ALMA, Hubble Space Telescope, and James Webb Space Telescope continue to test and refine its role in shaping stellar mass assembly across cosmic time.