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| Lick indices | |
|---|---|
| Name | Lick indices |
| Type | Spectral line-index system |
| Invented by | Allan R. Sandage, G. Worthey, S. M. Faber |
| Institution | Lick Observatory |
| First published | 1970s–1990s |
| Wavelength range | Visible (∼4000–6500 Å) |
Lick indices
Lick indices are a system of standardized spectral absorption-feature measurements developed to quantify stellar-population properties in unresolved systems such as galaxies and star clusters. Originating from observational programs at Lick Observatory and expanded through collaborations involving researchers at California Institute of Technology, University of California, Santa Cruz, and other institutions, the system links absorption strengths to age, metallicity, and abundance-pattern diagnostics for integrated-light spectra. They serve as empirical tools employed in comparative studies by teams at observatories and universities including Palomar Observatory, European Southern Observatory, Harvard College Observatory, Max Planck Institute for Astronomy, and large surveys like the Sloan Digital Sky Survey.
The genesis of the index system traces to mid-20th-century spectroscopic investigations by observers at Lick Observatory and theoreticians at California Institute of Technology. Early foundational work involved spectrophotometric libraries compiled by researchers affiliated with Mount Wilson Observatory, Royal Observatory, Edinburgh, and Kitt Peak National Observatory. Key formalization occurred in the 1980s–1990s as teams including investigators from University of California, Santa Cruz, Institute for Advanced Study, and Carnegie Institution for Science standardized bandpass definitions to create a reproducible system. Subsequent expansions incorporated model–data comparisons produced by groups at Yale University, University of Cambridge, Max Planck Institute for Astrophysics, and the Space Telescope Science Institute.
Each index is defined by central feature and adjacent pseudocontinuum bandpasses within specified wavelength intervals, measured in magnitudes or angstroms. The procedure requires flux-calibrated spectra obtained with instruments at facilities such as Keck Observatory, Very Large Telescope, Subaru Telescope, or Hobby–Eberly Telescope and often references libraries like those from Jones (1999), Pickles (1998), and the MILES spectral library assembled by European and Spanish groups. Spectra are brought to a common instrumental resolution—matching the original system set by the Lick observational setup—via convolution or rebinning, and indices are computed following prescriptions by teams at University of Oxford, University of Groningen, and University of Tokyo. Observers routinely correct for radial velocity shifts using algorithms pioneered at institutions such as National Optical Astronomy Observatory and apply broadening corrections derived from stellar kinematic studies at Princeton University and Columbia University.
The canonical set includes prominent atomic and molecular features: Balmer lines (Hβ), metal lines (Fe5270, Fe5335), magnesium features (Mg b, Mg2), and molecular bands (CN1, CN2, TiO1, TiO2). These indices probe stellar-atmosphere physics investigated by researchers at Lund Observatory, Universidad Complutense de Madrid, University of Toronto, and Australian National University. Additional indices target elements such as calcium (Ca4227), sodium (Na D), and carbon-sensitive features, with measurements compared to synthetic spectra generated by groups at Geneva Observatory, Los Alamos National Laboratory, and University of Michigan. The list has been extended and redefined by collaborative efforts involving Rutherford Appleton Laboratory, Institut d'Astrophysique de Paris, and Instituto de Astrofísica de Canarias.
Robust calibration requires observing standard stars with well-characterized parameters from catalogs maintained by institutions like Smithsonian Astrophysical Observatory, Geneva Observatory, and European Space Agency archives. Cross-calibration campaigns have been coordinated among teams at Lick Observatory, Palomar Observatory, Cerro Tololo Inter-American Observatory, and Las Campanas Observatory to tie modern high-resolution spectra to the original system. Standardization addresses instrumental resolution, flux calibration, and aperture effects via methods developed at University of Chicago, Rutgers University, and University of Pennsylvania. Users employ stellar population synthesis codes produced by groups at Padova Observatory, Bologna Observatory, Max Planck Institute for Astrophysics, and Institute of Astronomy, Cambridge to translate index measurements into physical parameters.
The indices are widely used to infer luminosity-weighted ages, total metallicities, and element-abundance ratios in early-type galaxies, bulges, and globular clusters by teams at Princeton University, University of California, Berkeley, Columbia University, Johns Hopkins University, and University of Oxford. They underpin studies of galaxy formation epochs conducted by collaborations involving European Southern Observatory, NASA Goddard Space Flight Center, Instituto de Astrofísica de Canarias, and survey consortia like DEEP2 Redshift Survey and Sloan Digital Sky Survey. Applications extend to interpreting stellar-population gradients in galaxies observed with instruments at Gemini Observatory, W. M. Keck Observatory, and Subaru Telescope, and to constraining chemical-enrichment histories examined by researchers at University of Bonn, University of St Andrews, and University of Padua.
Systematic uncertainties arise from non-solar abundance patterns, emission-line contamination, velocity-dispersion broadening, and imperfect flux calibration, issues explored by groups at University of California, Santa Cruz, Max Planck Institute for Astrophysics, Rutgers University, and Imperial College London. Model-dependence and degeneracies—such as age–metallicity degeneracy—limit unique solutions and motivate joint analyses combining indices with full-spectrum fitting methods developed by teams at Institute of Astronomy, Cambridge, University of Durham, and Leiden Observatory. Selection biases in stellar libraries maintained by ESO, Smithsonian Astrophysical Observatory, and MILES consortia introduce further uncertainties, while aperture and radial-coverage differences between instruments at Keck Observatory and VLT can affect comparisons across surveys.