NONLINEAR COLOR-METALLICITY RELATIONS of GLOBULAR CLUSTERS. VI. on CALCIUM II TRIPLET BASED METALLICITIES of GLOBULAR CLUSTERS in EARLY-TYPE GALAXIES

Chul Chung, Seok-Jin Yoon, Sang Yoon Lee, Young-Wook Lee

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4 Citations (Scopus)

Abstract

The metallicity distribution function of globular clusters (GCs) in galaxies is a key to understanding galactic formation and evolution. The calcium II triplet (CaT) index has recently become a popular metal abundance indicator thanks to its sensitivity to GC metallicity. Here we revisit and assess the reliability of CaT as a metallicity indicator using our new stellar population synthesis simulations based on empirical high-resolution fluxes. The model shows that the CaT strength of old (>10 Gyr) GCs is proportional to [Fe/H] below -0.5. In the modest metal-rich regime, however, CaT does not increase anymore with [Fe/H] due to the little contribution from coolest red giant stars to the CaT absorption. The nonlinear nature of the color-CaT relation is confirmed by the observations of GCs in nearby early-type galaxies. This indicates that the CaT should be used carefully when deriving metallicities of metal-rich stellar populations. Our results offer an explanation for the observed sharp difference between the color and CaT distributions of GCs in the same galaxies. We take this as an analogy to the view that metallicity-color and metallicity-Lick index nonlinearity of GCs is primarily responsible for their observed "bimodal" distributions of colors and absorption indices.

Original languageEnglish
Article number201
JournalAstrophysical Journal
Volume818
Issue number2
DOIs
Publication statusPublished - 2016 Feb 20

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metallicity
calcium
globular clusters
color
galaxies
metal
metals
red giant stars
nonlinearity
distribution functions
sensitivity
high resolution
synthesis
simulation
distribution
index

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

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title = "NONLINEAR COLOR-METALLICITY RELATIONS of GLOBULAR CLUSTERS. VI. on CALCIUM II TRIPLET BASED METALLICITIES of GLOBULAR CLUSTERS in EARLY-TYPE GALAXIES",
abstract = "The metallicity distribution function of globular clusters (GCs) in galaxies is a key to understanding galactic formation and evolution. The calcium II triplet (CaT) index has recently become a popular metal abundance indicator thanks to its sensitivity to GC metallicity. Here we revisit and assess the reliability of CaT as a metallicity indicator using our new stellar population synthesis simulations based on empirical high-resolution fluxes. The model shows that the CaT strength of old (>10 Gyr) GCs is proportional to [Fe/H] below -0.5. In the modest metal-rich regime, however, CaT does not increase anymore with [Fe/H] due to the little contribution from coolest red giant stars to the CaT absorption. The nonlinear nature of the color-CaT relation is confirmed by the observations of GCs in nearby early-type galaxies. This indicates that the CaT should be used carefully when deriving metallicities of metal-rich stellar populations. Our results offer an explanation for the observed sharp difference between the color and CaT distributions of GCs in the same galaxies. We take this as an analogy to the view that metallicity-color and metallicity-Lick index nonlinearity of GCs is primarily responsible for their observed {"}bimodal{"} distributions of colors and absorption indices.",
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AU - Chung, Chul

AU - Yoon, Seok-Jin

AU - Lee, Sang Yoon

AU - Lee, Young-Wook

PY - 2016/2/20

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N2 - The metallicity distribution function of globular clusters (GCs) in galaxies is a key to understanding galactic formation and evolution. The calcium II triplet (CaT) index has recently become a popular metal abundance indicator thanks to its sensitivity to GC metallicity. Here we revisit and assess the reliability of CaT as a metallicity indicator using our new stellar population synthesis simulations based on empirical high-resolution fluxes. The model shows that the CaT strength of old (>10 Gyr) GCs is proportional to [Fe/H] below -0.5. In the modest metal-rich regime, however, CaT does not increase anymore with [Fe/H] due to the little contribution from coolest red giant stars to the CaT absorption. The nonlinear nature of the color-CaT relation is confirmed by the observations of GCs in nearby early-type galaxies. This indicates that the CaT should be used carefully when deriving metallicities of metal-rich stellar populations. Our results offer an explanation for the observed sharp difference between the color and CaT distributions of GCs in the same galaxies. We take this as an analogy to the view that metallicity-color and metallicity-Lick index nonlinearity of GCs is primarily responsible for their observed "bimodal" distributions of colors and absorption indices.

AB - The metallicity distribution function of globular clusters (GCs) in galaxies is a key to understanding galactic formation and evolution. The calcium II triplet (CaT) index has recently become a popular metal abundance indicator thanks to its sensitivity to GC metallicity. Here we revisit and assess the reliability of CaT as a metallicity indicator using our new stellar population synthesis simulations based on empirical high-resolution fluxes. The model shows that the CaT strength of old (>10 Gyr) GCs is proportional to [Fe/H] below -0.5. In the modest metal-rich regime, however, CaT does not increase anymore with [Fe/H] due to the little contribution from coolest red giant stars to the CaT absorption. The nonlinear nature of the color-CaT relation is confirmed by the observations of GCs in nearby early-type galaxies. This indicates that the CaT should be used carefully when deriving metallicities of metal-rich stellar populations. Our results offer an explanation for the observed sharp difference between the color and CaT distributions of GCs in the same galaxies. We take this as an analogy to the view that metallicity-color and metallicity-Lick index nonlinearity of GCs is primarily responsible for their observed "bimodal" distributions of colors and absorption indices.

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