Metallicity effects on mid-infrared colors and the 8 μm PAH emission in galaxies

C. W. Engelbracht, K. D. Gordon, George H. Rieke, M. W. Werner, D. A. Dale, W. B. Latter

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Abstract

We examine colors from 3.6 to 24 μm as a function of metallicity (O/H) for a sample of 34 galaxies. The galaxies range over 2 orders of magnitude in metallicity. They display an abrupt shift in the 8 μm-to-24 μm color for metallicities between one-third and one-fifth of the solar value. The mean 8-to-24 μm flux density ratio below and above 12 + log (O/H) = 8.2 is 0.08 ± 0.04 and 0.70 ± 0.53, respectively. We use mid-IR colors and spectroscopy to demonstrate that the shift is primarily due to a decrease in the 8 μm flux density, as opposed to an increase in the 24 μm flux density. This result is most simply interpreted as being due to a weakening at low metallicity of the mid-IR emission bands usually attributed to PAHs (polycyclic aromatic hydrocarbons) relative to the small-grain dust emission. However, existing empirical spectral energy distribution models cannot account for the observed short-wavelength (below 8 μm) colors of the low-metallicity galaxies merely by reducing the strength of the PAH features; some other emission source (e.g., hot dust) is required.

Original languageEnglish (US)
JournalAstrophysical Journal
Volume628
Issue number1 II
DOIs
StatePublished - Jul 20 2005

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polycyclic aromatic hydrocarbons
metallicity
PAH
galaxies
color
flux density
dust
shift
spectral energy distribution
spectroscopy
wavelength
effect
energy
wavelengths

Keywords

  • Galaxies: ISM
  • Infrared: galaxies

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Engelbracht, C. W., Gordon, K. D., Rieke, G. H., Werner, M. W., Dale, D. A., & Latter, W. B. (2005). Metallicity effects on mid-infrared colors and the 8 μm PAH emission in galaxies. Astrophysical Journal, 628(1 II). https://doi.org/10.1086/432613

Metallicity effects on mid-infrared colors and the 8 μm PAH emission in galaxies. / Engelbracht, C. W.; Gordon, K. D.; Rieke, George H.; Werner, M. W.; Dale, D. A.; Latter, W. B.

In: Astrophysical Journal, Vol. 628, No. 1 II, 20.07.2005.

Research output: Contribution to journalArticle

Engelbracht, C. W. ; Gordon, K. D. ; Rieke, George H. ; Werner, M. W. ; Dale, D. A. ; Latter, W. B. / Metallicity effects on mid-infrared colors and the 8 μm PAH emission in galaxies. In: Astrophysical Journal. 2005 ; Vol. 628, No. 1 II.
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