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UV color-color relation of early-type galaxies

Published online by Cambridge University Press:  05 March 2015

Chang H. Ree
Affiliation:
Korea Astronomy and Space Science Institute, Daejeon 305-348, Republic of Korea email: chr@kasi.re.kr
Hyunjin Jeong
Affiliation:
Korea Astronomy and Space Science Institute, Daejeon 305-348, Republic of Korea email: chr@kasi.re.kr
Kyuseok Oh
Affiliation:
Dept. of Astronomy, Yonsei University, Seoul 120-749, Republic of Korea
Chul Chung
Affiliation:
Dept. of Astronomy, Yonsei University, Seoul 120-749, Republic of Korea
Joon Hyeop Lee
Affiliation:
Korea Astronomy and Space Science Institute, Daejeon 305-348, Republic of Korea email: chr@kasi.re.kr
Sang Chul Kim
Affiliation:
Korea Astronomy and Space Science Institute, Daejeon 305-348, Republic of Korea email: chr@kasi.re.kr
Jaemann Kyeong
Affiliation:
Korea Astronomy and Space Science Institute, Daejeon 305-348, Republic of Korea email: chr@kasi.re.kr
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Abstract

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The ultraviolet (UV) color-color relation of early-type galaxies (ETGs) in the nearby universe (0.05 < z < 0.12) is re-examined with the latest GALEX GR6 and SDSS DR7 data. By drawing the FUV – NUV (as a measure of UV temperature) versus FUV – r (as a measure of UV amplitude) color-color diagram for the morphologically-cleaned, spectroscopically-cleaned sample of ~3700 quiescent ETGs, we find that the “old and dead“ ETGs consist of a well-defined sequence in UV colors, the “UV red sequence”, so that the stronger UV excess galaxies should have a harder UV spectral shape systematically. However, the observed UV spectral slope is too steep to be reproduced by the canonical models in which the UV flux is mainly controlled by age or metallicity parameters. The observed data support the helium enhancement scenario in which the UV spectral shape of UV upturn (FUV – NUV < 0.9; FUV – r ~ 6) galaxies may be governed by the minority population of helium-enhanced horizontal-branch (HB) stars.

Type
Contributed Papers
Copyright
Copyright © International Astronomical Union 2015