J/A+A/471/795 Stellar population ages and metallicities from colors (Li+, 2007)
How young stellar populations affect the ages and metallicities of galaxies.
Li Z., Han Z.
<Astron. Astrophys. 471, 795 (2007)>
=2007A&A...471..795L 2007A&A...471..795L
ADC_Keywords: Populations, stellar ; Models, evolutionary
Keywords: galaxies: stellar content - galaxies: photometry -
galaxies: elliptical and lenticular, cD
Abstract:
Two important stellar-population parameters (age and metallicity) of
the dominant stellar populations (DSPs) of galaxies are usually
estimated by comparing the observed absorption line indices or colors
to predictions of some simple stellar population models. However, some
studies show that there is actually recent star formation in galaxies,
including early type ones. This suggests that we may not be obtaining
accurate the two stellar-population parameters for the DSPs of
galaxies. This is obvious when we estimate the two parameters by
colors, because the youngest populations dominate the light and make
the fitted stellar populations younger and richer in metal.
We study how young populations (YSPs) in composite stellar populations
(CSPs) affect the colors of star systems and to analyze how the
stellar ages and metallicities derived from colors possibly deviate
from those of the DSPs. It is found that the age and mass fraction of
a YSP affect colors of a mixed star system significantly, but the
former is stronger. In addition, our results show that the stellar
ages and metallicities derived directly from a pair of colors are
about 2.14Gyr younger, while 0.0027 more metal rich on average than
those of the DSPs of composite stellar systems. Some possible
distributions of the differences between stellar-population parameters
determined by colors and those of DSPs of CSPs are presented. The
possible distributions of the differences between colors of CSPs and
those of their DSPs are also shown. Stellar ages and metallicities
measured by colors and line-strength indices are compared in the work,
with a sample of 18 galaxies. Furthermore, the YSPs may affect the
fundamental plane and Kormendy relation of early type galaxies.
Description:
The deviations in stellar ages and metallicities determined by color
pairs of composite stellar populations (CSPs) are presented. The
results are derived from three color pairs: [(B-V),(V-K)], [(B-V),
(B-K)], and [(B-V), (I-H)]. The results can be used to correct for the
effects of YSPs on the stellar ages and metallicities. The (B-V) and
(B-K) colors, Hbeta and [MgFe] line-strength indices, and the stellar
ages and metallicities fitted using the Simple Stellar Populations
(SSPs) of Bruzual & Charlot (2003MNRAS.344.1000B 2003MNRAS.344.1000B) are also presented.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
dev.dat 48 30000 Deviations in stellar ages and metallicities
measured from color pairs
comp.dat 71 18 Two colors, two line-strength indices, and the
results fitted by SSPs, for 18 galaxies
(observational data are selected from a few
sources listed in our paper)
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Byte-by-byte Description of file: dev.dat
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Bytes Format Units Label Explanations
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1- 8 F8.4 Gyr delta(Age)1 Deviation in stellar age calculated
by (B-V) and (V-K)
10- 16 F7.4 --- delta(Z)1 Deviation in stellar metallicity calculated
by (B-V) and (V-K)
17- 24 F8.4 Gyr delta(Age)2 Deviation in stellar age calculated
by (B-V) and (B-K)
26- 32 F7.4 --- delta(Z)2 Deviation in stellar metallicity calculated
by (B-V) and (B-K)
33- 40 F8.4 Gyr delta(Age)3 Deviation in stellar age calculated
by (B-V) and (I-H)
42- 48 F7.4 --- delta(Z)3 Deviation in stellar metallicity calculated
by (B-V) and (I-H)
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Byte-by-byte Description of file: comp.dat
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Bytes Format Units Label Explanations
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1- 7 A7 --- Name Galaxy name
10- 15 F6.4 0.1nm Hbeta The Hbeta Lick/IDS index
18- 23 F6.4 0.1nm [MgFe] Composite index of Mgb, Fe5270 and Fe5335 (1)
25- 31 F7.4 [yr] log(Age)m Logarithmic stellar age measured from
Hβ and [MgFe] line-strength indices
33- 39 F7.4 [Sun] [Z/H]m Stellar metallicity measured from
Hβ and [MgFe] line-strength indices
42- 47 F6.4 mag (B-V) (B-V) color index
50- 55 F6.4 mag (B-K) (B-K) color index
58- 63 F6.4 [yr] log(Age)p Logarithmic stellar age measured from
(B-V) and (B-K) color indices
65- 71 F7.4 [Sun] [Z/H]p Stellar metallicity measured from
(B-V) and (B-K) color indices
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Note (1): [MgFe] was calculated by the definition of Gonzalez
(1993, Ph.D. Thesis, Univ. California, Santa Cruz).
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Acknowledgements:
Zhongmu Li, zhongmu.li(at)gmail.com
(End) Zhongmu Li [YNAO, China], Patricia Vannier [CDS] 08-Aug-2007