Checking the reliability of equivalent width R₂₃ for estimating the metallicities of galaxies

¹ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR China e-mail: ycliang@bao.ac.cn
² GEPI, Observatoire de Paris-Meudon, 92195 Meudon, France
³ Department of Physics, Hebei Normal University, Shijiazhuang 050016, PR China
⁴ Department of Physics, Harbin University, Haerbin 150086, PR China

Received: 15 May 2007
Accepted: 8 August 2007

Abstract

Aims.We verify whether the O/H abundances of galaxies can be derived from the equivalent width (EW) R₂₃ instead of the extinction-corrected flux R₂₃, and eventually propose a method of improving the reliability of this empirical method, which is often used for the non-flux calibrated spectra of galaxies.

Methods.We select 37 173 star-forming galaxies from the Second Data Release of the Sloan Digital Sky Survey (SDSS-DR2), which offers a wide range of properties to test the EW method.

Results.The EW-R₂₃ method brings with it a significant bias: for the bulk of SDSS galaxies, it may affect the determination of  (O/H) by factors ranging from –0.2 to 0.1 dex and for some galaxies by factors ranging from –0.5 to 0.2 dex. We characterize this discrepancy (or bias) by )/(, which is virtually independent of dust extinction, while tightly correlating with D_n(4000), although at a lower significance, with () colors.

Conclusions.The EW-R₂₃ method cannot be used as a proxy for the extinction-corrected flux R₂₃ method. From analytical third-order polynomial fits of α versus () colors, we have been able to correct the EW-R₂₃ method. With this additional and easy correction, the EW-R₂₃ method provides O/H abundance values similar to those derived from the extinction-corrected flux R₂₃ method with an accuracy of ±0.1 dex for >92% of the SDSS galaxies.