Star formation, metallicity gradient and ionized gas clues to the formation of the elliptic.pdf

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Star formation, metallicity gradient and ionized gas clues to the formation of the elliptic.pdf

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Star formation, metallicity gradient and ionized gas clues to the formation of the elliptic

a r X i v : 0 7 1 1 .3 6 0 7 v 1 [ a s t r o - p h ] 2 2 N o v 2 0 0 7 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 2 February 2008 (MN LATEX style file v2.2) Star formation, metallicity gradient and ionized gas: clues to the formation of the elliptical galaxies NGC 6868 and NGC5903 M.G. Rickes1, M.G. Pastoriza1 and C. Bonatto1 1Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Gonc?alves 9500, Porto Alegre, RS, Brazil. 2 February 2008 ABSTRACT The stellar population, metallicity distribution and ionized gas in the elliptical galaxies NGC6868 and NGC5903 are investigated in this paper by means of long-slit spec- troscopy and stellar population synthesis. Lick indices in both galaxies present a neg- ative gradient indicating an overabundance of Fe, Mg, Na and TiO in the central parts with respect to the external regions. We found that Mg2 correlates both with FeIλ5270 and FeIλ5335, suggesting that these elements probably underwent the same enrichment process in NGC6868. However, only a marginal correlation of Mg2 and FeIλ5270 occurs in NGC5903. The lack of correlation between computed galaxy mass and the Mg2 gradient suggests that these elliptical galaxies were formed by merger events. The stellar population synthesis shows the presence of at least two populations with ages of 13Gyr and 5Gyr old in both galaxies. We have estimated the metallicity of the galaxies using SSP models. The central region of NGC 6868 (|R| . 0.5 kpc) presents a deficiency of alpha elements with respect to iron and solar metallicity. The external parts, present a roughly uniform distribution of [α/Fe] ratios and metallicities ranging from [Z/Z⊙] = ?0.33 and solar. A similar conclusion applies to NGC5903. Concerning the emitting gas conspicuously detected in NGC6868, we test three hy- potheses as ionizing source: an H II region, post-AGB stars and an Active Galactic Nu- cleus (AGN). Diagnostic diagrams involving the ratios [NII]λ6584/Hα, [OI]λ630