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Highlights of Astronomy, Volume 15 XXVIIth IAU General Assembly, August 2009 Ian F. Corbett, ed.

c Intern atio nal A stro nom ical U nion 2010 doi:10.1017/S1743921310009282

Hot ISM in young elliptical galaxies
Dong-Wo o Kim
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Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA email: kim@cfa.harvard.edu

Abstract. Using Chandra and XMM-Newton X-ray observations of young, p ost-merger elliptical galaxies, we present X-ray characteristics of age-related observational results, by comparing with typical old elliptical galaxies in terms of X-ray prop erties of their low-mass X-ray binaries (LMXBs) and hot interstellar matter (ISM). Keywords. galaxies: elliptical and lenticular, cD, X-rays: galaxies, X-rays: ISM

Stellar age in an elliptical galaxy has b een measured and there are now a handful of elliptical galaxies with intermediate stellar age (< 5 Gyr) which could link b etween Antennae-like merging systems and typical old elliptical galaxies (e.g,. Thomas et al. 2005). We find that young elliptical galaxies host more luminous (> 5 в 1038 erg s-1 ) LMXBs which are often distributed in a non-uniform way, indicating a p ossible connection with recent mergers (Kim et al. 2009, in preparation). Young elliptical galaxies tend to have a small amount of hot gas, with Lx(gas) comparable to or smaller than Lx(LMXB). By comparison, old elliptical galaxies have a wide range of Lx(gas) and some of them (e.g., group or cluster dominant galaxies) have Lx(gas) 10 or 100 times higher than Lx(LMXB). One of the key observables to address the age effect is the abundance ratio of Fe to elements, b ecause of different pro duction yields of SNe Ia and I I. In the merger scenario of young elliptical galaxies, we would exp ect an enrichment of -elements from SN I I, asso ciated with the recent star formation. Based on the SN yields, [Si/Fe] would b e close to 0.5 if SNe Ia are dominating, while the ratio would b e close to 2.5 if SNe I I are dominating. Typical old giant elliptical galaxies have [Si/Fe] close to solar (e.g., Kim & Fabbiano 2004), indicating that the ejecta from b oth SNe I I (from the early star formation) and SNe Ia (continuously added later) are well mixed. With added contribution from SNe I I in young elliptical galaxies, we exp ect [Si/Fe] to b e b etween 1 and 2.5 solar. By carefully selecting young and old samples (e.g, similar * to avoid different [/Fe]*; weak AGN; deep Chandra observations to effectively remove LMXBs) and by rigorously analyzing the data (extracting X-rays only from r < 30" to avoid a background problem; do not arbitrarily tying different elements), we find that [Si/Fe] is close to 1.5-2 solar in young E's (NGC 720 and NGC 3923), while [Si/Fe] is very close to solar in old E's (e.g, N4472, N4649). Given the limited sample and related uncertainties, our result is consistent with the exp ectation. Finally, we note that while for X-ray faint galaxies the absolute abundances of individual elements are uncertain and controversial, the abundance ratio is b etter constrained b ecause of strong correlations b etween errors of different elements.

References Kim, D.-W. & Fabbiano, G. 2004, ApJ 613, 933 Thomas, D. Maraston, C., Bender, R., & de Oliveira, C. M. 2005, ApJ 621, 673

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