Takaya Ohashi — Page 4

arXiv1997

New Measurement of Metal Abundance in the Elliptical Galaxy NGC 4636 with Asca

High quality X-ray spectra of NGC 4636 are obtained with ASCA. Theoretical models are found unable to reproduce the data in the Fe-L line region. Spectral data above 1.4 keV indicate that Mg to Si abundance ratio is $\sim 1$ solar. Assuming that the abundance ratios among $α$-elements are the same with the solar ratios, spectral fit with increased systematic error in the 0.4--1.6 keV range gives abundances of $α$-elements and Fe to be both $\sim$ 1 solar by taking one solar of Fe to be 3.24$\times 10^{-5}$ by number. These new abundance results solve discrepancy between stellar and hot-gas metallicity, but still a low supernova rate is implied. We also detect strong abundance gradients for both $α$-elements and iron in a similar fashion. The abundance is $\sim 1$ solar within $4'$, and decreases outerwards down to $0.2\sim 0.3$ solar at $10'$ from the galaxy center. Dilution due to an extended hot gas is suggested.

astro-ph↗

arXiv1996

The Iron Discrepancy in Elliptical Galaxies after ASCA

We present estimates for the iron content of the stellar and diffused components of elliptical galaxies, as derived respectively from integrated optical spectra and from ASCA X-ray observations. A macroscopic discrepancy emerges between the expected iron abundances in the hot interstellar medium (ISM) and what is indicated by the X-ray observations, especially when allowance is made for the current iron enrichment by Type Ia supernovae. This strong discrepancy, that in some extreme instances may be as large as a factor of $\sim 20$, calls into question our current understanding of supernova enrichment and chemical evolution of galaxies. We discuss several astrophysical implications of the inferred low iron abundances in the ISM, including the chemical evolution of galaxies and cluster of galaxies, the evolution of gas flows in ellipticals, and the heating of the intracluster medium. Some of the consequences appear hard to accept, and in the attempt to avoid some of these difficulties we explore ways of hiding or diluting iron in the ISM of ellipticals. None of these possibilities appears astrophysically plausible, and we alternatively rise the question of the reliability of iron-L line diagonostic tools. Various thin plasma emission models are shown to give iron abundances that may differ significantly, especially at low temperatures ($kT \lsim 1$ keV). From a collection of ASCA and other X-ray observatory data, it is shown that current thin plasma codes tend to give very low iron abundances when the temperature of the objects is below $\sim 1$ keV. We conclude that -- besides rethinking the chemical evolution of galaxies -- one should also consider the possibility that existing thin plasma models may incorporate inaccurate atomic physics for the ions responsible for the iron-L complex.

astro-ph↗