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

Abstract

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 $\alpha$-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 $\alpha$-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 $\alpha$-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.