Suzaku observation of the metallicity in the hot interstellar medium of the isolated elliptical galaxy NGC 720

The metal abundances in the interstellar medium of the isolated elliptical galaxy NGC 720 were studied with the CCD X-ray Imaging Spectrometer (XIS) aboard the Suzaku satellite. The high detection efficiency along with the excellent spectral resolution of the XIS allowed the K-shell lines of 0 and Mg to be clearly resolved, and the abundances of O, Ne, Mg, Si, and Fe to be measured with good accuracy for the region within a 3 radius of NGC 720. The abundances of 0 and Mg were well determined to be 0.4 and 0.5 solar respectively, meaning stellar metallicity of mass-losing stars in NGC 720 was also well determined. The abundance of Fe was determined to be about 0.7 solar, while the O/Fe, Ne/Fe abundance ratios were found to be about 0.5 and Mg/Fe, Si/Fe were found to be 0.7 in solar units. Relative errors of 30% for the O/Fe, Ne/Fe, Mg/Fe abundance ratios are the lowest among isolated elliptical galaxies so far observed. The higher abundance of Fe than those of 0, Ne, and Mg supplied by stellar mass loss indicates that iron is enhanced by SN la in NGC 720. These abundance ratios are consistent with those in the ISM of NGC 1399, NGC 1404, and in the ICM of the Fornax cluster observed with Suzaku.