Metallicity of the Fossil Group NGC 1550 Observed with Suzaku

We studied the temperature and metal abundance distributions of the intra-cluster medium (ICM) in a group of galaxies, NGC 1550, observed with Suzaku. NGC 1550 is classified as a fossil group, which has few bright member galaxies, except for the central galaxy. Thus, such a type of galaxy is important to investigate how metals are enriched to the ICM. With the Suzaku XIS instrument, we directly measured not only the Si, S, and Fe lines, but also the O and Mg lines, and obtained those abundances to an outer region of similar to 0.5r(180) for the first time, and confirmed that the metals in the ICM of such a fossil group are indeed extending to a large radius. We found steeper gradients for the Mg, Si, S, and Fe abundances, while O showed an almost flat abundance distribution. Abundance ratios of alpha-elements to Fe were similar to those of the other groups and poor clusters. We calculated the number ratio of type II to type Ia supernovae for the ICM enrichment to be 2.9 +/- 0.5 within 0.1r(180); the value is consistent with those for the other groups and poor clusters observed with Suzaku. We also calculated the metal mass-to-light ratios (MLRs) for Fe, O, and Mg with the B-band and K-band luminosities of the member galaxies of NGC 1550. The derived MLRs are comparable to those of the NGC 5044 group in the r < 0.1r(180) region, while those of NGC 1550 are slightly higher than those of NGC 5044 in the outer region.