CHANDRA OBSERVATIONS OF THE THREE MOST METAL DEFICIENT BLUE COMPACT DWARF GALAXIES KNOWN IN THE LOCAL UNIVERSE, SBS 0335-052, SBS 0335-052W, AND Ⅰ Zw 18

摘要

We present an X-ray study of the three most metal deficient blue compact dwarf (BCD) galaxies known in the local universe, based on deep Chandra observations of SBS 0335-052 (0.025 Z_☉), SBS 0335-052W (0.02 Z_☉), and Ⅰ Zw 18 (0.02 Z_☉). All three are detected, with more than 90% of their X-ray emission arising from pointlike sources. The 0.5-10.0 keV luminosities of these point sources are in the range (1.3-8.5) x 10~(39) ergs s~(-1). We interpret them to be single or a collection of high-mass X-ray binaries, the luminosities of which may have been enhanced by the low metallicity of the gas. There are hints of faint extended diffuse X-ray emission in both SBS 0335—052 and Ⅰ Zw 18, probably associated with the superbubbles visible in both BCDs. The spectrum of Ⅰ Zw 18 shows an O Ⅷ hydrogen-like emission line. The best spectral fit gives an O overabundance of the gas in the X-ray point source by a factor of ～7 with respect to the Sun, or a factor of ～350 with respect to the O abundance determined for the H Ⅱ region.