SOFT X-RAY ABSORPTION DUE TO A FOREGROUND EDGE-ON SPIRAL GALAXY TOWARD THE CORE OF A2029

摘要

We have detected an X-ray absorption feature against the core of the galaxy cluster A2029 (z = 0.0767) that we identify with the foreground galaxy UZC J151054.6+054313 (z = 0.0221). Optical observations (B, V, R, and I) indicate that it is an Scd galaxy seen nearly edge-on at an inclination of 87°+- 3°. H I observations give a rotation velocity of 108 km s~(-1) and an atomic hydrogen mass of M_(H I) = 3.1 x 10~9 d_(90)~2 solar mass, where d_(90) is the distance to the galaxy in units of 90 Mpc. X-ray spectral fits to the Chandra absorption feature yield a hydrogen column density of (2.0 +- 0.4) x 10~(21) cm~(-2), assuming solar abundances. If the absorber is uniformly distributed over the disk of the galaxy, the implied hydrogen mass is M_H = (6.2 +- 1.2) x 10~8 d_(90)~2 solar mass. Since the absorbing gas in the galaxy is probably concentrated toward the center of the galaxy and the middle of the disk, this is a lower limit to the total hydrogen mass. On the other hand, the absorption measurements imply that the dark matter in UZC J151054.6+054313 is not distributed in a relatively uniform diffuse gas.