The Multiphase Absorption Systems toward PG 1206+459

Abstract

A high-resolution (R = 30,000) ultraviolet spectrum is presented, which covers Lyα and many low-, intermediate-, and high-ionization transitions in the three Mg II-selected absorption systems toward the quasar PG 1206+459. Three systems (A, B, and C), which are clustered within 1500 km s-1 at z ~ 0.93, were originally identified in a spectrum obtained with the High Resolution Spectrograph (HIRES) on the Keck I Telescope. A WIYN (Wisconsin-Indiana-Yale-NOAO) Gunn i-band image of the quasar field and spectroscopy of two galaxy candidates are presented. A multiphase medium is seen in all three systems, consistent with smaller, denser clouds producing low-ionization transitions (Mg II, Fe II, and Si II) and larger, diffuse, photoionized clouds giving rise to higher ionization transitions (C IV, N V, and/or O VI). (1) System A, a multicloud, weak Mg II absorber at z = 0.9254, requires a supersolar metallicity in both low- and high-ionization phases, unless an α-group enhancement is included. The low-ionization absorption is produced in clouds with sizes of 10-70 pc, which are surrounded in velocity space by broader, high-ionization components. With the unusually complex velocity structure resolved in the N V profiles, this system is unlikely to represent a traditional galaxy disk/corona. The most likely candidate host galaxy is a ~2L*, apparently warped, spiral at an impact parameter of 43 h-1 kpc. (2) System B, at z = 0.9276, has the strongest Mg II absorption and has an approximately solar metallicity in the low-ionization phase. The smooth, broad high-ionization profiles may indicate a coronal structure similar to that of the Milky Way. The redshift of an L* galaxy (z = 0.9289), at an impact parameter of 38 h-1 kpc, is consistent with the redshift of this system. (3) System C, at z = 0.9342, has a single component in Mg II, separated from the other two systems by ~+1000 km s-1. The Lyα profile is not aligned with the Mg II, requiring an additional velocity component offset by -40 km s-1. System C lacks the small, low-ionization cloud characteristic of an isolated single-cloud, weak Mg II absorber. Its absorption properties are similar to the satellite clouds of classic strong Mg II absorbers, so this could be a high-velocity cloud in the galaxy group responsible for the systems, possibly related to a 0.2L* galaxy at an impact parameter of 43 h-1 kpc.