H I 21 cm observations of the PG 1216+069 sub-damped Lyα absorber field at z = 0.00632

The Westerbork Synthesis Radio Telescope finds a weak 21 cm line emission feature at the coordinates (R.A., decl., velocity) of the sub-damped Lyα (DLA) absorber observed at z_abs = 0.00632 in the spectrum of PG 1216+069. The emission feature, WSRT J121921+0639, lies within 30″ of the quasar sight line, is detected at a 99.8% (3 σ) confidence level, has M_{H I} between 5 and 15 × 10⁶ M_⊙, and has a velocity spread between 20 and 60 km s^-1. Other H I emitters in the field include VCC 297 at a projected distance of 86 h₇₅ ^-1 kpc and a previously unreported H I cloud, WSRT J121919+0624, at 112 h₇₅^-1 kpc with M_{H I} ∼ 3 × 10 ⁸ M_⊙. The optically identified foreground galaxy that is closest to the quasar sight line appears to be VCC 339 (∼L*/25) at 29 h₇₅^-1 kpc with a velocity offset of 292 km s ^-1. A low surface brightness galaxy with the H I mass of the sub-DLA absorber WSRT J121921+0639 would likely have m_B ∼ 17, and its diffuse optical emission would need to compete with the light of both the background QSO and a brighter foreground star ∼10″ from the QSO sight line.