2012AJ....144...52L

The Green Bank Telescope has been used to search for 21 cm H I emission over a large area between the galaxies M31 and M33 in an attempt to confirm at 9'.1 angular resolution the detection by Braun & Thilker of a very extensive neutral gas "bridge" between the two systems at the level N_ H I_~ 10¹⁷/cm2. We detect H I emission at several locations up to 120 kpc in projected distance from M31, at least half the distance to M33, with velocities similar to that of the galaxies, confirming the essence of the Braun & Thilker discovery. The H I does not appear to be associated with the extraplanar high-velocity clouds of either galaxy. In two places we measure N_ H I_ > 3x10¹⁸/cm2, indicative of concentrations of H I with ∼10⁵ M_☉ on scales ≲ 2 kpc, but over most of the field we have only 5σ upper limits of N_ H I_ ≤ 1.4x10¹⁸/cm2. In very deep measurements in two directions H I lines were detected at a few 10¹⁷/cm2. The absence of emission at another location to a 5σ limit N_ H I_ ≤ 1.5x10¹⁷/cm2 suggests that the H I bridge is either patchy or confined to within ∼125 kpc of M31. The measurements also cover two of M31's dwarf galaxies, And II and And XV, but in neither case is there evidence for associated H I at the 5σ level of 1.4x10⁴ M_☉ for And II and 9.3x10³ M_☉ for And XV.