AGC198606: A gas-bearing dark matter minihalo?

¹ Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7900 AA Dwingeloo, The Netherlands
e-mail: adams@astron.nl
² Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Ramat Aviv 69978, Israel
³ Department of Astronomy, Indiana University, 727 East Third Street, Bloomington, IN 47405, USA
⁴ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AA Grongingen, The Netherlands
⁵ Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA
⁶ Department of Physics and Astronomy, Macalaster College, 1600 Grand Avenue, Saint Paul, MN 55105, USA
⁷ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile

Received: 24 October 2014
Accepted: 24 November 2014

Abstract

We present neutral hydrogen (H i) imaging observations with the Westerbork Synthesis Radio Telescope of AGC198606, an H i cloud discovered in the ALFALFA 21cm survey. This object is of particular note as it is located 16 km s^-1 and 1.̊2 from the gas-bearing ultra-faint dwarf galaxy Leo T while having a similar H i linewidth and approximately twice the flux density. The H i imaging observations reveal a smooth, undisturbed H i morphology with a full extent of 23′ × 16′ at the 5 × 10¹⁸ atoms cm^-2 level. The velocity field of AGC198606 shows ordered motion with a gradient of ~25 km s^-1 across ~20′. The global velocity dispersion is 9.3 km s^-1 with no evidence for a narrow spectral component. No optical counterpart to AGC198606 is detected. The distance to AGC198606 is unknown, and we consider several different scenarios: physical association with Leo T, a minihalo at a distance of ~150 kpc based on the models of Faerman et al. (2013, ApJ, 777, 119), and a cloud in the Galactic halo. At a distance of 420 kpc, AGC198606 would have an H i mass of 6.2 × 10⁵ M_⊙, an H i radius of 1.4 kpc, and a dynamical mass within the H i extent of 1.5 × 10⁸ M_⊙.