AGC198606: A gas-bearing dark matter minihalo?

TL;DR

This study presents HI imaging of AGC198606, a gas cloud near Leo T with undisturbed morphology and ordered motion, exploring its possible nature as a dark matter minihalo or Galactic halo cloud.

Contribution

The paper provides detailed HI observations of AGC198606 and discusses its potential classification as a gas-bearing dark matter minihalo or Galactic halo object.

Findings

AGC198606 has a smooth HI morphology with ordered velocity gradient.

No optical counterpart detected for AGC198606.

Estimated HI mass of 6.2x10^5 Msun at 420 kpc distance.

Abstract

We present neutral hydrogen (HI) imaging observations with the Westerbork Synthesis Radio Telescope of AGC198606, an HI cloud discovered in the ALFALFA 21cm survey. This object is of particular note as it is located 16 km/s and 1.2 degrees from the gas-bearing ultra-faint dwarf galaxy Leo T while having a similar HI linewidth and approximately twice the flux density. The HI imaging observations reveal a smooth, undisturbed HI morphology with a full extent of 23'x16' at the 5x10^18 atoms cm^-2 level. The velocity field of AGC198606 shows ordered motion with a gradient of ~25 km/s across ~20'. The global velocity dispersion is 9.3 km/s 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), and a cloud in the Galactic halo. At a distance of 420 kpc, AGC198606 would have an HI mass of 6.2x10^5 Msun, an HI radius of 1.4 kpc, and a dynamical mass within the HI extent of 1.5x10^8 Msun.