H I observations of two new dwarf galaxies: Pisces A and B with the SKA Pathfinder KAT-7^⋆

¹ Department of Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa
e-mail: ccarignan@ast.uct.ac.za
² Observatoire d’Astrophysique de l’Université de Ouagadougou, BP 7021, Ouagadougou 03, Burkina Faso
³ Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁵ Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA

Received: 7 December 2015
Accepted: 3 January 2016

Abstract

Context. Pisces A and Pisces B are the only two galaxies found via optical imaging and spectroscopy out of 22 Hi clouds identified in the GALFAHI survey as dwarf galaxy candidates.

Aims. We derive the Hi content and kinematics of Pisces A and B.

Methods. Our aperture synthesis Hi observations used the seven-dish Karoo Array Telescope (KAT-7), which is a pathfinder instrument for MeerKAT, the South African precursor to the mid-frequency Square Kilometre Array (SKA-MID).

Results. The low rotation velocities of ~5 km s^-1 and ~10 km s^-1 in Pisces A and B, respectively, and their Hi content show that they are really dwarf irregular galaxies (dIrr). Despite that small rotation component, it is more the random motions ~9−11 km s^-1 that provide most of the gravitational support, especially in the outer parts. The study of their kinematics, especially the strong gradients of random motions, suggest that those two dwarf galaxies are not yet in equilibrium.

Conclusions. These Hi- rich galaxies may be indicative of a large population of dwarfs at the limit of detectability. However, such gas-rich dwarf galaxies will most likely never be within the virial radius of MW-type galaxies and become subhalo candidates. Systems such as Pisces A and B are more likely to be found at a few Mpc s from MW-type galaxies.