The MeerKAT Fornax Survey. I. Survey description and first evidence of ram pressure in the Fornax galaxy cluster

The MeerKAT Fornax Survey maps the distribution and kinematics of atomic neutral hydrogen gas (H I) in the nearby Fornax galaxy cluster using the MeerKAT telescope. The 12 deg² survey footprint covers the central region of the cluster out to ∼R_vir and stretches south-west out to ∼2R_vir to include the NGC 1316 galaxy group. The H I column density sensitivity (3σ over 25 km s⁻¹) ranges from 5 × 10¹⁹ cm⁻² at a resolution of ∼10″ (∼1 kpc at the 20 Mpc distance of Fornax) down to ∼10¹⁸ cm⁻² at ∼1' (∼6 kpc), and slightly below this level at the lowest resolution of ∼100″ (∼10 kpc). The H I mass sensitivity (3σ over 50 km s⁻¹) is 6 × 10⁵ M_⊙. The H I velocity resolution is 1.4 km s⁻¹. In this paper, we describe the survey design and H I data processing, and we present a sample of six galaxies with long, one-sided, starless H I tails (only one of which was previously known) radially oriented within the cluster and with measurable internal velocity gradients. We argue that the joint properties of the H I tails represent the first unambiguous evidence of ram pressure shaping the distribution of H I in the Fornax cluster. The disturbed optical morphology of all host galaxies supports the idea that the tails consist of H I that was initially pulled out of the galaxies' stellar body by tidal forces. Ram pressure was then able to further displace the weakly bound H I and give the tails their current direction, length, and velocity gradient.