1994A&A...285..385B

The irregular (IBm) galaxy NGC 4449 has been observed in the 21 cm line using the Effelsberg 100-m telescope. The full extent of the giant H I halo of this galaxy has been mapped, its size being ∼ 14 times the optical diameter (at 25th magnitude). We confirm the existence of opposite velocity gradients in the outer H I envelope and the inner part of the galaxy. The S-shaped morphology of the H I halo as well as its velocity field provide evidence for a pronounced warp. The gravitational interaction with the nearby companion dwarf galaxy DDO 125 may be responsible for this warp. The systemic velocity of NGC 4449 is determined to be 214 km/s from which a distance of 3.7 h Mpc ( H₀ = 75/h km/s/Mpc) is derived. The H i mass then is 2.3.10⁹ h² M_☉, and the total mass (within a radius of 30') is 7.0. 10¹⁰ h M_☉.