Surface photometry and mass distributions of the interacting spiral galaxies NGC 5426 and 5427

Rotation curves of the interacting spiral galaxies NGC 5426 and 5427 have been measured using long slit spectrograms and ‘error free’ luminosity profiles in B, V and J passbands have been derived from sky limited plates. Both galaxies have flat rotation curves outside the nucleus. Neither has any large scale non-circular motions. Masses are $$3.5\times{10}^{10}\,{M}_{\odot}\,(r \lt8\,\text{kpc})\,\text{and}\,2.9\times{10}^{10}\,{M}_{\odot}\,(r\lt9\,\text{kpc})$$ for NGC 5426 and 5427 respectively for a distance of 26 Mpc. Total B and V magnitudes are 12.5, 11.9 and 12.1, 11.6 for NGC 5426 and 5427 respectively. The adjacent halves of the galaxies have been dimmed, apparently by the interaction, contributing only 38 per cent of the total flux. NGC 5426 has a red nucleus (B– V ~ 0.7) embedded in blue (B– V ~ 0.5) constant colour disc. NGC 5427 has a B–V colour of 0.5 throughout. The M/L distributions of both galaxies are essentially flat and, even when extrapolated to the limits of the photometry (corresponding to a B surface brightness of 27.8 mag arcsec^–2), provide no evidence for any ‘massive haloes’. The luminosity profile of NGC 5426 contains a distinct inner exponential component which is associated with the spiral arms. NGC 5427 has a ‘Freeman type II’ profile which is also caused by the spiral arms. Using the photometry as a guide, theoretical spiral patterns based on density wave theory and the observed rotation curves have been derived. These agree well with the observed spiral arms. The relative motion of the galaxies is such that no ‘missing mass’ is required for them to form a bound system.