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HI studies of the Sculptor group galaxies. VIII. The background galaxies: NGC 24 and NGC 45

In order to complete our HI survey of galaxies in the Sculptor group area, VLA observations of NGC 24 and NGC 45 are presented. These two galaxies of similar magnitude M_B ~ -17.4 lie in the background of the Sculptor group and are low surface brightness galaxies, especially NGC 45. The HI distribution and kinematics are regular for NGC 24 while NGC 45 exhibits a kinematical twist of its major axis. A tilted-ring model shows that the position angle of the major axis changes by ~25 degrees. A best-fit model of their mass distribution gives mass-to-light ratios for the stellar disk of 2.5 and 5.2 for NGC 24 and NGC 45 respectively. These values are higher than the ones expected from stellar population synthesis models. Despite the large dark matter contribution, the galaxy mass is still dominated by the stellar component in their very inner regions. These high mass-to-light ratios are typical of what is seen in low surface brightness galaxies and may indicate that, in those galaxies, disks are far from the maximum disk case. The halo parameters derived from the best-fit models are thus lower limits.