The central bar in M94

Abstract

Visual, NIR, and H_alpha surface photometry of the oval disk galaxy M94 (NGC 4736) was performed to study the distribution of mass in old stars and the distribution of the warm emission line gas. The radial profile of the inner stellar disk is steeper than exponential and displays twisted and deformed isophotes. Dust is not responsible for these deviations. The investigation of a number of morphological models showed that M94 has a weak central stellar bar of 0.7 kpc major axis length which comprises about 14 per cent of the total light within 20 arcsec. By means of longslit spectroscopy the kinematics of gas and stars in this region were investigated. The stellar kinematics reveal the existence of a small spheroidal bulge with v/sigma of about 0.8. The stellar velocity field in this region is approximately axisymmetric, showing that the effects of the bar on the kinematics of the hot component are relatively small. The warm gas in the bar region shows global and local deviations from the stellar kinematics, but outside the central bar fits into the general HI velocity field. Model calculations of closed orbits for gas in the potential of the bar perturbation and axisymmetric disk and bulge predict large non-circular motions for cold gas in equilibrium flow. These do not fit the central H_alpha kinematics; rather it appears that the H_alpha gas must have large random motions and is not in a steady state, and that hydrodynamical effects due to a recent starburst play a role in the central region.