A dynamical analysis of the H II galaxy II Zwicky 33 and its low surface brightness companion

We present radio and optical observations and an analysis of the dynamics of the H II galaxy II Zw 33 and its Low Surface Brightness companion, Both galaxies were observed in the radio with the VLA in its B- and C-array configurations and in the optical (B-band) with the Kitt Peak 4-m telescope, II Zw 33 is an example of an H II galaxy, i.e., a dwarf galaxy in which violent star formation is taking place. The absolute blue magnitude of II Zw 33 is M-B=-18.37 mag; the amount of observed H I mass is M-H I= 1.01CX10(9) M. leading to a H I mass to blue-light ratio of M-H I/L-B=0.31(M./L-B.) The companion, II Zw 33B, shows hardly any optical emission in the B-band (absolute blue magnitude M-B=-15.82 mag) and is considered to be a low surface brightness galaxy. It has, however, almost the same size and H I mass (M-H I=0.59X10(9) M.) as the parent galaxy II Zw 33, resulting in an H I mass to blue-light ratio of M-H I/L-B=1.83(M./L-B.). A dynamical study shows that no dark matter is needed to within the last measured point of the rotation curve (r<6.6 kpc) to explain the observed rotation curve of II Zw 33. This is a major difference to other dwarf galaxies studied so far which seem to be dark matter dominated throughout. The total (dynamical) mass for II Zw 33 is M-tot=5.1X10(9) M.. As it turns out, dark matter is not needed either to explain the rotation curve of the companion (total mass: M-tot=1.2X10(9) M.). There is a striking difference between the shape of the rotation curves of II Zw 33 and its companion which is attributed to the presence of stars in the former. in the course of our modelling we derive M*/L-B=1 (M./L-B.) for both objects. If we assume the system to be bound, however, this requires that at least 44X10(9) M. of mass must be present iii the system, presumably mostly in the form of dark matter surrounding the objects. (C) 1997 American Astronomical Society.