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 dynam ics of the H II galaxy II Zw 33 and its Low Surface Brightness compani on, Both galaxies were observed in the radio with the VLA in its B- an d C-array configurations and in the optical (B-band) with the Kitt Pea k 4-m telescope, II Zw 33 is an example of an H II galaxy, i.e., a dwa rf galaxy in which violent star formation is taking place. The absolut e 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 hard ly 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 h as, however, almost the same size and H I mass (M-H I=0.59X10(9) M.) a s the parent galaxy II Zw 33, resulting in an H I mass to blue-light r atio 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 cur ve (r<6.6 kpc) to explain the observed rotation curve of II Zw 33. Thi s is a major difference to other dwarf galaxies studied so far which s eem 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 n ot 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 a ttributed 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 f orm of dark matter surrounding the objects. (C) 1997 American Astronom ical Society.