IMAGING OF MBG STARBURSTS .2. THE NATURE OF THE SAMPLE

In this paper, broad-band imaging in BVRI is used in parallel with inf ormation from long-slit spectroscopy and IRAS data to study star forma tion processes in a sample of 15 MBG (Montreal blue galaxy) starbursts , in order to understand their nature more clearly. Most of these gala xies are early-type spirals with disturbed morphologies. The burst of star formation is concentrated in the nucleus, extending to a mean dis tance of 1.6 kpc from the centre. In the most active cases, ionized ga s could be detected up to a substantial fraction of the radius of the optical surface of the galaxy. We have found evidence suggesting that the enhancement of star formation in our galaxies is correlated to a h igher concentration of gas in the nucleus. No mechanism was clearly id entified to explain the accretion of gas in this region. Even though w e see MBGs at different levels of activity and with different morpholo gies, they present similar characteristics in terms of star formation processes. The peculiar morphologies, the infrared characteristics and the net excess of gas in the MBGs compared with galaxies of the same morphological type suggest that the bursts are related to some kind of interaction with other galaxies. We found near-constant star formatio n rates over a period of a few Gyr, which we interpret as an indicatio n of either long duration bursts (time-scale of the order 1 Gyr) or a succession of shorter bursts. The concentration of the bursts into the circumnuclear regions and their importance in terms of masses of star s created suggest that this particular phenomenon could represent an i mportant phase in the evolution of these galaxies.