The study of stellar populations, through multiband photometry and spectroscopy, together with HII regions and molecular gas distribution, and assisted by starburst evolutionary models, leads to the determination of the age and history of the star formation in a galaxy disk. It is possible to constrain the IMF, often found to be biased towards high-masses in starbursts, and to follow the propagation of the starburst radially.
In some cases, the star formation propagates inside out, a good example being the ring of NGC 1614 (Alonso-Herrero et al 2001): here a nuclear starburst is identified within 45pc, surrounded by a ring of HII regions of 600 pc in diameter, tracing a younger burst. These HII regions, about 10 times the intensity of 30 Doradus, lie inside a ring of molecular gas, as if the star formation wave was propagating radially outwards.
In the LINER galaxy NGC 5005, Sakamoto et al (2000) identify a stream of molecular gas, linking the inner ring of the bar to the nuclear disk, likely to correspond to the ILR. This stream represents a high rate of bar-driven inflow and they suggest that a major fueling event is in progress in this galaxy. The gas flow could then be episodic rather than continuous. Recurrent starbursts are then expected.
In other cases, the star formation appears to propagate
outside in: older star formation in a disk/ring of 200pc
in diameter surrounds a younger nuclear starburst in NGC 6764
(Schinnerer et al 2000):
two starbursts with decay times of 3 Myr occurred 3-5 Myr and 15 to 50
Myr ago. However, a constant star formation scenario over 1 Gyr
(at a rate of 0.3 M
/ yr) could also explain the data.
The ringed barred galaxy NGC 4314 also supports the outside in scenario: a ring of dense molecular gas is observed inside the radio-continuum ring (Combes et al 1992; Benedict et al 1996; Kenney et al 1998). The gas ring, inside the nuclear hot spots, evolves slowly, reducing its radius due to friction exerted by the background stars on the giant molecular clouds.
This shrinking ring of star formation is expected from the dynamical evolution of the gaseous nuclear ring. Alternatively, feedback processes from violent star formation, such as supernovae, bipolar gas outflows, etc ... are expected to compress the surrounding gas outwards, and to trigger star formation inside out.