Star formation properties of Universidad Complutense de Madrid survey galaxies

We present new near-infrared J and K imaging data for 67 galaxies from the Universidad Complutense de Madrid (UCM) survey used in the determination of the SFR density of the local Universe by Gallego ct al. This is a sample o f local star-forming galaxies with redshift lower than 0.045, and they cons titute a representative subsample of the galaxies in the complete UCM surve y. From the new data, complemented with our own Gunn-r images and lone-slit optical spectroscopy, we have measured integrated K-band luminosities, r - J and J - K colours, and H alpha luminosities and equivalent widths. Using a maximum likelihood estimator and a complete set of evolutionary synthesi s models, these observations allow us to estimate the strength of the curre nt (or most recent) burst of star formation, its age, the star formation ra te and the total stellar mass of the galaxies. An average galaxy in the sam ple has a stellar mass of 5 x 10(10) M-. and is undergoing (or has recently completed) a burst of star formation involving about 2 per cent of its tot al stellar mass. We identify two separate classes of star-forming galaxies in the UCM sample: low-luminosity, high-excitation galaxies (H II-like) and relatively luminous spiral galaxies (starburst disc-like). The former show higher specific star formation rates (SFRs per unit mass) and burst streng ths, and lower stellar masses than the latter. With regard to their specifi c star formation rates, the UCM galaxies are intermediate objects between n ormal quiescent spirals and the most extreme H II galaxies.