Star formation in blue compact dwarf galaxies: Mkn 104 and I Zw 97

Abstract

Two blue compact dwarf galaxies, Mkn 104 and I Zw 97, are studied photometrically and spectroscopically. Mkn 104 is found to contain three distinct bright star-forming regions, whereas I Zw 97 is found to contain three bright and two faint star-forming regions. Medium-resolution spectra of three bright HII regions in the two galaxies were obtained. Estimation of oxygen abundance in these regions yields a value equal to log(O/H) + 12 = 8.5 (Z = Zsolar/2.7). Star-formation rates in these star-forming regions are estimated. The highest star-formation rate for I Zw 97 is found to be 0.04Msolaryr-1, and for Mkn 104 it is 0.02Msolaryr-1. I Zw 97 is realized to be a cometary blue compact dwarf galaxy undergoing a strong burst of star formation. A U - B versus V - I colour-colour mixed population model is created using the Starburst99 evolutionary model curves. The spectrum of the bright star-forming knot of I Zw 97 does not show any strong signature of an underlying relatively older stellar population, but the U - B versus V - I two-colour diagram indicates the strong contribution of a ~500Myr population. A spectrum of the central region of Mkn 104 gives a hint about the underlying old stellar population. The age of this underlying population using the U - B versus V - I two-colour diagram is estimated to be ~ 500Myr. Surface-brightness profiles and colour profiles for these galaxies are presented. The surface-brightness profile of both the galaxies can be represented well by a two-component Sérsic profile consisting of a near-exponential distribution and a Gaussian nuclear starburst. To conclude, neither of these galaxies is a young system; instead they are undergoing episodic star formation superposed on a faint older component. I Zw 97 is a cometary blue compact dwarf galaxy where the underlying low-surface-brightness (LSB) galaxy is a dwarf irregular observed during a major stochastic enhancement of its otherwise moderate star-formation activity, a phenomenon widely accepted as self-propagating stochastic star formation. Both of these galaxies are very similar in their stellar content, showing an older 4-Gyr population, an intermediate 500-Myr population and the current burst of star formation of age 5-13 Myr.