Star formation in Blue Compact Dwarf Galaxies : Mkn 104 and I Zw 97

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 H ii 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 = Z⊙/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 0.04 M⊙yr −1 and for Mkn 104, it is 0.02 M⊙yr . I Zw 97 is realized to be a cometary blue compact dwarf galaxy undergoing a strong burst of star formation. A U −B vs 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 vs V −I two colour diagram indicates a strong contribution of a ∼ 500 Myr 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 vs V − I two colour diagram is estimated to be ∼ 500 Myr. 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, both of these galaxies are not young systems; 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 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.