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Star formation in Blue compact dwarf galaxies: Mkn 104 & 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 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=Z_sun/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_sun/yr and for Mkn 104, it is 0.02 M_sun/yr. 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. The age of the underlying population of Mkn 104 using the U-B vs V-I two colour diagram is estimated to be ~500 Myr. The surface brightness profile of both the galaxies can be represented well by a two-component Sersic profile consisting of a near exponential distribution and a Gaussian nuclear starburst. 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. 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.