We investigate the star formation history of the central regions of four luminous compact blue galaxies (LCBGs) at intermediate redshift using evolutionary population synthesis techniques. LCBGs are blue (B - V ≤ 0.6), compact (μB ≤ 21.0 mag arcsec-2) galaxies with absolute magnitudes MB brighter than -17.5. The LCBGs analyzed here are located at 0.436 ≤ z ≤ 0.525. They are among the most luminous (MB -20.5), blue (B - V ≤ 0.4), and high surface brightness (μB ≤ 19.0 mag arcsec-2) of this population. The observational data used were obtained with the Hubble Space Telescope (HST) STIS, the WFPC2, and the first NICMOS camera. We have disentangled the stellar generations found in the central regions of the observed targets using a very simple model. This is one of the first times this has been done for compact galaxies at this redshift using HST data, and it provides a comparison benchmark for future work on this kind of galaxy using instruments with adaptive optics in 10 m class telescopes. We find evidence for multiple stellar populations. One of them is identified as the ionizing population, and the other corresponds to the underlying stellar generation. The estimated masses of the inferred stellar populations are compatible with the dynamical masses, which are typically (2-10) × 109 M⊙. Our models also indicate that the first episodes of star formation these LCBGs underwent happened between 5 and 7 Gyr ago. We compare the stellar populations found in LCBGs with the stellar populations present in bright, local H II galaxies, nearby spheroidal systems, and blue compact dwarf galaxies. It turns out that the underlying stellar populations of LCBGs are similar to yet bluer than those of local H II galaxies. It is also the case that the passive color evolution of the LCBGs could convert them into local spheroidal galaxies if no further episode of star formation takes place. Our results help to impose constraints on evolutionary scenarios for the population of LCBGs found commonly at intermediate redshifts.
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