Their findings show that the three GPs display relatively low extinction, low oxygen abundances and high nitrogen-to-oxygen ratios. Also reported are the clear signatures of Wolf–Rayet stars, of which a population are found (between ~800 and ~1200). A combination of population and evolutionary synthesis models strongly suggest a formation history dominated by starbursts. These models show that these three GPs currently undergo a major starburst producing between ~4% and ~20% of their stellar mass. However, as these models imply, they are old galaxies having formed most of their stellar mass several billion years ago. The presence of old stars has been spectroscopically verified in one of the three galaxies by the detection of Magnesium. Surface photometry, using data from the Hubble Space Telescope archive, indicates that the three GPs possess an exponential low surface brightness envelope (see Low-surface-brightness galaxy). This suggests that GPs are identifiable with major episodes in the assembly history of local Blue Compact Dwarf galaxies.
