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tokens: values similar to the ratios in local SFGs with high EW(H α ). This increase results in high EW([O III ] 5007) > 1000 Å for bright z > 7 galaxies ( Roberts-Borsani et al. 2016 ). The properties of the high- z SFGs discussed above contrast with those of the z ∼ 0 main-sequence SFGs. The latter exhibit, on average, a much lower level of star formation activity and, thus, they are characterised by much lower SFRs, sSFRs, and EWs of emission lines at a fixed stellar mass. However, there is a small fraction of low- z SFGs with compact structure and other properties that are similar to those of high- z galaxies. Subsets of these compact star-forming galaxies (CSFGs) are variously called blue compact dwarf (BCD) galaxies, ‘green pea’ (GP) galaxies, and luminous compact galaxies (LCGs) in the literature. BCDs are nearby dwarf galaxies at redshift z typically ≤0.01 − 0.02, with strong bursts of star formation and low oxygen abundances of 12 + logO/H ≲ 7.9, extending down to 12 + logO/H ≲ 7.0 (e.g., Thuan Martin 1981 ; Izotov et al. 1994 , 2018c ). Typically, their stellar masses are low, M ⋆ ≲ 10 8 M ⊙ . A subset of the lowest-mass BCDs with M ⋆ ∼ 10 6 –10 7 M ⊙ and very high O 32 ≳ 10 has been named ‘blueberry’ galaxies by Yang et al. (2017) because of their compact structure and intense blue colour on the Sloan Digital Sky Survey (SDSS) composite images. Cardamone et al. (2009) considered ‘green pea’ (GP) galaxies in the redshift range of z = 0.112–0.360, with unusually strong [O III ] λ 5007 emission lines, similar to high- z UV-luminous galaxies, such as Lyman-break galaxies and LAEs, with sSFRs up to ∼10 Gyr −1 . The green colour of these galaxies on composite SDSS images is due to strong [O III ] λ 5007 line emission (with EWs up to 1000 Å), redshifted into the SDSS r -band. Izotov et al. (2011) studied a much larger sample of 803 star-forming luminous compact galaxies (LCGs) in a larger redshift range z = 0.02–0.63, with global properties similar to those of the GPs. The sSFRs in LCGs are extremely high and vary in the range ∼1–100 Gyr −1 , comparable to those derived in high- z SFGs. Similar high sSFRs were also found for low- z LyC leaking galaxies ( Izotov et al. 2016a , b , 2018a , b ; Schaerer et al. 2016 ) and some other low- z SFGs (e.g., Senchyna et al. 2017 ; Yuma et al. 2019 ). Izotov et al. (2016c) found sSFRs of up to 1000 Gyr −1 in a sample of ∼13 000 CSFGs selected from the SDSS. Compared to BCDs, both GPs and LCGs are characterised by higher stellar masses ∼10 9 M ⊙ and higher oxygen abundances ∼7.8–8.3. Izotov et al. (2015) selected ∼5200 low-redshift (0 z 1) CSFGs and studied the relations between the global characteristics of this sample, including absolute optical magnitudes, SFRs, stellar masses, and oxygen abundances. They found that for all relations, low- z and high- z SFGs are closely related, indicating a very weak dependence of metallicity on stellar mass, redshift, and star formation rate. This finding favours the