8.6 H II galaxies as distance indicators
It is established that giant extragalactic H II regions (GEHRs) display a correlation between their intrinsic luminosities and the width of their emission lines, the - L(H) relation (Terlevich & Melnick 1981). Melnick et al. (1987, 1988) have shown that the relation found for GEHRs holds also for H II galaxies. The scatter in the - L(H) relation is small enough that it can be used to determine distances.
Recent work with HIRES at the Keck (Koo et al. 1995, Guzmán et al. 1996) has shown that a large fraction of the numerous compact galaxies found at intermediate redshifts have kinematical properties similar to those of luminous local H II galaxies. They exhibit fairly narrow emission line widths ( = 28 to 157 km/s) rather than the 200 km/s typical for galaxies of similar luminosities. In particular galaxies with < 65 km/s seem to follow the same correlations in , MB and L(H) as the local ones. Recent infrared spectroscopy of Balmer emission lines of few Lyman break galaxies at z = 3 (Pettini et al. 1998) suggests that these adhere to the same relation, although this has to be confirmed for a larger sample. This opens the important possibility of applying the distance estimator and map the Hubble flow up to extremely high redshifts and simultaneously to study the behaviour of starbursts of similar luminosities over a huge redshift range. It may prove to be a useful method to measure q0 because the redshift interval of present day applicability considering the most luminous H II galaxies is larger than for methods involving SNe, allowing a good discrimination between deceleration (q0) and curvature (). Possible complications concern the treatment of the effects of metallicity and extinction in these systems.