On the universal validity of Case B recombination theory

Understand

In an ongoing search for low-mass extreme emission line galaxies, we identified a galaxy with a Ha/Hb Balmer line ratio of 2.620 +- 0.078.

• Ha/Hb Balmer ratios lower than the dust-free Case~B value appear relatively frequently in extreme emission line galaxies.
• These low values suggest that the Case~B assumption may not be valid in these objects.
• After ruling out the possibility that the low Ha/Hb ratio is due to systematic errors introduced by observational effects, we use constraints from the total Hb luminosity, the [OIII]/[OII] line ratio and the Balmer line equivalent widths, to suggest that the gas is optically thick to both Ha and Lya photons, and the geometry and orientation of the scattering gas causes Ha photons to be preferentially removed from the line of sight with respect to higher order Balmer series photons.