Anomalous Hydrogen Recombination Line Ratios in Ultraluminous Infrared Galaxies

Abstract

We conducted systematic observations of the H i Br α (4.05 μ m) and Br β (2.63 μ m) lines in 52 nearby ( z < 0.3) ultraluminous infrared galaxies (ULIRGs) with AKARI. Among 33 ULIRGs wherein the lines are detected, 3 galaxies show anomalous Br β /Br α line ratios (∼1.0), which are significantly higher than those for case B (0.565). Our observations also show that ULIRGs have a tendency to exhibit higher Br β /Br α line ratios than those observed in Galactic H ii regions. The high Br β /Br α line ratios cannot be explained by a combination of dust extinction and case B since dust extinction reduces the ratio. We explore possible causes for the high Br β /Br α line ratios and show that the observed ratios can be explained by a combination of an optically thick Br α line and an optically thin Br β line. We simulated the H ii regions in ULIRGs with the Cloudy code, and our results show that the high Br β /Br α line ratios can be explained by high-density conditions, wherein the Br α line becomes optically thick. To achieve a column density large enough to make the Br α line optically thick within a single H ii region, the gas density must be as high as n ∼ 10 8 cm −3 . We therefore propose an ensemble of H ii regions, in each of which the Br α line is optically thick, to explain the high Br β /Br α line ratio.