Anomalous Hydrogen Recombination Line Ratios in Ultraluminous Infrared Galaxies

We conducted systematic observations of the H i Br alpha (4.05 mu m) and Br beta (2.63 mu 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 beta/Br alpha line ratios (similar to 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 beta/Br alpha line ratios than those observed in Galactic H ii regions. The high Br beta/Br alpha 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 beta/Br alpha line ratios and show that the observed ratios can be explained by a combination of an optically thick Br alpha line and an optically thin Br beta line. We simulated the H ii regions in ULIRGs with the Cloudy code, and our results show that the high Br beta/Br alpha line ratios can be explained by high-density conditions, wherein the Br alpha line becomes optically thick. To achieve a column density large enough to make the Br alpha line optically thick within a single H ii region, the gas density must be as high as n similar to 10(8) cm(-3). We therefore propose an ensemble of H ii regions, in each of which the Br alpha line is optically thick, to explain the high Br beta/Br alpha line ratio.