Anomalous Hydrogen Recombination-Line Ratios in Ultraluminous Infrared Galaxies

Kenichi Yano; Shunsuke Baba; Takao Nakagawa; Matthew Malkan; Naoki Isobe; Mai Shirahata; Ryosuke Doi; Vanshree Bhalotia

doi:10.3847/1538-4357/ac26be

paper

Anomalous Hydrogen Recombination-Line Ratios in Ultraluminous Infrared Galaxies

arXiv:2109.06880 · doi:10.3847/1538-4357/ac26be

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, three galaxies show anomalous Br $β$ /Br $α$ line ratios ( $\sim 1.0$ ), 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 $n \sim 1 0^{8}$ $cm^{- 3}$ . 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.

arXiv comment28 pages, 15 figures, accepted for publication in ApJ

Anomalous Hydrogen Recombination-Line Ratios in Ultraluminous Infrared Galaxies

Abstract

References in corpus (31)

Cited by in corpus (3)