Anomalous Hydrogen Recombination-Line Ratios in Ultraluminous Infrared Galaxies

Abstract

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, three galaxies show anomalous Br$\beta$/Br$\alpha$ 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$\beta$/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 $n\sim 10^8$ ${\mathrm{cm}}^{-3}$. 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.

28 pages, 15 figures, accepted for publication in ApJ

References in corpus (5)

• Array Programming with NumPy
• Understanding hydrogen recombination line observations with ALMA and EVLA
• Turbulence in the Ionized Gas of the Orion Nebula
• Star formation traced by optical and millimeter hydrogen recombination lines and free-free emissions in the dusty merging galaxy NGC 3256 -- MUSE/VLT and ALMA synergy
• Star Formation in Ultraluminous Infrared Galaxies Probed with AKARI Near-Infrared Spectroscopy

Cited by in corpus (3)

• Supermassive black hole feeding and feedback observed on sub-parsec scales
• Measuring chemical abundances in AGN from infrared nebular lines: HII-CHI-Mistry-IR for AGN
• Chemical abundances and deviations from the solar S/O ratio in the gas-phase ISM of galaxies based on infrared emission lines