Anomalous Hydrogen Recombination-Line Ratios in Ultraluminous Infrared Galaxies

TL;DR

This study investigates anomalously high hydrogen recombination line ratios in ULIRGs, revealing that high-density, optically thick conditions in H II regions can explain these observations, challenging previous assumptions about line ratios.

Contribution

The paper presents the first systematic observation of Br$eta$/Br$ extalpha$ ratios in ULIRGs and explains the anomalies through high-density, optically thick H II region models.

Findings

High Br$eta$/Br$ extalpha$ ratios observed in some ULIRGs.

High-density conditions (n~10^8 cm^-3) cause optical thickness in Br$ extalpha$.

An ensemble of dense H II regions explains the line ratio anomalies.

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 ($\sim1.0$), 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 $n\sim10^8$ $\mathrm{cm}^{-3}$. 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.