Anomalous Hydrogen Recombination Line Ratios in Ultraluminous Infrared Galaxies

Abstract

We conducted systematic observations of the H i Brα (4.05 μm) and Brβ (2.63 μm) 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 Brβ/Brα 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 Brβ/Brα line ratios than those observed in Galactic H ii regions. The high Brβ/Brα 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 Brβ/Brα line ratios and show that the observed ratios can be explained by a combination of an optically thick Brα line and an optically thin Brβ line. We simulated the H ii regions in ULIRGs with the Cloudy code, and our results show that the high Brβ/Brα line ratios can be explained by high-density conditions, wherein the Brα line becomes optically thick. To achieve a column density large enough to make the Brα line optically thick within a single H ii region, the gas density must be as high as n ∼ 108 cm−3. We therefore propose an ensemble of H ii regions, in each of which the Brα line is optically thick, to explain the high Brβ/Brα line ratio.

Measuring chemical abundances in AGN from infrared nebular lines: HII-CHI-MISTRY-IR for AGN

“…With this upcoming data, further constraints can be established for the IR N/O-O/H relation, and for the systematic offset between IR and optical estimations. (Reunanen et al 2002), REU03 (Reunanen et al 2003), RIF06 (Riffel et al 2006), SEV01 (Severgnini et al 2001), SMA12 (Smajić et al 2012), SPI21 (Spinoglio et al 2021), VEI97 (Veilleux et al 1997), VEI09 (Veilleux et al 2009), YAN21 (Yano et al 2021), TW (This work).…”