Anomalous Hydrogen Recombination Line Ratios in Ultraluminous Infrared Galaxies

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.

η Carinae: high angular resolution continuum, H30α and He30α ALMA images

ABSTRACT We present images of η Carinae in the recombination lines H30α and He30α and the underlying continuum with 50 mas resolution (110 au), obtained with ALMA. For the first time, the 230 GHz continuum image is resolved into a compact core, coincident with the binary system position, and a weaker extended structure to the NW of the compact source. Iso-velocity images of the H30α recombination line show at least 16 unresolved sources with velocities between −30 and −65 km s−1 distributed within the continuum source. A NLTE model, with density and temperature of the order of 107 cm−3 and 104 K, reproduce both the observed H30α line profiles and their underlying continuum flux densities. Three of these sources are identified with Weigelt blobs D, C, and B; estimating their proper motions, we derive ejection times (in years) of 1952.6, 1957.1, and 1967.6, respectively, all of which are close to periastron passage. Weaker H30α line emission is detected at higher positive and negative velocities, extending in the direction of the Homunculus axis. The He30α recombination line is also detected with the same velocity of the narrow H30α line. Finally, the close resemblance of the H30α image with that of an emission line that was reported in the literature as HCO+(4–3) led us to identify this line as H40δ instead, an identification that is further supported by modelling results. Future observations will enable to determine the proper motions of all the compact sources discovered in the new high angular resolution data of η Carinae.

The impact of strong recombination on temperature determination in planetary nebulae

ABSTRACT The long-standing difference in chemical abundances determined from optical recombination lines and collisionally excited lines raises questions about our understanding of atomic physics, as well as the assumptions made when determining physical conditions and chemical abundances in astrophysical nebulae. Here, we study the recombination contribution of [O iii] 4363 and the validity of the line ratio [O iii] 4363/4959 as a temperature diagnostic in planetary nebulae with a high abundance discrepancy. We derive a fit for the recombination coefficient of [O iii] 4363 that takes into account the radiative and dielectronic recombinations, for electron temperatures from 200 to 30 000 K. We estimate the recombination contribution of [O iii] 4363 for the planetary nebulae Abell 46 and NGC 6778 by subtracting the collisional contribution from the total observed flux. We find that the spatial distribution for the estimated recombination contribution in [O iii] 4363 follows that of the O ii 4649 recombination line, both peaking in the central regions of the nebula, especially in the case of Abell 46 that has a much higher abundance discrepancy. The estimated recombination contribution reaches up to 70 and 40 per cent of the total [O iii] 4363 observed flux, for Abell 46 and NGC 6778, respectively.

How far actually is the Galactic Center IRS 13E3 from Sagittarius A*?

The Galactic Center IRS 13E cluster is a very intriguing infrared object located at ${\sim } 0.13$ pc from Sagittarius A$^\ast$ (Sgr A$^\ast$) in projection distance. There are arguments both for and against the hypothesis that a dark mass like an intermediate mass black hole (IMBH) exists in the cluster. We recently detected the rotating ionized gas ring around IRS 13E3, which belongs to the cluster, in the H30$\alpha$ recombination line using ALMA. The enclosed mass is derived to be $M_{\mathrm{encl.}}\simeq 2\times 10^{4}\, M_\odot$, which agrees with an IMBH and is barely less than the astrometric upper limit mass of an IMBH around Sgr A$^\ast$. Because the limit mass depends on the true three-dimensional (3D) distance from Sgr A$^\ast$, it is very important to determine it observationally. However, the 3D distance is indefinite because it is hard to determine the line-of-sight (LOS) distance by usual methods. We attempt here to estimate the LOS distance from spectroscopic information. The CH$_3$OH molecule is easily destroyed by the cosmic rays around Sgr A$^{\ast }$. However, we detected a highly excited CH$_3$OH emission line in the ionized gas stream associated with IRS 13E3. This indicates that IRS 13E3 is located at $r\gtrsim 0.4$ pc from Sgr A$^{\ast }$.