A systematic study of ULIRGs using near-infrared absorption bands reveals a strong UV environment in their star-forming regions

Abstract

We present a systematic study of the 3.0 µm H 2 O ice and the 3.4 µm aliphatic carbon absorption features toward 48 local ultraluminous infrared galaxies (ULIRGs) using spectra obtained by the AKARI Infrared Camera to investigate the UV environment in their star-forming regions. All the ULIRGs in our sample exhibit a ratio of optical depth of H 2 O ice to silicate dust (τ 3.0 /τ 9.7 ) that is lower than that in the Taurus dark cloud. This implies that ULIRGs cannot be described as an ensemble of low-mass star-forming regions and that a significant amount of high-mass star-forming regions contribute to star-forming clouds in local ULIRGs. The results also show that the ratios of optical depth of aliphatic carbon to silicate dust, τ 3.4 /τ 9.7 , exhibit diverse values. We investigate two effects that can affect this ratio: the geometric temperature gradient (which increases the ratio) and the intense UV environment (which decreases it).The geometric temperature gradient is typically considered as a sign of active galactic nuclei (AGN). ULIRGs with AGN signs (optical classification, near-infrared color, and a PAH emission strength of 3.3 µm) indeed tend to exhibit a large τ 3.4 /τ 9.7 ratio. However, we find that the presence of buried AGN is not the only cause of the geometric temperature gradient, because the enhancement of the ratio is also evident in pure starburst-like ULIRGs without these AGN signs. Regarding the intense UV environment in star-forming regions, the correlation between the aliphatic carbon ratio and the ratio of the [C II] 158 µm line luminosity to the far-infrared luminosity (L [C II] /L FIR ), which represents the UV environment in photodissociation regions, implies that the intense UV environment causes the decrease of the aliphatic carbon ratio.We find that an intense UV environment (G/n H > 3) in star-forming regions is needed for the aliphatic carbon ratio to be suppressed.

All the PAHs: an AKARI-Spitzer Cross Archival Spectroscopic Survey of Aromatic Emission in Galaxies

“…Since the profile of H 2 O absorption is temperature dependent and its wing extends towards longer wavelength as temperature increases (Spoon et al 2002), the upper limit of the FWHM is set to be 0.53 µm, slightly less than the even broader ice profile (FWHM = 0.56 µm) reported by Hammonds et al (2015) when analyzing a large number of Galactic objects using AKARI /IRC. We found no correlation between optical depths of H 2 O and silicate, similar to what was reported in Doi et al (2019), so τ H2O and τ Si were left uncoupled.…”

“…In particular, the H 2 O ice feature has an impact since its optical depth is not negligible even for some star-forming galaxies -the average H 2 O opacity for our bright-PAH sample is τ H2O =0.20. Note that in this study, we only consider the 3.4 µm aliphatic feature in emission, even though absorption feature can also be found in the ISM (Pendleton & Allamandola 2002;Doi et al 2019). For most standalone AKARI /IRC studies, the 3.3 µm PAH strength is measured by placing a linear continuum and integrating the band atop (Imanishi et al 2008(Imanishi et al , 2010Ichikawa et al 2014;Inami et al 2018).…”

The 3.3 $μ$m Infrared Emission Feature: Observational and Laboratory Constraints on Its Carrier

“…The ISO spectra of M82 and NGC 253 show that the Class A emission profile and central wavelength also holds on galaxy wide spatial scales and this can also be seen in the AKARI spectra of extragalactic objects (Inami et al 2018;Doi et al 2019). This shows that the Class A profile is ubiquitous and dominant in galaxies, and it highlights the invariance of the Class A 3.3 µm IEF.…”

Mysterious odd radio circle near the large magellanic cloud – an intergalactic supernova remnant?

“…Although, an interesting example of possible precessing jets is seen in the nearby (350 Mpc) ultraluminous infrared galaxy (ULIRG) WISEA J060253.98-710310.0 (Doi et al 2019), which is a merging pair of spiral galaxies (Filipović et al, in prep.). These authors show that merging spiral galaxies SMBH are most likely responsible for producing spirals of circular (precessing) jets spanning over 600 kpc in length.…”