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We implement a new observational method for mapping the aliphatic hydrocarbon content in the solid phase in our Galaxy, based on spectrophotometric imaging of the 3.4  |$\mu$| m absorption feature from interstellar dust. We previously demonstrated this method in a field including the Galactic Centre cluster. We applied the method to a new field in the Galactic Centre where the 3.4  |$\mu$| m absorption feature has not been previously measured and we extended the measurements to a field in the Galactic plane to sample the diffuse local interstellar medium, where the 3.4  |$\mu$| m absorption feature has been previously measured. We have analysed 3.4  |$\mu$| m optical depth and aliphatic hydrocarbon column density maps for these fields. Optical depths are found to be reasonably uniform in each field, without large source-to-source variations. There is, however, a weak trend towards increasing optical depth in a direction towards b  = 0° in the Galactic Centre. The mean value of column densities and abundances for aliphatic hydrocarbon were found to be about several |$\rm \times 10^{18} \, cm^{-2}$| and several tens × 10−6, respectively for the new sightlines in the Galactic plane. We conclude that at least 10–20 per cent of the carbon in the Galactic plane lies in aliphatic form. [ABSTRACT FROM AUTHOR]

HESS J1804−216 is one of the brightest yet most mysterious TeV γ-ray sources discovered to date. Previous arc-minute scale studies of the interstellar medium (ISM) surrounding this TeV γ-ray source revealed  HESS J1804−216 is likely powered by a mature supernova remnant (SNR) or pulsar, hence its origin remains uncertain. In this paper, we focus on the diffusive escape of cosmic ray protons from potential SNR accelerators. These cosmic rays interact with the ISM to produce TeV γ-rays. We utilize the isotropic diffusion equation solution for particles escaping from a shell, to model the energy-dependent escape and propagation of protons into the ISM. This work is the first attempt at modelling the spatial morphology of γ-rays towards  HESS J1804−216, using arc-minute ISM observations from both Mopra and the Southern Galactic Plane Survey. The spectral and spatial distributions of γ-rays for the two nearby potential SNR counterparts, SNR G8.7−0.1 and the progenitor SNR of PSR J1803−2137, are presented here. We vary the diffusion parameters and particle spectrum and use a grid search approach to find the best combination of model parameters. We conclude that moderately slow diffusion is required for both candidates. The most promising candidate to be powering the TeV  γ-rays from HESS J1804−216 in a hadronic scenario is the progenitor SNR of PSR J1803−2137. [ABSTRACT FROM AUTHOR]

In the interstellar medium, the cosmic elemental carbon abundance includes the total carbon in both gas and solid phases. The aim of the study was to trial a new method for measuring the amount and distribution of aliphatic carbon within interstellar dust over wide fields of view of our Galaxy. This method is based on the measurement of the 3.4- |$\mu$| m absorption feature from aliphatic carbonaceous matter. This can readily be achieved for single sources using infrared (IR) spectrometers. However, making such measurements over wide fields requires an imaging IR camera, equipped with narrow-band filters that are able to sample the spectrum. While this cannot produce as good a determination of the spectra, the technique can be applied to potentially tens to hundreds of sources simultaneously, over the field of view of the camera. We examined this method for a field in the centre of the Galaxy, and produced a map showing the variation of 3.4- |$\mu$| m optical depth across it. [ABSTRACT FROM AUTHOR]

The unidentified TeV γ-ray source HESS J1702–420 has no firm associations with any counterparts at other wavelengths. We present an analysis of the interstellar medium using results from the Mopra Southern Galactic Plane CO survey and archival H  i data from the Southern Galactic Plane Survey. We find significant components of molecular and atomic gas overlapping the TeV γ-ray source at several velocities along the line of sight, with masses ∼102–104 M⊙ and densities up to ∼103 cm−3. Potentially interesting and favourable morphological correspondences between the ISM and the TeV γ-ray emission were found, and in a hadronic scenario, we find that cosmic ray (CR) enhancement values of ∼100 times the local solar values are required to produce the TeV γ-rays. We investigate the separate scenarios in which HESS J1702–420 is powered by the nearby SNR G344.7−0.1, PSRJ1702−4128, and an unknown accelerator. No evidence in the ISM was found to link the SNR with the TeV source. While the morphology of the gas located at the dispersion distance of PSRJ1702−4128 (5.2 kpc, |$v$| LSR ∼ −70 km s−1) did not discount the possibility of a leptonic origin, the required energetics and the broad and hard TeV spectrum would likely disfavour such a scenario. Given the presence of potentially favourable ISM overlapping HESS J1702–420, we additionally speculate on the plausibility of scenarios involving undetected CR accelerators. [ABSTRACT FROM AUTHOR]

The Galactic supernova remnant $$\rm{HESS\,J1534}-{\rm 571}$$ (also known as G323.7 − 1.0) has a shell-like morphology in TeV gamma-ray emission and is a key object in the study of cosmic ray origin. Little is known about its distance and local environment. We examine Mopra 12CO/13CO(1–0) data, Australian Telescope Compact Array $$\rm{H\,{\small I}}$$, and Parkes $$\rm{H\,{\small I}}$$ data towards $$\rm{HESS\,J1534}-{\rm 571}$$. We trace molecular clouds in at least five velocity ranges, including clumpy interstellar medium structures near a dip in $$\rm{H\,{\small I}}$$ emission at a kinematic velocity consistent with the Scutum–Crux arm at ∼3.5 kpc. This feature may be a cavity blown-out by the progenitor star, a scenario that suggests $$\rm{HESS\,J1534}-{\rm 571}$$ resulted from a core-collapse event. By employing parametrizations fitted to a sample of supernova remnants of known distance, we find that the radio continuum brightness of $$\rm{HESS\,J1534}-{\rm 571}$$ is consistent with the ∼3.5 kpc kinematic distance of the Scutum–Crux arm $$\rm{H\,{\small I}}$$ dip. Modelling of the supernova evolution suggests a ∼8–24 kyr age for $$\rm{HESS\,J1534}-{\rm 571}$$ at this distance. [ABSTRACT FROM AUTHOR]

HESS J1825-137 is a pulsar wind nebula (PWN) whose TeV emission extends across~?1 deg. Its large asymmetric shape indicates that its progenitor supernova interacted with a molecular cloud located in the north of the PWN as detected by previous CO Galactic survey (e.g. Lemiere, Terrier & Djannati-Ataï). Here, we provide a detailed picture of the interstellar medium (ISM) towards the region north of HESS J1825-137, with the analysis of the dense molecular gas from our 7 and 12 mm Mopra survey and the more diffuse molecular gas from the Nanten CO(1-0) and GRS 13CO(1-0) surveys. Our focus is the possible association between HESS J1825-137 and the unidentified TeV source to the north, HESS J1826-130. We report several dense molecular regions whose kinematic distance matched the dispersion measured distance of the pulsar. Among them, the dense molecular gas located at (RA, Dec.) = (18h421h,-13.°282) shows enhanced turbulence and we suggest that the velocity structure in this region may be explained by a cloud.cloud collision scenario. Furthermore, the presence of a H α rim may be the first evidence of the progenitor supernova remnant (SNR) of the pulsar PSR J1826.1334 as the distance between the H α rim and the TeV source matched with the predicted SNR radius RSNR ~ 120 pc. From our ISM study, we identify a few plausible origins of the HESS J1826-130 emission, including the progenitor SNR of PSR J1826-1334 and the PWN G018.5-0.4 powered by PSR J1826-1256. A deeper TeV study however, is required to fully identify the origin of this mysterious TeV source. [ABSTRACT FROM AUTHOR]