Context : G326.3 - 1.8 ( also known as MSH 15 - 5 6 ) has been detected in radio as a middle-aged composite supernova remnant ( SNR ) consisting of an SNR shell and a pulsar wind nebula ( PWN ) , which has been crushed by the SNR ’ s reverse shock .
Previous \gamma -ray studies of SNR G326.3 - 1.8 revealed bright and extended emission with uncertain origin .
Understanding the nature of the \gamma -ray emission allows probing the population of high-energy particles ( leptons or hadrons ) but can be challenging for sources of small angular extent .

Aims : With the recent Fermi Large Area Telescope data release Pass 8 providing increased acceptance and angular resolution , we investigate the morphology of this SNR to disentangle the PWN from the SNR contribution .
In particular , we take advantage of the new possibility to filter events based on their angular reconstruction quality .

Methods : We perform a morphological and spectral analysis from 300 MeV to 300 GeV .
We use the reconstructed events with the best angular resolution ( PSF3 event type ) to separately investigate the PWN and the SNR emissions , which is crucial to accurately determine the spectral properties of G326.3 - 1.8 and understand its nature .

Results : The centroid of the \gamma -ray emission evolves with energy and is spatially coincident with the radio PWN at high energies ( E > 3 GeV ) .
The morphological analysis reveals that a model considering two contributions from the SNR and the PWN reproduces the \gamma -ray data better than a single-component model .
The associated spectral analysis using power laws shows two distinct spectral features , a softer spectrum for the remnant ( \Gamma = 2.17 \pm 0.06 ) and a harder spectrum for the PWN ( \Gamma = 1.79 \pm 0.12 ) , consistent with hadronic and leptonic origin for the SNR and the PWN respectively .
Focusing on the SNR spectrum , we use one-zone models to derive some physical properties and , in particular , we find that the emission is best explained with a hadronic scenario in which the large target density is provided by radiative shocks in H i clouds struck by the SNR .

Conclusions :