Context : We have obtained Herschel images at five wavelengths from 100 to 500 \mu m of a \sim 5.5 \times 2.5 degree area centred on the local galaxy M31 ( Andromeda ) , our nearest neighbour spiral galaxy , as part of the Herschel guaranteed time project “ HELGA ” .
The main goals of HELGA are to study the characteristics of the extended dust emission , focusing on larger scales than studied in previous observations of Andromeda at an increased spatial resolution , and the obscured star formation .

Aims : In this paper we present data reduction and Herschel maps , and provide a description of the far-infrared morphology , comparing it with features seen at other wavelengths .

Methods : We use high–resolution maps of the atomic hydrogen , fully covering our fields , to identify dust emission features that can be associated to M31 with confidence , distinguishing them from emission coming from the foreground Galactic cirrus .

Results : Thanks to the very large extension of our maps we detect , for the first time at far-infrared wavelengths , three arc-like structures extending out to \sim 21 , \sim 26 and \sim 31 kpc respectively , in the south-western part of M31 .
The presence of these features , hosting \sim 2.2 \times 10 ^ { 6 } M _ { \odot } of dust , is safely confirmed by their detection in HI maps .
Overall , we estimate a total dust mass of \sim 5.8 \times 10 ^ { 7 } M _ { \odot } , about 78 % of which is contained in the two main ring-like structures at 10 and 15 kpc , at an average temperature of 16.5 K. We find that the gas-to-dust ratio declines exponentially as a function of the galacto-centric distance , in agreement with the known metallicity gradient , with values ranging from 66 in the nucleus to \sim 275 in the outermost region .

Conclusions : Dust in M31 extends significantly beyond its optical radius ( \sim 21 kpc ) and what was previously mapped in the far-infrared .
An annular–like segment , located approximately at R _ { 25 } , is clearly detected on both sides of the galaxy , and two other similar annular structures are undoubtedly detected on the south-west side even further out .