The Herschel Exploitation of Local Galaxy Andromeda (HELGA) - I. Global far-infrared and sub-mm morphology

We have obtained Herschel images at five wavelengths from 100 to 500 micron of a ~5.5x2.5 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. In this paper we present the data reduction and provide a description of the far-infrared morphology, comparing it with features seen at other wavelengths. We use high-resolution maps of the atomic hydrogen, fully covering our fields, to identify dust emission features that genuinely belong to M31, distinguishing them from emission coming from the foreground Galactic cirrus. Thanks to the very large extension of our maps we detect, for the first time at far-infrared wavelengths, three ring-shaped structures extending out to ~21, ~26 and ~31 kpc respectively, in the south-western part of M31, with the innermost structure also having a counterpart at the opposite side. The presence of these features is safely confirmed by their detection in HI maps. Due to an unfortunate coincidence of M31's systemic velocity with the rotation speed in its outermost parts, together with the presence of a bright emission from the Galactic cirrus heavily contaminating the north-eastern side, the detection of the other fainter features on this side of the galaxy is more uncertain. We find that the dust in M31 significantly extends beyond the previously mapped far-infrared emission. An annular-like segment, extending beyond the already known 15-kpc ring, is clearly detected on both sides of the galaxy, and similar annular structures are undoubtedly detected on the south-west side as well.

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