We measure the extinction curve in the central 200 pc of M31 at mid-ultraviolet to near-infrared wavelengths ( from 1928Å to 1.5 \mu m ) , using Swift / UVOT and HST WFC3 / ACS observations in thirteen bands .
Taking advantage of the high angular resolution of the HST WFC3 and ACS detectors , we develop a method to simultaneously determine the relative extinction and the fraction of obscured starlight for five dusty complexes located in the circumnuclear region .
The extinction curves of these clumps ( R _ { V } =2.4-2.5 ) are steeper than the average Galactic one ( R _ { V } =3.1 ) , but are similar to optical and near-infrared curves recently measured toward the Galactic Bulge ( R _ { V } \sim 2.5 ) .
This similarity suggests that steep extinction curves may be common in the inner bulge of galaxies .
In the ultraviolet , the extinction curves of these clumps are also unusual .
We find that one dusty clump ( size < 2 pc ) exhibits a strong UV bump ( extinction at 2175Å ) , more than three standard deviation higher than that predicted by common models .
Although the high stellar metallicity of the M31 bulge indicates that there are sufficient carbon and silicon to produce large dust grains , the grains may have been destroyed by supernova explosions or past activity of the central super-massive black hole , resulting in the observed steepened extinction curve .