Archival HST /ACS data reveal details of a light echo around SN 2003gd in the galaxy M74 , only the fifth supernova around which resolved echoes have been reported .
An echo is detected 0 \farcs 3 from the supernova between PA 250°–360° , with fainter signal present at a few other position angles .
This material lies \sim 180 pc in front of the supernova , with a thickness of 60–120 pc , and may delineate the disk of M74 .
This structure has a gas density of 1–2 cm ^ { -3 } , typical of the interstellar medium , however , the dust grains are smaller than those found in our Galaxy , with maximum grain sizes around 0.25 \mu m. Since only one epoch of data exists , in two wavebands and with low signal-to-noise , deeper , annual visits should be made with HST and ground-based adaptive optics telescopes .