Discovery of an Exceptionally Large Quasar Light Echo Candidate

We report the discovery in [O III] survey images of a ~150,000-250,000 ly object near the AGN-candidate galaxy LEDA 388368. The object is visible in the DECaPS 2 sky survey data, most prominently in the g band and faintly in the r and z bands. Location and orientation of the object relative to LEDA 388368 and our analysis indicate that the object is not Galactic, but is rather associated with LEDA 388368. We present spectroscopic analysis of the object using the SALT telescope and visible-light observations using 43.2 cm and 100 cm reflecting instruments. The spectrum taken by the SALT telescope shows clear line emissions in [Hβ], [O III], [O I], [N II], [Hα], and [S II]. Results show high [S II] 6716, 6731 and strong [O I] 6300, 6364 emissions, both indicators of shocked material. The object additionally contains particularly strong [O III] 5007 and [N II] 6584 emission lines and a weak [Hβ] 4861 emission line. Analysis of the object using the BPT diagram and values obtained from the emission line ratios show that the object's properties lie far beyond the theoretical upper limit for star forming galaxies and within the range of AGNs and LINER galaxies. We argue therefore that the object is a result of emissions from these galaxy types, being either an extended narrow-line region or, more likely a quasar light ionization echo, making it by far the largest known of either of these object types.