With more institutions disappearing or threatened seemingly every day, it’s getting harder and harder to cling to the last vestiges of authenticity in downtown Manhattan. Dirty Old Town, a new “gonzo tone poem,” wholeheartedly embraces the last-gasp weirdness of the neighborhood and its longtime denizens; in short, the film follows Bowery Billy Leroy as he attempts to save his Junk Store, but the mix of real and fictional creates a love letter to a city that both never was and won’t be for much longer. The film will be screening three times tonight at BAM; the 6:50pm screening features a Q&A with directors Jenner Furst and Daniel B. Levin, along with co-writer Julia Willoughby Nason.