G107.0+9.0: A New Large Optically Bright, Radio and X-Ray Faint Galactic Supernova Remnant in Cepheus

Wide-field H alpha images of the Galactic plane have revealed a new supernova remnant (SNR) nearly 3 deg in diameter centred at l = 107.0, b = +9.0. Deep and higher resolution Ha and [O III] 5007 angstrom images show dozens of H alpha filaments along the remnant's northern, western, and southwestern limbs, but few [OIII] bright filaments. The nebula is well detected in the H alpha Virginia Tech Spectral-Line Survey images, with many of its brighter filaments even visible on Digital Sky Survey images. Low-dispersion spectra of several filaments show either Balmer dominated, non-radiative filaments, or the more common SNR radiative filaments with [S II]/H alpha ratios above 0.5, consistent with shock-heated line emission. Emission line ratios suggest shock velocities ranging from <= 70 km s(-1) along its western limb to similar or equal to 100 km s(-1) along its northwestern boundary. While no associated X-ray emission is seen in ROSAT images, faint 1420 MHz radio emission appears coincident with its western and northern limbs. Based on an analysis of the remnant's spatially resolved H alpha and [O III] emissions, we estimate the remnant's distance at similar to 1.5-2.0 kpc implying a physically large (dia. = 75-100 pc) and old (90-110 x 10(3) yr) SNR in its post-Sedov radiative phase of evolution expanding into a low-density interstellar medium (n(0) = 0.05-0.2 cm(-3)) and lying some 250-300 pc above the Galactic plane.