A Low-Frequency Survey of the Galactic Plane near l = 11°: Discovery of Three New Supernova Remnants

We have imaged a ∼1 deg2 field centered on the known Galactic supernova remnant (SNR) G11.2-0.3 at 74, 330, and 1465 MHz with the Very Large Array radio telescope and 235 MHz with the Giant Metrewave Radio Telescope. The 235, 330, and 1465 MHz data have a resolution of 25″, while the 74 MHz data have a resolution of ∼100″. The addition of this low-frequency data has allowed us to confirm the previously reported low-frequency turnover in the radio continuum spectra of the two known SNRs in the field, G11.2-0.3 and G11.4-0.1, with unprecedented precision. Such low-frequency turnovers are believed to arise from free-free absorption in ionized thermal gas along the lines of sight to the SNRs. Our data suggest that the 74 MHz optical depths of the absorbing gas is 0.56 and 1.1 for G11.2-0.3 and G11.4-0.1, respectively. In addition to adding much needed low-frequency integrated flux measurements for two known SNRs, we have also detected three new SNRs: G11.15-0.71, G11.03-0.05, and G11.18+0.11. These new SNRs have integrated spectral indices between -0.44 and -0.80. Because of confusion with thermal sources, the high resolution (compared with previous Galactic radio frequency surveys) and surface brightness sensitivity of our observations have been essential to the identification of these new SNRs. With this study we have more than doubled the number of SNRs within just a ∼1 deg2 field of view in the inner Galactic plane. This result suggests that future low-frequency observations of the Galactic plane of similar quality may go a long way toward alleviating the long-recognized incompleteness of Galactic SNR catalogs.