A Sino-German λ6 cm polarization survey of the Galactic plane VI. Discovery of supernova remnants G178.2-4.2 and G25.1-2.3

Context. Supernova remnants (SNRs) were often discovered in radio surveys of the Galactic plane. Because of the surface-brightness limit of previous surveys, more faint or confused SNRs await discovery. The Sino-German lambda 6 cm Galactic plane survey is a sensitive survey with the potential to detect new low-surface-brightness SNRs. Aims. We aim to identify new SNRs from the lambda 6 cm survey map of the Galactic plane. Methods. We searched for new shell-like objects in the lambda 6 cm survey maps and studied their radio emission, polarization, and spectra using the lambda 6 cm maps together with the lambda 11 cm and lambda 21 cm Effelsberg observations. Extended polarized objects with non-thermal spectra can be identified as SNRs. Results. We have discovered two new, large, faint SNRs, G178.2-4.2 and G25.1-2.3, both of which show shell structure. G178.2-4.2 has a size of 72' x 62' with strongly polarized emission being detected along its northern shell. The spectrum of G178.2-4.2 is non-thermal, with an integrated spectral index of alpha = -0.48 +/- 0.13. Its surface brightness is Sigma(1 GHz) = 7.2 x 10(-23) Wm(-2) Hz(-1) sr(-1), which makes G178.2-4.2 the second faintest known Galactic SNR. G25.1-2.3 is revealed by its strong southern shell, which has a size of 80' x 30'. It has a non-thermal radio spectrum with a spectral index of alpha = -0.49 +/- 0.13. Conclusions. Two new large shell-type SNRs have been detected at lambda 6 cm in an area of 2200 deg(2) along the Galactic plane. This demonstrates that more large and faint SNRs exist, but are very difficult to detect.