N70 and N185 are two large ( \geq 100 pc in diameter ) shell nebulae in the Large Magellanic Cloud ( LMC ) .
Their high [ S ii ] /H \alpha ratios rival those of supernova remnants ( SNRs ) , but they are not confirmed as SNRs .
To study their physical nature , we have obtained XMM-Newton X-ray observations and high-dispersion long-slit echelle spectroscopic observations of these two nebulae .
The X-ray spectra of both nebulae can be well interpreted with an optically thin thermal ( \sim 0.2 keV ) plasma with the average LMC abundance in a collisional ionization equilibrium .
N70 encompasses the OB association LH114 .
Although N70 has a modest expansion velocity and essentially thermal radio emission , its diffuse X-ray luminosity ( \sim 6.1 \times 10 ^ { 35 } erg s ^ { -1 } ) is higher than that from a quiescent superbubble with N70 ’ s density , size , and expansion velocity ; thus , N70 is most likely a superbubble that is recently energized by an interior SNR .
N185 does not contain any known OB association , and its X-ray luminosity is an order of magnitude lower than expected if it is a quiescent superbubble .
N185 has nonthermal radio emission and has high-velocity material expanding at nearly 200 km s ^ { -1 } , similar to many known SNRs in the LMC .
Its X-ray luminosity ( \sim 1.9 \times 10 ^ { 35 } erg s ^ { -1 } ) is also consistent with that of an evolved SNR .
We therefore suggest that N185 is energized by a recent supernova .