Supershells and chimneys: The giant ionized gas filaments in NGC 1800

The otherwise unremarkable dwarf galaxy NGC 1800 contains extraordinary structures in its ionized gas including an similar to 3 kpc web of filaments, fingers pointing out of the plane of the galaxy, and a supergiant shell, also apparently extending out of the plane. Emission-line ratios of the filamentary structures are similar to those observed in supershells in the LMC and M33, and they are consistent with direct photoionization as the dominant mechanism for ionization. We suggest that the web of filaments is fitted by the model of Mac Low, McCray, & Norman in which supershells from a previous generation of massive stars have expanded out of the disk of the galaxy and fragmented above the plane. The fingers are ''chimneys'' which provide paths along which UV photons from the current population of massive stars can travel the large distances to the filaments. Structures like these are not often found in late-type galaxies, and so this phenomenon is unlikely to be a common feature of galaxy evolution.