Discovery of a second giant low surface brightness galaxy: Further confirmation of slowly evolving disk galaxies

We report the discovery of another very large, gas-rich, low surface brightness (LSB) disk galaxy. Its overall properties are strikingly similar to those exhibited by Malin 1, which argues that the initial, accidental discovery of Malin 1 by Bothun et al. in 1987 was not a fluke and that the space density of these large, but LSB, galaxies is not negligible. Importantly, these two galaxies are at the top of the disk galaxy luminosity function (owing to their large scale lengths) but have essentially remained uncataloged until now. This again illustrates the importance of selection effects in our understanding of the general galaxy population because even very massive objects (up to 10¹² M_⊙) can be missed. Overall, the low surface brightnesses of these large disks argues for a relatively quiescent evolution which probably reflects their lower than average H I surface densities. However, some star formation is occurring in this disk as a few kpc size H II regions with luminosities in the Ha line approaching 10⁴¹ ergs s^-1 have been detected. On average, these H II regons are located at a distance of 40 kpc from the nucleus. This is an unprecedented situation for a noninteracting disk galaxy and is just one of the remarkable properties of this newly discovered galaxy. If the clockrate of disk galaxy evolution is fundamentally driven by the surface density of atomic hydrogen, then the recovery of LSB, gas-rich disk galaxies from the night sky background is important in the context of determining the true range of disk formation efficiencies.