Nicholas Mayall built a UV sensitive spectrograph for the Lick 36" reflector in 1935. For fun, he looked at a nearby early type galaxy and was surprised to see [O II] emission (figure from Mayall 1939). When first discovering these objects Mayall (1936) wrote:
The bright line λ3727 [O II] appears not only in NGC 1052, but also in the
spectra of about half of the other extra-galactic nebulae so far observed with this
instrument. This totally unexpected result is one of considerable interest, for it
raises a number of questions concerning the nature of the radiation from these
ob jects. However, the acquisition of further observations is patently the best course
to pursue before an attempt can be made to understand the phenomenon. From
this point of view, it is especially fortunate that the optical parts of the present
instrument are made of UV glass.
For my thesis, I studied massive early-type red galaxies (MERGs) in two different redshift bins (z ~ 0.1 and z ~ 0.8). The definition of the MERG term is best defined in the image below:
With two large samples of early type galaxies, I carefully stacked the spectra, you can see a pronounced [O II] line at λ3727 in the massive blue galaxies (MBG). Faint, but still there is the [O II] line in the massive early type galaxy samples:
Measurement of line strength, in terms of the luminosity density (power per unit volume) shows that the luminosity density in massive blue galaxies evolves differently than that of massive early type red galaxies. Even further, the ionizing source of [O II] in these massive early type red galaxies is not well understood at all.
In the future, I intend to use the instrument I'm building, MOSFIRE, to understand the nature of emission lines in MERGs at z ~ 1, and the origin of the above evolution.