When the first spectra of galaxies in distant clusters
were taken, the biggest surprise
was the finding of galaxies with strong Balmer lines in
absorption and no emission lines. First recognized and named "E+A"
by [46],
their spectra were first modeled in detail by
[47].
Now known also as "k+a" galaxies, they
can be explained if star formation was active in the recent past
and was halted at some point during the last 1-1.5 Gyr. The strongest
cases (equivalent width
EW(H
) > 5 Å)
require a strong starburst before the truncation of the star formation.
A large number of works have found and analyzed k+a spectra in distant
clusters
[46,
47,
48,
49,
50,
51,
52,
53,
54,
55,
11].
The Mass-to-Light ratios of (the few studied)
k+a galaxies appear to be much lower than that of early-type galaxies,
and are consistent with them having undergone a recent starburst
[33,
34].
In 10 clusters at z = 0.4-0.5, the MORPHS collaboration has found the k+a fraction to be significantly larger in clusters than in the field at similar redshifts [56, 31]. These cluster k+a's have mostly spiral morphologies and present a radial distribution within the cluster that is intermediate between the passive and the emission-line galaxy populations. In contrast, [57] interpret their results on the CNOC clusters concluding there is no significant excess of k+a's in the clusters at z = 0.3 compared to the field. The difference between the MORPHS and the CNOC results cannot be ascribed to the optical vs X-ray cluster selection: the brightest X-ray clusters in both samples have similar X-ray luminosities, the MORPHS clusters spanning a factor of 17 in X-ray luminosities while the CNOC clusters a factor of 4, but the most X-ray luminous MORPHS clusters tend to be those with the highest k+a fractions. A Principal Component Analysis of the CNOC spectra by [5] finds a Post-Starformation component which again is intermediate in dynamical state and stellar age between the old, passive population and the "field-like" starforming component. This component presents a (small) excess in clusters compared to the field, but the PCA cannot be easily translated into galaxy number fractions and thus be compared with the MORPHS results.
An excess of k+a galaxies in clusters as opposed to the field is a strong evidence for a quenching of star formation in galaxies as a consequence of the cluster environment. The implications of k+a spectra regarding the presence of starburts in clusters at high-z will be discussed in the next section. The fact that k+a's mostly have spiral morphologies indicates that spectrophotometric and morphological evolution are largely decoupled, and suggests that the timescale for morphological transformation must be longer than the k+a timescale (> 1.5 Gyr).