4.1. The role of dust

There are several lines of evidence indicating the importance of dust extinction in galaxies in distant clusters. In the local Universe, galaxies with very strong emission lines are usually faint very late-type galaxies (Sd, Irr), while massive and luminous starburst galaxies in most cases show a combination of strong early Balmer lines in absorption and weak-to-moderate [OII] emission ([65] and references therein). These spectral features are easily understood and reproduced by spectrophotometric models in which younger stars suffer a higher dust extinction than older stellar populations, as can be reasonably expected [31, 66, 67, 68]. Hence, searching for very strong emission lines is not an efficient method to identify starbursts with high SFRs.

Spectra resembling those of local massive starbursts with high dust extinction have been found in a significant fraction (~ 10 %) of both cluster and field galaxies at z = 0.5 [31]. Furthermore, the H line in emission has been detected in some otherwise k+a or passive spectra, again suggesting that the [OII] line in the blue might not have been detected due to dust obscuration [69, 70].

Additional evidence that SFRs can be strongly underestimated in the optical comes from studies at dust-free wavelengths: radio continuum emission has been detected in some of the strongest (= youngest) k+a galaxies at z = 0.4 [71] and some optically passive galaxies in low redshift clusters [70], suggesting they are undergoing a current star formation activity that is invisible in the optical. Mid-IR estimates of the SFR in star-forming galaxies in a cluster at z = 0.2 lead to values a factor 10 to 100 higher than those based on the [OII] line [72], while standard dust corrections usually adopted only account for a factor 2.5.