At a redshift of 0.18 , Abell 1689 is so far the most distant cluster of galaxies for which substantial mid–infrared ( MIR ) data have been published .
Its mapping with the ISOCAM camera onboard the ISO satellite allowed the detection of 30 cluster members at 6.75 \mu m ( LW2 filter ) and 16 cluster members at 15 \mu m ( LW3 filter ) within a clustercentric radius of 0.5 Mpc ( , Paper I ) .
We present here the follow–up optical photometric and spectroscopic observations which were used to study the individual properties of the galaxies members of A1689 .
We confirm the high fraction of blue galaxies initially reported in this rich cluster by , that was challenged by some subsequent studies .
We discuss the spectral and morphological properties of all cluster members in our spectroscopic sample , and of the MIR–detected galaxies in particular .
Sources with a low [ 15 \mu m ] / [ 6.75 \mu m ] flux ratio typically consist of luminous passive early–type galaxies while those with a high MIR color index are mainly luminous , blue , emission–line , morphologically disturbed spirals , i.e .
the star–forming galaxies usually associated with the ’ Butcher–Oemler ’ effect .
On the other hand , at least 30 % of the 15 \mu m sources have optical counterparts showing no evidence of current star–formation activity , while their 15 \mu m emission is most likely due to obscured star formation .
We argue that the LW3 luminosity measured in the cluster members is a reliable tracer of the total infrared luminosity which in A1689 galaxies peaks at \mbox { $L _ { \mbox { \tiny IR } } $ } = 6.2 \mbox { $ \times$ } 10 ^ { 10 } ~ { } \mbox { L$ { } _ { \odot } $ } .
We derive from L _ { \mbox { \tiny IR } } a star–formation rate free of dust extinction , SFR ( IR ) , which we compare with that determined in the optical from the flux of the [ OII ] emission line , SFR ( opt ) .
The highest total star formation rates ( 11 M _ { \odot } yr ^ { -1 } ) and dust extinction are measured in those galaxies exhibiting in their optical spectrum a signature of a dusty starburst .
In contrast , none of the galaxies with post-starburst optical spectra has been detected by ISOCAM down to a 15 \mu m flux limit corresponding to 1.4 M _ { \odot } yr ^ { -1 } .
We find a median SFR ( IR ) of the LW3–detected galaxies of 2 M _ { \odot } yr ^ { -1 } , that is ten times higher than the median SFR ( opt ) of the [ OII ] – detected galaxies .
The ratio SFR ( IR ) /SFR ( opt ) is in fact very high , ranging between 10 and 100 for LW3–detected galaxies with [ OII ] emission .
We conclude that a major part , at least 90 % , of the star formation activity taking place in Abell 1689 is hidden .
Whether the high extinction measured in the star-forming cluster members results from the cluster environment itself or reflects a comparable extinction in the coeval field is still unclear .