We estimate the star-formation rates and the stellar masses of the Extremely Red Objects ( EROs ) detected in a \approx 180 arcmin ^ { 2 } Ks -band survey ( Ks \approx 20 mag ) .
This sample is complemented by sensitive 1.4 GHz radio observations ( 12 \mu Jy 1 \sigma rms ) and multiwaveband photometric data ( UBVRIJ ) as part of the Phoenix Deep Survey .
For bright K < 19.5 mag EROs in this sample ( I - K > 4 mag ; total of 177 ) we use photometric methods to discriminate dust-enshrouded active systems from early-type galaxies and to constrain their redshifts .
Radio stacking is then employed to estimate mean radio flux densities of \approx 8.6 ( 3 \sigma ) and 6.4 \mu Jy ( 2.4 \sigma ) for the dusty and early-type subsamples respectively .
Assuming that dust enshrouded active EROs are powered by star-formation the above radio flux density at the median redshift of z = 1 translates to a radio luminosity of L _ { 1.4 } = 4.5 \times 10 ^ { 22 } W / Hz and a star-formation rate of SFR = 25 M _ { \odot } yr ^ { -1 } .
Combining this result with photometric redshift estimates we find a lower limit to the star-formation rate density of 0.02 \pm 0.01 M _ { \odot } yr ^ { -1 } Mpc ^ { -3 } for the K < 19.5 mag dusty EROs in the range z = 0.85 - 1.35 .
Comparison with the star-formation rate density estimated for previous ERO samples ( with similar selection criteria ) using optical emission lines , suffering dust attenuation , suggests a mean dust reddening of at least E ( B - V ) \approx 0.5 for this population .
We further use the Ks -band luminosity as proxy to stellar mass and argue that the dust enshrouded starburst EROs in our sample are massive systems , M \ga 5 \times 10 ^ { 10 } M _ { \odot } .
We also find that EROs represent a sizable fraction ( about 50 per cent ) of the number density of galaxies more massive than M = 5 \times 10 ^ { 10 } M _ { \odot } at z \approx 1 , with almost equal contributions from dusty and early type systems .
Similarly , we find that EROs contribute about half of the mass density of the Universe at z \approx 1 ( with almost equal contributions from dusty and early types ) , after taking into account incompleteness because of the magnitude limit K = 19.5 mag .