  We carried out the direct measurement of the fraction of dusty sources in a
sample of extremely red galaxies with (R - Ks) > 5.3mag and Ks < 20.2mag, using
24um data from the Spitzer Space Telescope. Combining deep 24um, Ks- and R-band
data over an area of 64sq.arcmin in ELAIS N1, we find that 50%+-6% of our ERO
sample have measurable 24um flux above the 3sigma flux limit of 40uJy. This
flux limit corresponds to a SFR of 12Msun/yr at z~1. The 24um-detected EROs
have 24-to-2.2 and 24-to-0.7um flux ratios consistent with infrared luminous,
dusty sources at z>1, and an order of magnitude too red to be explained by an
infrared quiescent spiral or a pure old stellar population at any redshift.
Keck optical spectroscopy of a sample of similarly selected EROs in the FLS
field suggests that most of the EROs in ELAIS N1 are probably at z~1. The mean
24um flux (167uJy) of the 24um-detected ERO sample roughly corresponds to the
total infrared luminosity of (3-10)x10^(11)Lsun at z=1-1.5, which is similar to
that of local LIRGs and ULIGs. The corresponding SFR is ~(50-170)Msun/yr. If
the time scale of this starbursting phase is on the order of 10^8yr as inferred
for the local LIRGs and ULIGs, the lower limit on the masses of these
24um-detected EROs is (5-20)x10^9Msun. It is plausible that some of the
starburst EROs are in the midst of violent transformation to become massive
early type galaxies at the epoch of z ~ 1-2.
