  We present a study of the classification of z ~1 extremely red objects
(EROs), using a combination of HST/ACS, Spitzer/IRAC, and ground-based images
of the COSMOS field. Our sample includes about 5300 EROs with i-Ks>2.45 (AB,
equivalently I-Ks=4 in Vega) and Ks<=21.1 (AB). For EROs in our sample, we
compute, using the ACS F814W images, their concentration, asymmetry, as well as
their Gini coefficient and the second moment of the brightest 20% of their
light. Using those morphology parameters and the Spitzer/IRAC [3.6]-[8.0]
color, the spectral energy distribution (SED) fitting method, we classify EROs
into two classes: old galaxies (OGs) and young, dusty starburst galaxies (DGs).
We found that the fraction of OGs and DGs in our sample is similar, about 48
percentages of EROs in our sample are OGs, and 52 percentages of them are DGs.
To reduce the redundancy of these three different classification methods, we
performed a principal component analysis on the measurements of EROs, and find
that morphology parameters and SEDs are efficient in segregating OGs and DGs.
The [3.6]-[8.0] color, which depends on reddening, redshift, and photometric
accuracy, is difficult to separate EROs around the discriminating line between
starburst and elliptical. We investigate the dependence of the fraction of EROs
on their observational properties, and the results suggest that DGs become
increasingly important at fainter magnitudes, redder colors, and higher
redshifts.
