Research Progress on Extremely Red Objects at z≈1 and

The extremely red objects (EROs) refer to one kind of galaxies selected by using the color indices in optical and near-infrared wavebands (such as I−K>4 mag). The studies indicate that the EROs are comprised of two classes: one stands for the dust-reddened star-forming galaxies with high redshifts, which are reddened by a large amount of dust, they are mainly the younger spiral galaxies and irregular galaxies with stars forming, and called the DGs, another one stands for the high-redshift elliptical galaxies, which are dominated by old stellar populations (≥ 1 Ga), with no or only with weak star formations, and simply called the OGs. The both classes of EROs are very possibly the precursors of nearby massive galaxies, and only represent the different evolutionary histories respectively. This paper has introduced the research progress on the physical properties of different classes of EROs, such as the morphology and structure, spectral feature, clustering, redshift distribution and galaxy count, as well as the direction of future studies in this field.