Spitzer 24 μm observations of optical/near-IR selected extremely red galaxies: Evidence for assembly of massive galaxies at z ∼ 1-2?

Optical/near-IR colors have been widely used to select z > 1 galaxies with old stellar populations. However, because a single color can not separate dust reddening from old stars, the relative fractions of dusty starbursts and old early type galaxies in an optical/near-IR selected sample are unknown. In this talk, I present the first, direct measurement of the fraction of dusty sources in a sample of extremely red galaxies with (R − Ks) ⩾ 5.3 mag and Ks < 20.2 mag, using 24 μm mid-infrared data from the Spitzer Space Telescope. Combining deep 24 μm, Ks- and R-band data over an area of ∼64 sq.arcmin in the ELAIS N1 field of the Spitzer First Look Survey (FLS), we find that 50 ± 6% of our ERO sample have measureable 24 μm flux above the 3σ flux limit of 40 μJy. This flux limit corresponds to a SFR of 12 M⊙/yr at z ∼ 1, much more sensitive than any previous long wavelength measurement. The significant implication is that our deep 24 μm observations have revealed for the first time galaxies being assembled together at z ∼ 1. 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.