The discovery of a young radio galaxy at z = 2.390: Probing initial star formation at z ≲ 3.0

We present the discovery of a weak radio galaxy from the Leiden Berkeley Deep Survey at a redshift of 2.390, the faint optical and IR counterpart of the steep-spectrum, compact radio source 53W002. Its λ-dependent optical continuum morphology is compact with linear size ∼ 10-35 kpc (H₀ = 50, q₀ = 0). In redshifted Lyα, the galaxy is somewhat more extended (≲ 67 kpc × 40 kpc). Its radio source is 7 times smaller than, and confined by, the Lyα gas. It shows alignment with the Lyα gas and the best seeing optical continuum images. We present nine-band photometry (Lyα U BgriJ H K) for the galaxy as well as surrounding objects. The source 53W002 is not variable on time scales of years, in either radio or optical. We compare its rest-frame UV continuum with IUE spectra of various nearby galaxies with relatively recent starbursts, and nearby active galactic nuclei (AGNs). The C IV/Lyα and N v/Lyα ratios suggest that 53W002 has a Seyfert 1-like AGN, and constrain the nonthermal component to be ∼35% of the observed UV continuum. Several independent age estimates yield a consistent value of 0.25-0.32 Gyr: (1) its small 4000 Å break or UV-visual continuum amplitude compared with nearby galaxies; (2) a best model fit to the downturn of its UV spectrum below ∼2000 Å; (3) limits from the lack of detected stellar absorption features; (4) its total stellar mass (from its V- and K-band luminosity) compared with its SFR [from W_λ(Lyα, z = 0)]. These parameters together suggest that at z = 2.390, 53W002 had processed only a few times 10¹¹ M_⊙ into stars, significantly less than most powerful radio galaxies have converted into stars at z ∼ 2-3.8. Hence, star formation in 53W002 has proceeded slower, and started at a later epoch, than in the most powerful high-redshift radio galaxies. The available data are consistent with 53W002 being a genuinely young (radio) galaxy seen at z = 2.390 during its first major starburst. It likely started forming most of its current stars at redshifts 2.5-3.0 (for H₀ = 50-100, q₀ = 0.0-0.5). This suggests that (radio) galaxies do not form the bulk of their stars coevally, but start doing so over a wide range of cosmic time.