The discovery of a young radio galaxy at Z = 2.390 - Probing initial star formation at Z less than approximately 3.0

Abstract

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 (H_0_= 50, q_0_ = 0). In redshifted Lyα, the galaxy is somewhat more extended (&lt;~67 kpc x 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 UBgriJHK) 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 A break or UV-visual continuum amplitude compared with nearby galaxies; (2) a best model fit to the downturn of its UV spectrum below ~2000 A; (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<SUB>lambda</SUB>(Lyα, z = 0)]. These parameters together suggest that at z = 2.390, 53W002 had processed only a few times 10^11^ M_sun_ 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 H_sun_ = 50-100, q_0_ = 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.