Starburst and old stellar populations in two z=3.8 radio galaxies 4C 41.17 and TN J2007-1316

TL;DR

This study uses spectral synthesis modeling to analyze the stellar populations of two high-redshift radio galaxies, revealing evidence of massive starbursts and older stellar components, which inform galaxy formation theories.

Contribution

Introduces the Pegase.3 spectral synthesis code to model and interpret the complex stellar populations in high-z radio galaxies, highlighting the presence of both recent starbursts and older stars.

Findings

Massive starburst occurred ~30 Myrs after formation

Older (~1 Gyr) stellar population detected

High stellar mass (~10^12 solar masses) confirmed at z=4

Abstract

The new evolutionary code Pegase.3 is used for the spectral synthesis of continuous optical-Spitzer-Herschel-submm spectral energy distributions (SEDs) of two z=3.8 radio galaxies 4C 41.17 and TN J2007-1316. Both galaxies are selected from the Herschel Radio Galaxy Evolution Project for their faint AGN contribution and because they show evidence of a large stellar contribution to their bolometric luminosity. Pegase.3 coherently models the reprocessing of the stellar luminosity to dust emission, allowing to build UV to IR-submm libraries by types. These templates allow to predict SEDs at redshifts z in the observer's frame by using z=0 templates corrected for cosmology and evolution. Our best fits are a sum of two distinct evolving stellar populations: i) a very massive starburst observed ~30 Myrs after formation from optical and far-IR peaks ii) a significantly older (~1Gyr old) massive early-type population revealed from mid-IR Spitzer data. The AGN emission fits a roughly thermal model. These results confirm that many of the stellar populations in high-z radio galaxies were already formed by massive (a few thousands of billion solar masses) starbursts at z=4, confirming the previous K-z diagram interpretation. Gas-rich mergers and/or jet-cloud interactions are favored for triggering the intense star formation, possibly in relation with massive super black holes of these radio galaxies. These multiple stellar populations may be a generic feature of the luminous infrared radio galaxy population as a whole and strongly constrain mass accumulation and galaxy formation models.