Herschel-ATLAS/GAMA: a difference between star-formation rates in strong-line and weak-line radio galaxies

TL;DR

This study compares star-formation activity in strong-line and weak-line radio galaxies, finding that strong-line ones exhibit higher star-formation rates and dust temperatures, indicating more active star formation in their hosts.

Contribution

It provides a large-scale analysis of star formation differences between high- and low-excitation radio galaxies using Herschel and GAMA data, highlighting the link to host galaxy evolution.

Findings

Strong-line radio galaxies have ~4 times higher Herschel luminosity.

Strong-line sources have higher dust temperatures.

Star-formation rates are 3-4 times higher in strong-line radio galaxies.

Abstract

We have constructed a sample of radio-loud objects with optical spectroscopy from the Galaxy and Mass Assembly (GAMA) project over the Herschel-ATLAS Phase 1 fields. Classifying the radio sources in terms of their optical spectra, we find that strong-emission-line sources (`high-excitation radio galaxies') have, on average, a factor ~4 higher 250-micron Herschel luminosity than weak-line (`low-excitation') radio galaxies and are also more luminous than magnitude-matched radio-quiet galaxies at the same redshift. Using all five H-ATLAS bands, we show that this difference in luminosity between the emission-line classes arises mostly from a difference in the average dust temperature; strong-emission-line sources tend to have comparable dust masses to, but higher dust temperatures than, radio galaxies with weak emission lines. We interpret this as showing that radio galaxies with strong nuclear emission lines are much more likely to be associated with star formation in their host galaxy, although there is certainly not a one-to-one relationship between star formation and strong-line AGN activity. The strong-line sources are estimated to have star-formation rates at least a factor 3-4 higher than those in the weak-line objects. Our conclusion is consistent with earlier work, generally carried out using much smaller samples, and reinforces the general picture of high-excitation radio galaxies as being located in lower-mass, less evolved host galaxies than their low-excitation counterparts.