3D spectroscopy of the radio galaxy 3C171 ( z=0.238) with OASIS/CFHT

Abstract

Distant radio galaxies, observed up to the highest redshifts ( z≃6), are characterized by intense emission lines. Measuring emission line ratios bring clues on ionizing processes. When massive stars are ionizing sources, they give the information on the primeval star formation activity. However analyses are difficult for several reasons. Surface brightness and spatial resolution are rapidly decreasing with redshift. Moreover various emission processes (photoionization by the central AGN, shocks due to jet-cloud interactions or the coupling of ionized gas with radio emitting plasmas) may partly obliterate the photoionization by massive stars. The contribution of star formation at high redshift can only be derived after substraction of other contributing processes. The solution is to locally disentangle the various contributions to emission lines with the help of integral field spectroscopy. We firstly select, with the help of models, the emission line ratio diagnostics able to separate ionizing processes by reproducing at best the observations. Then we observe distant radio galaxies by using integral field units (IFUs) to build maps of emission line ratios. Models are applied by zones, allowing a comparative interpretation between zones. Results give the main sources of ionization in each zone. We present results of 3D spectroscopy on the FR-II radio galaxy 3C171 ( z=0.238) observed with OASIS/CFHT. The H α emission line map represents a typical ionized cocoon, as predicted by theory. The [OIII] λ5007/H β map shows some striking features. Preliminary results from the zone by zone analysis of the [OIII] λ5007/H β map with our recent modelling ( Moy and Rocca-Volmerange, 2002) is also highlighted.