Letter to the Editor

The Na D profiles of nearby low-power radio sources: jets powering outflows^⋆

¹ GEPI, Observatoire de Paris, UMR 8111, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
e-mail: matthew.lehnert@obspm.fr
² Institut d’Astrophysique Spatiale, UMR 8617, CNRS, Université Paris-Sud, Bâtiment 121, 91405 Orsay Cedex, France
³ SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

Received: 23 May 2011
Accepted: 2 July 2011

Abstract

We have analyzed the properties of the Na D doublet lines at λλ5890, 5896 Å in a large sample of 691 radio galaxies using the Sloan Digital Sky Survey (SDSS). These radio galaxies are resolved in the FIRST survey, have redshifts less that 0.2 and radio flux densities at 1.4 GHz higher than 40 mJy. The sample is complete within the main spectroscopic magnitude limits of the SDSS. Approximately 1/3 of the sources show a significant excess (above that contributed by their stellar populations) of Na D absorption that can be robustly fitted with two Voigt profiles representing the Na D doublet. A further 1/6 of the sources show residual absorption, for which the fits were not well constrained though while  ~50% of the sample show no significant residual absorption. The residual absorption is modestly blueshifted, typically by  ~50 km s^-1, but the velocity dispersions are high, generally  ~500 km s^-1. Assuming that the size of the absorbing region is consistent with  ~1 kpc for dust lanes in a sample of generally more powerful radio sources, assuming a continuous constant velocity flow (continuity equation), we estimate mass and energy outflow rates of about 10 M_⊙ yr^-1 and few  ×10⁴² erg s^-1. These rates are consistent with those in the literature based on H i absorption line observations of radio galaxies. The energy required to power these outflows is on the order of 1–10% of the jet mechanical power and we conclude that the radio jet alone is sufficient. The mass and energy outflow rates are consistent with what is needed to heat/expel the mass returned by the stellar populations as well as the likely amount of gas from a cooling halo. This suggests that radio-loud AGN play a key role in energizing the outflow/heating phase of the feedback cycle. The deposition of the jet mechanical energy could be important for explaining the ensemble characteristics of massive early type galaxies in the local universe.