Far-UV to mid-IR properties of nearby radio galaxies

TL;DR

This study examines the far-UV to mid-IR properties of nearby radio galaxies, revealing correlations between UV excess, emission lines, IR emission, and black hole mass, and identifying a transition in properties at a specific radio power.

Contribution

It introduces the $XUV$ parameter to quantify UV excess and links it to radio power, emission lines, IR properties, and black hole mass in low luminosity radio galaxies.

Findings

UV excess is absent in low-power sources but appears above a certain radio power.

$XUV$ correlates strongly with $H\alpha$ emission and IR slope.

Objects with UV excess have less massive black holes.

Abstract

We investigate whether the far-UV continuum of nearby radio galaxies reveals evidence for the presence of star forming or non-stellar components. If a UV excess due to an extra radiation component exists we compare this with other properties such as radio power, optical spectral type and the strength of the emission lines. We also discuss the possible correlation between the ultra-violet flux, IR properties and central black hole mass. We use two sampes of low luminosity radio galaxies with comparable redshifts ($z < 0.2$). Spectral Energy Distributions are constructed using a number of on-line databases: GALEX, SDSS, 2MASS, and WISE. The parameter $XUV$ is introduced, which measures the excess slope of the UV continuum between 4500 and 2000 \AA, with respect to the UV radiation produced by the underlying old galaxy component. We find that the UV excess is usually small or absent in low luminosity sources, but sets in abruptly at the transition radio power above which we find mostly FRII sources. $XUV$ behaves very similarly to the strength of the optical emission lines (in particular $H\alpha$). Below $P_{1.4 GHz} < 10^{24}$ WHz$^{-1}$ $XUV$ is close to zero. $XUV$ correlates strongly with the $H\alpha$ line strength, but only in sources with strong $H\alpha$ emission. There is a strong correlation between $XUV$ and the slope of the mid-IR, as measured by the WISE bands in the interval 3.4 to 22 $\mu$m, in the sense that sources with a strong UV excess also have stronger IR emission. There is an inverse correlation between $XUV$ and central black hole mass: strong UV excess objects have, on average, $M_{BH}$ about 2-3 times less massive than those without UV excess. Low luminosity radio galaxies tend to be more massive and contain more massive black holes.