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

Abstract

Aims. We investigate whether the far-UV continuum of nearby radio galaxies is due solely to the parent galaxy that passively evolves, or if it reveals evidence for the presence of other star-forming or non-stellar components. If the UV excess is due to an additional radiation component, we compare this with other properties such as radio power, optical spectral type (e.g. high-and low-excitation galaxies), and the strength of the emission lines. We also discuss the possible correlation between the ultraviolet flux, IR properties, and the central black hole mass. Methods. We used a sample of low-luminosity B2 radio galaxies and a small sample of higher luminosity 3C radio galaxies at comparable redshift (z < 0.2). Spectral energy distributions (SEDs) were constructed using a number of on-line databases that are freely available now: GALEX, SDSS, 2MASS, and WISE. These were compared with model SEDs of early-type galaxies with passively evolving stellar populations at various ages (typically 0.5−1.3×10 9 years). We established whether a second component was needed to obtain a satisfactory fit with the observed overall SED. We introduce the parameter XUV , which measures the excess slope of the UV continuum between 4500 and 2000 Å with respect to the UV radiation produced by the underlying old galaxy component. Results. We find that the UV excess as measured by XUV is usually small or absent in low-luminosity (FR I) 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 Hα). Below P 1.4 GHz < 10 24 WHz −1 XUV is close to zero. XUV correlates strongly with the Hα line strength, but only in sources with strong Hα emission. We discuss whether the line emission might be due to photoionization by radiation from the parent galaxy, possibly with additional star formation, or if it requires the presence of a non-stellar active galactic nucleus component. XUV and the slope of the mid-IR are strongly correlated, as measured by the WISE bands in the interval 3.4 to 22 μm, 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: the M BH of objects with strong UV excess is on average two to three times less massive than that of objects without UV excess. Low-luminosity radio galaxies tend to be more massive and contain more massive black holes.

Multi-frequency characterisation of remnant radio galaxies in the Lockman Hole field

“…Radio core flux densities are observed to correlate with the total flux density of a source. This trend is also illustrated by the anti-correlation between the ratio of radio luminosity of the core and extended emission, and total radio luminosity found by de Ruiter et al (1990) for a sample of low luminosity active radio galaxies. FRI radio galaxies have higher CP values than FRII radio galaxies, while the most extreme CP values have been found in compact FR0 radio galaxies (CP > 0.1, see Baldi et al 2015).…”

“…3. To further investigate these CP values and how they relate to the total radio luminosities of our sources, we put them in the context of the relation derived by de Ruiter et al (1990) as described below.…”

“…In Fig. 4 we show the derived relation presented by de Ruiter et al (1990) and include our measurements of the ratio of radio luminosity of the core and extended emission 5 as a function of total radio luminosity. The green line represents the relation found by de Ruiter et al (1990) for 3C and B2 sources, scaled for the different H 0 parameter used in this work.…”

“…Concerning the third criterion, we decided to confirm as remnants not only those sources lacking a core detection (as done by MB17 and Mahatma et al 2018) but also sources that have a very low CP for their total power, as presented in Fig. 4 with the relation by de Ruiter et al (1990). Indeed, the simultaneous presence of USS emission or emission with a strong SPC, combined with a low-luminosity radio core suggests that the nucleus is still active even if at a low level, but the large scale radio jets might have switched off, resulting in the ageing of the large scale emission.…”

Monitoring radio galaxies at TeV energies with HAWC

“…Its equatorial coordinates are α = 176.271°and δ = 19.606°(J2000). It harbors a SMBH with a mass of ∼ 5 × 10 8 M [21]. It has a relativistic jet that reaches kiloparsec scales, in which…”