Starbursts and the triggering of the activity in nearby powerful radio galaxies

Abstract

We present high‐quality long‐slit spectra for three nearby powerful radio galaxies – 3C 293, 3C 305 and PKS 1345+12. These were taken with the aim of characterizing the young stellar populations (YSP), and thereby investigating the evolution of the host galaxies, as well as the events that triggered the activity. Isochrone spectral synthesis modelling of the wide wavelength coverage spectra of nuclear and off‐nuclear continuum‐emitting regions have been used to estimate the ages, masses and luminosities of the YSP component, taking full account of reddening effects and potential contamination by activity‐related components. We find that the YSP make a substantial contribution to the continuum flux in the off‐nuclear regions on a radial scale of 1–20 kpc in all three objects. Moreover, in two objects we find evidence for reddened post‐starburst stellar populations in the near‐nuclear regions of the host galaxies. The YSP are relatively old (0.1–2 Gyr), massive (109 < MYSP < 2 × 1010 M⊙) and make up a large proportion (∼1–50 per cent) of the total stellar mass in the regions of the galaxies sampled by the observations. Overall, these results are consistent with the idea that the nuclear activity of active galactic nuclei in some radio galaxies is triggered by major gas‐rich mergers. Therefore, these radio galaxies form part of the subset of early‐type galaxies that is evolving most rapidly in the local Universe. Intriguingly, the results also suggest that the radio jets are triggered relatively late in the merger sequence, and that there is an evolutionary link between radio galaxies and luminous/ultraluminous infrared galaxies.

Energetics of the molecular gas in the H2luminous radio galaxy 3C 326: Evidence for negative AGN feedback

“…We note that this is more than a factor 10 greater than the upper limit on the current star-formation rate of ≤0.07 M yr −1 estimated from the infrared emission. For 3C 293 we find a significantly more complex starformation history (see also Tadhunter et al 2005). Namely ∼80% of the stellar mass of 3C 293 was formed at high redshift, ∼20% were formed in a more recent star-formation episode about 1−2 × 10 9 yrs ago , found similar results).…”

Evidence for Merger Remnants in Early‐Type Host Galaxies of Low‐Redshift QSOs1

“…From two-dimensional fitting of a wellmatched sample of 33 AGNs (radio-loud and radio-quiet QSOs, as well as radio galaxies), using images obtained with the Wide Field and Planetary Camera 2 (WFPC2) on board HST, D03 concluded that ''for nuclear luminosities M V < À23:5, the hosts of both radio-loud and radio-quiet AGNs are virtually all massive elliptical galaxies with basic properties that are indistinguishable from those of quiescent, evolved, low-redshift ellipticals of comparable mass.'' Such conclusions conflict with recent findings of significant amounts of dust and cold molecular gas, as well as young stellar populations, in many radio-loud and radio-quiet QSOs (e.g., Martel et al 1999;Canalizo & Stockton 2001;Evans et al 2001;Scoville et al 2003;Tadhunter et al 2005;Barthel 2006). Although the hosts of most luminous AGNs are bulge-dominated, they seem to be significantly bluer than inactive elliptical galaxies, and indeed show evidence for starbursts in the relatively recent past (1-2 Gyr; e.g., Kauffmann et al 2003;Sánchez et al 2004;Canalizo et al 2006;Schweitzer et al 2006;Jahnke et al 2007).…”

Starbursts and black hole masses in X-shaped radio galaxies: Signatures of a merger event?

“…This peak suggests that there has been enhanced star formation due to a merger event, which implies a time delay of 1−3 Gyr between the peak of starburst activity and the end of the merger. This timescale agrees with the time delay of ∼2 Gyr found by hydrodynamical simulations of galaxy mergers (Lotz et al 2008), and with the timescales of 0.5−2 Gyr found observationally for individual objects (e.g., Tadhunter et al 2005;Emonts et al 2006). On the other hand, the jet dynamic age of 10 6 yrs derived for the X-shaped sources also agrees with the time delay between the merger event and the onset of the radio-AGN triggered activity found by other studies to be 10 6 years (e.g., Emonts et al 2006).…”