SPITZERMID-IR SPECTROSCOPY OF POWERFUL 2 JY AND 3CRR RADIO GALAXIES. I. EVIDENCE AGAINST A STRONG STARBURST-AGN CONNECTION IN RADIO-LOUD AGN

Abstract

We present deep Spitzer/IRS spectra for complete samples of 46 2Jy radio galaxies (0.05<z<0.7) and 19 3CRR FRII radio galaxies (z<0.1), and use the detection of polycyclic aromatic hydrocarbon (PAH) features to examine the incidence of contemporaneous star formation and radio-loud AGN activity. Our analysis reveals PAH features in only a minority (30%) of the objects with good IRS spectra. Using the wealth of complementary data available for the 2Jy and 3CRR samples we make detailed comparisons between a range of star formation diagnostics: optical continuum spectroscopy, mid-to far-IR (MFIR) color, far-IR excess and PAH detection. There is good agreement between the various diagnostic techniques: most candidates identified to have star formation activity on the basis of PAH detection are also identified using at least two of the other techniques. We find that only 35% of the combined 2Jy and 3CRR sample show evidence for recent star formation activity (RSFA) at optical and/or MFIR wavelengths. This result argues strongly against the idea of a close link between starburst and powerful radio-loud AGN activity, reinforcing the view that, although a large fraction of powerful radio galaxies may be triggered in galaxy interactions, only a minority are triggered at the peaks of star formation activity in major, gas-rich mergers. However, we find that compact radio sources (D < 15 kpc) show a significantly higher incidence of RSFA (>75%) than their more extended counterparts (≈15 -25%). We discuss this result in the context of a possible bias towards the selection of compact radio sources triggered in gas-rich environments.

Star formation inz> 1 3CR host galaxies as seen byHerschel

“…Second, our result that many small radio sources are hosted by strongly star-forming galaxies is consistent with the observational/theoretical finding that radio jets may induce bursts of star formation (positive feedback, e.g. Silk 2013) in the hosts of high-z (Dey et al 1997;van Breugel et al 1998) and low-z radio-loud AGN (Tadhunter et al 2011;Dicken et al 2012). Best et al (1996) found that smaller radio sources show stronger alignment effect 5 providing further support for the incidence of positive feedback in hosts of small radio sources.…”

Observational evidence that positive and negative AGN feedback depends on galaxy mass and jet power

“…Seymour et al 2011;Dicken et al 2012;Magliocchetti et al 2014;Podigachoski et al 2015;Magliocchetti et al 2016). It also confirms and extends the high star-forming activity observed in the hosts of radio-active AGN selected by different methods (e.g.…”

Strong Molecular Hydrogen Emission and Kinematics of the Multiphase Gas in Radio Galaxies With Fast Jet-Driven Outflows

“…Most of these sources are bright compact radio sources, with sizes ranging from 0.01 to 40 kpc. These H i-outflow objects appear to belong to the minority of powerful radio galaxies (≈35%) that show evidence for recent star formation at optical wavelengths , based on spectral synthesis modeling) and/or mid-IR wavelengths (Dicken et al 2012, based on PAH detection). Most such objects are compact radio sources (e.g., CSS/GPS/CSO) or have unusually strong compact steep spectrum cores .…”