In this chapter I have studied the FIR properties and the star-formation of matched samples of radio-loud and radio-quiet quasars. The main result of our study is that RLQs have higher SFRs than RQQs at low optical luminosities. This result is in agreement with my previous work (Kalfountzou et al., 2012) where the [OII] emission was used as a tracer of the star- formation.
controlling the effect of redshift, in the case of RLQs and especially at low optical luminosi- ties and redshifts. This correlation supports the idea of the jet-induced star-formation.
The possible differences I found between the two populations regarding the dust mass and dust temperature could explain the differences in star-formation rate, but they also point the way forwards further investigation of the evolution of their host galaxies and their envi- ronment and their correlation with AGN activity.
the graveyard’s zigzag mountain path, some saw him leap on rocks that edged the savage shore, some visionaries saw him in the dead of night swimming and talking secretly with the sea-demons, but only a small boy saw him in a lonely dream sit crouched and weeping by the dark sea’s foaming edge
[The Odyssey: A Modern Sequel (1938)] - Nikos Kazantzakis
4
The star formation rate of
z ∼ 1 AGN
This chapter is reproduced from the paper: The star formation rate of z ∼ 1 AGN by E.
Kalfountzou, J. Stevens, M. J. Jarvis, D. Wilner, M. J. Hardcastle, M. Elvis, M. Trichas et
al. to be submitted for publication in MNRAS, 2015
Abstract
Several studies support the existence of a link between AGN activity and the star formation in their host galaxy. In addition, radio jets have been argued to be an ideal mechanism for the direct interaction between the AGN and the host galaxy. However, a drawback of previous surveys is that they are fundamentally limited by the degeneracy between redshift and luminosity in flux-density limited samples. To overcome this limitation, I present FIR Herschel observations of 74 RLQs, 72 RQQs and 27 RGs, selected at a single cosmic epoch of0.9 < z < 1.1 which span over two
decades in optical luminosity. Submillimeter Array (SMA) observations at 1300 µm for the RLQ
sample were obtained to estimate the level of the synchrotron contamination of the FIR bands. By decoupling luminosity from evolutionary effects and using the FIR luminosity as a tracer of the SFR, I investigate how the SFR depends on AGN luminosity, radio-loudness and orientation. I find that: 1) the SFR shows a weak correlation with the bolometric luminosity for all AGN sub-samples, 2) the RLQs show a SFR excess of about300 M⊙yr−1 (a factor of 1.4) compared to the RQQ sample, matched in terms of black hole mass and bolometric luminosity, suggesting either positive radio-jet feedback or radio AGN triggering being linked to the star formation triggering and 3) RGs have lower SFRs by a factor of 2.5 than the RLQs sub-sample with the same BH mass and bolometric luminosity but not jet power. I suggest that there is some jet power threshold at which radio-jet feedback switches from enhancing star formation (by compressing gas) to suppressing it (by ejecting gas). I expect that this threshold depends on both galaxy mass and jet power.
4.1
Decoupling the evolution effects
Observationally, the fact that at high redshift AGN offer the only opportunity to directly investigate the evolution of the BH - galaxy relation has re-invigorated interest in studies of AGN host-galaxy evolution. However, a drawback of previous works is that they are fundamentally limited by the strong correlation between redshift and luminosity, i.e., only the most powerful sources are observed at high redshifts and, due to the much smaller volume probed, only the less luminous, more abundant populations are found at lower redshifts.
In Chapter 3, I investigated the FIR properties of a large RLQ and RQQs sample, over a wide range of optical, radio luminosity and redshift. My results of higher SFRs in RLQs than RQQs particularly at low optical luminosities supports the idea of the jet-induced star formation. Although it is apparent that some form of feedback is needed to explain these results, we have to keep in mind that many parameters could cause the observed differences. Specifically, evolution effects could be strong as low and high optical luminosity sources have significantly different redshift,z ≈ 0.9 and z ≈ 2.5, respectively.
While fundamental questions about the relation between radio-loud and radio-quiet AGN, and how they affect the host galaxy, are in principle soluble with multiwavelength surveys, with already interesting results, most of them remain intractable until we have a compre- hensive AGN sample in which influence of cosmological evolution have been decoupled from the effects of luminosity, radio-loudness and orientation. The sheer size of the SDSS QSO sample (Schneider et al., 2005) makes it possible to generate a homogeneous sam- ple of quasars covering a large range in luminosity at a single epoch. The redshift range 0.9 < z < 1.1 is ideal for this study because this allows us to probe over two decades in op- tical luminosity. Importantly, this redshift is the minimum at which we have a large enough sample of high luminosity quasars with which to compare to the bright quasars found at higher redshifts.
The Herschel Space Observatory (Pilbratt et al.,2010), with its unprecedented FIR sen- sitivity and wavelength coverage, offers the only way of measuring the instantaneous SFR with the minimal AGN contamination (e.g.Netzer et al.,2007;Hatziminaoglou et al.,2010; Mullaney et al., 2011b). To do this, we obtained five-band Herschel photometry for a z 1 benchmark sample of 173 AGN using the Photodetector Array Camera (PACS) at 70 and 160µm and the Spectral and Photometric Imaging Receiver (SPIRE) at 250, 350 and 500 µm on-board the Herschel Space Observatory. We have additionally obtained millimeter ob- servations of the RLQs sub-sample using Submillimeter Array (SMA) radio interferometer at 1300µm to investigate the radio-jets synchrotron contamination to the FIR emission.
This chapter is structured as follows. In Section 4.2 I describe the sample selection, the Herschel and SMA observations carried out, and the steps used for measuring the flux densities in the observed bands. BH and host galaxy properties and analysis are presented in Section4.3. In Sections4.4-4.6I present our results about the star formation dependence on
AGN luminosity, radio jets and orientation, respectively. In Section4.7I list and discuss the conclusions of this work.