Ongoing and Future work - Far-Infrared constraints on the ongoing Star Formation of distant gal

and mass dependencies, and the AGN contribution to the IR emission, the trends of the mean SFR as a function of AGN luminosity are consistent with being flat with evidence of a slight upturn at the highest AGN luminosities. The general form of the trends can be reproduced by empirical models that include short term variability of the AGN luminosity while assuming a long-term correlation of SFR with AGN luminosity. Subsequently a correlation of the two processes through their mutual dependence on the cold gas supply can be flattened due to the short-term variability of the AGN. Even though this suggests that the investigation of trends in the SFR–LAGN plane is not ideal to investigate the inter-

action of the two activities, the results of Chapter 3 and simulation work by Veale et al. (2014) demonstrate that it can be used as a diagnostic to test the assumptions made in models of galaxy evolution, such as the underlying Eddington rate distribution. Repro- ducing the results presented in this thesis will also be an important test on cosmological simulations. On average, AGN show no evidence of suppressing the star-formation of their hosts when investigated through their mean SFRs.

We find that including sensitive ALMA photometry at 870µm in our SED fitting pro- cedure provides significantly improved constraints on the SFRs, reducing upper limit constraints by up to an order of magnitude, in comparison to the SFR constraints when using only Spitzer and Herschel photometry (Chapter 5). Additionally, we have demonstrated how the combination of sensitive 870µm photometry from ALMA with MIPS-24µm can be used as an AGN identification tool.

6.3 Ongoing and Future work

The work in this thesis has defined a base for the understanding of the relationship between star formation and AGN luminosity and has resulted in a number of follow-up projects. In the next few paragraphs I outline ongoing and future work based on the methods and results presented in this thesis and the prospects of understanding the connection between star formation and AGN activity with new and future facilities and state of the art cosmological simulations of galaxy evolution.

6.3.1 Extending the SFR–L

_AGN

plane to lower redshifts

In Chapter 3 we found that the mean SFR of X-ray AGN follows a flat trend as a function of LAGN when split into redshift ranges. For that work we concentrated on moderate

to high redshifts of 0.2 < z < 2.5, that include the epoch where AGN activity and star formation where at their peak.

In the local Universe however, extreme sources such as luminous AGN (or QSOs) are more of a rarity and have properties that characterise them as outliers to the general population. This leads to the question, what would the relation between the mean SFR and LAGN look like at these low redshifts, and is it any different to that observed at the

epoch of peak activity? The result would demonstrate any potential for an evolution of the observed trends with redshift.

The SWIFT-BAT telescope (Gehrels et al. 2004) provides an all-sky hard X-ray (14– 195keV) survey, ideal for constructing a sample of local AGN almost completely unaf- fected by the presence of absorption. We have made use of the available catalogue from Shimizu et al. (2015) containing 313 SWIFT-BAT AGN selected at z_{. 0.05 from the 70-} month catalogue (Baumgartner et al. 2013) with WISE and Herschel counterparts. The sample covers the AGN bolometric luminosity range of 1043< LAGN

erg s−1 < 5 × 10

45_{. Using}

the SED fitting method described in Chapter 2, we fit and decompose the IR SED into the AGN and star-forming components for all sources. More than 90% of the sample have a well constrained measurement on their SFR, a significant improvement over our higher redshift samples. Following Stanley et al. (2015; Chapter 3), the sample is split in bins of AGN luminosity, and for each bin the mean SFR is calculated (Murray, S. et al. in prep). From the preliminary results the flat trend of the mean SFRs as a function of AGN luminosity that is observed at higher redshifts (Chapter 3) is also seen for this sample of local X-ray AGN.

6.3.2 Constraining the faint end of the SFR distribution of AGN

With the new constraints on individual SFRs using ALMA observations presented in Chapter 5, we can start determining the distributions of SFRs of X-ray AGN. The mode and shape of the distribution can reveal possible differences between the AGN population

6.3. Ongoing and Future work 171 and the overall star-forming galaxy population beyond the simple mean, as distributions of different shape can have similar means (i.e., Gaussian distributions of different widths but with the same center). A well constrained SFR distribution will also enable important tests on galaxy evolution models. Based on the SFR measurements of Chapter 5, J. Scholtz (in prep) will define the SFR distributions of AGN in two AGN luminosity ranges, following and developing further the methods of Mullaney et al. (2015). With the resulting distributions we will explore underlying differences in the SFR distribution of AGN at different luminosities, as well as differences to the overall star-forming galaxy population.

An important extension in this analyses when investigating the faint-end of the SFR distributions, will be the use of multi-wavelength SFR indicators. As the IR luminosity covers the re-emission of the obscured light of star-forming regions, and obscuration is possibly less significant in galaxies of low SFRs, it will be necessary to also have good constraints on the overall emission from both un-obscured and obscured star formation. Deep fields, such as COSMOS, have a plethora of multi-wavelength data available, including optical and rest-frame UV photometry. However, a number of challenges occur at these wavelengths, including varying sensitivity, and successfully disentangling the emission from: the AGN, the older stellar population and the ongoing star-forming emission.

In the era of the James Webb Space Telescope (JWST) we will be able to observe the rest-frame NIR and MIR emission of AGN and their host galaxies to lower sensitivities than previously achieved. By combining these observations with sensitive ALMA observations that can place strong constraints on the Submm end of the SED we will be able to disentangle the total emission due to star formation and that of the AGN. With such measurements it will be possible to finally constrain the full distributions of SFRs for AGN samples.

6.3.3 Testing models of galaxy evolution

In this thesis, and in the ongoing work described above, we have placed strong constraints on the trends of the SFR as a function of LAGN, and in Chapter 3 we have demonstrated

how empirical toy models can produce different predictions of these trends for different assumptions on the Eddington ratio (or accretion rate) distributions. Furthermore, Veale

et al. (2014) also showed that the SFR–LAGNplane is more sensitive than luminosity func-

tions, to the initial conditions assumed, such as the Eddington ratio distribution. Overall, our results can be a useful tool for simulations of galaxy evolution to be tested.

Recently two major cosmological scale hydrodynamical models of galaxy evolution were released, the Illustris (Vogelsberger et al. 2014) and the EAGLE (Schaye et al. 2015) simulations. In these simulations a set of initial conditions and physical equations are set and the simulated universe evolves through cosmic time. Simulations such as these have the advantage of allowing us to trace individual or a set of galaxies throughout cosmic time, something particularly advantageous for understanding the interaction of the AGN with its host galaxy and how it changes with time. In both models the AGN feedback can regulate/quench the star-formation in the host, as well as the accretion on to the BH itself, mainly through ejecting and heating the cold-gas of the galaxy. By extracting the information for galaxies with active AGN at different redshifts in the simulations, it is possible to calculate the mean SFRs in bins of AGN luminosity, following the methods of this thesis. The results from the simulated AGN sample could then be compared to the observations as a function of mass and redshift. If the model reproduces the observed trends, we can use it to work backwards and better understand what drives this trend, and how the mechanism of AGN feedback can affect it, and what the underlying distribution looks like. Already, the results presented in Chapter 3 are being used to test the results of the EAGLE cosmological hydrodynamical simulation (McApline, S. et al. in prep). As we struggle to observationally constrain the quiescent fraction of AGN and how it compares proportionally to the general galaxy population (i.e, is the fraction of quiescent AGN hosts larger than the general quiescent fraction of galaxies), predictive results could help point to the right answer.

6.3.4 Radio powerful AGN and their impact on star formation

In this thesis we have placed strong constraints on the mean SFRs of the overall AGN population, and have shown that there is no evidence for an influence of the AGN on the average star-formation rates. However, it is possible for AGN going through extreme phases of activity to show different results, and so samples of extreme sources could show different SFRs. Extreme AGN samples can be selected through their optical, radio

6.4. Final remarks 173

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