Teoría pronominalización y orden de constituyentes

There are numerous evidence that support a global evolutionary connection between the star formation and AGN activity e.g. (1) the differential redshift evolution of the AGN luminosity function, or ‘AGN downsizing’ is also found for the star-forming galaxy pop- ulation (e.g.Hasinger et al.,2005;Hopkins & Beacom,2006;Aird et al.,2010), (2) the red- shift distribution of the most strongly star-forming galaxies follows that of powerful AGN (e.g. Willott et al., 2001; Chapman et al., 2005; Wardlow et al., 2011; Miyaji et al., 2015), (3) the star formation rate density as a function of redshift is broadly similar with the black hole accretion rate density (e.g. Boyle & Terlevich, 1998; Merloni, 2004; Silverman et al., 2008a;Aird et al.,2010;Madau & Dickinson, 2014) and (4) the tight correlation of the BH and stellar mass of the host galaxy bulge (e.g. Magorrian et al., 1998; McConnell & Ma, 2013;Graham & Scott,2013;Kormendy & Bender,2013) with both ongoing AGN and star- formation activity. There are several examples of composite objects showing both AGN and star formation activity, in the literature (e.g. Page et al., 2001, 2004; Alexander et al., 2005), particularly at_{z ≈ 1, close to the peak of the AGN luminosity density in the Universe} (Barger et al.,2005;Hasinger et al.,2005). The AGN and star-formation histories show sim- ilar evolution up to_{z ≈ 2, when the mass accretion history is scaled up by a factor ≈ 5000}

(e.g.Silverman et al., 2008a;Aird et al., 2010). However, at higher redshifts there are sig- nificant differences between the two evolution histories with the AGN slope being much steeper. As an example, at_{z ≈ 6 the star formation per unit volume is found to be 1 − 2} orders of magnitude higher than that of the the mass accretion.

The first studies on the identification of the star-formation and accretion activity had at- tempted to determine the star-formation activity in AGN host galaxies using a broad variety of indicators such as optical colours or spectroscopy, mid-IR data and submillimetre obser- vations (e.g. Isaak et al., 2002; Stevens et al., 2005;Alexander et al.,2005; Mullaney et al., 2010;Lutz et al.,2010;Kalfountzou et al.,2011). These results suggested that, globally, the SFRs of AGNs of all classes are found to increase with redshift while the SFRs cover a wide range, up to_{≈ 5 orders of magnitude for a fixed AGN luminosity (e.g.}Mullaney et al.,2010, 2011a;Lutz et al.,2010;Seymour et al.,2011). The main drawbacks of these studies was the significant uncertainties in the estimation of the SFRs due to potential contamination from the AGN, underestimation of the SFR due to obscuration by dust and uncertain extrapolation from the observed wavelength to the total SFR.

These issues are best addressed by FIR observations which are shown to be dominated by emission from dust in the host galaxy, except in the most extreme cases (e.g.Netzer et al., 2007;Mullaney et al., 2011a), and to be a proxy of its star formation activity that is largely uncontaminated by the AGN (e.g.Hatziminaoglou et al.,2010). Consequently, the launch of the Herschel Space Observatory (Pilbratt et al.,2010), with its high FIR sensitivity and wave- length coverage, offers a powerful way of measuring the instantaneous SFR with minimal AGN contamination (e.g. Netzer et al., 2007;Hatziminaoglou et al., 2010; Mullaney et al., 2011b;Bonfield et al., 2011; Hardcastle et al., 2013). Due to a large number of studies un- dertaken using Herschel observations, we have concluded that the average SFRs of the AGN host galaxies increase strongly with redshift from_{z < 0.1 to z = 2 − 3 (e.g.}Mullaney et al., 2011a;Harrison et al., 2012;Rosario et al., 2012; Rovilos et al., 2012). A similarly strong SFR increase is also seen in the overall star-forming galaxy population (_{≈ (1 + z)}4; e.g. Daddi et al., 2007; Pannella et al., 2009; Rodighiero et al., 2010; Elbaz et al., 2011). This good agreement probably suggests that both star formation and AGN activity are driven by the same fuel supplies.

Apart from the strong increase with redshift it is also found that, for both star-forming galaxies and AGN, this increase is independent of galaxy mass. Specifically, the specific star formation rate (sSFR) evolves strongly with redshift across all stellar masses, and is thought to be driven by the availability of a cold-gas supply (i.e., the distant galaxies are more gas rich than the nearby galaxies (Daddi et al., 2010; Genzel et al., 2010; Tacconi et al., 2013). The relatively tight relation between the galaxy stellar mass and its SFR is often referred to as the ‘main sequence’ of star formation, implying that galaxies spend most of their life in this stage of growth. Fig. 1.8 shows a comparison of sSFR between X-ray AGNs and star-forming galaxies out to_{z ≈ 3.}

FIGURE 1.8: Top: Mean60µm (FIR) luminosity vs. AGN luminosity in 5 different redshift bins

(as labelled) over_{z ≈ 0 − 2.5, showing the observed relationship between AGN activity and star} formation. The solid curves are functional fits to the mean measurements, based on a two-component model using a flat line (the constant value is determined by the mean SFR of low-luminosity AGN hosts and is unrelated to accretion activity) and a straight line with a positive non-zero slope. The dashed line is the correlation line shown by AGN-dominated systems inNetzer(2009), and the shaded region corresponds to the estimated1σ range exhibited by empirical pure-AGN SEDs (see Section 3.1

ofRosario et al.,2012for details). LAGNcorresponds to the bolometric AGN luminosity:LX = 1042

and1044 _{erg s}−1 _{correspond toL}

AGN = 5.7 × 1042and 3.4 × 1045erg s−1, respectively. Bottom: Individually FIR-detected (left) and average (right) sSFR vs. redshift for X-ray AGNs withLX =

1042_{− 10}44erg s−1 (as labelled) over_{z = 0.5 − 3. The AGNs are compared to FIR-detected star-} forming galaxies not hosting AGN activity (non AGNs) and the tracks trace the evolution in sSFR found for star-forming galaxies with redshift, as defined by Pannella et al. (2009) and Elbaz et al.

(2011). Overall, the X-ray AGNs broadly trace the evolution in SFR and sSFR found for the star- forming galaxy population. However, the observed relationship between the AGN and star-formation luminosity is complex and is probably, at least partially, driven by the different timescales of stability between star formation and AGN activity. Adapted fromMullaney et al.(2012a) andRosario et al.

Although most studies agree on the strong increase of SFR with redshift, a broad range of results have been found regarding the dependence on AGN luminosity with researchers arguing that either the average SFR increases with both redshift and X-ray luminosity, in- creases only with redshift (similarly to moderate-luminosity AGN), or decreases with X-ray luminosity (e.g.Harrison et al., 2012; Page et al., 2012;Rosario et al., 2012;Rovilos et al., 2012). Part of the disagreement between the different studies for high-luminosity AGNs pos- sible arises from the facts that: (1) luminous AGNs are more rare than moderate-luminosity AGNs, limiting the statistics especially in small to moderate area surveys and (2) AGN con- tribution to the FIR wavelengths increases for luminous AGN reducing the reliability of the SFR measurements. Indeed, studies performed in large-area fields with good source statis- tics find that the average SFR of luminous AGNs is either constant with X-ray luminosity or increases with X-ray luminosity. The transaction from a constant to a rising trend is found to be a function of redshift (e.g.Harrison et al.,2012;Rosario et al.,2012,2013; Fig.1.8).

In distant galaxies, the AGN fraction has been found to be driven by the star-formation rate. Recently, Rafferty et al. (2011) found that moderate-luminosity X-ray AGN (LX >

1043_{erg s}−1_{) are at least∼ 5 − 10 times more common in systems with high star-formation}

(> 300 M⊙ yr−1; equivalent to LIRGs) than in systems with lower star-formation rate

(< 30 M⊙yr−1). At the highest star-formation rates (∼ 1000 M⊙yr−1; equivalent to

ULIRGs) the AGN fraction rises to _{≈ 30 per cent. These results are in agreement with} previous studies (Alexander et al., 2005; Symeonidis et al., 2010). The high AGN fraction at the highest SFRs indicates an intimate connection between BH growth and star-formation phases during periods of vigorous growth. However, the overall AGN fractions are generally consistent with those for intense star-forming galaxies in the local Universe (e.g., the fraction of nearby ULIRGs hosting AGNs withLX > 1043erg s−1is≈ 40 per cent;Alexander et al.,

2008), implying a constant ‘duty cycle’ of BH growth in in star-forming galaxies over a wide range of redshift (_{z ≈ 0 − 3).}

Although these results imply a general connection between observed AGN luminosi- ties and the properties of AGN hosts, the observed correlations AGN and star-formation luminosity appear relatively weak or absent especially at lower and moderate-luminosity AGNs where the average SFR is flat across at least 2 orders of magnitude in AGN lu- minosity (Fig. 1.8). Several models (e.g. Gabor & Bournaud, 2013; Hickox et al., 2014; Neistein & Netzer, 2014) have explored the possibility that the lack of correlation is caused by significant AGN variability on timescales shorter (. Myrs) than those characteristic of star formation which is is assumed to be relatively stable over long periods (of order_{≈ 100} Myr). For instance,Hickox et al.(2014) constructed a simple model which assumes that the long-term growth rate of BHs is exactly proportional to the star formation rate in the host galaxy but it allows the observed AGN luminosity to vary over a wide dynamic range on short timescales on the basis of an assumed Eddington-ratio distribution. This model, and the other models referenced above, reproduce the broad trends seen between AGN activity and SFR

over galaxy evolution timescales and demonstrate how instantaneous luminosity of an AGN is a weak indicator of the average BH accretion rate on the timescales of the galaxy evolution processes that may be expected to drive the long-term growth of BHs (Hickox et al., 2014). However, it is not yet clear which model provides the best physical description of the ob- served trends and how additional parameters such as BH mass, AGN fueling and variability , Eddington limit would affect them. Further observational diagnostics and more sophisticated versions required to account for the joint distribution of BH masses and SFRs and provide greater diagnostic power.