We have derived broad band optical/infrared SEDs for a sample of 28 SWIRE X- ray sources with detections in 70µm (and 160µm for 25% of them) using Rowan-
Robinson’s SED fitting method [282]. Our sample was selected to lie within
0.5 < z < 1.3 close to the peak of the AGN luminosity density. 26 out of 28 of our sources have X-ray luminosities indicative of AGN domination in the X-ray, 19
of which exhibit high obscuration column densities NH > 1022 cm−2 with a mixture
of hardness ratios. This is further supported by Figure 6.9 which shows that all of our sources are AGN dominated in X-ray. The number of sources in this redshift interval with X-ray/70µm detections far exceeds the expected number of sources and it is representative of the 70µm population since it consists of ∼ 50% of all
70µm SWIRE sources with X-ray counterparts found within ∼ 2 deg2. Broad band
SEDs show that 19/28 sources are composite, with strong contributions both from a starburst and AGN activity, while in total 27/28 show the presence of a very strong star-formation component which dominates in 19 sources. 12 of these starburst dom-
inated sources have hard X-ray luminosities LX > 1043. Only 9 sources are fitted
with a torus template and the rest are fitted with a starburst template except one source that is fitted with a cirrus template which nonetheless show evidence of star- burst activity contributing up to 20% in infrared emission. An intriguing finding is that most of our sources (25/28) are fitted with a galaxy template rather than with a QSO template. This is in agreement with the transition phase from the blue to red cloud agreeing with the fact that triggered AGN activity in 0.5-1 Gyr quenches star formation causing galaxies to transit from the blue to the red cloud passing from the green cloud where most of the X-ray sources lie (Figure 6.2). The fact that
Figure 6.16: 2-10 keV X-ray luminosity versus the ratio of of the torus lumi- nosity/optical luminosity (top) and versus the torus luminosity/X-ray luminosity
(bottom). Open black circles are sources with logNH > 22. Solid lines are from
Figure 6.17: Star formation rate in M yr−1versus redshift. Red dots is the en-
tire SWIRE population of galaxies. Green dots are galaxies with 70µm detections. Blue large circles are galaxies within the sample of 70µm/X-ray sources.
9 of our sources are fitted with an early type galaxy with evidence of ongoing star formation is also in agreement with results from local samples that conclude that the prerequisites for an AGN is a central massive black hole and a gas supply that fuels both star formation and accretion. This if further supported by the fact that most of the sources in this sample have composite SEDs.
All of our sources have high X/O ratios. Fiore postulates that this is because the nuclear emission is obliterated by dust [105]. This is in agreement with IRAC/MIPS color-color plots which are indicative of the presence of high masses of dust that ob- scure the central engine. The X-ray to mid-IR diagram in Figure 6.10 can provide additional information on which sources have large intrinsic columns with the highly absorbed sources showing a low ratio of absorbed X-ray to mid-IR flux. According to Piccinotti’s diagnostic half of our sources appear to be unobscured although they have large column densities. This scattering can be due to the fact that infrared and X-ray emission originate from a single dominant physical process which can be the large contribution of star-formation [215]. This is evidence that gas is needed to feed the black hole and this is possibly the same gas that is used to form stars. It is also possible that the same mechanism that is responsible for the formation of the stars is also responsible for triggering accretion.
Figure 6.18: Dust mass versus stellar mass in M yr−1 . Colours and symbols
from Figure 6.17. Both axis in logarithmic scale.
In the mid-IR, AGN are characterised by red and almost featureless spectra. These properties make their IRAC colors unique among other IR sources providing a pow- erful tool to identify them. However, this diagnostic is effective only when the AGN is the dominant energy source. In cases where thermal radiation produced by dust in star forming regions is more luminous than that produced by the AGN then the infrared emission of the AGN cannot be distinguishable. In these sources the AGN can either manifest itself in longer infrared wavelengths or in X-rays. Lacy’s diag- nostics in Figure 6.14 and 6.15 suggest that all of our sources, except one which is a pure starburst have relatively red colours compared with normal galaxies possibly as a result of high obscuring columns of dust.
An interpretation for the above findings is to suggest that star formation is intercon- nected with AGN activity with the starburst activity feeding the central black hole. The fact that most of of our sources are obscured and half of the sources have a hard spectrum (HR > −0.2) is further supporting Hopkins models which predict that there is a stage of AGN evolution where obscuration and star-formation coexist. The existence of soft spectra within our sample implies that the same gas that is used for star-formation is also used for the AGN activity. On the other hand, the existence of large number of QSOs with no starburst activity suggests that star-formation is not a requirement for AGN activity. An alternative explanation is that AGN activity
supress star formation in galaxies leaving behind a red remnant and an unobscured AGN. This is supported by the fact that all of our X-ray/70µm sources show evi- dence of an ongoing co-evolution of the two components. The higher fraction (20%) of the composite objects compared to the general population of 70µm sources might suggest that these sources are in the epoch of transition between the red and the blue region (Figure 6.2) in agreement with the QSO mode merger models where an initial merger provides gas to feed both the black hole and star-formation but with time QSO expells all the gas via strong relativistic winds.