1. INTRODUCCIÓN
1.4. Marco Teórico
1.4.3. Turismo en la Costa del Pacífico
The initial motivation of both CEx and MEx AGN selection techniques was to target type 2 AGN at higher redshifts than the original BPT technique. The results of this analy- sis suggest the CEx classification technique performs well in this regard. The obscured fraction of the X-ray detected CEx AGN is consistent with the obscured fraction of local type 2 AGN (e.g. Kirhakos and Steiner 1990). Yan et al. (2011) observe that the detected fraction of CEx AGN is consistent with the X-ray detected fraction predicted for a sample of type 2 AGN at 0.3 ≤ z ≤ 0.8. Given that the X-ray detected fraction of CEx AGN
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found in this work is consistent with that of Yan et al. (2011), it stands to reason that this prediction should still hold true. The observed Compton-thick AGN fraction among the X-ray detected CEx AGN alone falls well short of local observations, but many of the Compton thick AGN are likely to remain undetected in the 800ks observations. Stacking the X-ray undetected AGN reveals a hard signal, likely generated by obscured and Comp- ton thick AGN emission. Therefore the “missing” Compton thick AGN are probably in the X-ray undetected CEx AGN sample. Yan et al. (2011) acknowledge that in some cases the AGN emission lines may be drowned out by emission from star-formation. The X-ray detected CEx SFG in this analysis are a prime example of this exact issue, with their ab- sorption corrected X-ray luminosities exceeding those predicted from their optical SFRs. This issue appears to be confined to the X-ray detected CEx SFG (2.6 ± 0.6% of the CEx SFGs) because the stacked emission of X-ray undetected CEx SFG does not exhibit any sign of AGN activity.
The MEx AGN selection does not appear to be identifying type 2 (obscured AGN) with the same accuracy as the CEx AGN selection, at least for the optical sample presented in this work. The MEx AGN selection identifies nearly all the X-ray detected AGN that do not reside in QGs. This implies that the MEx diagnostic is fairly complete, ie it is efficient at selecting AGN as identified by their X-ray emission (except for the XBONGs). On the other hand the efficiency of MEx for selecting obscured sources is less clear. X-ray spectral fitting analysis demonstrates that there is little evidence for excess obscuration among the X-ray detected MEx AGN. Additionally stacking X-ray undetected MEx AGN reveals there is very little evidence to support a significant population of X-ray undetected heavily obscured and/or Compton thick AGN. A large population of low luminosity unobscured AGN appears to be present in the stacked emission of the X-ray undetected MEx AGN instead. Imposing the stricter optical emission line requirements (SN>3) used by Juneau et al. (2011) does not improve the accuracy with which the MEx AGN selection identifies obscured/type 2 AGN. Juneau et al. (2011) argue that the MEx technique has the potential to identify heavily reddened, obscured AGN more effectively than the CEx technique. On the basis of the X-ray analysis presented in this work, however, there appears to be little evidence to support this.
The CEx AGN selection identifies obscured AGN in this sample with greater accuracy than the MEx AGN selection (see Tables 5.5 and 5.7). The lingering question is why is this the case? It is most likely because the CEx technique is dependant upon optical colour (U-B)0. The threshold [OIII]/Hβ ratio of the CEx technique falls as the U-V colour gets
redder. An unobscured AGN, however, is likely to contribute significantly to the blue end of the spectrum (optical and UV). This means galaxies hosting unobscured AGN are likely be driven beyond the AGN parameter space defined by the CEx technique (unless their [OIII]/Hβ ratio is high). This culls the majority of the unobscured AGN population from the CEx selection. The observations of Nandra et al. (2007a) are consistent with this picture, with galaxies on the “red-sequence” of the colour-magnitude diagram tending to
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Figure 5.14: Plot of (U-B)0 colour versus galaxy stellar mass with galaxies classified
using the CEx diagnostic. Red crosses represent emission line AGN and blue crosses are SFGs. Obscured and unobscured X-ray sources has also been overlaid onto the plot.
exhibit hard X-ray emission while galaxies in the “blue cloud” tending to exhibit soft X-ray emission. A similar effect is not observed in the MEx selection because the galaxy mass estimation is independent of the galaxy colour. Therefore the MEx selection identifies nearly all unobscured and obscured AGN.
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Figure 5.15: Plot of (U-B)0 colour versus galaxy stellar mass with galaxies classified
using the MEx diagnostic. Red crosses represent emission line AGN, green crosses are AGN-SF transition galaxies and blue crosses are SFGs. Obscured and unobscured X-ray sources has also been overlaid onto the plot.
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Table 5.10: Obscured and Compton thick fractions for subsamples of X-ray undetected galaxies binned by stellar mass, (U-B)0 and L[OIII].
Galaxies lacking [OIII] and Hβ emission (ie QGs) have been omitted from this analysis. Column (1): emission line diagnostic classification; column (2): number of optical sources; column (3): number of optical sources LR matched to X-ray counterparts; column (4): percentage of optical sources with X-ray counterparts; column (5): number of X-ray detected galaxies with signatures of obscured emission; column (6): percentage of X-ray detected galaxies with obscured emission (ie best fit by models C, D or E), with binomial error; column (7): number of X-ray detected galaxies exhibiting Compton thick emission; column (8): percentage of X-ray detected galaxies exhibiting Compton thick emission.
Type Nsrc NX FX Nobs Fobs NCT FCT
(1) (2) (3) (4) (5) (6) (7) (8) M∗ < 1010.0 285 1 0.3±0.3 0 ... 0 ... 1010.0 ≤ M∗ < 1010.5 300 3 1.0±0.6 0 ... 0 ... 1010.5 ≤ M∗ < 1011.0 266 26 9.8±1.8 11 42.3±9.7 4 15.4±7.1 M∗ ≥ 1011.0 104 22 21.2±4.0 13 59.1±10.5 1 .4.5±4.4. (U-B)0 < 0.6 275 4 1.5±0.7 0 ... 0 ... 0.6 ≤ (U-B)0< 0.8 342 5 1.5±0.6 0 ... 0 ... 0.8 ≤ (U-B)0< 1.1 235 24 10.2±2.0 11 45.8±10.2 2 8.3±5.6 (U-B)0 ≥ 1.1 103 19 18.4±3.8 13 68.4±10.7 3 15.8±8.4 L[0III]< 1040.0 155 6 3.9±1.5 0 ... 0 ... 1040.0≤ L [0III]< 1040.5 310 9 2.9±1.0 3 33.3±15.7 0 ... 1040.5≤ L[0III]< 1041.0 312 15 4.8±1.2 9 60.0±12.6 1 6.7±6.4 L[0III]≥ 1041.0 178 22 12.4±2.5 12 54.5± 10.6 4 18.2±8.2
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To gain further insight into the relative merits of using galaxy colour and galaxy mass as tracers of AGN activity, the CEx and MEx identifications have been overlaid onto plots of (U-B)0 versus galaxy mass (Figures 5.14 and 5.15). The QG subsample
has been omitted from these plots for the purpose of this analysis as they are essentially independent of the CEx and MEx classified galaxies. There is a clear positive correlation between stellar mass and (U-B)0 of a galaxy, with the most massive galaxies possessing
the reddest (U-B)0 colours. A similar correlation between galaxy mass and colour was
observed by Weiner et al. (2009; Figure 10) out to z ∼ 1.4. As a result the X-ray detected fractions of galaxies binned by (U-B)0 and galaxy mass (see Table 5.10) are both positively
correlated. The X-ray detected fraction increases as galaxy stellar mass increases because more massive galaxies tend to contain more massive black holes (Magorrian et al., 1998; McLure et al., 2006) which are in turn more luminous and thus more likely to be detected. A similar increase is thus observed in the X-ray detected fraction as the (U-B)0 colour
becomes redder. There is also a strong increase in the obscured fraction of X-ray detected obscured AGN as stellar mass and (U-B)0 increase.
There is a fairly clean separation of CEx AGN and SFGs in Figure 5.14; the majority of the CEx AGN can be isolated within a region M∗ > 1010.8 M⊙ and (U-B)0 > 1.0, with
minimal contamination from CEx SFG. Compare this to Figure 5.15 which is far more chaotic, with the various MEx subsamples contaminating one another to a substantial extent. The most severe contamination occurs between the MEx AGN and the MEx Transition galaxy subsamples. There is no way to disentangle these populations using the (U-B)0 colour or the stellar mass of the galaxy either in concert or isolation. If the MEx
Transition galaxies were predominantly SF-AGN composites, as observed by Juneau et al. (2011), then this cross-contamination would be understandable. There is, however, no evidence to support significant AGN activity within the MEx Transition galaxy subsample used in this work. Evidently, (U-B)0 is providing a cleaner separation for CEx classified
galaxies than stellar mass does for MEx classified galaxies. This makes the MEx selection technique far more sensitive to the calibration of its own AGN and Transition galaxy boundaries than the CEx technique, thus increasing the risk of false identifications if these are calibrated incorrectly.
It might be possible to target obscured AGN in the MEx plot by redefining the MEx AGN cutoff region. Figure 5.16 is a reproduction of the CEx diagnostic plot for this sample with obscured and unobscured X-ray detected sources overlaid. This plot clearly demonstrates the CEx AGN selection targeting obscured AGN. Superimposing the CEx classifications onto a MEx diagnostic plot (see Figure 5.17), the MEx AGN region is seen to select all the CEx AGN, but it also suffers from heavy contamination by CEx SFGs. The majority of the CEx SFGs, as well as most of the unobscured low luminosity X-ray sources, lie close to the MEx AGN boundary. This suggests that the [OIII]/Hβ requirement imposed on MEx AGN is generally lower than that imposed on CEx AGN. While this allows the detection of lower luminosity AGN, the majority of these sources
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Figure 5.16: CEx diagnostic plot of optical emission line galaxies (0.3 ≤ z ≤ 0.8) with obscured and unobscured X-ray sources identified. Red crosses represent emission line AGN, blue crosses are SFGs and the grey crosses are QGs with no optical emission lines. An upwards arrow highlights sources for which log10 [OIII]/Hβ is a lower limit (ie no
[OIII] detected) whereas a downwards arrow is used when log10 [OIII]/Hβ is an upper
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Figure 5.17: MEx diagnostic plot of optical emission line galaxies (0.3 ≤ z ≤ 0.8) which have been classified using the CEx diagnostic. Red crosses represent emission line AGN, blue crosses are SFGs and the grey crosses are QGs with no optical emission lines. An upwards arrow highlights sources for which log10 [OIII]/Hβ is a lower limit (ie no [OIII]
detected) whereas a downwards arrow is used when log10 [OIII]/Hβ is an upper limit (ie
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appear to be unobscured. Lowering the [OIII]/Hβ also leaves the MEx AGN selection more vulnerable to contamination from SFGs. Revising the MEx AGN boundary upwards would greatly improve the agreement between the CEx and MEx AGN selections and hopefully increase the accuracy with which obscured AGN are identified.