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Appendix A: Brighter AGN properties

For completeness we state the relevant properties of the AGN sample from G+09 in Table A.1.

Appendix B: Additional notes on individual objects

B.1. IC 1459

Detailed X-ray modeling of Chandra observations of this object are presented in González-Martín et al. (2009b). Satyapal et al. (2004) give comparable model parameters for the same data set. For the X-ray luminosity the mean of both works was used. There is no [OIII] λ5007   Å  or [OIV] λ25.89   μm luminosity measurement in the literature. The X-ray data suggest that the object is not Compton-thick. In addition, a compact radio core was detected by Slee et al. (1994), confirming the AGN nature of this object. Accordingly, the non-detection with VISIR may be explainable by the bad weather conditions during the observations of IC 1459.

B.2. NGC 676

The only redshift-independent distance measurement for NGC 676 available in the literature is by Tully (1988) employing the Tully-Fisher relation, and this is used in this work. This object is included in the Palomar optical spectroscopic survey (Ho et al. 1995). But it was not detected in radio by Ho & Ulvestad (2001). The used X-ray data based on XMM-Newton is published in Panessa et al. (2006). They interpret this object as Compton-thick in accordance with Akylas & Georgantopoulos (2009) based on the same data. This is also supported by the relatively strong [OIII] emission (Panessa et al. 2006). Unfortunately, there are no Spitzer data available, and therefore no [OIV] measurement.

B.3. NGC 1052

The X-ray properties of NGC 1052 are described in González-Martín et al. (2009b) based on Chandra data. The derived X-ray luminosity is very similar to the one given in Satyapal et al. (2004) for the same observation after correction for the different distances. The mean of both was used in this work. Counting as a prototypical LINER, NGC 1052 is surprisingly bright and obscured in X-rays. While the [OIII] luminosity agrees with the X-ray luminosity (González-Martín et al. 2009a), no [OIV] emission line has been detected (Satyapal et al. 2004). Very recently Brightman & Nandra (2011) published high S/NXMM-Newton data of this object, stating a higher intrinsic L_{2 − 10   keV} = 10^41.5 erg/s, which would bring this object closer to the correlation.

B.4. NGC 1097

The X-ray luminosity is taken from Nemmen et al. (2006) using Chandra observations. It is interesting to note that the measured [OIV] brightness would imply a much higher X-ray luminosity (L_X ~ 10^42.2 erg/s) according to the correlation found between both (Diamond-Stanic et al. 2009), while the [OIII] luminosity implies a lower value (L_X ~ 10^40.0 erg/s) using the [OIII]:X-ray correlation (Panessa et al. 2006). At a larger scale (~10″) the well-known starburst ring is evident in both filters but is only partly visible in the small VISIR field of view. The infrared properties of the nucleus have already been investigated in detail by Mason et al. (2007). They find the absence of PAH 3.3 μm emission at subarcsec scale, which seems to be typical for strong radiation fields. On the other hand, attempts to fit the infrared data with torus models failed, which led them to the conclusion that the torus is absent or weak. Instead, a nuclear star-forming region, co-existing with the LLAGN, is assumed to dominate the observed MIR emission. The likely PAH 11.3 μm emission in the VISIR data indicates that the nucleus is indeed dominated by emission coming from such a star-forming region. This also matches the finding of Storchi-Bergmann et al. (2005) of UV evidence for a nuclear star cluster inside the innermost 9 parsec of NGC 1097. But on the other hand, the MIR excess relative to the luminosity correlation is consistent with the co-existence of star-forming region and a torus inside the nucleus of NGC 1097. To distinguish these components better, additional MIR observations are needed.

B.5. NGC 1386

NGC 1386 is one of the objects for which the different methods used to determine the distance show the largest discrepancies (~50% variation). Because this object is very likely Compton-thick it is difficult to derive a reliable X-ray luminosity. We followed the approach of Levenson et al. (2006) and estimated the luminosity by using the [OIII] line strength. The derived value is consistent with the Fe Kα emission line properties observed by Chandra that are analyzed in the same work. It also agrees with the [OIV] brightness stated in Diamond-Stanic et al. (2009) and LaMassa et al. (2010). Therefore, all available evidence favors a Compton-thick nature for this object.

B.6. NGC 1404

This elliptical galaxy is falling into the center of the Fornax cluster, which leads to large differences in the different distance measurement methods. It is classified as NELG in the optical. No evidence for AGN activity was found in the literature. Nevertheless, it is relatively bright in X-rays with a luminosity of L_X ~ 10⁴¹ erg/s reported by Kim et al. (2006) based on Chandra observations. Because this value was calculated for the whole galaxy including soft X-ray emission, it has to be regarded as an upper limit for any Compton-thin AGN in this object. Very recently, Grier et al. (2011) re-examined the Chandra data and found a clear detection of a nuclear point source with L_X ~ 10⁴⁰ erg/s, which makes a LLAGN in this object likely. However, we treated NGC 1404 as a non-AGN galaxy and comparison object only.

B.7. NGC 1566

Similar to NGC 1386, NGC 1566 shows huge discrepancies in the different distance measurement methods. The only published X-ray data of this objects are from ROSAT (Liu & Bregman 2005), where the emphasis was on off-nuclear ultra-luminous X-ray sources. The nuclear point-source was fitted only by a simple power-law. In the lack of any other available data, we still used these values. This X-ray luminosity agrees well with the predicted values from the [OIII] and [OIV] emission line measurements stated in Diamond-Stanic et al. (2009).

B.8. NGC 1667

Bianchi et al. (2005) found an X-ray flux variation of a factor of 100 for this object and interpreted the data in a way that the object is Compton-thick. Unfortunately, they give no intrinsic X-ray luminosity or flux derived from the used XMM-Newton data. Therefore we used the value of Panessa et al. (2006) based on ASCA observations. These authors also interpreted the data in favor of Compton-thickness. This is supported by the appearance of a strong Fe Kα line (Bassani et al. 1999). On the other hand, the same ASCA data were originally used by Pappa et al. (2001) for a Compton-thin fit, giving an X-ray luminosity that is two orders of magnitude lower for a similar observed flux. The stated [OIII] measurements in Panessa et al. (2006) support a Compton-thick nature. The [OIV] emission (Diamond-Stanic et al. 2009; LaMassa et al. 2010) also predicted an intrinsic X-ray luminosity similar to the one for the Compton-thick case. The VISIR observations support this scenario because NGC 1667 is much closer to the MIR-X-ray correlation than in the unabsorbed scenario.

B.9. NGC 3312

NGC 3312 was classified as a LINER by Carrillo et al. (1999). The X-ray data were taken from Hudaverdi et al. (2006), who describe XMM-Newton observations of this object. No other X-ray data or Spitzer or [OIII] observations are available.

B.10. NGC 4235

We used the XMM-Newton data that was analyzed by Papadakis et al. (2008) and Bianchi et al. (2009) and yielded similar results. Panessa et al. (2006) derived a luminosity 0.6 dex higher but based on ASCA observations, which we treated with a lower priority because of their poorer spatial resolution. It is noteworthy that the properties of this type 1 LLAGN agree well with those of brighter objects, and it is therefore likely that NGC 4235 accretes in radiatively efficient mode as well (Papadakis et al. 2008).

B.11. NGC 4261 – 3C 270

González-Martín et al. (2009b) and Satyapal et al. (2005) arrived at very similar X-ray luminosities based on the same Chandra observation. It is interesting to note that van der Wolk et al. (2010) reports a non-detection with VISIR in the SiC filter for this source. We re-analyzed these observations and found a weak but clear detection. The measured flux matches the new observations presented here, and is six times higher than their reported upper limit (2 mJy). Additional evidence for MIR emission of hot dust in the nucleus of this galaxy has been found by Leipski et al. (2009). Therefore a dusty torus is likely present in NGC 4261, which agrees with the fact that the object aligns well with the MIR-X-ray correlation.

B.12. NGC 4472

This object was included in Horst et al. (2008) erroneously because the nuclear X-ray luminosity was ambiguous owing to many off-nuclear X-ray sources. Only an upper limit for the AGN (L_X < 10^39.32 erg/s) derived from Chandra observations is given in Panessa et al. (2006). On the other hand, in a very detailed analysis Maccarone et al. (2003) found a nuclear X-ray source with L_X ~ 10³⁹ erg/s in the same data. This agrees very well with the [OIII] emission line brightness stated in Diamond-Stanic et al. (2009) while the [OIV] data indicate a much higher luminosity (L_X ~ 10^41.4 erg/s). This seems to be the only indication of Compton-thickness, which is also not supported by the non-detection with VISIR. NGC 4472 might be a so-called “true” Seyfert 2 candidate – meaning that the broad emission lines are intrinsically absent rather than obscured by a torus (Nicastro et al. 2003). This was discussed in Akylas & Georgantopoulos (2009), but the XMM-Newton data presented there are heavily contaminated by off-nuclear emission and therefore are unreliable. Unfortunately, we can reach no conclusion about the existence of a torus in NGC 4472.

B.13. NGC 4486 – M 87

We used the mean of the X-ray luminosities of González-Martín et al. (2009b) and Satyapal et al. (2004). Both were using the same Chandra data and state very comparable results. The flux visible in the Spitzer IRS spectrum matches the VISIR data well. Interestingly, Perlman et al. (2007) explained all observed MIR emission with synchrotron emission by the jet and conclude that there is probably no torus present in NGC 4486. Very recent Herschel/PACS and SPIRE observations (Baes et al. 2010) showed the absence of significant thermal components in the MIR to far-infrared SED on large-scale (>10″): a single power law can roughly fit the overall SED from MIR all way to the radio with a slope of  ~−0.7. But because of the comparably low spatial resolution no conclusion about the presence of a torus can be drawn from these data.

B.14. NGC 4594 – M 104

The X-ray properties are taken from González-Martín et al. (2009b) and Ho et al. (2001) based on Chandra with marginally different results. This object was detected in the N-band by Grossan (2004) with the Keck LWS. The reported nuclear flux at 10.2 μm (11.1 mJy) is consistent with the upper limit we derived and would place this object close to the correlation. The Spitzer/IRS spectrum is very noisy and the much higher flux implies heavy non-nuclear contamination at this lower spatial resolution. Similar to NGC 4472, NGC 4594 is a so-called “true” Sy 2 candidate as well and was investigated in detail in Shi et al. (2010). They found strong silicate emission, indicating that this object possesses a face-on dusty torus. The combined multiwavelength properties, including X-ray and optical/UV imply that NGC 4594 indeed lacks broad emission lines, which contradicts the simple unification models.

B.15. NGC 4698

Cappi et al. (2006) and González-Martín et al. (2009b) give X-ray spectral fits for NGC 4698 with low absorption differing by 0.4 dex derived from XMM-Newton and Chandra data (L_X = 10^39.39 erg/s and 10^38.97 erg/s), respectively. But González-Martín et al. (2009a) interpreted NGC 4698 as Compton-thick using the same Chandra data. This conclusion was based on the [OIII] to X-ray flux ratio. Similarly high [OIII] fluxes were stated in Panessa et al. (2006) and Diamond-Stanic et al. (2009). In addition, a high X-ray obscuration is consistent with the [OIV] emission line brightness of the latter work. Therefore, the object is very likely Compton-thick, but with a large luminosity uncertainty. However, the MIR upper limit of NGC 4698 is consistent with both the Compton-thin or -thick scenario.

B.16. NGC 5363

Initial X-ray studies by González-Martín et al. (2009b) and Sansom et al. (2006) applied unobscured spectral fits to the available XMM-Newton observations and calculated a low X-ray luminosity (L_X = 10^39.68 erg/s and 10^40.30 erg/s) for NGC 5363. However, in the most recent analysis González-Martín et al. (2009a) give a much higher luminosity from the same data but with a Compton-thick model. This is supported by the [OIII] brightness and a relatively flat spectrum above 2 keV. Unfortunately, there is no Spitzer/IRS spectrum and no available [OIV] measurement. Consequently we adopted the Compton-thick fit.

B.17. NGC 5813

We used the X-ray luminosity of González-Martín et al. (2009b) based on Chandra for NGC 8813. The X-ray image shows extremely diffuse soft emission. But no hard emission core is visible, which makes an AGN in this object highly unlikely. On the other hand, it could be a Compton-thick object as argued in González-Martín et al. (2009a). But the [OIII] luminosity given in the same work predicts a only slightly higher X-ray luminosity than in the Compton-thin fit. The AGN nature is supported by the detection of a compact radio core by Nagar et al. (2005). But even if the Compton-thick luminosity (L_X = 10^40.55 erg/s) is adopted, the non-detection with VISIR is not surprising and the derived upper limit does not constrain the correlation.

B.18. NGC 7213

The X-ray measurements by XMM-Newton (e.g., Starling et al. 2005), Chandra (Shu et al. 2010b) and Suzaku (Lobban et al. 2011) available in the literature give very similar X-ray luminosities for NGC 7213. We used the mean value. Because these come from different satellites and epochs, it seems unlikely that this object varies strongly in the X-rays. Therefore we adopted a smaller uncertainty for this object (0.1 dex).

B.19. NGC 7590

XMM-Newton data revealed that the X-ray emission of NGC 7590 is dominated by an off-nuclear source and extended soft emission from the host galaxy (Shu et al. 2010a). Both dominate the older ASCA observation by Bassani et al. (1999), which has a poorer spatial resolution. Unfortunately, there are no Chandra observations available yet for this object. We used the estimated nuclear hard X-ray luminosity of Shu et al. (2010a). But note that based on the [OIII] to 2–10 keV flux ratio, the object is likely Compton-thick, which would imply a much higher intrinsic value. This contradicts the hypothesis that this object is a “true” Sy 2 candidate (compare also Shi et al. 2010). The upper MIR flux limit derived from the VISIR observation does not help to constrain its nature because even an intrinsic 2–10 keV luminosity higher by 2 orders of magnitude would be consistent with the correlation found. Similarly, the IRS spectrum is too noisy for a more detailed analysis and it remains unclear if the nucleus was detected at all.

B.20. NGC 7626

The Chandra observation of NGC 7626 has only been published by Humphrey & Buote (2006) for the whole galaxy in the 0.1–10 keV band. The [OIII] flux measured by Ho et al. (1997) predicts a much lower value (L_X = 10^39.56 erg/s) and will be used in this study. Dudik et al. (2009) analyzed the forbidden emission lines visible in Spitzer data and concluded that this object might not be an AGN based on the non-detection of [NeV] and broad Hα emission. But the upper limit for the predicted X-ray luminosity (L_X < 10^41.03 erg/s) from the upper-limit of the [OIV] non-detection is consistent with the [OIII] prediction. In addition, a point source was detected in radio (Nagar et al. 2005). Accordingly, the nuclear nature of NGC 7626 remains uncertain. The non-detection with VISIR favors a low X-ray luminosity but the upper-limit would be consistent with the MIR-X-ray correlation in any case.

B.21. NGC 7743

The properties of this object are very similar to NGC 5363: it could either be highly absorbed or not absorbed at all: based on XMM-Newton González-Martín et al. (2009b) give an unobscured fit (L_X = 10^39.5 erg/s), while Panessa et al. (2006), González-Martín et al. (2009a), and Akylas & Georgantopoulos (2009) interpreted the X-ray data as Compton-thick. The [OIII] data (e.g., González-Martín et al. 2009a; Panessa et al. 2006) favor Compton-thickness in agreement with [OIV] measurements stated in Diamond-Stanic et al. (2009). Although there is no broad Hα line detected (Terashima et al. 2000), a compact radio core is present (Ho & Ulvestad 2001). In summary, the true nature of this source remains unclear but it is likely Compton-thick. Therefore we used the X-ray luminosity given by González-Martín et al. (2009b). Note however that a Compton-thin nature would agree better with the MIR-X-ray correlation.

© ESO, 2011