XMM-Newton observations of Seyfert galaxies from the Palomar spectroscopic survey: the X-ray absorption distribution
We present XMM-Newton spectral analysis of all 38 Seyfert galaxies from the Palomar spectroscopic sample of galaxies. These are found at distances of up to 67 Mpc and cover the absorbed 2-10 keV luminosity range . Our aim is to determine the distribution of the X-ray absorption in the local Universe. Three of these are Compton-thick with column densities just above and high equivalent width FeK lines ( eV). Five more sources have low values of the X-ray to [OIII] flux ratio suggesting that they could be associated with obscured nuclei. Their individual spectra show neither high absorbing columns nor flat spectral indices. However, their stacked spectrum reveals an absorbing column density of . Therefore the fraction of absorbed sources ( ) could be as high as %. A number of Seyfert-2 appear to host unabsorbed nuclei. These are associated with low-luminosity sources . Their stacked spectrum again shows no absorption while inspection of the Chandra images, where available, shows that contamination from nearby sources does not affect the XMM-Newton spectra in most cases. Nevertheless, such low luminosity sources are not contributing significantly to the X-ray background flux. When we consider only the brighter, , 21 sources, we find that the fraction of absorbed nuclei rises to % while that of Compton-thick sources to 15-20%. The fraction of Compton-thick AGN is lower than that predicted by the X-ray background synthesis model in the same luminosity and redshift range.
Key Words.:Surveys – X-rays: galaxies – X-rays: general
The moderate to high redshift Universe has been probed at unparallelled depth with the most sensitive observations performed at X-ray wavelengths in the Chandra Deep fields. The Chandra 2Ms observations (Alexander et al. 2003, Luo et al. 2008) resolved about 80 per cent of the extragalactic X-ray light in the hard 2-10 keV band (see Brandt & Hasinger 2005 for a review). These deep surveys find a sky density of 5000 sources per square degree, the vast majority of which are found to be AGN through optical spectroscopy (e.g. Barger et al. 2003). In contrast, the optical surveys for QSOs (e.g. the COMBO-17 survey) reach only a surface density about an order of magnitude lower (e.g. Wolf et al. 2003). This clearly demonstrates the power of X-ray surveys for detecting AGN. This is because hard X-rays can penetrate large amounts of gas without suffering from significant absorption. Indeed detailed spectral analysis on X-ray selected AGN reveals large amount of obscuration (e.g. Akylas et al. 2006, Tozzi et al. 2006, Georgantopoulos et al. 2007). In particular, about two thirds of the X-ray sources, over all luminosities, present column densities higher than . These high absorbing columns are believed to originate in a molecular torus surrounding the nucleus.
However, even the efficient 2-10 keV X-ray surveys may be missing a fraction of highly obscured sources. This is because at very high obscuring column densities ( , corresponding to an optical reddening of ), the X-ray photons with energies between 2 and 10 keV are absorbed. These are the Compton-thick AGN (see Comastri 2004 for a review) where the Compton scattering on the bound electrons becomes significant. Despite the fact that Compton-thick AGN are abundant in our vicinity (e.g. NGC1068, Circinus), only a few tens of Compton-thick sources have been identified from X-ray data (Comastri 2004). Although the population of Compton-thick sources remains elusive there is concrete evidence for its presence. The X-ray background synthesis models can explain the peak of the X-ray background at 30-40 keV, where most of its energy density lies, (Frontera et al. 2007, Churazov et al. 2007) only by invoking a large number of Compton-thick AGN (Gilli, Comastri & Hasinger 2007). Additional evidence for the presence of a Compton-thick population comes from the directly measured space density of black holes in the local Universe. It is found that this space density is a factor of two higher than that predicted from the X-ray luminosity function (Marconi et al. 2004). This immediately suggests that the X-ray luminosity function is missing an appreciable number of obscured AGN.
In recent years there have been many efforts to uncover heavily obscured and in particular Compton-thick AGN in the local Universe by examining IR or optically selected, [OIII], AGN samples. This is because both the IR and the narrow-line region originate beyond the obscuring region and thus represent an isotropic property of the AGN. Risaliti et al. (1999) examine the X-ray properties of a large sample of [OIII] selected Seyfert-2 galaxies whose X-ray spectra were available in the literature. They find a large fraction of Compton-thick sources (over half of their sample). Their estimates are complemented by more recent XMM-Newton observations of local AGN samples (Cappi et al. 2006, Panessa et al. 2006, Guainazzi et al. 2005). All these authors also claim a large Compton-thick AGN fraction exceeding 30 per cent of the Seyfert-2 population. The advent of the SWIFT and INTEGRAL missions which carry X-ray detectors with imaging capabilities (e.g. Barthelmy et al. 2005, Ubertini et al. 2003) in ultra-hard X-rays (15-200 keV) try to shed new light on the absorption properties of AGN in the local Universe. In principle, at these ultra-hard X-rays obscuration should play a negligible role, at least up to column densities as high as . However, because of the limited effective area the above surveys can provide X-ray samples, down to very bright fluxes , with limited quality spectra. Again XMM-Newton observations are often required to determine the column density in each source. Interestingly, these surveys find only a limited number of Compton-thick sources (Markwardt et al. 2005, Bassani et al. 2006, Malizia et al. 2007, Ajello 2008, Winter et al. 2008, Tueller et al. 2008, Sazonov et al. 2008).
Here, we present XMM-Newton observations of all 38 Seyfert galaxies in the Palomar spectroscopic sample of nearby galaxies (Ho et al. 1997). This is the largest complete optically selected AGN sample in the local Universe analyzed so far. 23 of the Seyfert galaxies presented here have already been discussed in previous works (e.g. Cappi et al. 2006). For 5 of them newer XMM-Newton observations are available and are presented here. The current work should provide the most unbiased census of the AGN column density distribution at low redshifts and luminosities.
2 The sample
The Seyfert sample used in this study is derived from the Palomar optical spectroscopic survey of nearby galaxies (Ho, Fillipenko, & Sargent 1995). This survey has taken high quality spectra of 486 bright ( mag), northern () galaxies selected from the Revised Shapley-Ames Catalogue of Bright Galaxies (RSAC, Sandage & Tammann 1979) and produced a comprehensive and homogeneous catalogue of nearby Seyfert galaxies. The catalogue is 100% complete to mag and 80% complete to mag (Sandage, Temmann & Yahil 1981).
For the purpose of this work we consider all the Seyfert galaxies from the Palomar survey. Sources lying in-between the Seyfert-Liner or the Seyfert-Transient boundary have been excluded. Furthermore seven Seyfert galaxies (i.e. NGC1068, NGC1358, NGC1667, NGC2639, NGC3185, NGC4235, NGC5548), which have been included in the Palomar survey for various reasons (see Ho et al. 1995), even though they did not satisfy the survey selection criteria, are also excluded.
There are 40 Seyfert galaxies comprising the optical sample. 9 sources are classified as type-1 (contains types 1, 1.2, 1.5) and 31 as type-2 (contains types 1.8,1.9,2) Seyfert galaxies. However NGC4051, NGC4395 and NGC4639 which have been initially classified as Seyfert 1.2, 1.8 and 1 by Ho et al. (1997) has been re-classified as type-1.5, 1 and 1.5 respectively (see Cappi et al. 2006, Panessa et al. 2006, Baskin & Laor 2008). Moreover NGC185 which is classified as a Seyfert-2 may not contain an active nucleus since it presents line intensity ratios possibly produced by stellar processes (Ho & Ulvestad 2001).
The main characteristics of these sources, taken from Ho et al. (1997), are listed in Table 2. Some galaxies listed here present fainter than the formal limit of the Palomar survey. According to Ho et al. (1995) this discrepancy can be attributed to errors in the apparent magnitudes given in the RSAC.