[OIII] λ5007, near IR and X-ray properties of Seyfert 2s: A new means of estimating the torus induced extinction?

Vistas in Astronomy Vol. 40. No. 1, pp. 221-226, 1996 Copyright @ 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0083-6656/96 $32.00 + 0.00 0083-6656(95)00130-l

[OIII]h5007,NEARIRANDX-RAY PROPERTIESOFSEYFERT2s:ANEWMEANS OFESTIMATINGTHETORUSINDUCED EXTINCTION? ALMUDENA

ALONSO-HERRERO, I MARTIN JAR1 KOTILAINEN 3,*

WARD 2 and

1 Departamento

de Astrofisica, Universidad Complutense de Madrid, 28040 Madrid, Spain 2 Astrophysics, Nuclear Physics Building, Keble Road, Oxford OX1 3RH, England 3 European Southern Observatory, Karl Schwarzschild-Str. 2, D-8046 Garching, Germany

Abstract- It is known that hard X-rays emission is a good indicator of the intrinsic luminosity of Seyfert 2s in those cases where they are transmitted through the torus, rather than seen purely by reflection as in NGC 1068. It has recently been demonstrated that the [OIII] h5007 line emission and hard X-rays emission in Seyfert 2s follow the same correlation as found for Seyfert Is, and hence that [0111] flux can also be used to infer the intrinsic power. Recent models predict that the inner regions of the torus have hot dust with a black body peak around 3.5 pm (L-band). We use comparisons of the hard X-ray, [0111] and L-band properties of PG quasars, Seyfert 1 and 2 galaxies, to infer the amount of reddening in the latter. Our values for the extinction are compared with those inferred from the hard X-ray column densities. Copyright 01996 Published by Elsevier Science Ltd.

1. INTRODUCTION According to the Unified Model for Active Galactic Nuclei (AGN), Seyfert 2 nuclei are fundamentally identical to Seyfert 1 nuclei, with the observed differences due to the degree of obscuration and our viewing angle of the active nuclei, and not to intrinsic, physical * Also at: Tuorla Observatory, University of Turku, FIN-21500 Piikkio, Finland 221

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differences. In the dusty torus model only the highly penetrating photons (hard X-rays) can pass through the torus and escape isotropically, at least for column densities NH 5 10z5cm-*. Therefore, for most Seyfert 2s detected in hard X-rays (2-10 keV), this is a good indicator of their intrinsic power. A recent compilation of emission properties of a sample of Seyfert type 1 and 2 galaxies (Mulchaey et al., 1994) has found that the distribution functions of the [0111] emission line, hard X-ray and far IR continua are similar for both types of Seyfert, consistent with these properties being isotropic, i.e. not dependent on orientation effects. Theoretical models of the dusty obscuring tori (Pier and Krolik, 1993) predict that dust located in the inner regions with a higher temperature (T,m = 1300 K) than the main part of the torus, will dominate the near IR emission in type 1 AGNs, but might be obscured by the outer parts in narrow line objects. Hence, this additional source of IR radiation would be directly visible in the L-band (3.5 pm) in unobscured objects, but attenuated to some degree, depending on the extinction to the nucleus of the galaxy, in obscured objects (Seyfert 2s). In this paper we compare the hard X-ray, [0111] (both believed to be isotropic properties) and near IR data to search for evidence of the dusty torus. If the model predictions are correct for Seyfert 2s, only a reduced fraction of the near IR emission is able to penetrate the torus. Therefore, we can infer the extinction in obscured objects by comparison of their properties with those of unobscured objects. The L-band spectral region was selected for several reasons. It is less affected by stellar contribution from the host galaxy in Seyfert 1s and 2s than the shorter near-IR wavelengths [i.e. J, H and K bands, see Alonso-Herrero et al. (1996), hereafter AWK]. Furthermore, the L-band is sensitive to the emission from hot dust and it can still be detected through large columns (AL = 0.04 x Av). This method has been applied previously to individual objects, e.g. IC 5063 (Simpson et al., 1994) but this is the first attempt to apply it to a sample of Seyfert 2s. For its application to radio galaxies, see the contribution by C. Simpson in this issue.

2. DATA FOR THE SAMPLES The Seyfert 2 sample is heterogeneous, consisting of galaxies for which we could obtain the fluxes of L-band, hard X-rays and [OIII]. Specifically for about half the Seyfert 2 sample the L-band (3.5 pm) fluxes were obtained from images using a projected 3 arcsec aperture (see AWK). In addition, for those galaxies without images we have chosen the smallest aperture fluxes available in the literature, ranging from 3 to 8 arcsec (see Glass and Moorwood, 1985; Kotilainen et al., 1992; and Ward et al., 1987). All the [0111] fluxes are given in Mulchaey et al. (1994) and references therein. The hard X-ray data are from Mulchaey et al. (1994) Awaki (1992) and GINGA data re-analysed by Smith and Done (1996) who give both the observed fluxes and those corrected for the derived column densities, see their contribution in this issue. For the Seyfert 1 sample the small aperture L-band fluxes are from Kotilainen et al. (1992), Glass and Moorwood (1985) Ward et al. (1987) and McAlary et al. (1983). The [OIIII are from Mulchaey et al. (1994) and the hard X-ray fluxes are from the latter paper and Nandra and Pounds (1994). The PG quasars are redshift selected (z < 0.15). The [OIII] fluxes are from Miller et al. (1992) and 5.5 arcsec L’-band (3.7 pm) fluxes are from Neugebauer et al. (1987) and Sanders et al. (1989) and the hard X-ray fluxes are from Williams et al. (1992) and Comastri et al. (1992).

[OZZZ] h5007, Near ZR and X-ray Properties of Seyfert 2s

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4

log

f([OIII])

(erg

crnW2 s-l)

Fig. I. [0111] vs L-band flux correlation. *, Represent PC quasars; 0, Seyfert 1s; 0, Seyfert 2s; arrows are upper limits for L fluxes (large apertures). The straight line is the best-fit for the unobscured objects.

3. RESULTS AND DISCUSSION Our analysis relies on the assumption that the [0111] line and hard X-ray fluxes are isotropic properties of AGN. Support for this hypothesis comes from the fact that the hard X-ray and the [OIII] luminosities are similar in both types of Seyferts (Mulchaey et al., 1994), indicating that the X-rays arise from the compact nucleus and that the [0111] emission comes predominantly from regions above and bellow the torus. In Fig. 1 we present a [0111] 5007 versus L-band flux diagram (with a correlation factor r = 0.86). It can be seen from this figure that for Seyfert 1s and PG quasars there is a correlation, whereas in general for Seyfert 2s there is not. Those Seyfert 2s which do lie close to the correlation line are known to have low extinctions, i.e. MCG-5-23-16 (Blanc0 et af., 1990). Figure 2 shows a plot of hard X-ray versus L-band fluxes (correlation factor r = 0.90). The similarity with Fig. I follows from the good correlation between [OIII] and hard X-rays. In principle, both [0111] versus L-band and hard X-rays versus L-band correlations can be used to estimate the reddening. If the near IR radiation arises from the inner regions of the torus, then this property would be inclination dependent. In Seyfert 1s and quasars this component would be viewed directly, whilst in Seyfert 2s it would be attenuated. Since we are assuming that the [0111] fluxes are not affected by orientation effects, it is then possible to compute the intrinsic near

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I

-10

-12 log

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(erg

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-2

S-l)

Fig. 2. Hard X-rays vs L-band correlation in fluxes, symbols as in Fig. 1

IR fluxes for obscured objects by using the [0111] 5007 versus L-band correlation found for unobscured objects and hence, estimate the visual extinction. The same method can be applied using the hard X-ray versus L-band data. The results obtained for Seyfert 2s are presented in Table 1, where column (1) gives the galaxy name. The upper part is the AWK sample for which we have calculated small aperture L-band fluxes. Those listed in the lower part of the table are Seyfert 2s for which the L-band data were obtained through large apertures. Column (2) gives the AV extinction estimated using the [OIII] 5007 versus L-band correlation, column (3) lists the AV estimated from the X-ray versus L-band correlation and finally column (4) gives the extinction obtained directly from the column densities derived from the X-ray spectra. A standard dust-to-gas ratio, AV = 5 x lo-**NH, has been used. The two numbers in columns (2) and (3) are the extinctions obtained from using the total L fluxes and stellar subtracted L fluxes, respectively (AWK). As can be seen from Table 1, in general we find values of the visual extinction smaller than those obtained from the X-ray column densities, but there is some correlation between columns (2) and (4). It should be noted that IR photometry measures the extinction due to dust, while columns from X-ray spectra measure the attenuation due to an atomic metal K edge. These different values will not necessarily agree, since they depend on the dust-to-gas ratio and metallicity of the atomic gas. A systematic effect that could reduce the derived reddening from these methods would be the presence in Seyfert 2s of a nuclear starburst component emitting at L, in which case the listed values should be considered as lower limits. Goodrich et al. (1994) obtained IR spectroscopy of several Seyfert 2s and estimated the visual extinctions

[OIII] h.5007, Near IR and X-ray Properties of Seyfert 2s Fable I. Visual extinctions Galaxy

A,([OIII]-L)

NGC NGC NGC NGC Mkn Mkn Mkn

1386 2110 4388 5728 3 78 273

Mkn Mkn Mkn MCG

348 533 573 -5-23-16

NGC NGC

526A 2992

NGC 4507 IC 5063 NGC 4945 NGC 7172 NGC 7314

32153 9126 11113 56172 62169 65179 26134 20125 17121 66178

for the Seyfert &(X-ray-L) No 35154 32134 No 57164 No No 17122

>31 34 No No 22

2s AV(NH)

No I I .4+‘,’ -1.0 104.5’;;:~

No I8124

43No$ No No 58’;’ <7.9 No 9.3’1.;

>46 24 >38 18 29

x.3:;:; 7.5:;:; 289:;; 154.5:;: > 350.0 54.5::‘: 2.9’;:;

2

225

Note: “No” indicates no data for the galaxy and “-” indicates that an estimation of the extinction was not feasible. When two numbers are given in columns 2 and 3, the first number is the estimated extinction using the total 3” L-band flux and the second number is the extinction obtained using the stellar subtrdted 3” L-band flux.

from the hydrogen line ratios. They also found that these were lower than those based on X-ray columns. We have four galaxies in common with the Goodrich et al. (1994) study, i.e. NGC 4388, Mkn 3, MCG-5-23-16 and NGC 2992. In all the cases our extinctions are larger than those found by them from the hydrogen recombination ratios. There is a general tendency for those objects with a large estimated IR Av and which have been observed in hard X-rays either to be undetected or to show very large column densities, i.e. Mkn 3, Mkn 78, Mkn 573.

4. SUMMARY We have estimated the visual extinctions for a sample of Seyfert 2s by using correlations between [0111] line emission, hard X-ray and near IR (3.5 pm) fluxes. We find a tendency for Seyfert 2 galaxies to have lower IR extinctions than those derived from hard X-ray columns, although they are correlated. Reddenings obtained using small aperture L-band data are in the range Av = 20-80 mag.

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