Infrared Spectroscopy of NGC 1068: Probing the Obscured Ionizing AGN Continuum

Abstract

The ISO-SWS 4 infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC 4151, which are described in a companion paper, are used together with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow line emitting gas in the active galactic nucleus. We present a new method to determine the best fitting photoionizing continuum and emission line cloud model from a heterogeneous set of emission line data.For NGC 4151, we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average, and which is consistent with the observed geometry of the optical Narrow Line Region (NLR). Our model consists of a clumpy, optically thick (ionization bounded) gas distribution, with a hydrogen gas density of ∼ 1000 cm −3 and a volume filling factor of 6.5 10 −4 .Our best fitting spectral energy distribution (SED) falls sharply beyond the Lyman limit and then rises sharply again towards 100 eV. In particular, it does not display a 'Big Blue Bump' signature of a hot accretion disk. We find that this SED, which best reproduces the NLR line emission on the 100-500 pc scale, does not have enough UV photons to produce the observed BLR recombination emission from the inner 1 pc. This suggests that the BLR is photoionized by the intrinsic continuum source, which does have a strong UV component (perhaps a Big Blue Bump), but that this UV component is absorbed by material located between the NLR and BLR. Our analysis suggests that the absorber consists of ∼ 5 10 19 cm −2 of neutral hydrogen. Such an absorber was independently detected by UV absorption lines (Kriss et al. 1992(Kriss et al. , 1995.Using our new method, we confirm our previous conclusion that a Big Blue Bump is present in the SED of the Seyfert 2 galaxy Circinus.4 Based on observations made with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, The Netherlands and the United Kingdom) and with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.-2 -

Resolved Spectroscopy of the Narrow‐Line Region in NGC 1068. III. Physical Conditions in the Emission‐Line Gas

“…Snow & Witt 1996) : C, 65% ; O, 50% ; Si, Mg, and Fe, 100%. Although Alexander et al (2000) report evidence of the presence of a big blue bump in the SED of the ionizing continuum, in Paper II we demonstrated that the emissionline spectrum from the hot spot could be produced by a simple power-law continuum. Here we assume the same ionizing continuum used in Paper II, speciÐcally a broken power law of the form as follows (also, see…”

“…Notably, if there is mix of optically thin and thick components among the emission-line gas, as is the case in the NE-blue quadrant, the lower value of the He II j4686/Hb ratio is evidence for a paucity of photons above the He II Lyman limit, which can be caused by an intervening absorber (Alexander et al 1999 ;. As mentioned in°1, Alexander et al (2000) found strong evidence that the narrow-line gas in NGC 1068 is irradiated by continuum which has been partially absorbed by a neutral hydrogen column of D6 ] 1019 cm~2. Similar to the situation for NGC 4151 , we modeled the absorber as a large column of highly ionized gas (an "" X-ray ÏÏ absorber) and a thinner, outer layer of low ionization gas (a "" UV ÏÏ absorber).…”

“…On the other hand, using HST Faint Object Spectrograph data, Kraemer, Ruiz, & Crenshaw (1998) examined conditions in the inner D150 pc of the NLR and determined that the observed emission lines could generally arise in gas photoionized by the hidden AGN (although there appeared to be some additional collisional excitation in the region nearest the radio jet). Recently, Alexander et al (2000) found evidence that the ionizing radiation incident upon the NLR was heavily absorbed by an intervening layer of gas (although it should be noted that the spectra these authors analyzed were from an area encompassing nearly the entire NLR, and thus any local e †ects were diluted). In summary, although it is likely that the NLR is photoionized, both the spectral energy distribution (SED) of the ionizing radiation incident on the emission-line clouds and the physical process(es) in which this radiation arises have yet to be Ðrmly established.…”

N‐Band Imaging of Seyfert Nuclei and the Mid‐Infrared–X‐Ray Correlation

“…More speciÐcally, it is overluminous in hard X-rays by about an order of magnitude. Possibly, the dust extinction toward the nucleus of NGC 6240 is much higher than assumed based on NIR colors (Tecza et al 2000 ;Alexander et al 2000). Indeed, NGC 6240 is the most distant object of our sample, and the observations sample larger spatial scales (approximately kiloparsecs).…”

Spectroscopic Cosmological Surveys in the Far-IR

“…In order to do so, we predicted the line intensities as a function of redshift (in the range z=0.1-5) for three local template objects: NGC1068 (Alexander et al, 2000;Spinoglio et al, 2005), a prototypical Seyfert 2 galaxy containing both an AGN and a starburst; NGC6240 (Lutz et al, 2003), a bright starburst with an obscured AGN and M82 (Forster Schreiber et al, 2001;Colbert et al, 1999), the prototypical starburst galaxy. We assumed that the line luminosities scale as the bolometric luminosity and adopted a luminosity evolution proportional to (z+1) 2 , consistent with recent Spitzer results at least up to redshift z=2 (Perez-Gonzalez et al, 2005 For simplicity 1 , we adopted an Einstein-De Sitter model Universe, with Ω Λ = 0 and Ω M = 1, H 0 =75 km s −1 Mpc −1 .…”