Photoionization models for extreme Lyα\alpha λ\lambda1216 and HeII λ\lambda1640 ratios in quasar halos, and PopIII vs AGN diagnostics

We explore mechanisms to produce extremely high Ly-alpha/HeII flux ratios, or to enhance the observed number of Ly-alpha photons per incident ionizing photon, in extended AGN-photoionized nebulae at high-redshift. Using photoionization models, we explore the impact of ionization parameter, gas metallicity, ionizing spectrum, electron energy distribution, and cloud viewing angle on the relative fluxes of Ly-alpha, HeII and other lines, and on the observed number of Ly-alpha photons per incident ionizing photon. We find that low ionization parameter, a relatively soft or filtered ionizing spectrum, low gas metallicity, kappa-distributed electron energies, or reflection of Ly-alpha photons by HI can all result in significantly enhanced Ly-alpha relative to other lines (>10%), with log Ly-alpha/HeII reaching values up to 4.6. In the cases of low gas metallicity, reflection by HI, or a hard or filtered ionizing spectrum, the observed number of Ly-alpha photons per incident ionizing photon is itself significantly enhanced above the nominal Case B value of 0.66 due to collisional excitation, reaching values up to 5.3 in our 'extreme case' model. At low gas metallicity (e.g. 0.1 x Solar), the production of Ly-alpha is predominantly via collisional excitation rather than recombination. In addition, we find that collisional excitation of Ly-alpha becomes more efficient if the ionizing continuum is pre-filtered through an optically thin screen of gas closer to the AGN. We also show that Ly-alpha / HeII ratios of the z~3.5 quasars studied by Borisova et al. (2016) are consistent with AGN-photoionization of gas with moderate to low metallicity and/or low ionization parameter, without requiring exotic ionization/excitation mechanisms such as strong line-transfer effects. We also present UV-optical diagnostic diagrams to distinguish between photoionization by Pop III stars and AGN photoionization.Comment: Accepted for publication in A&A. 14 pages, 9 figures. Abstract slightly shortened to meet arxiv character limi

Extended High-Ionization Nuclear Emission-Line Region in the Seyfert Galaxy NGC 4051

We present an optical spectroscopic analysis of the well-known Seyfert galaxy NGC 4051. The high-ionization nuclear emission-line region (HINER) traced by [Fe X]6374 is found to be spatially extended to a radius of 3a rcseconds (150 pc) west and southwest from the nucleus; NGC 4051 is the third example which has an extended HINER. The nuclear spectrum shows that the flux of [Fe X]6374 is stronger than that of [Fe VII] 6087 in our observation. This property cannot be interpreted in terms of a simple one-zone photoionization model. In order to understand what happens in the nuclear region in NGC 4051, we investigate the physical condition of the nuclear emission-line region in detail using new photoionization models in which the following three emission-line components are taken into account; (1) optically thick, ionization-bounded clouds; (2) optically thin, matter-bounded clouds; and (3) a contamination component which emits H$\alpha$ and H$\beta$ lines. Here the observed extended HINER is considered to be associated with the low-density, matter-bounded clouds. Candidates of the contamination component are either the broad-line region (BLR) or nuclear star forming regions or both. The complexity of the excitation condition found in NGC 4051 can be consistently understood if we take account of these contamination components.Comment: 16 pages, including figures. To Appear in the Astronomical Journal February 2000 Issu

Deep spectroscopy of the FUV-optical emission lines from a sample of radio galaxies at z~2.5: metallicity and ionization

We present long-slit NIR spectra, obtained using the ISAAC instrument at the Very Large Telescope, for nine radio galaxies at z~2.5. One-dimensional spectra have been extracted and cross calibrated with optical spectra from the literature to produce line spectra spanning a rest wavelength of ~1200-7000 A. We have also produced a composite of the rest-frame UV-optical line fluxes of powerful, z~2.5 radio galaxies. We have investigated the relative strengths of Ly-alpha, H-beta, H-alpha, HeII 1640 and HeII 4687, and we find that Av can vary significantly from object to object. In addition, we identify new line ratios to calculate electron temperature: [NeV] 1575/[NeV] 3426, [NeIV] 1602/[NeIV] 2423, OIII] 1663/[OIII] 5008 and [OII] 2471/[OII]3728. We model the emission line spectra and conclude they are best explained by AGN-photoionization with the ionization parameter U varying between objects. Single slab photoionization models are unable to reproduce the high- and the low-ionization lines simultaneously: this may be alleviated either by combining two or more single slab photoionization models with different U, or by using mixed-medium models such as those of Binette, Wilson & Storchi-Bergmann (1996). On the basis of NV/NIV] and NIV]/CIV we argue that shocks make a fractional contribution to the ionization of the EELR. We find that in the EELR of z~2 radio galaxies the N/H abundance ratio is close to its solar value. We conclude that N/H and metallicity do not vary by more than a factor of two in our sample. This is consistent with the idea that massive ellipticals are assembled very early in the history of the universe, and then evolve relatively passively up to the present day.Comment: Accepted for publication by MNRA

PKS2250-41 and the role of jet-cloud interactions in powerful radio galaxies

We have studied the effects of the interaction between the radio jet and the ambient gas in the powerful radio galaxy PKS2250-41 (z=0.31). Our results show that the gas has been accelerated, compressed, heated and collisionally ionized by the shock. This study helps us to understand the processes which determine the observed properties of many high redshift radio galaxies.Comment: 4 pages, 1 ps figure. Accepted to be published in Astrophysics & Space Science. Conference proceedings of the III National Astronomical Meeting of Spai

The Temperature of Extended Gas in Active Galaxies -- Evidence for Matter-Bounded Clouds

We report measurements of the electron temperature at about a dozen locations in the extended emission-line regions of five active (Seyfert and radio) galaxies. Temperatures (T_{[OIII]} and T_{[NII]}) have been determined from both the I([OIII]\lambda4363)/I([OIII]\lambda5007) and I([NII]\lambda5755)/I[NII]\lambda6583) ratios. T_{[OIII]} lies in the range (1.0 -- 1.7) \times 10^{4}K. We find a strong trend for T_{[OIII]} to be higher than T_{[NII]}, with the difference typically being \approx 5,000K. Because the critical density for collisional de-excitation of the ^{1}D{_2} level in NII is lower than that of the same level in OIII, the deviations of the measured intensity ratios from those expected for T_{[OIII]} = T_{[NII]} in the low density limit are unlikely to result from collisional de-excitation. The measured values of T_{[OIII]} and the differences between T_{[OIII]} and T_{[NII]} are very similar to those found in Galactic planetary nebulae. It is argued that the dominant form of energy input to the clouds is photoionization, but detailed modelling indicates that the temperature difference is too large to be accounted for in terms of photoionization of ionization-bounded clouds. We propose instead that both matter- and ionization-bounded clouds are present in the extended emission-line regions, with most of the [OIII] emission originating from a hot zone in the matter-bounded clouds and essentially all of the [NII] from the ionization-bounded clouds.Comment: 14 pages, Latex, 3 postscript figures Astrophysical Journal (Letters), in press for June 20 1997 issue, Volume 48

Physical Conditions in the Narrow-Line Region of M51

We have investigated the physical conditions in the narrow-line region (NLR) of M51 using long-slit spectra obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST) and 3.6 cm radio continuum observations obtained with the Very Large Array (VLA). Emission-line diagnostics were employed for nine NLR clouds, which extend 2.5" (102 pc) from the nucleus, to examine the electron density, temperature, and ionization state of the NLR gas. The emission-line ratios are consistent with those typically found in Seyfert nuclei and indicate that within the inner near-nuclear region (r ~< 1") the ionization decreases with increasing radius. Upper-limits to the [O III] electron temperature (T ~< 11,000 K) for the inner NLR clouds indicate that photoionization is the dominant ionization mechanism close to the nucleus. The emission-line fluxes for most of the NLR clouds can be reproduced reasonably well by simple photoionization models using a central power-law continuum source and supersolar nitrogen abundances. Shock+precursor models, however, provide a better fit to the observed fluxes of an NLR cloud ~2.5" south of the nucleus that is identified with the extra-nuclear cloud (XNC). The large [O III] electron temperature of this cloud (T = 24,000 K) further suggests the presence of shocks. This cloud is straddled by two radio knots and lies near the location where a weak radio jet, ~2.5" (102pc) in extent, connects the near-nuclear radio emission with a diffuse lobe structure spanning \~4" (163 pc). It is plausible that this cloud represents the location where the radio jet impinges on the disk ISM.Comment: 25 pages, 26 figures (9 color), 7 tables. Accepted for publication in the Astrophysical Journa