The CaFe project: Optical Fe II and near-infrared Ca II triplet emission in active galaxies: simulated EWs and the co-dependence of cloud size and metal content

Abstract

Aims. Modelling the low ionization lines (LILs) in active galactic nuclei (AGNs) still faces problems in explaining the observed equivalent widths (EWs) when realistic covering factors are used and the distance of the broad-line region (BLR) from the centre is assumed to be consistent with the reverberation mapping measurements. We re-emphasize this problem and suggest that the BLR “sees” a different continuum compared to a distant observer. This change in the continuum reflected in the change in the net bolometric luminosity from the AGN is then able to resolve this problem. Methods. We carefully examine the optical Fe ii and near-infrared Ca ii triplet (CaT) emission strengths, i.e. with respect to Hβ emission, using the photoionization code CLOUDY using a range of physical parameters, prominent among which are (a) the ionization parameter (U), (b) the local BLR cloud density (nH), (c) the metal content in the BLR cloud, and (d) the cloud column density. Using an incident continuum for I Zw 1 a prototypical Type-1 narrow-line Seyfert galaxy, our basic setup is able to recover the line ratios for the optical Fe ii (i.e. RFeII) and for the NIR CaT (i.e. RCaT) in agreement to the observed estimates. Although, the pairs of (U,nH) that reproduce the conforming line ratios, unfortunately, do not relate to agreeable line EWs. We thus propose a way to mitigate this issue the low ionization line (LIL) region of the BLR cloud doesn’t see the same continuum seen by a distant observer that is emanated from the accretion disk, rather it sees a filtered version of the original continuum which brings the radial sizes in agreement with the reverberation mapped estimates for the extension of the BLR. This is achieved by scaling the radial distance of the emitting regions from the central continuum source using the photoionization method in correspondence with the reverberation mapping estimates for I Zw 1. Taking inspiration from past studies, we suggest that this collimation of the incident continuum can be explained by the anisotropic emission from the accretion disk that modifies the spectral energy distribution (SED), such that the BLR receives a much cooler continuum with a reduced number of line-ionizing photons that allows mitigating the issue with the lines’ EWs. Results. (1) The assumption of the filtered continuum as the source of BLR irradiation recovers realistic EWs for the LIL species, such as the Hβ, Fe ii and CaT. However, our study finds that to account for the adequate RFeII (Fe ii/Hβ flux ratio) emission, the BLR needs to be selectively overabundant in iron. On the other hand, the RCaT (CaT/Hβ flux ratio) emission spans a broader range from solar to super-solar metallicities. In all these models the BLR cloud density is found to be consistent with our conclusions from prior works, i.e. nH ∼ 1012 cm−3 is required for the sufficient emission of Fe ii, as well as for CaT. (2) We extend our modelling to test and confirm the co-dependence between the metallicity and the cloud column density for these two ionic species (Fe ii and CaT), further allowing us to constrain the physical parameter space for the emission of these LILs. Adopting the estimates from line ratios that diagnose the metallicity in these gas-rich media that suggest super-solar values (& 5-10 Z ), we arrive at cloud columns that are of the order of 1024 cm−2. (3) Finally, we test the effect of inclusion of a micro-turbulent velocity within the BLR cloud which informs us that the Fe ii emission is positively affected by the inclusion of the microturbulence. An interesting result obtained here is the reduction in the value of the metallicity by up to a factor of 10 for the RFeII cases when the microturbulence is invoked, suggesting that microturbulence can act as an apparent metallicity controller for the Fe ii. On the contrary, the RCaT cases are rather unaffected by the effect of microturbulence.