The Near-Infrared Coronal Line Spectrum of 54 Nearby Active Galactic Nuclei

Abstract

The relationship between the emission of coronal lines (CLs) and nuclear activity in 36 Type 1 and 18 Type 2 active galactic nuclei (AGNs) is analyzed, for the first time, based on near-infrared (0.8-2.4 μm) spectra. The eight CLs studied, of Si, S, Fe, Al, and Ca elements and corresponding to ionization potentials (IPs) in the range 125-450 eV, are detected (3σ ) . We found that the non-detection is largely associated with either loss of spatial resolution or increasing object distance: CLs are essentially nuclear and easily lose contrast in the continuum stellar light for nearby sources or get diluted by the strong AGN continuum as the redshift increases. Yet, there are AGNs where the lack of coronal emission, i.e., lines with IP 100 eV, may be genuine. The absence of these lines reflects a non-standard AGN ionizing continuum, namely, a very hard spectrum lacking photons below a few Kev. The analysis of the line profiles points out a trend of increasing FWHM with increasing IPs up to energies around 300 eV, where a maximum in the FWHM is reached. For higher IP lines, the FWHM remains nearly constant or decreases with increasing IPs. We ascribe this effect to an increasing density environment as we approach the innermost regions of these AGNs, where densities above the critical density of the CLs with IPs larger than 300 eV are reached. This sets a strict range limit for the density in the boundary region between the narrow and the broad region of 10 8 -10 9 cm −3 . A relationship between the luminosity of the CLs and that of the soft and hard X-ray emission and the soft X-ray photon index is observed: the coronal emission becomes stronger with both increasing X-ray emission (soft and hard) and steeper X-ray photon index, i.e., softer X-ray spectra. Thus, photoionization appears as the dominant excitation mechanism. These trends hold when considering Type 1 sources only; they get weaker or vanish when including Type 2 sources, very likely because the X-ray emission measured in the latter is not the intrinsic ionizing continuum.

The complex, dusty narrow-line region of NGC 4388: gas–jet interactions, outflows and extinction revealed by near-IR spectroscopy

“…For example, NGC 1068 shows sulfur coronal line ratios rather similar to those of NGC 4388 in its nuclear and extended NLR (Martins et al 2010). Moreover, the observed sulfur ratios in NGC 4388 are also well within the distribution of values for that ratio presented by Rodríguez-Ardila et al (2011) in their study of coronal lines in 54 AGNs. Thus, sulfur coronal line emission posses a challenge to photoionization by the central source.…”

Star formation inside a galactic outflow

“…In particular, none of the coronal lines observable in good regions of atmospheric transmission ([SVIII]0.9913µm, [FeXIII]1.0747µm, [SIX]1.252µm, [AlIX]2.045µm) is detected. Although coronal lines are not always seen in AGNs, most of the powerful AGN do show evidence for coronal lines in the near-IR (which is the wavelength range hosting many of the most intense coronal lines), especially if observed with appropriate signal-to-noise and for those AGNs not suffering dilution from star formation in the host galaxy 35,39,40,41,42,43,44 . For instance, in the prototypical Sy2 galaxy Ciricinus, [AlIX]2.045µm is three times stronger than then nearby HeI2.058µm line 42 , while in IRAS2318-59 [AlIX]2.045µm is completely undetected (despite being in a region of good atmospheric transmission and unaffected by OH sky lines), implying that it is at least 20 times fainter than the HeI2.058µm line (Fig.1b).…”

VLT-SINFONI integral field spectroscopy of low-zluminous and ultraluminous infrared galaxies

“…However, the [CaVIII] line is too faint (typically ×4 fainter than [SiVI], Rodríguez-Ardila et al 2011) and to close to CO (3−1) to be measured easily. Given the high ionisation potential of 167 eV for [SiVI] and 128 eV for [CaVIII], the outskirts of the broadline region and extended narrow-line regions have been proposed as possible locations for the formation of these lines in AGNs.…”

“…Given the high ionisation potential of 167 eV for [SiVI] and 128 eV for [CaVIII], the outskirts of the broadline region and extended narrow-line regions have been proposed as possible locations for the formation of these lines in AGNs. Although which mechanism is responsible for the emission remains unclear, there are two main processes proposed: photoionisation due to the central source and shocks due to high-velocity clouds and the NLR gas (see Rodríguez-Ardila et al 2011, and references therein).…”