Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths

Abstract

In the context of observations of the rest-frame ultraviolet and optical emission from distant galaxies, we explore the emission-line properties of photoionization models of active and inactive galaxies. Our aim is to identify new line-ratio diagnostics to discriminate between gas photoionization by active galactic nuclei (AGN) and star formation. We use a standard photoionization code to compute the emission from AGN narrow-line regions and compare this with calculations of the nebular emission from star-forming galaxies achieved using the same code. We confirm the appropriateness of widely used optical spectral diagnostics of nuclear activity versus star formation and explore new diagnostics at ultraviolet wavelengths. We find that combinations of a collisionally excited metal line or line multiplet, such as C iv λλ1548, 1551,iii]λ1892 and [C iii]λ1907+C iii]λ1909, with the He ii λ1640 recombination line are individually good discriminants of the nature of the ionizing source. Diagrams involving at least 3 of these lines allow an even more stringent distinction between active and inactive galaxies, as well as valuable constraints on interstellar gas parameters and the shape of the ionizing radiation. Several line ratios involving Ne-based emission lines, such as [Ne iv]λ2424, [Ne iii]λ3343 and [Ne v]λ3426, are also good diagnostics of nuclear activity. Our results provide a comprehensive framework to identify the sources of photoionization and physical conditions of the ionized gas from the ultraviolet and optical nebular emission from galaxies. This will be particularly useful to interpret observations of high-redshift galaxies with future facilities, such as the James Webb Space Telescope and extremely large ground-based telescopes.

Ultraviolet/Optical Emission of the Ionized Gas in AGN: Diagnostics of the Ionizing Source and Gas Properties

“…The nebular emission of the AGN NLR is computed combining the AGN ionizing spectrum, described as a series of broken power laws (Equation 5 of Feltre et al, 2016), with the photoionization code CLOUDY (version c13.03, latest described in Ferland et al, 2013). The gas in the NLR is modeled with clouds of single type using the approach described in Feltre et al (2016) 1 .…”

“…The gas in the NLR is modeled with clouds of single type using the approach described in Feltre et al (2016) 1 . The models are parametrized in terms of (i) the ultraviolet spectral index, α, of the incident radiation field, (ii) the ionization parameter, (iii) the hydrogen gas density of the clouds, (iv) the gas metallicity, and (v) the dust-to-heavy element mass ratio.…”

“…The models are parametrized in terms of (i) the ultraviolet spectral index, α, of the incident radiation field, (ii) the ionization parameter, (iii) the hydrogen gas density of the clouds, (iv) the gas metallicity, and (v) the dust-to-heavy element mass ratio. A more detailed explanation of the physical parameters is provided in Feltre et al (2016). Note that we have improved the original model grid by adding two new adjustable parameters, namely the inner radius of the NLR and the internal microturbulence velocity of the gas cloud.…”

“…Hα and Hβ) are commonly used to distinguish between stellar and nuclear activity (e.g., Baldwin et al, 1981;Veilleux and Osterbrock, 1987). In addition to optical, we explored new diagnostics at ultraviolet wavelengths for three reasons: (i) future facilities will provide high quality rest-ultraviolet spectra of the most distant sources, (ii) models are usually calibrated on optical observations of the local Universe and this might not always be appropriated to study the emission from star-formation and interstellar gas at high redshift, and (iii) standard optical diagnostic diagrams might fail to distinguish between stellar and AGN activity at higher redshift (e.g., in the case of low metallicity Groves et al, 2006;Coil et al, 2015;Feltre et al, 2016;Hirschmann et al, 2017). Diagrams involving combinations of a collisionally excited metal line or line multiplet, such as C IV λλ1548, 1551,…”

“…, with the He II λ1640 recombination line have been found to allow a good distinction of the nature of the ionizing source as well as valuable constraints on interstellar gas parameters and the shape of the ionizing radiation (Feltre et al, 2016). Figure 1 shows the predictions from photoionization models of both AGN and star-forming galaxies, for different values of metallicity and ionization parameter in two diagnostic diagrams, namely C III]λ1909/He II λ1640 vs. C IV λ1550/He II λ1640 (left panel) and C IV λ1550/C III]λ1909 vs. C IV λ1550/He II λ1640 (right panel).…”