With JWST it will be possible to extend the near- and mid-IR metallicity diagnostics used for nearby galaxies to objects at z ~ 1-2, with important implications for our understading of the evolution of the stellar and interstellar-medium (ISM) metallicity in the Universe.
The near-IR spectroscopic diagnostics discussed in the previous section are clearly detected in the spectra of nearby galaxies (e.g. Boisson et al. 2004; Dasyra et al. 2006), and have been widely used to determine stellar metallicities, especially in dusty systems. However, atmospheric absorption bands and lack of sensitivity have limited the use of these diagnostic tools to only up to z ~ 0.1. Instead, with NIRSpec it will be possible to observe the several H-band metal absorption features (Figure 23.2) up to z ~ 2. For a typical ULIRG at z ~ 2, m(4.5 |im)AB ~ 20.7 (Egami et al. 2004; Frayer et al. 2004), NIRSpec will detect the continuum with a signal-to-noise ratio of ~20 with an integration time of about three hours at a spectral resolution of R = 2,700. The stellar features in the J-band, which is rich in Fe lines, will be observable by NIRSpec up to z ~ 3.
The mid-IR band of star-forming galaxies displays a number of emission lines that can be used to constrain the gas-phase metallicity. In particular, the strength of the fine-structure transitions of Ne+ and Ne2+ at 12.8 |im and 15.6 |im, relative to the hydrogen recombination lines (Bra and Brp), is a good indicator of the Ne
Figure 23.3. Dust absorption due to C—H stretching vibration in hydrocarbon dust grains observed in the ULIRG galaxy IRAS 19254-7245 at z = 0.06. From Risaliti et al. (2003). Individual dips due to the —CH3 and —CH2 vibrational transitions in hydrocarbon grains are marked.
abundance, which in turn is a good tracer of heavy elements produced by Type-II supernovae (Willner & Nelson 2002). In contrast to the more commonly used optical gaseous diagnostic, these mid-IR tracers have the advantage of being much less affected by dust extinction and also less sensitive to effects of temperature gradients within the H ii regions. Spitzer-IRS has detected these lines in several objects, but still within the local Universe or in exceptionally luminous sources. MIRI will allow us to extend these metallicity mid-IR diagnostic tools easily up to z ~ 1. It will then be most important to compare the metallicity evolution inferred from MIRI observations with the results obtained from optical surveys, which are certainly biased against dusty galaxies and have provided some contradictory results (e.g. Savaglio et al. 2005; Liang et al. 2006).
The near-to-mid-IR spectral range also includes several dusty features that can be used to constrain the chemical composition of the ISM solid phase in galaxies. In particular, the 3-25- |vm spectrum of galaxies is rich in polyaromatic hydrocarbon (PAH) emission features (Genzel & Cesarsky 2000; Brandl et al. 2004; Smith et al. 2004), and it often presents absorption (but also emission) by silicates at 10-20 |vm (Spoon et al. 2004; Shi et al. 2006; Bressan et al. 2006) and absorption by ice-covered grains at 3.4 |vm and by hydrocarbon grains at 3.3 |vm (Imanishi et al. 2006; Risaliti et al. 2003). As an example, Figure 23.3 shows the hydrocarbon grains absorption observed in an infrared luminous galaxy at z = 0.06, together with the identification of the individual chemical dust components responsible for the individual absorption features. With NIRSpec and MIRI it will be possible to extend to high redshift such studies on the dust composition. Since the dusty absorption and emission features are generally broad, a moderate spectral resolution (R ~ 100) is enough for their detection and identification. For instance, MIRI will be able to detect the 3.3^m dusty feature with a signal-to-noise ratio of ~20 through R ~ 100 spectroscopy in a luminous infrared galaxy at z ~ 2 with three hours of integration. Therefore, JWST will offer the unique opportunity to investigate the evolution of the chemical composition of dust with redshift, which will provide important information for understanding the origin of dust through the cosmic ages.
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