4.2.3. High-Redshift Applications
Since the spectral signature in Figure 4 applies to the majority of star-forming galaxies, it can be used as a template to obtain redshifts of highly extincted galaxies with the next infrared observatory, NASA's SIRTF (Space InfraRed Telescope Facility). For instance, a galaxy at a redshift z = 3 with a flux density average of 0.5 mJy in the range 19-27µm and a total infrared luminosity comparable to Mkn 231 at ~ 3 x 1012 L would be detected by the SIRTF IRS (Infrared Spectrometer; Roellig et al. 1998) in roughly 1000 seconds of integration (D.W. Weedman, private communication).
In surveys with fixed spectral bands, the Aromatic Features will result in a unique K-correction as redshift takes them in and out of the bands. The detection probability would be enhanced or reduced for certain redshift intervals, causing ripples in source counts as a function of flux density; Xu et al. (1998) have modelled this effect for NASA's WIRE (Wide-Field InfraRed Explorer; Hacking et al. 1999) mission. However, this effect has already been manifested with ISO, in deep 15µm imaging data in the direction of galaxy clusters. These observations were aimed at detecting background objects boosted by gravitational lensing, thus allowing the survey to penetrate further in space and time. In the Barvainis et al. (1999) survey, one out of every five galaxies detected in the direction of each of Abell 2218 and Abell 2219 turns out to be at a reshift near 1, the others being at redshifts of 0.3 or less. A redshift of 1 places the main AFE clump squarely in the 15µm band, enhancing the probability of detection. Similar results from a more extensive survey are reported by Metcalfe et al. (1999), and by Fadda et al. (1999).
As the early Universe opens up for mid-infrared exploration, the properties of low metallicity star-forming galaxies become more relevant as a template for high-redshift galaxies. ISO has provided the data for constructing such templates, both from studies of low metallicity dwarf galaxies (Sauvage & Thuan 1999), and the study of nearby galaxy disks with significant metallicity gradients such as M 101 (Vigroux et al. 1999).