Starburst galaxy contributions to extragalactic source counts

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Abstract
Using detailed models of galaxy spectra, luminosity functions defined at 60 µm and pure luminosity evolution, the number—flux relation for extragalactic sources is constructed for wavelengths ranging from the submillimetre, through the infrared, to the K, I and B photometric bands. Such accurate source counts are vital for the preparation of possible survey strategies for the next generation of ground-based instruments and space-borne observatories such as SCUBA, ISO and FIRST. The model consists of non-evolving spiral and elliptical components mixed with an evolving population of starburst galaxies, active galactic nuclei and a hyperluminous galaxy component. Pure luminosity evolution of the form L (z) = L (0)(1 + z)3.1 is used for all the evolving components. We show that, with this form of universal evolution, an excellent fit is found for both the source counts at 60 µm and the faint (S < 1 mJy) radio counts at 1.4 GHz where the starburst galaxies are dominant. A new calculation of the infrared (IR) background comes interestingly close to the most recent COBE limits at 500 µm. By extending our models to the near-IR we find that starburst galaxies only contribute to the K and I bands at the faintest magnitudes and that an open universe provides a better fit than closed world models, although this conclusion is highly dependent on the assumed form of the evolution. In order to build a more complete picture of the extragalactic populations at various wavelengths, we directly connect the 60-µm and optical luminosity functions for spiral galaxies. For normal spiral galaxies emitting in the IR via IR ‘cirrus’ (reradiation of starlight by interstellar dust), we confirm earlier findings that 〈LIR/LB〉 ≈ 0.3, supporting the idea that spiral galaxies are on average optically thin todust. For starburst galaxies we constrain the escape fraction of optical light to 5–10 per cent, and suggest that a strongly evolving starburst component could explain the faint-blue galaxy excess and the increasing number of emission-line objects found in redshift surveys to B ≈ 22–24, depending on the world model.

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