Integrated spectral energy distributions (SEDs) are our primary source of information about the properties of unresolved galaxies. Indeed, the different physical processes occurring in galaxies all leave their imprint on the global and detailed shape of the spectrum, each dominating at different wavelengths. Detailed analysis of the SED of a galaxy should therefore, in principle, allow us to fully understand the properties of that galaxy. SED fitting is thus the attempt to analyze a galaxy SED and to derive one or several physical properties simultaneously from fitting models to an observed SED. This is in contrast to searching a single feature that could constrain a single parameter (a prominent example would be the use of the H line to derive the star formation rate, SFR).
The aim of this review is to present the state of the art in the area of fitting the ultraviolet (UV) to far infrared (FIR) SEDs of galaxies. It grew from and presents a summary of a workshop held at the Lorentz center in Leiden in November 2008. As the field is extremely large we attempt here a somewhat novel approach to the process of writing a review. We consider this text as a starting point. The text will then be made available at www.sedfitting.org and we invite the community to sent us corrections and additions. We particularly hope that this will help in covering the work that we might have missed. We also made no attempt (yet) to cover the new results of the last year, in particular from Herschel.
Progress in many areas that affect SED fitting has been made recently. A major development in the last decade has been the advent of new observing facilities and large surveys at all wavelengths of the spectrum, enabling astronomers for the first time to observe the full SEDs of galaxies at wavelengths from the X-rays to the radio. The same surveys have also pushed the distance of the farthest galaxy whose SED is amenable to study to redshifts higher than 6. At the same time, tools and models have been created that aim to extract the complex information imprinted in the SEDs. Also, useful semi-analytic models of galaxy formation have appeared that provide realistic predictions for the properties of galaxies as they would be formed in the current cosmological standard model. Not only are astronomers exploiting the available data, but the next generation of surveys is now in the planning phase.
For this review we concentrate on observations from the ultraviolet to the far infrared, including both multi-band photometric and spectral data. We thus treat the light emitted by stars, either directly or processed by the gas and dust of the surrounding interstellar medium. At wavelengths outside the regime considered here, such as the X-ray and radio wavelengths, non-stellar processes (or at least those not directly associated with stellar light) such as shocks, accretion onto compact objects, etc. dominate. While many of these can be associated with the star formation history of a galaxy (e.g. supernova rate and recent star formation), these processes require a higher order of complexity generally not considered in most current models of galaxy spectra, and hence we do not discuss these wavelengths in the rest of this review. We also do not treat the contribution of active galactic nuclei to the SEDs of galaxies.
Our initial aim of both the workshop and this review was to set up a basic framework to answer the main question relating to SED fitting: considering the difficulties with the models, considering the limitations of the data and considering the fitting technique, what is the true uncertainty and limitations on the properties that can be determined from galaxy SED fitting?
This review is structured as follows: in Section 2 we review the basics of galaxy ultraviolet to infrared SED modelling, from galaxy formation to the production of and processing of the radiation from these galaxies. We especially mention some of the current issues and main uncertainties of the modelling of galaxy SEDs. In Section 3 we provide a short overview of the intricacies of assembling multi-wavelength SEDs. In Section 4 we present techniques and algorithms for SED fitting, and - most importantly - efforts at validating the results from the SED fitting procedure with independent data. Section 5 presents a review of photometric redshift determinations, a special case of a physical property derived from SED fitting, as it can be compared to and calibrated on independently determined data, spectroscopic redshifts. Finally, Section 6 showcases some recent results from application of the SED fitting procedures, where we hope to emphasize the variety of problems to which SED fitting can significantly contribute.