Fitting the integrated Spectral Energy Distributions of Galaxies

Fitting the spectral energy distributions (SEDs) of galaxies is an almost universally used technique that has matured significantly in the last decade. Model predictions and fitting procedures have improved significantly over this time, attempting to keep up with the vastly increased volume and quality of available data. We review here the field of SED fitting, describing the modelling of ultraviolet to infrared galaxy SEDs, the creation of multiwavelength data sets, and the methods used to fit model SEDs to observed galaxy data sets. We touch upon the achievements and challenges in the major ingredients of SED fitting, with a special emphasis on describing the interplay between the quality of the available data, the quality of the available models, and the best fitting technique to use in order to obtain a realistic measurement as well as realistic uncertainties. We conclude that SED fitting can be used effectively to derive a range of physical properties of galaxies, such as redshift, stellar masses, star formation rates, dust masses, and metallicities, with care taken not to over-interpret the available data. Yet there still exist many issues such as estimating the age of the oldest stars in a galaxy, finer details ofdust properties and dust-star geometry, and the influences of poorly understood, luminous stellar types and phases. The challenge for the coming years will be to improve both the models and the observational data sets to resolve these uncertainties. The present review will be made available on an interactive, moderated web page (sedfitting.org), where the community can access and change the text. The intention is to expand the text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics & Space Scienc

Observed and Model-Calculated NO2/NO Ratios in Tropospheric Air Sampled During the NASA GTE/CITE II Field Study

Data gathered during the NASA GTE/CITE 2 airborne field campaign were analyzed and compared with diagnostically derived parameters to study the NOx photostationary state in the troposphere and the processes that control this photostationary state. Our analysis focussed on two sets of NO2/NO ratios derived from the data; these were based on overlapping NO and NO2 measurements made by two independent techniques; i.e., a chemiluminescent technique and a technique based on two-photon, laser-induced-fluorescence. While for any given 6- to 10-min time interval the two observed NO2/NO ratios often exhibited significant discrepancies, these discrepancies appeared to be mostly random rather than systematic, and as a result, the average difference for all time intervals with overlapping NOx measurements was only 12%. One notable exception, however, was the block of data gathered during the last three CITE 2 missions; during these three missions the ratios observed by the chemiluminescent technique were systematically larger than those observed by the laser-induced fluorescence technique by a factor of 1.6. When the data from these three missions were omitted from the analysis, the averages of the observed ratios agreed to within 1%. In contrast to a number of previous studies, the ratios predicted from photochemical model calculations were found to be reasonably consistent with the observed ratios, although on average they tended to fall about 20 – 25% below the observations. This agreement between observations and theory provides strong evidence in support of the importance of peroxy radicals in the fast photochemical cycling of NOx (and the concomitant photochemical production of O3) in both the marine and continental troposphere