How to Build a Synthetic Galaxy Sample Clauses

How to Build a Synthetic Galaxy. To derive the physical properties of an observed galaxy, the shape of its observed SED, found by measuring its flux in several filters, is compared to a template, usually a synthetically constructed spectrum. Such a synthetic spectrum has several key ingredients described in the paragraphs below and shown in Fig. 2.1. The process of creating SED models begins by modelling a coeval stellar population (i.e. one which formed at the same time) which evolves according to a set of specified stellar evolutionary tracks. This creates a simple stellar population (SSP), which can then be combined with an assumed star formation history to produce a composite stellar population (CSP). Modifying the CSP by incorporating the effects of dust attenuation and nebular emission thus creates the final synthetic SED at a given age and for specific input parameters. The Initial Mass Function The initial mass function (IMF) provides a measure of the initial distribution of stellar masses along the main-sequence. It has been studied extensively for decades, with the canonical Salpeter [Salpeter, 1955] IMF taking the form dN/dM ∝ M −2.35 for stellar masses of up to ∼ 100 M⊙. Other IMF models, such as the Xxxxxx [2001] and Xxxxxxxx [2003] models, have a shallower distribution at sub-Solar masses, hence producing fewer low-mass stars than the Salpeter IMF. By indicating how many stars of a given mass are present within a galaxy, the IMF determines the overall normalisation of the mass-to-light ratio of the synthetic stellar population. Knowing the IMF of a stellar population also makes it possible to calculate the luminosity evolution of the population, assuming that star- formation has ceased. When modelling the SED of a composite stellar population, the IMF affects the integrated light of the system. While, for a coeval population, the integrated light is dominated by stars at the turnoff mass, a composite population contains a range of turnoff masses which all contribute to the bolometric luminosity of the galaxy. While low-mass stars do not make a significant contribution to a galaxy’s luminosity, they dominate the stellar mass within it. The Interstellar Medium Emission and absorption due to atoms and molecules in the ISM needs to be taken into account. There are two components to nebular emission, the continuum emission due to free-free, free-bound, or 2-photon emission, and recombination lines. The effect of nebular emission on the spectral energy distri- bution of a system i...
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