Introduction to SLUG
This is a guide for users of the SLUG software package. If you use SLUG in any published work, please cite the following papers:
What Does SLUG Do?
SLUG (Stochastically Lighting Up Galaxies) is a stellar population synthesis (SPS) code, meaning that, for a specified stellar initial mass function (IMF), star formation history (SFH), cluster mass function (CMF), cluster lifetime function (CLF), metallicity distribution [Fe/H], and (optionally) a distribution of extinctions (A_V), it predicts the spectra, photometry, nucleosynthetic yields, and feedback of both individual star clusters (treated as simple stellar populations) and galaxies (composite populations). In this regard, SLUG operates much like any other SPS code. The main difference is that SLUG regards the functions describing the stellar population as probability distributions, and the resulting stellar population as being the result of a draw from them. SLUG performs a Monte Carlo simulation to determine the PDF of the outputs produced by the stellar populations that are drawn from these distributions. The remainder of this section briefly describes the major conceptual pieces of a SLUG simulation. Please refer to the papers references above for a full description of the theoretical underpinning of SLUG.
Cluster Simulations and Galaxy Simulations
SLUG can simulate either a simple stellar population (i.e., a group of stars all born at one time) or a composite stellar population, consisting of stars born at a distribution of times. We refer to the former case as a “cluster” simulation, and the latter as a “galaxy” simulation, since one can be thought of as approximating the behavior of a single star cluster, and the other as approximating a whole galaxy. In a galaxy simulation, stars can either form as part of clusters, or as “field” stars that are not part of any cluster. Clusters can also disrupt over time, leaving their stars to contribute to the integrated light of the galaxy, but no longer appearing as a visible cluster.
Probability Distribution Functions and Monte Carlo Sampling
As mentioned above, SLUG regards many of the characteristics describing a stellar population – for example the initial mass function (IMF), the star formation history (SFH), and the metallicity ([Fe/H]) – as probability distribution functions. These PDFs can be described by a very wide range of possible functional forms; see Probability Distribution Functions for details on the exact functional forms allowed, and on how they can be specified in the code. In its default operating mode, when SLUG generates a stellar population it draws from these PDFs to produce a stellar population star-by-star, drawing stellar masses from the IMF (and, in a galaxy simulation, stellar ages from the PDF associated with the star formation rate), and similarly drawing other parameters such as the amount of extinction from their own PDFs. It then calculates the composite spectra and photometry of the resulting population by using one of several possible sets of stellar evolutionary tracks and stellar atmosphere models. Since each draw from the PDFs is random, the resulting stellar population will be different each time SLUG is run. By running many trials, one can build up a probability distribution function for the outputs of interest that arises from the stochastic nature of star formation.
However, users also have the option to disable some or all of the stochastic sampling in SLUG, for example by specifying that the IMF should be fully sampled, or that the SFH should be treated as a continuous function rather than a probability distribution. In this case, SLUG will determine the outputs of interest by numerically integrating over the various distributions, exactly as happens in other, non-stochastic SPS codes. Simulations can also be “partially stochastic”, where some parts of the distributions are handled stochastically and others deterministically – for example, it is possible to do stochastic sampling only for stars above a certain mass. See Probability Distribution Functions for details on how to control the stochasticity of the various distributions.
SLUG Physics
SLUG includes a number of physics modules that control different aspects of the stellar population synthesis. The main physics modules are:
Stellar Tracks: SLUG can use a variety of different sets of stellar evolutionary tracks, which determine the evolution of stars of different masses and metallicities.
Stellar Atmospheres and Spectral Synthesis: SLUG can use a variety of different sets of stellar atmosphere models, which determine the spectra and photometry of stars of different masses, metallicities, and evolutionary stages.
Photometry and Filters: SLUG can calculate the photometry of stars and stellar populations in a wide range of different filters and using a range of photometric systems.
Nebular Emission: SLUG can calculate the contribution of nebular emission to the spectra and photometry of stellar populations, using a variety of different models for the ionized gas.
Extinction: SLUG can calculate the effects of dust extinction on the spectra and photometry of stellar populations, using a variety of different extinction laws.
Yields: SLUG can calculate the nucleosynthetic yields of stellar populations, i.e., the masses of individual isotopes returned to the interstellar medium by stars, using a variety of different yield tables.
Feedback: SLUG can calculate feedback from stellar populations in the form of supernovae and stellar winds, using a variety of models to predict supernovae and wind power.