Periodic Reporting for period 1 - EHMVSIMF (INVESTIGATING EVIDENCE OF HIGH-MASS VARIATIONS OF THE STELLAR INITIAL MASS FUNCTION) Reporting period: 2017-01-23 to 2019-01-22 Summary of the context and overall objectives of the project Star clusters comprise thousands, even millions, of stars, some like the sun, some are giants, and some are dwarfs. Many millions of star clusters together form a galaxy, and the Universe contains billions of galaxies. Therefore star clusters, in a way, are a fundamental constituent of a galaxy. One of the defining questions in the study of galaxy evolution is the assumption that stars formed within star clusters with a preferred mass distribution independent of time and environment. This preferred birth mass distribution of stars, called the stellar initial mass function (IMF), is intimately related to the star formation processes within a galaxy, underlying much of its evolution; therefore it is essential to confirm the true nature of the IMF to advance the field. While significant advances in understanding mainly the low-mass stellar IMF variations have been made in the past decade, relatively a little attention has been paid to the high-mass IMF. We aimed to utilise innovative techniques, the latest high-resolution observations, stellar population and photoionisation codes and simulations to study starbursting regions within galaxies that are hosts to high gas pressure and turbulent environments. These types of environments are thought to be responsible for determining the form of the high-mass IMF, so studying them will allow us to disentangle many degeneracies that generally affect star formation studies. The overall objectives include developing robust techniques to tightly constrain star formation properties and histories of starbursting regions to characterise the evolution and progression of star formation within galaxies, and comparisons with simulated observations. The data for the project was drawn from the Antennae observing programme of the MUSE (Multi Unit Spectroscopic Explorer) consortium as well as from other European and Australasian-led surveys such as GAMA (Galaxy And Mass Assembly) and SAMI (Sydney-AAO Multi-Object IFU). One of the primary outcomes of the project is that we have developed a comprehensive and self-consistent suit of high-resolution models specifically tailored to fit spectroscopic observations of starbursting regions. The model fitting process is designed to use a wide range of information in a given spectrum of a starbursting region to simultaneously converge on the best-fitting physical parameters, such as star formation history, the metallicity of gas and stars, the age of the dominant young and old stellar populations, and electron densities and temperatures. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far One of the results is the full exploitation of stellar and nebular features of spectra to constrain characteristic properties of individual star-forming regions (e.g. gas temperature, electron density, the metallicity of stars and gas, IMF etc.). In contrast to what is typically done, which is that these properties are derived individually by using a few spectral features, we took a modelling approach where the full spectrum is fitted with a comprehensive, high-resolution suite of self-consistently modelled stellar and nebular templates. The fitting yields probability density functions for different characteristic properties as well as its star formation history. Figure 1 describes the functionality of the model library and model-fitting process. Another focus of the project was investigating the `spatial' correlations between dust and other galaxy properties (e.g. star formation rate or SFR, SFR surface density, SFR per unit stellar mass, stellar mass, gas-phase metallicity, morphology etc.) using spatial spectroscopic data drawn from the SAMI survey of ~3000 galaxies. While dust is a crucial component that both affects all observed properties galaxies and needs to be incorporated into models of galaxy formation and evolution, there is no clear consensus on whether SFR or stellar mass correlates the strongest with dust reddening in star-forming regions, and to-date spatial datasets have not been used to study this aspect. The results of this part of the project demonstrate that, on spatial scales, dust obscuration in galaxies is strongly correlated with the local star formation rate surface density with, quite surprisingly, no correlation observed with local stellar mass surface density. This suite of models and the model-fitting techniques developed during the project are currently state-of-the-art and present a novel way of extracting characteristic physical properties from the spectra of highly star-forming regions within galaxies. The simultaneous estimation of different physical properties of a star-forming region allows this method to provide much-needed insights into the degeneracies between star formation history, dust, metallicity, and stellar initial mass function. Therefore there are many prospects of exploiting them through application to on-going and up-coming large galaxy surveys (e.g. MOONS, 4-MOST and MUSE surveys). These models can further be exploited through radiative transfer models that are widely used within the theoretical community to process galaxy simulation data. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) One of the expected impacts of the project results is future joint research projects and collaborations. For example, the scientific community has shown a lot of interest in adopting the model libraries developed during the project - there are already some plans in place for starting new projects. The models and the data products that resulted from the project (e.g. catalogues of physical properties of star-forming regions in the Antennae galaxy, gas fractions and star formation histories etc.) will be made available to the worldwide scientific community in due course. Self-consistent modelling of young stellar populations in starbursting HII regions
