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Stellar Populations and Dynamics: a comprehensive IMF analysis

Periodic Reporting for period 2 - SPanD (Stellar Populations and Dynamics: a comprehensive IMF analysis)

Reporting period: 2019-05-01 to 2020-04-30

The stellar initial mass function (IMF) dictates the mass distribution of stars at birth. Despite of this simple definition, the IMF is the Rosetta Stone of astronomers, as it allows us to understand the formation and evolution of galaxies through their spectro-photoemtric properties. The total mass of galaxies and black holes, the number of new stars that are formed, their ages and chemical composition, all these fundamental quantities ultimately depend on the IMF.

For more than fifty years the IMF was assumed to be universal which greatly simplified the analysis of astronomical measurements. However, over the last decade this traditional scenario has been challenged by growing observational evidence showing that the relative number of low-mass stars, i.e. the slope of the IMF, is enhanced in the center of the most massive galaxies. If the IMF is not universal, how is it varying? What is the physical origin explaining the enhancement of low-mass stars in the central regions of the most massive galaxies?

The development of the SPanD project has lead to two main results regarding the IMF variations. First, and contrary to the wide-spread belief, metallicity is not the best predictor for local IMF variations as significant departures emerge in the two-dimensional structure of galaxies. Second, the comparison between stellar populations and dynamical properties has revealed that, at least for some galaxies, stellar population exhibiting the most extreme IMF variations are in rather warm orbits, but not necessarily associated to the hotter, more centrally concentrated ones traditionally linked to the central bulge. These two observational results challenge our understanding of the origin of IMF variations, and have opened new research lines which are now under development.
The SPanD project has been focused on two complementary research lines. The first one is a rather direct approach to constrain and study IMF variations in nearby galaxies. I joined the F3D team as an expert on stellar population analysis with the goal of characterize the IMF variations using state-of-the-art spectroscopic data. I have developed faster and more robust analysis tools, which are now fully tested. The outcome of this efforts are summarized in Martin-Navarro 2019a, where we showed for the first time the full, two-dimensional IMF map of a nearby massive galaxy. This unprecedented analysis has also allowed us to demonstrate that, contrary to what it was believed, the local metallicity is not the main driver of the observed IMF variations. Moreover, a striking connection between the IMF and the orbital structure of the galaxy was found, further probing into the connection between stellar dynamics and IMF studies. Period 2 has been mostly dedicated to apply these newly developed tools to a large sample of galaxies. This work is currently on its final stages and it will be published by the end of the year. Additionally, we have upgraded our modelling techniques to deal with more complex stellar populations, which has lead to the first simultaneous measurement of the high- and low-mass end of the IMF in a star-forming galaxy.

The second research line developed within the SPanD project is related to one of the most fundamental yet open questions in Astronomy: why do galaxies stop forming new stars? Within the SPanD framework I was able to develop a new observational approach that lead to the first direct evidence supporting the effect of black hole feedback in driving the evolution of massive galaxies (Martin-Navarro et al. 2018b, Nature). We expanded this idea to low-mass galaxies, a regime where the effect of the IMF should become more important, finding indeed that black holes do not seem to regulate star formation in these low-mass systems. The SPanD project has lead to two more paper where I further studied how stellar and black hole feedback processes alter the stellar population properties and the evolution of galaxies. I have also shown the relation between black hole growth and therefore feedback on galaxy cluster scales. In Period 2 these ideas have been further developed, finding a striking dependence of the star formation rate of satellites and their position with respect to the plane defined by the central galaxy. This signal is also seen in cosmological simulations but, interestingly, as an emerging process, at subgrid physics in simulations is fundamentally isotropic.

The dissemination of all these results has been done through two main channels. First, among colleagues, the results of the SPanD project have been presented internationally in numerous conferences, workshops, and invited talks. I have recently organized a conference where we brought together the expertise of different communities to understand the chemical enrichment of galaxies, which is ultimately set by the slope of the IMF. Complementary, a bi-yearly collaboration with a Spanish newspaper was established and four articles have been published so far. In addition to this, two press releases have been published. These press releases led two several interviews for newspapers and outreach journals both in the United States of America, Spain, and France. Moreover, I also participated in public outreach talks, and I also try to engage younger generations of high-schoolers. I was also involved in activities to promote scientific career path among under-represented groups, in particular the Hispanic community in the Bay Area.
The impact of the SPanD project beyond expand and will keep growing beyond the initial expectations. In particular, my results based on MUSE spectroscopic data will be benchmark studies in the upcoming years, probing IMF variations with unprecedented detail and their connection with both the local conditions and the internal dynamics of galaxies. Moreover, as part of the TIMER collaboration, I have also developed tools to expand IMF studies to star-forming galaxies, opening the door to completely unexplored territory. These new tools are already leading to unprecedented discoveries, as for the first time I have simultaneously measure IMF variations across a large range of stellar masses in a post-starburst galaxy combining radio and optical data.

The study of IMF variations in younger stellar populations is crucial to pave the way for high-redshift studies. Similarly, and thanks to the robust analysis tools developed within SPanD, I will explore the connection between weak gravitational lensing, galaxy assembly, and IMF variations, in collaboration with world-leading experts in deep photoemtric studies. this collaboration started during the outgoing phase of the SPanD project and it brings together American and European expertises. Within this collaboration I wrote an observational proposal that has been awarded with 16 hours of telescope time in the highly competitive Very Large Telescope of the European Southern Observatory. Data are been currently taken.

The collaboration in with the Spanish newspaper will help to reduce the gap between astronomers and the general public. This is particularly important in places where big telescopes are located, as their environmental impact may have a societal impact (as for example in the case of the Thirty Meter Telescope in Hawaii). Maintaining this collaboration is therefore crucial for the SPanD project, as the public outreach articles have received very positive feedback.
FCC167 IMF map