Periodic Reporting for period 2 - SPanD (Stellar Populations and Dynamics: a comprehensive IMF analysis)
Reporting period: 2019-05-01 to 2020-04-30
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 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 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.