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...
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?
During the first two years, 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 Fornax 3D team as an expert on stellar population analysis with the goal of characterize the IMF variations using state-of-the-art spectroscopic data acquired with the MUSE integral field unit. This instrument allows for an unprecedented view of nearby galaxies given its efficiency and spatial resolution. The advent of this new generation of spectrographs has revolutionized the field, as the amount of available data has increased, per galaxy, by more than a hundred times. In order to match this technological leap, 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 (Lyubenova, Martin-Navarro et al. 2016). Finally, in another work based on integral field unit data, a suggestive evidence was found linking the IMF with the observed chemical properties of massive galaxies (Martin-Navarro et al. 2018a). As detailed bellow, a series of papers will follow over the next year.
The second research line developed within the SPanD project is related to one of the most fundamental yet open questions in Astronomy: why do massive galaxies stop forming new stars? In the absence of so-called feedback processes, the predicted properties of massive galaxies radically differ from what is observed, and two main mechanisms has been proposed. Stellar feedback, directly related to the number of massive stars and therefore to the IMF, is expected to dominate in low-mass galaxies. In massive galaxies, feedback from the central super-massive black hole is supposed to regulate star formation. The critical aspect of feedback in our understanding of the Universe is, however, almost entirely based on theoretical arguments. 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 (Martin-Navarro & Mezcua, 2018). Over a similar line, the SPanD project has lead to two more paper (Martin-Navarro et al. 2019b,c) where I further studied how stellar and black hole feedback processes alter the stellar population properties and the evolution of galaxies.
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 with six invited talks in Spain, Italy, Germany and the United States, in addition to a numerous of more informal presentations. Moreover, I have also given four contributed talks in international conferences in France (two), Spain, and the Netherlands. Finally, 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 (Metals in galaxies, near and far; Leiden, The Netherlands, 2019). 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 (follow
In the last year of the SPaND action, many of the ongoing projects will be completed thanks to the work carried out during the first two years. Among these projects, arguably the most important ones are those based on MUSE spectroscopic data as they 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. 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.
From a wider perspective, 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. In fact, I have been contacted by a theater director in order to explore the possibility of creating a play around one of this articles where I explain the concept of the IMF and its critical role in astronomy.
More info: http://www.nmartinn.com/spand.html.