What is the Initial Mass Function in star-forming regions of external galaxies?

The stellar initial mass function (IMF) is the mass distribution of stars at birth and is a critical function to measure, given its importance in our understanding of galaxy evolution. Measurements of the IMF are generally restricted to the Milky Way or non-star-forming external galaxies. The exquisite TIMER data provides an opportunity to study the IMF in star-forming regions of external galaxies. This was done by Ignacio Martín-Navarro, who, for the first time, provides consistent IMF measurements across stellar populations with different ages and complex star-formation histories (arXiv:2312.13355). The measurements are consistent with a Milky Way IMF, but also show changes in a systematic and spatially coherent manner. He also found peculiar regions, associated with star-forming bursts, that are metal-poor but with a bimodal [Mg/Fe] distribution. He interprets these regions as enriched by core-collapse supernovae.

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When in cosmic history do disc galaxies develop their bars? Because bars are one of

the major drivers of galaxy evolution, knowing when the bar was formed in a galaxy is a

crucial piece of information to understand the evolution of galaxies. This is one of the

main goals of TIMER (see paragraph at the bottom of this page) and after much

work we are now producing the first bar age measurements. In a recent paper, Camila

de Sá-Freitas developed a new methodology to derive bar formation epochs and

demonstrated the method by applying it on the TIMER galaxy NGC 1433

(arXiv:2211.07670; see galaxy picture above). She found that the bar in NGC 1433

formed around 7.5 billion years ago, or at a redshift z ~ 1. In another paper

(arXiv:2308.04482), Camila found the smallest nuclear discs discovered thus far, with

sizes similar to the nuclear disc in the Milky Way. These new results show that the

process of disc settling is still happening in galaxies today. She is now applying her

methodology on all TIMER galaxies, so stay tuned for more very soon!

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Nuclear discs in massive disc galaxies. The central region of massive disc galaxies is often dominated either by a kinematically hot (i.e., dispersion supported) stellar spheroid (aka a classical bulge), or by a kinematically cold (i.e., rotation supported) nuclear disc (aka a pseudo-bulge). While a classical bulge is plausibly formed through violent processes such as mergers, nuclear discs are thought to form from bar-driven secular evolution processes. In two recent papers, one led by Dimitri Gadotti (arXiv:2009.01852) and the other led by Adrian Bittner (arXiv:2009.01856), we show that indeed nuclear discs have all the physical properties expected from a bar-driven formation scenario. In addition, we find no evidence for a large-scale classical bulge in any of the 21 TIMER galaxies studied. This is puzzling in the context of the hierarchical picture of galaxy formation, given that the TIMER galaxies are all in a mass range where classical bulges are expected to occur often. Our own galaxy, the Milky Way, may as well host a nuclear disc, indicating a quiet merger history. You can check the beautiful maps of stellar kinematics and population properties we published in these papers for all TIMER galaxies in our Gallery.

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Stellar populations in bars.  How do the ages of stars vary across bars? It turns out that idealised models of isolated galaxies had predicted that stars should be younger close the bar major axis. Given the excellent quality of the TIMER data, we were able to test and confirm this prediction (again for the first time!) in the work led by Justus Neumann (arXiv:2003.08946). Further, we show that cosmological simulations are currently also able to reproduce this behaviour. In this study we also discovered that most bars are more metal-rich than their surroundings.

 

Inner bars. Most disc galaxies host bars, a central stellar structure where stars move along elongated orbits around the galaxy centre. It turns out that many such galaxies hosts as well an inner bar. We studied in detail two such inner bars in the galaxies NGC 1291 and NGC 5850, and we found that such inner bars appear to form just like main bars, i.e., from dynamical instabilities in a stellar disc (where stars move along near circular orbits). In addition, we found evidence that inner bars are also long lived, just as main bars. These results can be found in the paper by Adriana de Lorenzo Cáceres (arXiv:1901.06394). In a related study led by Jairo Méndez-Abreu (arXiv:1811.03855), we discovered by chance the first box/peanut in an inner bar. Box/peanuts are structures that form from dynamical instabilities in the central parts of bars, changing stellar orbits such that the structure looks like a peanut when seen from certain directions. Box/peanuts are common in main bars, but this is the first time it is observed in an inner bar. This discovery strengthens the case that inner bars are just like main bars, following the same orbital structure and dynamical evolution. In a more recent paper by Adrian Bittner (arXiv:2012.04661), we found more similarities between inner and main bars, not only from the dynamical point of view but also concerning their impact in the spatial distribution of stars with different metallicities and ages. This suggests the picture of a `galaxy within a galaxy', with inner bars in nuclear discs being equivalent to main bars in main galaxy discs.

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Stellar feedback from nuclear rings. One of the main processes driven by bars is the funnelling

of gas to the central regions of the galaxy. Shocks on the leading edges of bars remove

angular momentum from gas in the interstellar medium, which then streams down

along the bar. The in-falling gas stops at the region where the orbital structure of

the bar leads to more circular orbits perpendicular to the bar, which often results

in star-forming nuclear rings. In the galaxy NGC 3351, new stars are being

formed through this process at a very high rate. Combining our MUSE TIMER

data with data from ALMA, we found that the young stars in the nuclear ring

not only heat up the gas around them, but also blows it away from the galaxy

centre at speeds of 70 km/s. We also see that cold molecular gas surrounding the

nuclear ring gets pushed back by the warm gas. In the picture on the right, the

expanding, warm gas detected with MUSE is shown in orange shades, while the

cold gas detected with ALMA is shown in blue shades. The start-bursting nuclear

ring is the whitish structure that is elongated vertically in the picture. These results

were published in the paper led by Ryan Leaman (arXiv:1907.13142) and advertised in a

ESO Picture of the Week release.

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The ages of bars and the settling of disc galaxies. When the MUSE instrument was still a newcomer at the La Silla Paranal Observatory, our team has published a pilot study led by Dimitri Gadotti (arXiv:1509.00032), where we showed that we can provide an estimate to when a bar has formed in a galaxy by studying the star formation history of the nuclear ring formed by the bar. Essentially, since the stars in the ring form from gas brought to the central regions by the bar, the oldest star therein have an age that is a lower limit to the age of the bar. In fact, in this study we found a very old bar, with an age of about 10 Gyr, which implies that (i), bars can be robust structures, and (ii), some bars are formed at redshifts as high as z ~ 2. The TIMER project (see paper 1) was born from that pilot study, with the aim of estimating the ages of bars for 24 nearby barred galaxies. An important aspect of this work is that a bar can only develop in a galaxy when the main disc is dynamically settled. Therefore, our work will also provide estimates for the time when discs settle, which is a very important observational constraint to models of galaxy formation and evolution. In addition, as our sample spans a range in stellar mass, we will also be able to test the downsizing scenario, in which discs settle (and bars form) in more massive galaxies first.