Cuspy no more: cored dwarf galaxies in high resolution hydrodynamical simulations

The current ΛCDM standard model for structure formation provides a very successful description of our universe on large scales, but (“There is always a but in this imperfect world“, to quote Anne Bronte) it has long had difficulty reconciling theoretical predictions with observations of dwarf galaxies.

In particular, the dwarf fellows are less numerous and have shallower central density profiles than their peers in numerical cosmological simulations. The first of these small-scale problems is known as the “missing satellites problem” (“missing” from the cosmologists point of view, of course!) and it can be improved by hydrodynamical simulations which properly account for the low efficiency of gas cooling and star formation in dwarf dark halos (have a look at Dan’s astrobite). The second problem, addressed in this paper, is commonly referred to as the “core-cusp problem:” dark matter-only simulations predict that dark halos follow a quasi–universal Einasto or NFW profile, characterized by steep (or “cuspy”) inner density profiles, while many different observations of dwarf galaxies show shallower (or “cored”) central density profiles. In practice, if we assume that the density is proportional to r^(alpha), numerical simulations predict that alpha<-1, while observations find values of the logarithmic density slope alpha closer to zero.

Among other things, this discrepancy prompted the development of several alternative models for dark matter: warm DM, meta–DM, Self Interacting DM. . . and also the investigation of alternative theories of gravitation, such as MOND. However, there is convincing evidence that the inconsistency between numerical simulations and observations of dwarf galaxies can be ascribed to our poor understanding of the baryonic processes involved in galaxy formation, without necessitating refinement of the ΛCDM model.

Today, Governato et al. present new self-consistent dark matter+gas simulations in a full ΛCDM cosmological scenario. A  spectacular 3D rendering of the simulations used in the work, showing the formation and evolution of several dwarf galaxies from around 400 million years to around 5 billion years after the Big Bang, is worth watching here.

Such numerical simulations include cooling from metal lines and H2, and a physically motivated energy feedback from supernovae in star forming regions, which can be identified only thanks to the adopted high-resolution. Feedback from supernovae generates repeated and fast gas outflows, which are able to remove efficiently large fraction of gas from the inner regions of proto-galaxies and, in smaller galaxies, from the galaxy themselves, thus forming central cores.

The study focuses on the properties of field galaxies (to avoid the effects of tidal interactions), and compares the simulations of small galaxies with measurements of dark matter profiles derived from observational data from the THINGS and the LITTLE THINGS HI nearby galaxy surveys. The excellent agreement between the simulation and the real galaxies is shown  in Fig.1: the simulations reproduce both the value of the density slope alpha and its trend with stellar (or total) mass. Of course, in the numerical simulations it is easy to spot the dark matter particles, compute their density profile and its logarithmic slope. For the observed galaxies, instead, the dark matter density profiles are measured by subtracting the contribution of the (shining) baryons from the total mass profile, inferred from rotation curves (see this paper for more details).

Figure 1: The slope of the dark matter density profile as a function of the stellar mass measured at 500 parsec and redshift z=0, for all the resolved halos in the simulation. Solid line: prediction for dark matter-only simulations. Large crosses: resolved halos with more than 0.5 × 10^6 particles. Small crosses: more than 5 × 10^4 particles. Small squares: 22 observational data points from galaxies of the THINGS and LITTLE THINGS surveys.

This work highlights the crucial importance of a correct modeling of baryon–dark matter interactions in galaxy formation simulations. It is remarkable that the main tensions between simulations and observations can be alleviated taking into account feedback processes correctly.

 

About Lucia Morganti

I am a fourth-year graduate student at the Ludwig Maximilians Universitaet of Munich, Germany. I am doing my PhD at the Max Planck Institute for Extraterrestrial Physics, where I belong to the Dynamics group led by Ortwin Gerhard. I am interested in galactic dynamics and modelling of galaxies.

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  1. Too many galaxy pairs in the Local Group? | astrobites - [...] in simulations, also known as the “Missing Satellites problem” (have a look at this astrobites). Another prediction of galaxy formation…
  2. Astrobites at AAS 229: Day 3 | astrobites - […] as the NFW profile; actual dwarfs have much less of their mass in their centers than expected (the “core-cusp”…

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