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Register a new userCusp-core transformations in dwarf galaxies: observational predictions
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Authors
Romain Teyssier
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The presence of a dark matter core in the central kiloparsec of many dwarf galaxies has been a long standing problem in galaxy formation theories based on the standard cold dark matter paradigm. Recent cosmological simulations, based on Smooth Particle Hydrodynamics and rather strong feedback recipes have shown that it was indeed possible to form extended dark matter cores using baryonic processes related to a more realistic treatment of the interstellar medium. Using adaptive mesh refinement, together with a new, stronger supernovae feedback scheme that we have recently implemented in the RAMSES code, we show that it is also possible to form a prominent dark matter core within the well-controlled framework of an isolated, initially cuspy, 10 billion solar masses dark matter halo. Although our numerical experiment is idealized, it allows a clean and unambiguous identification of the dark matter core formation process. Our dark matter inner profile is well fitted by a pseudo-isothermal profile with a core radius of 800 pc. The core formation mechanism is consistent with the one proposed recently by Pontzen & Governato. We highlight two key observational predictions of all simulations that find cusp-core transformations: (i) a bursty star formation history with peak to trough ratio of 5 to 10 and a duty cycle comparable to the local dynamical time; and (ii) a stellar distribution that is hot with v/sigma=1. We compare the observational properties of our model galaxy with recent measurements of the isolated dwarf WLM. We show that the spatial and kinematical distribution of stars and HI gas are in striking agreement with observations, supporting the fundamental role played by stellar feedback in shaping both the stellar and dark matter distribution.
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In a recent study (Martizzi et al. 2012), we used cosmological simulations to show that active galactic nuclei (AGN) feedback on the gas distribution in clusters of galaxies can be important in determining the spatial distribution of stars and dark matter in the central regions of these systems. The hierarchical assembly of dark matter, baryons and black holes obscures the physical mechanism behind the restructuring process. Here we use idealized simulations to follow the response of a massive dark matter halo as we feed the central black hole with a controlled supply of cold gas. This removes most of the complexity taking place in the cosmological simulations that may have biased our previous study. We confirm our previous results: gas heated and expelled from the central regions of the halo by AGN feedback can return after cooling; repeated cycles generate gravitational potential fluctuations responsible for irreversible modifications of the dark matter mass profile. The main result is the expulsion of large amounts of baryons and dark matter from the central regions of the halo. According to the work presented here, outflow induced fluctuations represent the only mechanism able to efficiently create dark matter cores in clusters of galaxies.
Context. The cusp-core discrepancy is one of the major problems in astrophysics. It results from comparing the observed mass distribution of galaxies with the predictions of Cold Dark Matter simulations. The latter predict a cuspy density profile in the inner parts of galaxies, whereas observations of dwarf and low surface brightness galaxies show a constant density core. Aims. We want to determine the shape of the dark matter potential in the nuclear regions of a sample of six nearby irregular dwarf galaxies. Methods. In order to quantify the amount of non-circular motions which could potentially affect a mass decomposition, we first perform a harmonic decomposition of the HI Hermite velocity fields of all sample galaxies. We then decompose the HI rotation curves into different mass components by fitting NFW and pseudo-isothermal halo models to the HI rotation curves using a chi^2 minimisation. We model the minimum-disc, the minimum-disc+gas, and the maximum-disc cases. Results. The non-circular motions are in all cases studied here of the order of only a few km/s (generally corresponding to less than 25% of the local rotation velocity), which means that they do not significantly affect the rotation curves. The observed rotation curves can better be described by the cored pseudo-isothermal halo than by the NFW halo. The slopes of the dark matter density profiles confirm this result and are in good agreement with previous studies. The quality of the fits can often be improved when including the baryons, which suggests that they contribute significantly to the inner part of the density profile of dwarf galaxies.
This paper gives an overview of the attempts to determine the distribution of dark matter in low surface brightness disk and gas-rich dwarf galaxies, both through observations and computer simulations. Observations seem to indicate an approximately constant dark matter density in the inner parts of galaxies, while cosmological computer simulations indicate a steep power-law-like behaviour. This difference has become known as the core/cusp problem, and remains one of the unsolved problems in small-scale cosmology.
We performed a series of high-resolution $N$-body simulations to examine whether dark matter candidates in the form of primordial black holes (PBHs) can solve the cusp-core problem in low-mass dwarf galaxies. If some fraction of the dark matter in low-mass dwarf galaxies consists of PBHs and the rest is cold dark matter, dynamical heating of the cold dark matter by the PBHs induces a cusp-to-core transition in the total dark matter profile. The mechanism works for PBHs in the 25-100 M$_{sun}$ mass window, consistent with the LIGO detections, but requires a lower limit on the PBH mass fraction of 1$%$ of the total dwarf galaxy dark matter content. The cusp-to-core transition time-scale is between 1 and 8 Gyr. This time-scale is also a constant multiple of the relaxation time between cold dark matter particles and PBHs, which depends on the mass, the mass fraction and the scale radius of the initial density profile of PBHs. We conclude that dark matter cores occur naturally in halos comprised of cold dark matter and PBHs, without the need to invoke baryonic processes.
Cosmological numerical simulations of galaxy formation have led to the cuspy density profile of a pure cold dark matter halo toward the center, which is in sharp contradiction with the observations of the rotation curves of cold dark matter-dominated dwarf and low surface brightness disk galaxies, with the latter tending to favor mass profiles with a flat central core. Many efforts have been devoted to resolve this cusp-core problem in recent years, among them, baryon-cold dark matter interactions are considered to be the main physical mechanisms erasing the cold dark matter (CDM) cusp into a flat core in the centers of all CDM halos. Clearly, baryon-cold dark matter interactions are not customized only for CDM-dominated disk galaxies, but for all types, including giant ellipticals. We first fit the most recent high resolution observations of rotation curves with the Burkert profile, then use the constrained core size-halo mass relation to calculate the lensing frequency, and compare the predicted results with strong lensing observations. Unfortunately, it turns out that the core size constrained from rotation curves of disk galaxies cannot be extrapolated to giant ellipticals. We conclude that, in the standard cosmological paradigm, baryon-cold dark matter interactions are not universal mechanisms for galaxy formation, and therefore, they cannot be true solutions to the cusp-core problem.
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