The ΛCDM model at small scales: Milky Way' satellites and cusp-core crisis

Tesis doctoral inédita cotutelada por la Sapienza Universitá di Roma y la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica. Fecha de lectura: 21-07-201

Signatures of dark matter halo expansion in galaxy populations

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reservedDark matter cores within galaxy haloes can be formed by energy feedback from star-forming regions: an energy balance suggests that the maximum core formation efficiency arises in galaxies with Mstar ~ 108.5M⊙. We show that a model population of galaxies, in which the density profile has been modified by such baryonic feedback, is able to explain the observed galaxy velocity function and Tully-Fisher relations significantly better than a model in which a universal cuspy density profile is assumed. Alternative models, namely warm or self-interacting dark matter, also provide a better match to these observed relations than a universal profile model does, but make different predictions for how halo density profiles vary with mass compared to the baryonic feedback case. We propose that the expected signatures of the mass dependence of core formation generated by baryonic feedbackCB thanks the MICINN (Spain) for the financial support through the MINECO grant AYA2012-31101 and the Ramon y Cajal program. ADC is supported by the DARK independent fellowship program

Expanded haloes, abundance matching and too-big-to-fail in the Local Group

Observed kinematical data of 40 Local Group (LG) members are used to derive the dark matter halo mass of such galaxies. Haloes are selected from the theoretically expected LG mass function and two different density profiles are assumed, a standard universal cuspy model and a mass dependent profile which accounts for the effects of baryons in modifying the dark matter distribution within galaxies. The resulting relations between stellar and halo mass are compared with expectations from abundance matching. Using a universal cuspy profile, the ensemble of LG galaxies is fit in relatively low mass haloes, leaving "dark" many massive haloes of \mhalo$\gtrsim$10$^{10}$\msun: this reflects the "too big to fail" problem and results in a \mstar-\mhalo\ relation that differs from abundance matching predictions. Moreover, the star formation efficiency of isolated LG galaxies increases with decreasing halo mass when adopting a cuspy model. By contrast, using the mass dependent density profile, dwarf galaxies with \mstar$\gtrsim$10$^{6}$\msun are assigned to more massive haloes, which have a central cored distribution of dark matter: the "too big to fail" problem is alleviated, the resultant \mstar-\mhalo\ relation follows abundance matching predictions down to the completeness limit of current surveys, and the star formation efficiency of isolated members decreases with decreasing halo mass, in agreement with theoretical expectations. Finally, the cusp/core space of LG galaxies is presented, providing a framework to understand the non-universality of their density profiles.Comment: Accepted in MNRAS, 15 pages 7 figures. section 3.3 has been added after report

Constraining gas metal mixing strength in simulations using observations of the Milky Way's disc

This work explores the mixing rate of metals in the interstellar medium (ISM), comparing observational constraints from our solar neighbourhood to high resolution cosmological hydrodynamical simulations of Milky Way (MW)-like galaxies. The mixing rate, described by the coefficient C, is varied in simulations between 0 and 0.05, with resultant simulated galaxies compared to observations of metallicity dispersion in young star clusters, HII regions and neutral gas in the disc of the MW. A value of C between 0.003125 and 0.0125 is found to self-consistently match a range of observables, with a best estimate of C=0.0064$\pm$0.0004. We demonstrate that the relationship between metal dispersion in young stars, HII regions and neutral gas, versus the coefficient C, can be described by a power law. These constrained mixing rates infer a comparatively well mixed ISM in the solar neighbourhood, at odds with some recent observations that have reported a highly inhomogeneous ISM. The degree of mixing suggested by this work is lower than what often employed in many hydrodynamical simulations. Our results have implications for studying the metallicity distribution of stars as well as of gas in the interstellar and circumgalactic media.Comment: 7 pages, 3 figures, accepted for publication in MNRA

On the anti-correlation between pericentric distance and inner dark matter density of Milky Way's dwarf spheroidal galaxies

An anti-correlation between the central density of the dark matter halo ($\rho_{150,\ {\rm DM}}$) and the pericentric distances ($r_{p}$) of the Milky Way's (MW's) dwarf spheroidal galaxies (dSphs) has been reported in the literature. The existence and origin of such anti-correlation is however controversial, one possibility being that only the densest dSphs can survive the tidal field towards the centre of our Galaxy. In this work, we place particular emphasis on quantifying the statistical significance of such anti-correlation, by using available literature data in order to explore its robustness under different assumptions on the MW gravitational potential, and for various derivations of $\rho_{150}$ and $r_{p}$. We consider models in which the MW is isolated and has a low ($8.8\times10^{11}\,M_{\odot}$) and high ($1.6\times10^{12}\, M_{\odot}$) halo mass, respectively, as well as configurations in which the MW's potential is perturbed by a Large Magellanic Cloud (LMC) infall. We find that, while data generally support models in which the dSphs' central DM density decreases as a function of their pericentric radius, this anti-correlation is statistically significant at $3\sigma$ level only in $\sim$12$\%$ of the combinations of $\rho_{150}$ and $r_{p}$ explored. Moreover, including the impact of the LMC's infall onto the MW weakens or even washes away this anti-correlation, with respect to models in which the MW is isolated. Our results suggest that the strength and existence of such anti-correlation is still debatable: exploring it with high-resolution simulations including baryonic physics and different DM flavours will help us to understand its emergence.Comment: 10 pages, 3 figures. Accepted for publication in MNRA

Testing Feedback-Modified Dark Matter Haloes with Galaxy Rotation Curves: Estimation of Halo Parameters and Consistency with Λ\LambdaCDM

Cosmological $N$-body simulations predict dark matter (DM) haloes with steep central cusps (e.g. NFW, Navarro et al. 1996). This contradicts observations of gas kinematics in low-mass galaxies that imply the existence of shallow DM cores. Baryonic processes such as adiabatic contraction and gas outflows can, in principle, alter the initial DM density profile, yet their relative contributions to the halo transformation remain uncertain. Recent high resolution, cosmological hydrodynamic simulations (Di Cintio et al. 2014, DC14) predict that inner density profiles depend systematically on the ratio of stellar to DM mass (M$_*$/M$_{\text{halo}}$). Using a Markov Chain Monte Carlo approach, we test the NFW and the M$_*$/M$_{\text{halo}}$-dependent DC14 halo models against a sample of 147 galaxy rotation curves from the new {\it Spitzer} Photometry and Accurate Rotation Curves (SPARC) data set. These galaxies all have extended H{\small I} rotation curves from radio interferometry as well as accurate stellar mass density profiles from near-infrared photometry. The DC14 halo profile provides markedly better fits to the data compared to the NFW profile. Unlike NFW, the DC14 halo parameters found in our rotation curve fits naturally fall within two standard deviations of the mass-concentration relation predicted by $\Lambda$CDM and the stellar mass-halo mass relation inferred from abundance matching with few outliers. Halo profiles modified by baryonic processes are therefore more consistent with expectations from $\Lambda$ cold dark matter ($\Lambda$CDM) cosmology and provide better fits to galaxy rotation curves across a wide range of galaxy properties than do halo models that neglect baryonic physics. Our results offer a solution to the decade long cusp-core discrepancy.Comment: 23 Pages, 18 Figures, MNRAS Accepte

NIHAO XI: Formation of Ultra-Diffuse Galaxies by outflows

We address the origin of Ultra-Diffuse Galaxies (UDGs), which have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects. Their formation mechanism, and whether they are failed $\rm L_{\star}$ galaxies or diffuse dwarfs, are challenging issues. Using zoom-in cosmological simulations from the NIHAO project, we show that UDG analogues form naturally in medium-mass haloes due to episodes of gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses $10^{10-11}\rm M_{\odot}$, have stellar masses of $10^{7-8.5}\rm M_{\odot}$, effective radii larger than 1 kpc and dark matter cores. They show a broad range of colors, an average S\'ersic index of 0.83, a typical distribution of halo spin and concentration, and a non-negligible HI gas mass of $10^{7-9}\rm M_{\odot}$, which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes that form UDGs: feedback driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low surface brightness galaxies and should exist in the field. The largest isolated UDGs should contain more HI gas than less extended dwarfs of similar $\rm M_{\star}$.Comment: matches accepted version, MNRAS Letter 2016-10-1