Yohan Dubois

Cusp-core transformations in dwarf galaxies: observational predictions

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 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 renement, 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 identication of the dark matter core formation process. Our dark matter inner prole is well tted by a pseudo-isothermal prole 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 nd cusp-core transformations: (i) a bursty star formation history (SFH) 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= 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.

Dancing in the dark: galactic properties trace spin swings along the cosmic web

A large-scale hydrodynamical cosmological simulation, Horizon-AGN , is used to investigate the alignment between the spin of galaxies and the large-scale cosmic filaments above redshift one. The analysis of more than 150 000 galaxies with morphological diversity in a 100h −1 Mpc comoving box size shows that the spin of low-mass, rotationdominated, blue, star-forming galaxies is preferentially aligned with their neighbouring filaments. High-mass, dispersion-dominated, red, quiescent galaxies tend to have a spin perpendicular to nearby filaments. The reorientation of the spin of massive galaxies is provided by galaxy mergers which are significant in the mass build up of high-mass galaxies. We find that the stellar mass transition from alignment to misalignment happens around 3×10 10 M⊙. This is consistent with earlier findings of a dark matter mass transition for the orientation of the spin of halos (5 × 10 11 M⊙ at the same redshift from Codis et al. 2012). With these numerical evidence, we advocate a scenario in which galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the physical and morphological properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as Active Galactic Nuclei is mandatory to quench in situ star formation in massive galaxies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping galaxy spins misaligned with cosmic filaments. It also promotes diversity amongst galaxy properties.

On the onset of galactic winds in quiescent star forming galaxies

Context. The hierarchical model of galaxy formation, despite its many successes, still overpredicts the baryons fraction locked in galaxies as a condensed phase. The efficiency of supernovae feedback, proposed a long time ago as a possible solution for this so-called “overcooling” problem, is still under debate, mainly because modelling supernovae explosions within a turbulent interstellar medium, while capturing realistic large scale flows around the galaxy is a very demanding task. Aims. Our goal is to study the effect of supernovae feedback on a disc galaxy, taking into account the impact of infalling gas on both the star formation history and the corresponding outflow structure, the apparition of a supernovae-driven wind being highly sensitive to the halo mass, the galaxy spin and the star formation efficiency. Methods. We model our galaxies as cooling and collapsing NFW spheres. The dark matter component is modelled as a static external potential, while the baryon component is described by the Euler equations using the AMR code RAMSES. Metal-dependent cooling and supernovae-heating are also implemented using state-of-the-art recipes coming from cosmological simulations. We allow for three parameters to vary: the halo circular velocity, the spin parameter and the star formation efficiency. Results. We found that the ram pressure of infalling material is the key factor limiting the apparition of galactic winds. We obtain a very low feedback efficiency, with supernovae to wind energy conversion factor around one percent, so that only low circular velocity galaxies give rise to strong winds. For massive galaxies, we obtain a galactic fountain, for which we discuss the observational properties. Conclusions. We conclude that for quiescent isolated galaxies, galactic winds appear only in very low mass systems. Although this can quite efficiently enrich the IGM with metals, they do not carry away enough cold material to solve the overcooling problem.

Connecting the cosmic web to the spin of dark haloes: implications for galaxy formation

We investigate the alignment of the spin of dark matter halos relative (i) to the surrounding large-scale filamentary structure, and (ii) to the tidal tensor eigenvectors using the Horizon 4π dark matter simulation which resolves over 43 million dark matter halos at redshift zero. We detect a clear mass transition: the spin of dark matter halos above a critical massM s 0 ≈ 5(±1) �10 12 M⊙ tends to be perpendicular to the closest large scale filament (with an excess probability up to 12%), and aligned with the intermediate axis of the tidal tensor (with an excess probability of up to 40%), whereas the spin of low-mass halos is more likely to be aligned with the closest filament (with an excess probability up to 15%). Furthermore, this critical mass is redshift-dependent, scaling as M s (z) ≈ M s �(1 +z) −γs with γs = 2.5 ± 0.2. A similar fit for the redshift evolution of the tidal tensor transition mass yields M t ≈ 8(±2) �10 12 M⊙ and γt = 3 ± 0.3. This critical mass also varies weakly with the scale defining filaments. We propose an interpretation of this signal in terms of large-scale cosmic flows. In this picture,most low-mass halos areformed through the winding offlows embedded in misaligned walls; hence they acquire a spin parallel to the axis of the resulting filaments forming at the intersection of these walls. On the other hand, more massive halos are typically the products of later mergers along such filaments, and thus they acquire a spin perpendicular to this direction when their orbital angular momentum is converted into spin. We show that this scenario is consistent with both the measured excess probabilities of alignment w.r.t. the eigen-directions of the tidal tensor, and halo merger histories. On a more qualitative level, it also seems compatible with 3D visualization of the structure of the cosmic web as traced by “smoothed” dark matter simulations or gas tracer particles. Finally, it provides extra support to the disc forming paradigm presented by Pichon et al. (2011) as it extends it by characterizing the geometry of secondary infall at high redshift.

AGN-driven quenching of star formation: morphological and dynamical implications for early-type galaxies

In order to understand the physical mechanisms at work during the formation of massive early-type galaxies, we performed six zoomed hydrodynamical cosmological simulations of halos in the mass range 4.3 10^12 < M_vir < 8.0 10^13 M_sun at z=0, using the Adaptive Mesh Refinement code RAMSES. These simulations explore the role of Active Galactic Nuclei (AGN), through jets powered by the accretion onto supermassive black holes on the formation of massive elliptical galaxies. In the absence of AGN feedback, large amounts of stars accumulate in the central galaxies to form overly massive, blue, compact and rotation-dominated galaxies. Powerful AGN jets transform the central galaxies into red extended and dispersion-dominated galaxies. This morphological transformation of disc galaxies into elliptical galaxies is driven by the efficient quenching of the in situ star formation due to AGN feedback, which transform these galaxies into systems built up by accretion. For galaxies mainly formed by accretion, the proportion of stars deposited farther away from the centre increases, and galaxies have larger sizes. The accretion is also directly responsible for randomising the stellar orbits, increasing the amount of dispersion over rotation of stars as a function of time. Finally, we find that our galaxies simulated with AGN feedback better match the observed scaling laws, such as the size-mass, velocity dispersion-mass, fundamental plane relations, and slope of the total density profiles at z~0, from dynamical and strong lensing constraints.

Self-regulated growth of supermassive black holes by a dual jet/heating AGN feedback mechanism: methods, tests and implications for cosmological simulations

We develop a new sub-grid model for the growth of supermassive Black Holes (BHs) and their associated Active Galactic Nuclei (AGN) feedback in hydrodynamical cosmological simulations. Assuming that BHs are created in the early stages of galaxy formation, they grow by mergers and accretion of gas at a Eddington-limited Bondi accretion rate. However this growth is regulated by AGN feedback which we model using two different modes: a quasar-heating mode when accretion rates onto the BHs are comparable to the Eddington rate, and a radio-jet mode at lower accretion rates. In other words, our feedback model deposits energy as a succession of thermal bursts and jet outflows depending on the properties of the gas surrounding the BHs. We assess the plausibility of such a model by comparing our results to observational measurements of the coevolution of BHs and their host galaxy properties, and check their robustness with respect to numerical resolution. We show that AGN feedback must be a crucial physical ingredient for the formation of massive galaxies as it appears to be the only physical mechanism able to efficiently prevent the accumulation of and/or expel cold gas out of halos/galaxies and significantly suppress star formation. Our model predicts that the relationship between BHs and their host galaxy mass evolves as a function of redshift, because of the vigorous accretion of cold material in the early Universe that drives Eddington-limited accretion onto BHs. Quasar activity is also enhanced at high redshift. However, as structures grow in mass and lose their cold material through star formation and efficient BH feedback ejection, the AGN activity in the low-redshift Universe becomes more and more dominated by the radio mode, which powers jets through the hot circum-galactic medium.

Jet-regulated cooling catastrophe

We present the first implementation of active galactic nuclei (AGN) feedback in the form of momentum-driven jets in an adaptive mesh refinement (AMR) cosmological resimulation of a galaxy cluster. The jets are powered by gas accretion on to supermassive black holes (SMBHs) which also grow by mergers. Throughout its formation, the cluster experiences different dynamical states: both a morphologically perturbed epoch at early times and a relaxed state at late times allowing us to study the different modes of black hole (BH) growth and associated AGN jet feedback. BHs accrete gas efficiently at high redshift (z > 2), significantly pre-heating proto-cluster haloes. Gas-rich mergers at high redshift also fuel strong, episodic jet activity, which transports gas from the proto-cluster core to its outer regions. At later times, while the cluster relaxes, the supply of cold gas on to the BHs is reduced leading to lower jet activity. Although the cluster is still heated by this activity as sound waves propagate from the core to the virial radius, the jets inefficiently redistribute gas outwards and a small cooling flow develops, along with low-pressure cavities similar to those detected in X-ray observations. Overall, our jet implementation of AGN feedback quenches star formation quite efficiently, reducing the stellar content of the central cluster galaxy by a factor of 3 compared to the no-AGN case. It also dramatically alters the shape of the gas density profile, bringing it in close agreement with the β model favoured by observations, producing quite an isothermal galaxy cluster for gigayears in the process. However, it still falls short in matching the lower than universal baryon fractions which seem to be commonplace in observed galaxy clusters.

Rigging dark haloes: why is hierarchical galaxy formation consistent with the inside-out build-up of thin discs?

State-of-the-art hydrodynamical simulations show that gas inflow through the virial sphere of dark matter haloes is focused (i.e. has a preferred inflow direction), consistent (i.e. its orientation is steady in time) and amplified (i.e. the amplitude of its advected specific angular momentum increases with time). We explain this to be a consequence of the dynamics of the cosmic web within the neighbourhood of the halo, which produces steady, angular momentum rich, filamentary inflow of cold gas. On large scales, the dynamics within neighbouring patches drives matter out of the surrounding voids, into walls and filaments before it finally gets accreted on to virialized dark matter haloes. As these walls/filaments constitute the boundaries of asymmetric voids, they acquire a net transverse motion, which explains the angular momentum rich nature of the later infall which comes from further away. We conjecture that this large-scale driven consistency explains why cold flows are so efficient at building up high-redshift thin discs inside out.

Mass Distribution in Galaxy Clusters: the Role of AGN Feedback

We use 1-kpc resolution cosmological Adaptive Mesh Refinement (AMR) simulations of a Virgo-like galaxy cluster to investigate the effect of feedback from supermassive black holes on the mass distribution of dark matter, gas and stars. We compared three different models: (i) a standard galaxy formation model featuring gas cooling, star formation and supernovae feedback, (ii) a ‘quenching’ model for which star formation is artificially suppressed in massive haloes and finally (iii) the recently proposed active galactic nucleus (AGN) feedback model of Booth and Schaye. Without AGN feedback (even in the quenching case), our simulated cluster suffers from a strong overcooling problem, with a stellar mass fraction significantly above observed values in M87. The baryon distribution is highly concentrated, resulting in a strong adiabatic contraction (AC) of dark matter. With AGN feedback, on the contrary, the stellar mass in the brightest cluster galaxy (BCG) lies below observational estimates and the overcooling problem disappears. The stellar mass of the BCG is seen to increase with increasing mass resolution, suggesting that our stellar masses converge to the correct value from below. The gas and total mass distributions are in better agreement with observations. We also find a slight deficit (∼10 per cent) of baryons at the virial radius, due to the combined effect of AGN-driven convective motions in the inner parts and shock waves in the outer regions, pushing gas to Mpc scales and beyond. This baryon deficit results in a slight adiabatic expansion of the dark matter distribution that can be explained quantitatively by AC theory.

Lyman-alpha emission properties of simulated galaxies: interstellar medium structure and inclination effects

Aims. This paper is the first of a series investigating Lyman-alpha (hereafter Ly ) radiation transfer through hydrodynamical simulations of galaxy formation. Its aim is to assess the impact of the interstellar medium (ISM) physics on Ly radiation transfer and to quantify how galaxy orientation with respect to the line of sight alters observational signatures. Methods. We compare the results of Ly radiation transfer calculations through the ISM of a couple of idealized galaxy simulations in a dark matter halo of 10 10 M . In the first one, G1, this ISM is modeled using physics typical of large scale cosmological hydrodynamics simulations of galaxy formation, where gas is prevented from radiatively cooling below 10 4 K. In the second one, G2, gas is allowed to radiate away more of its internal energy via metal lines and consequently fragments into dense star-forming clouds. Results. First, as expected, the small-scale structuration of the ISM plays a determinant role in shaping a galaxy’s Ly properties. The artificially warm, and hence smooth, ISM of G1 yields an escape fraction of 50% at the Ly line center, and produces symmetrical double-peak profiles. On the contrary, in G2, most young stars are embedded in thick star-forming clouds, and the result is a 10 times lower escape fraction. G2 also displays a stronger outflowing velocity field, which favors the escape of red-shifted photons, resulting in an asymmetric Ly line. Second, the Ly properties of G2 strongly depend on the inclination at which it is observed: From edge-on to face-on, the line goes from a double-peak profile with an equivalent width of 5A to a 15 times more luminous red-shifted asymmetric line with EW 90A. Conclusions. The remarkable discrepancy in the Ly properties we derived for two ISM models raises a fundamental issue. In e ect, it demonstrates that Ly radiation transfer calculations can only lead to realistic properties in simulations where galaxies are resolved into giant molecular clouds. Such a stringent requirement translates into severe constraints both in terms of ISM physics modeling and numerical resolution, putting these calculations out of reach of current large scale cosmological simulations. Finally, we find inclination e ects to be much stronger for Ly photons than for continuum radiation. This could potentially introduce severe biases in the selection function of narrow-band Ly emitter surveys, and in their interpretation, and we predict these surveys could indeed miss a significant fraction of the high-z galaxy population.