Building Late-Type Spiral Galaxies by In-Situ and Ex-Situ Star Formation

We analyze the formation and evolution of the stellar components in "Eris", a 120 pc-resolution cosmological hydrodynamic simulation of a late-type spiral galaxy. The simulation includes the effects of a uniform UV background, a delayed-radiative-cooling scheme for supernova feedback, and a star formation recipe based on a high gas density threshold. It allows a detailed study of the relative contributions of "in-situ" (within the main host) and "ex-situ" (within satellite galaxies) star formation to each major Galactic component in a close Milky Way analog. We investigate these two star-formation channels as a function of galactocentric distance, along different lines of sight above and along the disk plane, and as a function of cosmic time. We find that: 1) approximately 70 percent of today's stars formed in-situ; 2) more than two thirds of the ex-situ stars formed within satellites after infall; 3) the majority of ex-situ stars are found today in the disk and in the bulge; 4) the stellar halo is dominated by ex-situ stars, whereas in-situ stars dominate the mass profile at distances < 5 kpc from the center at high latitudes; and 5) approximately 25% of the inner, r < 20 kpc, halo is composed of in-situ stars that have been displaced from their original birth sites during Eris' early assembly history.Comment: 12 pages, 8 figures; submitted to Ap

Shape of Dark Matter Haloes in the Illustris Simulation: Effects of Baryons

We study the effect of baryonic processes on the shapes of dark matter (DM) haloes from Illustris, a suite of hydrodynamical (Illustris) and DM-only (Illustris-Dark) cosmological simulations performed with the moving-mesh code {\sc arepo}. DM halo shapes are determined using an iterative method based on the inertia tensor for a wide range of $z=0$ masses ($M_{200} = 1 \times 10^{11} - 3 \times 10^{14} M_\odot$). Convergence tests shows that the local DM shape profiles are converged only for $r > 9\epsilon$, $\epsilon$ being the Plummer-equivalent softening length, larger than expected. Haloes from non-radiative simulations (i.e. neglecting radiative processes, star formation, and feedback) exhibit no alteration in shapes from their DM-only counterparts: thus moving-mesh hydrodynamics alone is insufficient to cause differences in DM shapes. With the full galaxy-physics implementation, condensation of baryons results in significantly rounder and more oblate haloes, with the median minor-to-major axis ratio \left \approx 0.7, almost invariant throughout the halo and across halo masses. This somewhat improves the agreement between simulation predictions and observational estimates of the Milky Way halo shape. Consistently, the velocity anisotropy of DM is also reduced in Illustris, across halo masses and radii. Within the inner halo ($r=0.15 R_{200}$), both $s$ and $q$ (intermediate-to-major axis ratio) exhibit non-monotonicity with galaxy mass, peaking at $m_* \approx 10^{10.5-11} M_\odot$, which we find is due to the strong dependence of inner halo shape with galaxy formation efficiency. Baryons in Illustris affect the correlation of halo shape with halo properties, leading to a positive correlation of sphericity of MW-mass haloes with halo formation time and concentration, the latter being mildly more pronounced than in Illustris-Dark.Comment: 18 pages, 14 figure

Galaxies with Shells in the Illustris Simulation: Metallicity Signatures

Stellar shells are low surface brightness arcs of overdense stellar regions, extending to large galactocentric distances. In a companion study, we identified 39 shell galaxies in a sample of 220 massive ellipticals ($\mathrm{M}_{\mathrm{200crit}}>6\times10^{12}\,\mathrm{M}_\odot$) from the Illustris cosmological simulation. We used stellar history catalogs to trace the history of each individual star particle inside the shell substructures, and we found that shells in high-mass galaxies form through mergers with massive satellites (stellar mass ratios $\mu_{\mathrm{stars}}\gtrsim1:10$). Using the same sample of shell galaxies, the current study extends the stellar history catalogs in order to investigate the metallicity of stellar shells around massive galaxies. Our results indicate that outer shells are often times more metal-rich than the surrounding stellar material in a galaxy's halo. For a galaxy with two different satellites forming $z=0$ shells, we find a significant difference in the metallicity of the shells produced by each progenitor. We also find that shell galaxies have higher mass-weighted logarithmic metallicities ([Z/H]) at $2$-$4\,\mathrm{R}_{\mathrm{eff}}$ compared to galaxies without shells. Our results indicate that observations comparing the metallicities of stars in tidal features, such as shells, to the average metallicities in the stellar halo can provide information about the assembly histories of galaxies.Comment: 15 pages, 5 figures. Article published in a special issue of MDPI Galaxies after the conference "On the Origin (and Evolution) of Baryonic Galaxy Halos", Galapagos Islands, 201

Formation and Incidence of Shell Galaxies in the Illustris Simulation

Shells are low surface brightness tidal debris that appear as interleaved caustics with large opening angles, often situated on both sides of the galaxy center. In this paper, we study the incidence and formation processes of shell galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We identify shells at redshift z=0 using stellar surface density maps, and we use stellar history catalogs to trace the birth, trajectory and progenitors of each individual star particle contributing to the tidal feature. Out of a sample of the 220 most massive galaxies in Illustris ($\mathrm{M}_{\mathrm{200crit}}>6\times10^{12}\,\mathrm{M}_{\odot}$), $18\%\pm3\%$ of the galaxies exhibit shells. This fraction increases with increasing mass cut: higher mass galaxies are more likely to have stellar shells. Furthermore, the fraction of massive galaxies that exhibit shells decreases with increasing redshift. We find that shell galaxies observed at redshift $z=0$ form preferentially through relatively major mergers ($\gtrsim$1:10 in stellar mass ratio). Progenitors are accreted on low angular momentum orbits, in a preferred time-window between $\sim$4 and 8 Gyrs ago. Our study indicates that, due to dynamical friction, more massive satellites are allowed to probe a wider range of impact parameters at accretion time, while small companions need almost purely radial infall trajectories in order to produce shells. We also find a number of special cases, as a consequence of the additional complexity introduced by the cosmological setting. These include galaxies with multiple shell-forming progenitors, satellite-of-satellites also forming shells, or satellites that fail to produce shells due to multiple major mergers happening in quick succession.Comment: 27 pages, 18 figures. Accepted for publication in MNRAS (new figures 3 and D1 + additional minor changes to match accepted version

Halo mass function and scale-dependent bias from N-body simulations with non-Gaussian initial conditions

We perform a series of high-resolution N-body simulations of cosmological structure formation starting from Gaussian and non-Gaussian initial conditions. We adopt the best-fitting cosmological parameters from the third- and fifth-year data releases of the Wilkinson Microwave Anisotropy Probe, and we consider non-Gaussianity of the local type parametrized by eight different values of the non-linearity parameter fNL. Building upon previous work based on the Gaussian case, we show that, when expressed in terms of suitable variables, the mass function of friends-of-friends haloes is approximately universal (i.e. independent of redshift, cosmology and matter transfer function) to good precision (nearly 10 per cent) also in non-Gaussian scenarios. We provide fitting formulae for the high-mass end (M > 1013 h−1 M⊙) of the universal mass function in terms of fNL, and we also present a non-universal fit in terms of both fNL and z to be used for applications requiring higher accuracy. For Gaussian initial conditions, we extend our fit to a wider range of halo masses (M > 2.4 × 1010 h−1 M⊙) and we also provide a consistent fit of the linear halo bias. We show that, for realistic values of fNL, the matter power spectrum in non-Gaussian cosmologies departs from the Gaussian 1 by up to 2 per cent on the scales where the baryonic-oscillation features are imprinted on the two-point statistics. Finally, using both the halo power spectrum and the halo-matter cross spectrum, we confirm the strong k-dependence of the halo bias on large scales (k < 0.05 h Mpc−1) which was already detected in previous studies. However, we find that commonly used parametrizations based on the peak-background split do not provide an accurate description of our simulations which present extra dependencies on the wavenumber, the non-linearity parameter and, possibly, the clustering strength. We provide an accurate fit of the simulation data that can be used as a benchmark for future determinations of fNL with galaxy survey

Formation and Incidence of Shell Galaxies in the Illustris Simulation

Shells are low surface brightness tidal debris that appear as interleaved caustics with large opening angles, often situated on both sides of the galaxy center. In this paper, we study the incidence and formation processes of shell galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We identify shells at redshift z=0 using stellar surface density maps, and we use stellar history catalogs to trace the birth, trajectory and progenitors of each individual star particle contributing to the tidal feature. Out of a sample of the 220 most massive galaxies in Illustris ($\mathrm{M}_{\mathrm{200crit}}>6\times10^{12}\,\mathrm{M}_{\odot}$), $18\%\pm3\%$ of the galaxies exhibit shells. This fraction increases with increasing mass cut: higher mass galaxies are more likely to have stellar shells. Furthermore, the fraction of massive galaxies that exhibit shells decreases with increasing redshift. We find that shell galaxies observed at redshift $z=0$ form preferentially through relatively major mergers ($\gtrsim$1:10 in stellar mass ratio). Progenitors are accreted on low angular momentum orbits, in a preferred time-window between $\sim$4 and 8 Gyrs ago. Our study indicates that, due to dynamical friction, more massive satellites are allowed to probe a wider range of impact parameters at accretion time, while small companions need almost purely radial infall trajectories in order to produce shells. We also find a number of special cases, as a consequence of the additional complexity introduced by the cosmological setting. These include galaxies with multiple shell-forming progenitors, satellite-of-satellites also forming shells, or satellites that fail to produce shells due to multiple major mergers happening in quick succession.Comment: 27 pages, 18 figures. Accepted for publication in MNRAS (new figures 3 and D1 + additional minor changes to match accepted version

Cosmological perturbation theory in a matter-dominated universe: the gradient expansion.

La teoria delle perturbazioni dello spazio-tempo si basa sulla stessa idea delle teorie perturbative di ogni tipo: si vuole trovare una soluzione approssimata di una qualche equazione di campo (le Equazioni di Einstein), considerandola come una piccola deviazione da una certa soluzione nota di riferimento (il background: di solito la metrica di Friedmann-Robertson-Walker). In Relatività Generale le complicazioni nascono dal fatto che ciò che si deve perturbare non sono solo i campi in una certa geometria -campi relativi al contenuto di materia in senso stretto o campi scalari come l'inflatone e quelli dei modelli di Dark Energy-, ma la geometria stessa, ovvero la metrica. Nella tesi noi ci limitiamo allo studio delle perturbazioni in universi dominati da un fluido perfetto con pressione nulla, detto di "polvere", assunto irrotazionale. In gauge sincrona e comovente, presentiamo il calcolo al primo e al secondo ordine delle funzioni perturbative in quella che è nota come espansione in gradienti e confrontiamo tale tecnica di calcolo con la procedura perturbativa standard. La teoria standard prevede di perturbare una metrica di background di FRW intorno alle correzioni (piccole) a tale metrica, contenenti a priori tutti e tre i modi perturbativi: scalare, vettoriale e tensoriale. In altre parole, si assume FRW come una buona approssimazione all'ordine zero per descrivere il nostro universo. La perturbazione è implementata attraverso funzioni dello spazio e del tempo, la cui forma in funzione del potenziale gravitazionale peculiare si determina ai vari ordini risolvendo iterativamente le Equazioni di Einstein (ci si ferma al secondo ordine). Nella tesi il punto di partenza è lo stesso di quello standard: si introducono le due variabili fisiche "espansione volumetrica" e "shear" e si scrivono le Equazioni di Einstein in formalismo ADM. La procedura di perturbazione però diversa. Si parte da una metrica spaziale contenente le funzioni perturbative Ψ e Χ della teoria standard, a loro volta contenenti tutti gli ordini di questa espansione: al tempo iniziale si ha a che fare con una metrica "seed" conforme a FRW per un fattore esponenziale dipendente dal punto. Quindi si considera come parametro perturbativo non la grandezza dello scostamento dal background ma il contenuto di gradienti spaziali, cosicchè la metrica all'ordine zero (così come ogni altro campo) è quella non contenente derivate spaziali. Conteggiare il contenuto dei gradienti nei diversi ordini equivale a considerare le lunghezze scala delle variazioni spaziali della metrica (e degli altri campi) più grandi, in diversa approssimazione, rispetto al tempo scala delle variazioni temporali delle stesse quantità: ne risulta un metodo di approssimazione non lineare per studiare come le disomogeneità cosmologiche crescono a partire da perturbazioni iniziali, il nostro "seed" (generato dalle perturbazioni inflazionarie) su scale inizialmente molto maggiori del raggio di Hubble. Nella tesi, quindi, dopo aver descritto la dinamica della polvere irrotazionale, commentato la nostra scelta di gauge e riassunto le idee generali delle perturbazioni cosmologiche, otteniamo Ψ e Χ fino al secondo ordine risolvendo rispettivamente l'equazione di evoluzione dello scalare di espansione volumetrica e del tensore di shear, verifichiamo i vincoli sull'energia e sul momento, proseguiamo nel confronto con i risultati standard attraverso un'opportuna procedura e infine mostriamo la forma che la parte magnetica del tensore di Weyl assume in questo approccio

Feedback reshapes the baryon distribution within haloes, in halo outskirts, and beyond: the closure radius from dwarfs to massive clusters

We explore three sets of cosmological hydrodynamical simulations, IllustrisTNG, EAGLE, and SIMBA, to investigate the physical processes impacting the distribution of baryons in and around haloes across an unprecedented mass range of $10^8<M_{\rm 200c}/{\rm M_{\odot}}<10^{15}$, from the halo centre out to scales as large as $30\,R_{\rm 200c}$. We demonstrate that baryonic feedback mechanisms significantly redistribute gas, lowering the baryon fractions inside haloes while simultaneously accumulating this material outside the virial radius. To understand this large-scale baryonic redistribution and identify the dominant physical processes responsible, we examine several variants of TNG that selectively exclude stellar and AGN feedback, cooling, and radiation. We find that heating from the UV background in low-mass haloes, stellar feedback in intermediate-mass haloes, and AGN feedback in groups ($10^{12} \leq M_{\rm 200c}/{\rm M_{\odot}}<10^{14}$) are the dominant processes. Galaxy clusters are the least influenced by these processes on large scales. We introduce a new halo mass-dependent characteristic scale, the closure radius $R_{\rm c}$, within which all baryons associated with haloes are found. For groups and clusters, we introduce a universal relation between this scale and the halo baryon fraction: $R_{\rm c}/R_{\rm 200c,500c}-1=\beta(z)(1-f_{\rm b}(<R_{\rm 200c,500c})/f_{\rm b,cosmic})$, where $\beta(z)=\alpha\,(1+z)^\gamma$, and $\alpha$ and $\gamma$ are free parameters fit using the simulations. Accordingly, we predict that all baryons associated with observed X-ray haloes can be found within $R_{\rm c}\sim 1.5-2.5 R_{\rm 200c}$. Our results can be used to constrain theoretical models, particularly the physics of supernova and AGN feedback, as well as their interplay with environmental processes, through comparison with current and future X-ray and SZ observations.Comment: Submitted to MNRA