Constrained Local UniversE Simulations Research Articles

CLUES about M33: the reversed radial stellar age gradient in the outskirts of Triangulum galaxy

HST/ACS observations along the major axis of M33 show that the mean age of its stars decreases with increasing distance from the galaxy center. Such a behavior is consistent with an inside-out growth of the disc. However, in the outermost observed field, at r$\simeq$11.6 kpc, a reversal of this gradient is detected, with old stars found in high percentages beyond this radius. In this work we investigate the origin of such a reversal in stellar age gradient, by using a simulated M33 analogue from the Constrained Local UniversE Simulations (CLUES). The simulated M33 is similar to the observed one in terms of mass, rotation velocity, surface brightness and, similar to what has been reported in observations, shows a stellar age turnaround at large radii. We demonstrate that this reversal is mostly a result of stellar accretion from old satellite galaxies and, to a lesser extent, of stellar migration of in-situ stars. The old accreted stars, with formation times tf < 4 Gyrs, are kinematically hot and can be differentiated from the in-situ stars by their high velocity dispersion and the fact that they do not have rotationally-supported orbits. In the future, obtaining kinematic information of the stars in the outskirt of M33 will help to verify this scenario.

Reionization of the Milky Way, M31, and their satellites – I. Reionization history and star formation

Observations of the Milky Way (MW), M31, and their vicinity, known as the Local Group (LG), can provide clues about the sources of reionization. We present a suite of radiative transfer simulations based on initial conditions provided by the Constrained Local UniversE Simulations (CLUES) project that are designed to recreate the Local Universe, including a realistic MW-M31 pair and a nearby Virgo. Our box size (91 Mpc) is large enough to incorporate the relevant sources of ionizing photons for the LG. We employ a range of source models, mimicking the potential effects of radiative feedback for dark matter haloes between $10^{8}-10^{9}$ M$_{\odot}$. Although the LG mostly reionizes in an inside-out fashion, the final 40 per cent of its ionization shows some outside influence. For the LG satellites, we find no evidence that their redshift of reionization is related to the present-day mass of the satellite or the distance from the central galaxy. We find that less than 20 per cent of present-day satellites for MW and M31 have undergone any star formation prior to the end of global reionization. Approximately five per cent of these satellites could be classified as fossils, meaning the majority of star formation occurred at these early times. The more massive satellites have more cumulative star formation prior to the end of global reionization, but the scatter is significant, especially at the low-mass end. Present-day mass and distance from the central galaxy are poor predictors for the presence of ancient stellar populations in satellite galaxies.

How did the Virgo cluster form?

While the Virgo cluster is the nearest galaxy cluster and therefore the best observed one, little is known about its formation history. In this paper, a set of cosmological simulations that resemble the Local Universe is used to shed the first light on this mystery. The initial conditions for these simulations are constrained with galaxy peculiar velocities of the second catalog of the Cosmicflows project using algorithms developed within the Constrained Local UniversE Simulation project. Boxes of 500 Mpc/h on a side are set to run a series of dark matter only constrained simulations. In each simulation, a unique dark matter halo can be reliably identified as Virgo's counterpart. The properties of these Virgo halos are in agreement at a 10-20% level with the global properties of the observed Virgo cluster. Their zero-velocity masses agree at one-sigma with the observational mass estimate. In all the simulations, the matter falls onto the Virgo objects along a preferential direction that corresponds to the observational filament and the slowest direction of collapse. A study of the mass accretion history of the Virgo candidates reveals the most likely formation history of the Virgo cluster, namely a quiet accretion over the last 7 Gigayears.

Constrained Local UniversE Simulations: a Local Group factory

Near field cosmology is practiced by studying the Local Group (LG) and its neighbourhood. The present paper describes a framework for simulating the near field on the computer. Assuming the LCDM model as a prior and applying the Bayesian tools of the Wiener filter (WF) and constrained realizations of Gaussian fields to the Cosmicflows-2 (CF2) survey of peculiar velocities, constrained simulations of our cosmic environment are performed. The aim of these simulations is to reproduce the LG and its local environment. Our main result is that the LG is likely a robust outcome of the LCDM scenario when subjected to the constraint derived from CF2 data, emerging in an environment akin to the observed one. Three levels of criteria are used to define the simulated LGs. At the base level, pairs of halos must obey specific isolation, mass and separation criteria. At the second level the orbital angular momentum and energy are constrained and on the third one the phase of the orbit is constrained. Out of the 300 constrained simulations 146 LGs obey the first set of criteria, 51 the second and 6 the third. The robustness of our LG factory enables the construction of a large ensemble of simulated LGs. Suitable candidates for high resolution hydrodynamical simulations of the LG can be drawn from this ensemble, which can be used to perform comprehensive studies of the formation of the LG

Cosmicflows Constrained Local UniversE Simulations

This paper combines observational datasets and cosmological simulations to generate realistic numerical replicas of the nearby Universe. These latter are excellent laboratories for studies of the non-linear process of structure formation in our neighborhood. With measurements of radial peculiar velocities in the Local Universe (cosmicflows-2) and a newly developed technique, we produce Constrained Local UniversE Simulations (CLUES). To assess the quality of these constrained simulations, we compare them with random simulations as well as with local observations. The cosmic variance, defined as the mean one-sigma scatter of cell-to-cell comparison between two fields, is significantly smaller for the constrained simulations than for the random simulations. Within the inner part of the box where most of the constraints are, the scatter is smaller by a factor 2 to 3 on a 5 Mpc/h scale with respect to that found for random simulations. This one-sigma scatter obtained when comparing the simulated and the observation-reconstructed velocity fields is only 104 +/- 4 km/s i.e. the linear theory threshold. These two results demonstrate that these simulations are in agreement with each other and with the observations of our neighborhood. For the first time, simulations constrained with observational radial peculiar velocities resemble the Local Universe up to a distance of 150 Mpc/h on a scale of a few tens of megaparsecs. When focusing on the inner part of the box, the resemblance with our cosmic neighborhood extends to a few megaparsecs (< 5 Mpc/h). The simulations provide a proper Large Scale environment for studies of the formation of nearby objects.

The distribution of gas in the Local Group from constrained cosmological simulations: the case for Andromeda and the Milky Way galaxies

We study the gas distribution in the Milky Way and Andromeda using a constrained cosmological simulation of the Local Group (LG) within the context of the CLUES (Constrained Local UniversE Simulations) project. We analyse the properties of gas in the simulated galaxies at $z=0$ for three different phases: `cold', `hot' and HI, and compare our results with observations. The amount of material in the hot halo ($M_{hot}\approx 4-5\times10^{10}\,$M$_{\odot}$), and the cold ($M_{cold}(r\lesssim10\,$kpc$)\approx10^{8}\,$M$_{\odot}$) and HI ($M_{HI}(r\lesssim50\,$kpc$)\approx 3-4\times10^8\,$M$_{\odot}$) components display a reasonable agreement with observations. We also compute the accretion/ejection rates together with the HI (radial and all-sky) covering fractions. The integrated HI accretion rate within $r=50\,$kpc gives $\sim$$0.2-0.3\,$M$_{\odot}\,$yr$^{-1}$, i.e. close to that obtained from high-velocity clouds in the Milky Way. We find that the global accretion rate is dominated by hot material, although ionized gas with $T\lesssim10^5\,$K can contribute significantly too. The $net$ accretion rates of $all$ material at the virial radii are $6-8\,$M$_{\odot}\,$yr$^{-1}$. At $z=0$, we find a significant gas excess between the two galaxies, as compared to any other direction, resulting from the overlap of their gaseous haloes. In our simulation, the gas excess first occurs at $z\sim1$, as a consequence of the kinematical evolution of the LG.

THE STELLAR-TO-HALO MASS RELATION FOR LOCAL GROUP GALAXIES

We contend that a single power law halo mass distribution is appropriate for direct matching to the stellar masses of observed Local Group dwarf galaxies, allowing the determination of the slope of the stellar mass-halo mass relation for low mass galaxies. Errors in halo masses are well defined as the Poisson noise of simulated local group realisations, which we determine using constrained local universe simulations (CLUES). For the stellar mass range 10$^7$<M*<10$^8$M$_\odot$, for which we likely have a complete census of observed galaxies, we find that the stellar mass-halo mass relation follows a power law with slope of 3.1, significantly steeper than most values in the literature. The steep relation between stellar and halo masses indicates that Local Group dwarf galaxies are hosted by dark matter halos with a small range of mass. Our methodology is robust down to the stellar mass to which the census of observed Local Group galaxies is complete, but the significant uncertainty in the currently measured slope of the stellar-to halo mass relation will decrease dramatically if the Local Group completeness limit was $10^{6.5}$M$\odot$ or below, highlighting the importance of pushing such limit to lower masses and larger volumes.

Weighing the Local Group in the presence of dark energy

Abstract We revise the mass estimate of the Local Group (LG) when dark energy (in the form of the cosmological constant) is incorporated into the timing argument (TA) mass estimator for the LG. Assuming the age of the Universe and the cosmological constant according to the recent values from the Planck cosmic microwave background experiment, we find the mass of the LG to be MTAΛ = (4.73 ± 1.03) × 1012 M⊙ which is 13 per cent higher than the classical TA mass estimate. This partly explains the discrepancy between earlier results from Lambda cold dark matter simulations and the classical TA. When a similar analysis is performed on 16 LG-like galaxy pairs from the Constrained Local Universe Simulations, we find that the scatter in the ratio of the virial to the TA estimated mass is given by Mvir/MTAΛ = 1.04 ± 0.16. Applying it to the LG mass estimation we find a calibrated Mvir = (4.92 ± 1.08(obs.) ± 0.79(sys.)) × 1012 M⊙.

Size matters: the non-universal density profile of subhaloes in SPH simulations and implications for the Milky Way’s dSphs

We use dark matter only and full hydrodynamical Constrained Local UniversE Simulations (CLUES) of the formation of the Local Group to study the density profile of subhaloes of the simulated Milky Way and Andromeda galaxies. We show that the Einasto model provides the best description of the subhaloes' density profile, as opposed to the more commonly used NFW profile or any generalisation of it. We further find that the Einasto shape parameter \nEin\ is strongly correlated with the total subhalo mass, pointing towards the notion of a non-universality of the subhaloes' density profile. We observe that the effect of mass loss due to tidal stripping, in both the dark matter only and the hydrodynamical run, is the reduction of the shape parameter \nEin\ between the infall and the present time. Assuming now that the dSphs of our Galaxy follow the Einasto profile and using the maximum and minimum values of \nEin\ from our hydrodynamical simulation as a gauge, we can improve the observational constraints on the \Rmax-\Vmax\ pairs obtained for the brightest satellite galaxies of the Milky Way. When considering only the subhaloes with $-13.2\lesssim M_V\lesssim-8.8$, i.e. the range of luminosity of the classical dwarfs, we find that all our simulated objects are consistent with the observed dSphs if their haloes follow the Einasto model with $1.6\lesssim n_{\rm E} \lesssim5.3$. The numerically motivated Einasto profile for the observed dSphs will alleviate the recently presented "massive failures" problem.

Galaxies going MAD: the Galaxy-Finder Comparison Project

With the ever increasing size and complexity of fully self-consistent simulations of galaxy formation within the framework of the cosmic web, the demands upon object finders for these simulations has simultaneously grown. To this extent we initiated the Halo Finder Comparison Project that gathered together all the experts in the field and has so far led to two comparison papers, one for dark matter field haloes (Knebe et al. 2011), and one for dark matter subhaloes (Onions et al. 2012). However, as state-of-the-art simulation codes are perfectly capable of not only following the formation and evolution of dark matter but also account for baryonic physics (e.g. hydrodynamics, star formation, feedback) object finders should also be capable of taking these additional processes into consideration. Here we report on a comparison of codes as applied to the Constrained Local UniversE Simulation (CLUES) of the formation of the Local Group which incorporates much of the physics relevant for galaxy formation. We compare both the properties of the three main galaxies in the simulation (representing the MW, M31, and M33) as well as their satellite populations for a variety of halo finders ranging from phase-space to velocity-space to spherical overdensity based codes, including also a mere baryonic object finder. We obtain agreement amongst codes comparable to (if not better than) our previous comparisons, at least for the total, dark, and stellar components of the objects. However, the diffuse gas content of the haloes shows great disparity, especially for low-mass satellite galaxies. This is primarily due to differences in the treatment of the thermal energy during the unbinding procedure. We acknowledge that the handling of gas in halo finders is something that needs to be dealt with carefully, and the precise treatment may depend sensitively upon the scientific problem being studied.

Applying scale-free mass estimators to the Local Group in Constrained Local Universe Simulations

We use the recently proposed scale-free mass estimators to determine the masses of the Milky Way (MW) and Andromeda (M31) galaxy in a dark matter only Constrained Local UniversE Simulation (CLUES). While these mass estimators work rather well for isolated spherical host systems, we examine here their applicability to a simulated binary system with a unique satellite population similar to the observed satellites of MW and M31. We confirm that the scale-free estimators work also very well in our simulated Local Group galaxies with the right number of satellites which follow the observed radial distribution. In the isotropic case and under the assumption that the satellites are tracking the total gravitating mass, the power-law index of the radial satellite distribution $N(<r)\propto r^{3-\gamma}$ is directly related to the host's mass profile $M(<r)\propto r^{1-\alpha}$ as $\alpha=\gamma-2$. The use of this relation for any given $\gamma$ leads to highly accurate mass estimations which is a crucial point for observer, since they do not know a priori the mass profile of the MW and M31 haloes. We discuss possible bias in the mass estimators and conclude that the scale-free mass estimators can be satisfactorily applied to the real MW and M31 system.

Local Group progenitors: Lyman Alpha bright?

Abstract We present a novel approach of identifying the Milky Way (MW) and Andromeda (M31) progenitors that could be visible as Lyman Alpha emitters (LAEs) at z ∼ 6: we couple a snapshot from the Constrained Local UniversE Simulations (CLUES) project, which successfully reproduces the MW and M31 galaxies situated in their correct environment, to a LAE model. Exploring intergalactic medium (IGM) ionization states ranging from an almost neutral to a fully ionized one, we find that including (excluding) the effects of clustered sources the first Local Group progenitor appears as a LAE for a neutral hydrogen fraction . This number increases to five progenitors each of the MW and M31 being visible as LAEs for ; the contribution from clustered sources is crucial in making many of the progenitors visible in the Lyα for all the ionization states considered. The stellar mass of the Local Group LAEs ranges between 107.2 and 108 M⊙, the dust mass is between 104.6 and 105.1 M⊙ and the colour excess E(B−V) = 0.03–0.048. We find that the number density of these LAEs is higher than that of general field LAEs (observed in cosmological volumes) by about two (one) orders of magnitude for . Detections of such high LAE number densities at z ∼ 6 would be a clear signature of an overdense region that could evolve and resemble the Local Group volume at z = 0.

Renegade subhaloes in the Local Group

Abstract Using a dark matter only Constrained Local UniversE Simulation (CLUES) we examine the existence of subhaloes that change their affiliation from one of the two prominent hosts in the Local Group (i.e. the Milky Way and the Andromeda galaxy) to the other, and call these objects ‘renegade subhaloes’. In light of recent claims that the two Magellanic Clouds (MCs) may have originated from another region (or even the outskirts) of the Local Group or that they have been spawned by a major merger in the past of the Andromeda galaxy, we investigate the nature of such events. However, we cannot confirm that renegade subhaloes enter as deep into the potential well of their present host nor that they share the most simplest properties with the MCs, namely mass and relative velocity. Our simulation rather suggests that these renegade subhaloes appear to be flying past one host before being pulled into the other. A merger is not required to trigger such an event, it is rather the distinct environment of our simulated Local Group facilitating such behaviour. Since just a small fraction of the full z= 0 subhalo population are renegades, our study indicates that it will be intrinsically difficult to distinguish them despite clear differences in their velocity, radial distribution, shape and spin parameter distributions.

Too small to succeed? Lighting up massive dark matter subhaloes of the Milky Way

Abstract Using Constrained Local UniversE Simulations (CLUES) of the formation of the Local Group in a cosmological context, we investigate the recently highlighted problem that the majority of the most massive dark subhaloes of the Milky Way (MW) are too dense to host any of its bright satellites. In particular, we examine the influence of baryonic processes and find that they leave a twofold effect on the relation between the peak of the rotation curve and its position (Vmax and Rmax). Satellites with a large baryon fraction experience adiabatic contraction, thus decreasing Rmax while leaving Vmax more or less unchanged. Subhaloes with smaller baryon fractions undergo a decrease in Vmax possibly due to outflows of material. Furthermore, the situation of finding subhaloes in simulations that lie outside the confidence interval for possible hosts of the bright MW dwarf spheroidals appears to be far more prominent in cosmologies with a high σ8 normalization and depends on the mass of the host. We conclude that the problem cannot be simply solved by including baryonic processes and hence demands further investigations.

FORMATION OF DWARF SPHEROIDAL GALAXIES VIA MERGERS OF DISKY DWARFS

We perform collisionless N-body simulations to investigate whether binary mergers between rotationally-supported dwarfs can lead to the formation of dwarf spheroidal galaxies (dSphs). Our simulation campaign is based on a hybrid approach combining cosmological simulations and controlled numerical experiments. We select merger events from a Constrained Local UniversE (CLUES) simulation of the Local Group (LG) and record the properties of the interacting dwarf-sized halos. This information is subsequently used to seed controlled experiments of binary encounters between dwarf galaxies consisting of exponential stellar disks embedded in cosmologically-motivated dark matter halos. These simulations are designed to reproduce eight cosmological merger events, with initial masses of the interacting systems in the range ~ (5-60) x 10^7 Mo, occurring quite early in the history of the LG, more than 10 Gyr ago. We compute the properties of the merger remnants as a distant observer would and demonstrate that at least three of the simulated encounters produce systems with kinematic and structural properties akin to those of the classic dSphs in the LG. Tracing the history of the remnants in the cosmological simulation to z=0, we find that two dSph-like objects remain isolated at distances larger than 800 kpc from either the Milky Way or M31. These systems constitute plausible counterparts of the remote dSphs Cetus and Tucana which reside in the LG outskirts, far from the tidal influence of the primary galaxies. We conclude that merging of rotationally-supported dwarfs represents a viable mechanism for the formation of dSphs in the LG and similar environments.

Disentangling the dark matter halo from the stellar halo

The outer haloes of the Milky Way and Andromeda galaxies contain as much important information on their assembly and formation history as the properties of the discs resident in their centres. In this paper we have used the Constrained Local UniversE Simulation project to disentangle the stellar and DM component of three galaxies that resemble the MW, M31 and M33 using both DM only and DM + gas-dynamical simulations. Stars that are accreted in substructures and then stripped follow a completely different radial distribution than the stripped DM: the stellar halo is much more centrally concentrated than DM. In order to understand how the same physical process can lead to different z=0 radial profiles, we examined the potential at accretion of each stripped particle. We found that star particles sit at systematically higher potentials than DM, making them harder to strip. We then searched for a threshold in the potential of accreted particles \phi_[th], above which DM particles behave as star particles. We found such a threshold at >16 \phi_{subhalo}, where \phi_{subhalo} is the potential at a subhaloes edge at the time of accretion. Thus a rule as simple as selecting particles according to their potential at accretion is able to reproduce the effect that the complicated physics of star formation has on the stellar distribution. This niversal result reproduces the stellar halo to an accuracy of within ~2%. Studies which make use of DM particles as a proxy for stars will undoubtedly miscalculate their proper radial distribution and structure unless particles are selected according to their potential at accretion. Furthermore, we have examined the time it takes to strip a given star or DM particle after accretion. We find that, owing to their higher binding energies, stars take longer to be stripped than DM. The stripped DM halo is thus considerably older than the stripped stellar halo.

The luminosities of backsplash galaxies in constrained simulations of the Local Group

We study the differences and similarities in the luminosities of bound, infalling and the so-called backsplash (Gill et al. 2005) galaxies of the Milky Way and M31 using a hydrodynamical simulation performed within the Constrained Local UniversE Simulation (CLUES) project. The simulation models the formation of the Local Group within a self-consistent cosmological framework. We find that even though backsplash galaxies passed through the virial radius of their host halo and hence may have lost a (significant) fraction of their mass, their stellar populations are hardly affected. This leaves us with comparable luminosity functions for infalling and backsplash galaxies and hence little hope to decipher their past (and different) formation and evolutionary histories by luminosity measurements alone. Nevertheless, due to the tidal stripping of dark matter we find that the mass-to-light ratios have changed when comparing the various populations against each other: they are highest for the infalling galaxies and lowest for the bound satellites with the backsplash galaxies in-between.

The preferred direction of infalling satellite galaxies in the Local Group

Using a high resolution DM simulation of the Local Group (LG), conducted within the framework of the Constrained Local UniversE Simulation (CLUES) project, we investigate the nature of how satellites of the MW and M31 are accreted. Satellites of these galaxies are accreted anisotropically, entering the virial radius of their hosts from specific "spots" with respect to the large scale structure. Furthermore, material which is tidally stripped from these satellites is also, at z=0, distributed anisotropically and is characterized by an ellipsoidal sub-volume embedded in the halo. The angular pattern created by the locus of satellite infall points and the projected stripped DM is investigated within a coordinate system determined by the location of the LG companion and the simulated Virgo cluster across concentric shells ranging from 0.1 to 5 r_vir. Remarkably, the principal axis of the ellipsoidal sub-volume shows a coherent alignment extending from well within the halo to a few r_vir. A spherical harmonics transform applied to the angular distributions confirms the visual impression: namely, the angular distributions of both the satellites entry points and stripped DM is dominated by the l=2 quadrupole term, whose major principal axis is aligned across the shells considered. It follows that the structure of the outer (r>0.5 r_vir) regions of the main halos is closely related to the cosmic web within which it is embedded. Given the hypothesis that a large fraction of the stellar halo of the Milky Way has been accreted from satellites, the present results can be applied to the stellar halo of the MW and M31. We predict that the remnants of tidally stripped satellites should be embedded in streams of material composed of dark matter and stars. The present results can therefore shed light on the existence of satellites embedded within larger streams of matter, such as the Segue 2 satellite.

The grouping, merging and survival of subhaloes in the simulated Local Group

We use a simulation performed within the Constrained Local UniversE Simulation (CLUES) project to study a realistic Local Group-like object. We employ this group as a numerical laboratory for studying the evolution of the population of its subhaloes from the point of view of the effects it may have on the origin of different types of dwarf galaxies. We focus on the processes of tidal stripping of the satellites, their interaction, merging and grouping before infall. The tidal stripping manifests itself in the transition between the phase of mass accretion and mass loss seen in most subhaloes, which occurs at the moment of infall on to the host halo, and the change of the shape of their mass function with redshift. Although the satellites often form groups, they are loosely bound within them and do not interact with each other. The infall of a large group could however explain the observed peculiar distribution of the Local Group satellites, but only if it occurred recently. Mergers between prospective subhaloes are significant only during an early stage of evolution, i.e. more than 7 Gyr ago, when they are still outside the host haloes. Such events could thus contribute to the formation of more distant early type Milky Way companions. Once the subhaloes enter the host halo the mergers become very rare.

The impact of baryonic physics on the shape and radial alignment of substructures in cosmological dark matter haloes

We use two simulations performed within the Constrained Local UniversE Simulation (CLUES) project to study both the shape and radial alignment of (the dark matter component of) subhaloes; one of the simulations is a dark matter only model while the other run includes all the relevant gas physics and star formation recipes. We find that the involvement of gas physics does not have a statistically significant effect on either property -- at least not for the most massive subhaloes considered in this study. However, we observe in both simulations including and excluding gasdynamics a (pronounced) evolution of the dark matter shapes of subhaloes as well as of the radial alignment signal since infall time. Further, this evolution is different when positioned in the central and outer regions of the host halo today; while subhaloes tend to become more aspherical in the central 50% of their host's virial radius, the radial alignment weakens in the central regime while strengthening in the outer parts. We confirm that this is due to tidal torquing and the fact that subhaloes at pericentre move too fast for the alignment signal to respond.