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Register a new userThe APOSTLE simulations: solutions to the Local Groups cosmic puzzles
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Till Sawala
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The Local Group of galaxies offer some of the most discriminating tests of models of cosmic structure formation. For example, observations of the Milky Way (MW) and Andromeda satellite populations appear to be in disagreement with N-body simulations of the Lambda Cold Dark Matter ({Lambda}CDM) model: there are far fewer satellite galaxies than substructures in cold dark matter halos (the missing satellites problem); dwarf galaxies seem to avoid the most massive substructures (the too-big-to-fail problem); and the brightest satellites appear to orbit their host galaxies on a thin plane (the planes of satellites problem). Here we present results from APOSTLE (A Project Of Simulating The Local Environment), a suite of cosmological hydrodynamic simulations of twelve volumes selected to match the kinematics of the Local Group (LG) members. Applying the Eagle code to the LG environment, we find that our simulations match the observed abundance of LG galaxies, including the satellite galaxies of the MW and Andromeda. Due to changes to the structure of halos and the evolution in the LG environment, the simulations reproduce the observed relation between stellar mass and velocity dispersion of individual dwarf spheroidal galaxies without necessitating the formation of cores in their dark matter profiles. Satellite systems form with a range of spatial anisotropies, including one similar to that of the MW, confirming that such a configuration is not unexpected in {Lambda}CDM. Finally, based on the observed velocity dispersion, size, and stellar mass, we provide new estimates of the maximum circular velocity for the halos of nine MW dwarf spheroidals.
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We examine the spatial distribution of the oldest and most metal poor stellar populations of Milky Way-sized galaxies using the APOSTLE cosmological hydrodynamical simulations of the Local Group. In agreement with earlier work, we find strong radial gradients in the fraction of the oldest (tform < 0.8 Gyr) and most metal poor ([Fe/H]< -2.5) stars, both of which increase outwards. The most metal poor stars form over an extended period of time; half of them form after z = 5.3, and the last 10% after z = 2.8. The age of the metal poor stellar population also shows significant variation with environment; a high fraction of them are old in the galaxys central regions and an even higher fraction in some individual dwarf galaxies, with substantial scatter from dwarf to dwarf. Overall, over half of the stars that belong to both the oldest and most metal-poor population are found outside the solar circle. Somewhat counter-intuitively, we find that dwarf galaxies with a large fraction of metal poor stars that are very old are systems where metal poor stars are relatively rare, but where a substantial old population is present. Our results provide guidance for interpreting the results of surveys designed to hunt for the earliest and most pristine stellar component of our Milky Way.
The APOSTLE cosmological hydrodynamical simulation suite is a collection of twelve regions $sim 5$ Mpc in diameter, selected to resemble the Local Group of galaxies in terms of kinematics and environment, and re-simulated at high resolution (minimum gas particle mass of $10^4,{rm M}_odot$) using the galaxy formation model and calibration developed for the EAGLE project. I select a sample of dwarf galaxies ($60 < V_{rm max}/{rm km},{rm s}^{-1} < 120$) from these simulations and construct synthetic spatially- and spectrally-resolved observations of their 21-cm emission. Using the $^{3{rm D}}$BAROLO tilted-ring modelling tool, I extract rotation curves from the synthetic data cubes. In many cases, non-circular motions present in the gas disc hinder the recovery of a rotation curve which accurately traces the underlying mass distribution; a large central deficit of dark matter, relative to the predictions of cold dark matter N-body simulations, may then be erroneously inferred.
We detected 10 compact galaxy groups (CGs) at $z=0$ in the semi-analytic galaxy catalog of Guo et al. (2011) for the milli-Millennium Cosmological Simulation (sCGs in mGuo2010a). We aimed to identify potential canonical pathways for compact group evolution and thus illuminate the history of observed nearby compact groups. By constructing merger trees for $z=0$ sCG galaxies, we studied the cosmological evolution of key properties, and compared them with $z=0$ Hickson CGs (HCGs). We found that, once sCG galaxies come within 1 (0.5) Mpc of their most massive galaxy, they remain within that distance until $z=0$, suggesting sCG birth redshifts. At $z=0$ stellar masses of sCG most-massive galaxies are within $10^{10} lesssim M_{ast}/M_{odot} lesssim 10^{11}$. In several cases, especially in the two 4- and 5-member systems, the amount of cold gas mass anti-correlates with stellar mass, which in turn correlates with hot gas mass. We define the angular difference between group members 3D velocity vectors, $Deltatheta_{rm vel}$, and note that many of the groups are long-lived because their small values of $Deltatheta_{rm vel}$ indicate a significant parallel component. For triplets in particular, $Deltatheta_{rm vel}$ values range between $20^{circ}$ and $40^{circ}$ so that galaxies are coming together along roughly parallel paths, and pairwise separations do not show large pronounced changes after close encounters. The best agreement between sCG and HCG physical properties is for $M_{ast}$ galaxy values, but HCG values are higher overall, including for SFRs. Unlike HCGs, due to a tail at low SFR and $M_{ast}$, and a lack of $M_{ast}gtrsim 10^{11}M_{odot}$ galaxies, only a few sCG galaxies are on the star-forming main sequence.
We use a large sample of isolated dark matter halo pairs drawn from cosmological N-body simulations to identify candidate systems whose kinematics match that of the Local Group of Galaxies (LG). We find, in agreement with the timing argument and earlier work, that the separation and approach velocity of the Milky Way (MW) and Andromeda (M31) galaxies favour a total mass for the pair of $sim 5times 10^{12} ,M_{odot}$. A mass this large, however, is difficult to reconcile with the small relative tangential velocity of the pair, as well as with the small deceleration from the Hubble flow observed for the most distant LG members. Halo pairs that match these three criteria have average masses a factor of $sim 2$ times smaller than suggested by the timing argument, but with large dispersion. Guided by these results, we have selected $12$ halo pairs with total mass in the range $1.6$-$3.6 times 10^{12},M_{odot}$ for the APOSTLE project (A Project Of Simulating The Local Environment), a suite of hydrodynamical resimulations at various numerical resolution levels (reaching up to $sim10^{4},M_{odot}$ per gas particle) that use the subgrid physics developed for the EAGLE project. These simulations reproduce, by construction, the main kinematics of the MW-M31 pair, and produce satellite populations whose overall number, luminosities, and kinematics are in good agreement with observations of the MW and M31 companions. The APOSTLE candidate systems thus provide an excellent testbed to confront directly many of the predictions of the $Lambda$CDM cosmology with observations of our local Universe.
We use cosmological simulations from the FIRE (Feedback In Realistic Environments) project to study the baryon cycle and galaxy mass assembly for central galaxies in the halo mass range $M_{rm halo} sim 10^{10} - 10^{13} M_{odot}$. By tracing cosmic inflows, galactic outflows, gas recycling, and merger histories, we quantify the contribution of physically distinct sources of material to galaxy growth. We show that in situ star formation fueled by fresh accretion dominates the early growth of galaxies of all masses, while the re-accretion of gas previously ejected in galactic winds often dominates the gas supply for a large portion of every galaxys evolution. Externally processed material contributes increasingly to the growth of central galaxies at lower redshifts. This includes stars formed ex situ and gas delivered by mergers, as well as smooth intergalactic transfer of gas from other galaxies, an important but previously under-appreciated growth mode. By $z=0$, wind transfer, i.e. the exchange of gas between galaxies via winds, can dominate gas accretion onto $sim L^{*}$ galaxies over fresh accretion and standard wind recycling. Galaxies of all masses re-accrete >50% of the gas ejected in winds and recurrent recycling is common. The total mass deposited in the intergalactic medium per unit stellar mass formed increases in lower mass galaxies. Re-accretion of wind ejecta occurs over a broad range of timescales, with median recycling times ($sim 100-350$ Myr) shorter than previously found. Wind recycling typically occurs at the scale radius of the halo, independent of halo mass and redshift, suggesting a characteristic recycling zone around galaxies that scales with the size of the inner halo and the galaxys stellar component.
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