Browsing by Subject "Local Group"

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Now showing items 1-12 of 12
  • Schewtschenko, J. A.; Baugh, C. M.; Wilkinson, R. J.; Boehm, C.; Pascoli, S.; Sawala, T. (2016)
    In the thermal dark matter (DM) paradigm, primordial interactions between DM and Standard Model particles are responsible for the observed DM relic density. In Boehm et al., we showed that weak-strength interactions between DM and radiation (photons or neutrinos) can erase small-scale density fluctuations, leading to a suppression of the matter power spectrum compared to the collisionless cold DM (CDM) model. This results in fewer DM subhaloes within Milky Way-like DM haloes, implying a reduction in the abundance of satellite galaxies. Here we use very high-resolution N-body simulations to measure the dynamics of these subhaloes. We find that when interactions are included, the largest subhaloes are less concentrated than their counterparts in the collisionless CDM model and have rotation curves that match observational data, providing a new solution to the 'too big to fail' problem.
  • Gaia Collaboration; Helmi, A.; Muinonen, K.; Fedorets, G.; Granvik, M.; Siltala, L. (2018)
    Aims. The goal of this paper is to demonstrate the outstanding quality of the second data release of the Gaia mission and its power for constraining many different aspects of the dynamics of the satellites of the Milky Way. We focus here on determining the proper motions of 75 Galactic globular clusters, nine dwarf spheroidal galaxies, one ultra-faint system, and the Large and Small Magellanic Clouds. Methods. Using data extracted from the Gaia archive, we derived the proper motions and parallaxes for these systems, as well as their uncertainties. We demonstrate that the errors, statistical and systematic, are relatively well understood. We integrated the orbits of these objects in three different Galactic potentials, and characterised their properties. We present the derived proper motions, space velocities, and characteristic orbital parameters in various tables to facilitate their use by the astronomical community. Results. Our limited and straightforward analyses have allowed us for example to (i) determine absolute and very precise proper motions for globular clusters; (ii) detect clear rotation signatures in the proper motions of at least five globular clusters; (iii) show that the satellites of the Milky Way are all on high-inclination orbits, but that they do not share a single plane of motion; (i v) derive a lower limit for the mass of the Milky Way of 9.1(-2.6)(+6.2) x 10(11) M-circle dot based on the assumption that the Leo I dwarf spheroidal is bound; (v) derive a rotation curve for the Large Magellanic Cloud based solely on proper motions that is competitive with line-of-sight velocity curves, now using many orders of magnitude more sources; and (v i) unveil the dynamical effect of the bar on the motions of stars in the Large Magellanic Cloud. Conclusions. All these results highlight the incredible power of the Gaia astrometric mission, and in particular of its second data release.
  • Lovell, Mark R.; Bose, Sownak; Boyarsky, Alexey; Crain, Robert A.; Frenk, Carlos S.; Hellwing, Wojciech A.; Ludlow, Aaron D.; Navarro, Julio F.; Ruchayskiy, Oleg; Sawala, Till; Schaller, Matthieu; Schaye, Joop; Theuns, Tom (2017)
    We study galaxy formation in sterile neutrino dark matter models that differ significantly from both cold and from 'warm thermal relic' models. We use the EAGLE code to carry out hydrodynamic simulations of the evolution of pairs of galaxies chosen to resemble the Local Group, as part of the APOSTLE simulations project. We compare cold dark matter (CDM) with two sterile neutrino models with 7 keV mass: one, the warmest among all models of this mass (LA120) and the other, a relatively cold case (LA10). We show that the lower concentration of sterile neutrino subhaloes compared to their CDM counterparts makes the inferred inner dark matter content of galaxies like Fornax (or Magellanic Clouds) less of an outlier in the sterile neutrino cosmologies. In terms of the galaxy number counts, the LA10 simulations are indistinguishable from CDM when one takes into account halo-to-halo (or 'simulation-to-simulation') scatter. In order for the LA120 model to match the number of Local Group dwarf galaxies, a higher fraction of low-mass haloes is required to form galaxies than is predicted by the EAGLE simulations. As the census of the Local Group galaxies nears completion, this population may provide a strong discriminant between cold and warm dark matter models.
  • Sawala, Till; Pihajoki, Pauli; Johansson, Peter H.; Frenk, Carlos S.; Navarro, Julio F.; Oman, Kyle A.; White, Simon D. M. (2017)
    The predicted abundance and properties of the low-mass substructures embedded inside larger dark matter haloes differ sharply among alternative dark matter models. Too small to host galaxies themselves, these subhaloes may still be detected via gravitational lensing or via perturbations of the Milky Way's globular cluster streams and its stellar disc. Here, we use the APOSTLE cosmological simulations to predict the abundance and the spatial and velocity distributions of subhaloes in the range 10(6.5)-10(8.5)M(circle dot) inside haloes of mass similar to 10(12) M-circle dot in Lambda cold dark matter. Although these subhaloes are themselves devoid of baryons, we find that baryonic effects are important. Compared to corresponding dark matter only simulations, the loss of baryons from subhaloes and stronger tidal disruption due to the presence of baryons near the centre of the main halo reduce the number of subhaloes by similar to 1/4 to 1/2, independently of subhalo mass, but increasingly towards the host halo centre. We also find that subhaloes have non-Maxwellian orbital velocity distributions, with centrally rising velocity anisotropy and positive velocity bias that reduces the number of low-velocity subhaloes, particularly near the halo centre. We parametrize the predicted population of subhaloes in terms of mass, galactocentric distance and velocities. We discuss implications of our results for the prospects of detecting dark matter substructures and for possible inferences about the nature of dark matter.
  • Siilin, Kasper (Helsingin yliopisto, 2022)
    I use hydrodynamic cosmological N-body simulations to study the effect that a secondary period of inflation, driven by a spectator field, would have on the Local Group substructures. Simulations of the Local Group have been widely adopted for studying the nonlinear structure formation on small scales. This is essentially because detailed observations of faint dwarf galaxies are mostly limited to within the Local Group and its immediate surroundings. In particular, the ∼ 100 dwarf galaxies, discovered out to a radius of 3 Mpc from the Sun, constitute a sample that has the potential to discriminate between different cosmological models on small scales, when compared to simulations. The two-period inflaton-curvaton inflation model is one such example, since it gives rise to a small-scale cut-off in the ΛCDM primordial power spectrum, compared to the power spectrum of the ΛCDM model with single field power-law inflation. I investigate the substructures that form in a simulated analogue of the Local Group, with initial conditions that incorporate such a modified power spectrum. The most striking deviation, from the standard power-law inflation, is the reduction of the total number of subhalos, with v_max > 10 km/s, by a factor of ∼ 10 for isolated subhalos and by a factor of ∼ 6 for satellites. However, the reduction is mostly in the number of non-star-forming subhalos, and the studied model thus remains a viable candidate, taking into account the uncertainty in the Local Group total mass estimate. The formation of the first galaxies is also delayed, and the central densities of galaxies with v_max < 50 km/s are lowered: their circular velocities at 1 kpc from the centre are decreased and the radii of maximum circular velocity are increased. As for the stellar mass-metallicity and the stellar mass-halo mass relations, or the selection effects from tidal disruption, I find no significant differences between the models.
  • Fattahi, Azadeh; Navarro, Julio F.; Sawala, Till; Frenk, Carlos S.; Oman, Kyle A.; Crain, Robert A.; Furlong, Michelle; Schaller, Matthieu; Schaye, Joop; Theuns, Tom; Jenkins, Adrian (2016)
    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 (LG) of galaxies. 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 similar to 5 x 10(12) M-circle dot. 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 similar to 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 x 10(12) M-circle dot for the APOSTLE project (A Project Of Simulating The Local Environment), a suite of hydrodynamical resimulations at various numerical resolution levels (reaching up to similar to 10(4) M-circle dot 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 cold dark matter cosmology with observations of our local Universe.
  • Sawala, Till; Teeriaho, Meri Tuuli; Johansson, Peter (2023)
    The total mass of the Local Group and the masses of its primary constituents, the Milky Way (MW) and M31, are important anchors for several cosmological questions. Recent independent measurements have consistently yielded halo masses close to 10(12)M(circle dot) for the MW, and 1-2 x 10(12)M(circle dot) for M31, while estimates derived from the pair's kinematics via the timing argument' have yielded a combined mass of around 5 x 10(12)M(circle dot). We analyse the extremely large UCHUU simulation to constrain the mass of the Local Group and its two most massive members. First, we demonstrate the importance of selecting pairs whose kinematics reflect their mutual interactions. Adopting the observed separation and radial velocity, we obtain a weighted posterior of 75 (+ 65) (-40) km s (-1) for the uncertain transv erse v elocity. Via Gaussian process regression, we infer a total mass of 3.2 (+ 1 . 2) (-0 . 9) x 10(12)M(circle dot), significantly below the timing argument value. Importantly, the remaining uncertainty is not rooted in the analysis or observational errors, but in the irreducible scatter in the kinematics-mass relation. We further find a mass for the less massive halo of 0.9 (+ 0 . 6) (-0 . 3) x 10(12)M(circle dot) and for the more massive halo of 2.3 (+ 1 . 0) (-0 . 9) x 10(12)M(circle dot) consistent with independent measurements of the masses of MW and M31, respectively. Incorporating the MW mass as an additional prior let us constrain all measurements further and determine that the MW is very likely less massive than M31.
  • Starkenburg, Else; Oman, Kyle A.; Navarro, Julio F.; Crain, Robert A.; Fattahi, Azadeh; Frenk, Carlos S.; Sawala, Till; Schaye, Joop (2017)
    We examine the spatial distribution of the oldest and most metal-poor stellar populations of Milky Way-sized galaxies using the A Project Of Simulating The Local Environment (APOSTLE) cosmological hydrodynamical simulations of the Local Group. In agreement with earlier work, we find strong radial gradients in the fraction of the oldest (t(form) <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 per cent 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 galaxy's central regions and an even higher fraction in some individual dwarf galaxies, with substantial scatter from dwarf to dwarf. We investigate the dependence of these results on the assumptions made for metal mixing. 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.
  • Benitez-Llambay, Alejandro; Navarro, Julio F.; Frenk, Carlos S.; Sawala, Till; Oman, Kyle; Fattahi, Azadeh; Schaller, Matthieu; Schaye, Joop; Crain, Robert A.; Theuns, Tom (2017)
    We examine the baryon content of low-mass A cold dark matter (ACDM) haloes (10(8) <M-200/M-circle dot <5 x 10(9)) using the APOSTLE cosmological hydrodynamical simulations. Most of these systems are free of stars and have a gaseous content set by the combined effects of cosmic reionization, which imposes a mass-dependent upper limit, and of ram-pressure stripping, which reduces it further in high-density regions. Haloes mainly affected by reionization (RELHICS; REionization-Limited H I Clouds) inhabit preferentially low-density regions and make up a population where the gas is in hydrostatic equilibrium with the dark matter potential and in thermal equilibrium with the ionizing UV background. Their thermodynamic properties are well specified, and their gas density and temperature profiles may be predicted in detail. Gas in RELHICs is nearly fully ionized but with neutral cores that span a large range of HI masses and column densities and have negligible non-thermal broadening. We present predictions for their characteristic sizes and central column densities; the massive tail of the distribution should be within reach of future blind HI surveys. Local Group RELHICs (LGRs) have some properties consistent with observed Ultra Compact High Velocity Clouds (UCHVCs) but the sheer number of the latter suggests that most UCHVCs are not RELHICS. Our results suggest that LGRs (i) should typically be beyond 500 kpc from the Milky Way or M31; (ii) have positive Galactocentric radial velocities; (iii) H I sizes not exceeding 1 kpc, and (iv) should be nearly round. The detection and characterization of RELHICS would offer a unique probe of the small-scale clustering of CDM.
  • Sawala, Till; McAlpine, Stuart; Jasche, Jens; Lavaux, Guilhem; Jenkins, Adrian; Johansson, Peter; Frenk, Carlos S. (2021)
    We introduce ‘Simulations Beyond The Local Universe’ (SIBELIUS) that connect the Local Group (LG) to its cosmic environment. We show that introducing hierarchical small-scale perturbations to a density field constrained on large scales by observations provides an efficient way to explore the sample space of LG analogues. From more than 60 000 simulations, we identify a hierarchy of LG characteristics emanating from different scales: the total mass, orientation, orbital energy, and the angular momentum are largely determined by modes above λ = 1.6 comoving Mpc (cMpc) in the primordial density field. Smaller scale variations are mostly manifest as perturbations to the MW–M31 orbit, and we find that the observables commonly used to describe the LG – the MW–M31 separation and radial velocity – are transient and depend on specifying scales down to 0.2 cMpc in the primordial density field. We further find that the presence of M33/LMC analogues significantly affects the MW–M31 orbit and its sensitivity to small-scale perturbations. We construct initial conditions that lead to the formation of an LG whose primary observables precisely match the current observations.
  • Wang, M. -Y.; Fattahi, Azadeh; Cooper, Andrew P.; Sawala, Till; Strigari, Louis E.; Frenk, Carlos S.; Navarro, Julio F.; Oman, Kyle; Schaller, Matthieu (2017)
    We use the APOSTLE (A Project Of Simulating The Local Environment) cosmological hydrodynamic simulations to examine the effects of tidal stripping on cold dark matter subhaloes that host three of the most luminous Milky Way dwarf satellite galaxies: Fornax, Sculptor and Leo I. We identify simulated satellites that match the observed spatial and kinematic distributions of stars in these galaxies, and track their evolution after infall. We find similar to 30 per cent of subhaloes hosting satellites with present-day stellar mass 10(6)-10(8) M-circle dot experience >20 per cent stellar mass-loss after infall. Fornax analogues have earlier infall times compared to Sculptor and Leo I analogues. Star formation in Fornax analogues continues for similar to 3-6 Gyr after infall, whereas Sculptor and Leo I analogues stop forming stars
  • Fattahi, Azadeh; Navarro, Julio F.; Frenk, Carlos S.; Oman, Kyle A.; Sawala, Till; Schaller, Matthieu (2018)
    The shallow faint-end slope of the galaxy mass function is usually reproduced in Lambda cold dark matter (Lambda CDM) galaxy formation models by assuming that the fraction of baryons that turn into stars drops steeply with decreasing halo mass and essentially vanishes in haloes with maximum circular velocities Vmax <20-30 km s(-1). Dark-matter-dominated dwarfs should therefore have characteristic velocities of about that value, unless they are small enough to probe only the rising part of the halo circular velocity curve (i.e. half-mass radii, r(1/2)