94 research outputs found
The Orbital Ellipticity of Satellite Galaxies and the Mass of the Milky Way
We use simulations of Milky Way-sized dark matter haloes from the Aquarius
Project to investigate the orbits of substructure haloes likely, according to a
semi-analytic galaxy formation model, to host luminous satellites. These tend
to populate the most massive subhaloes and are on more radial orbits than the
majority of subhaloes found within the halo virial radius. One reason for this
(mild) kinematic bias is that many low-mass subhaloes have apocentres that
exceed the virial radius of the main host; they are thus excluded from subhalo
samples identified within the virial boundary, reducing the number of subhalos
on radial orbits. Two other factors contributing to the difference in orbital
shape between dark and luminous subhaloes are their dynamical evolution after
infall, which affects more markedly low-mass (dark) subhaloes, and a weak
dependence of ellipticity on the redshift of first infall. The ellipticity
distribution of luminous satellites exhibits little halo-to-halo scatter and it
may therefore be compared fruitfully with that of Milky Way satellites. Since
the latter depends sensitively on the total mass of the Milky Way we can use
the predicted distribution of satellite ellipticities to place constraints on
this important parameter. Using the latest estimates of position and velocity
of dwarfs compiled from the literature, we find that the most likely Milky Way
mass lies in the range , with a best fit value of . This value is consistent with Milky Way mass estimates based on
dynamical tracers or the timing argument.Comment: 10 pages, 9 figures, Accepted by MNRA
The missing dwarf galaxies of the Local Group
We study the Local Group (LG) dwarf galaxy population predicted by the APOSTLE ΛCDM cosmological hydrodynamics simulations. These indicate that: (i) the total mass within 3 Mpc of the Milky Way-Andromeda midpoint (M3Mpc) typically exceeds ∼3 times the sum of the virial masses (M200crit) of the two primaries and (ii) the dwarf galaxy formation efficiency per unit mass is uniform throughout the volume. This suggests that the satellite population within the virial radii of the Milky Way and Andromeda should make up fewer than one third of all LG dwarfs within 3 Mpc. This is consistent with the fraction of observed LG galaxies with stellar mass M* > 107 M⊙ that are satellites (12 out of 42; i.e., 28 per cent). For the APOSTLE galaxy mass-halo mass relation, the total number of such galaxies further suggests a LG mass of M3Mpc ∼ 1013 M⊙. At lower galaxy masses, however, the observed satellite fraction is substantially higher (42 per cent for M* > 105 M⊙). If this is due to incompleteness in the field sample, then ∼50 dwarf galaxies at least as massive as the Draco dwarf spheroidal must be missing from the current LG field dwarf inventory. The incompleteness interpretation is supported by the pronounced flattening of the LG luminosity function below M* ∼ 107 M⊙, and by the scarcity of low-surface brightness LG field galaxies compared to satellites. The simulations indicate that most missing dwarfs should lie near the virial boundaries of the two LG primaries, and predict a trove of nearby dwarfs that await discovery by upcoming wide-field imaging surveys
Can we really pick and choose? Benchmarking various selections of Gaia Enceladus/Sausage stars in observations with simulations
Large spectroscopic surveys plus Gaia astrometry have shown us that the inner stellar halo of the Galaxy is dominated by the debris of Gaia Enceladus/Sausage (GES). With the richness of data at hand, there are a myriad of ways these accreted stars have been selected. We investigate these GES selections and their effects on the inferred progenitor properties using data constructed from APOGEE and Gaia. We explore selections made in eccentricity, energy-angular momentum (E-Lz), radial action-angular momentum (Jr-Lz), action diamond, and [Mg/Mn]-[Al/Fe] in the observations, selecting between 144 and 1279 GES stars with varying contamination from in-situ and other accreted stars. We also use the Auriga cosmological hydrodynamic simulations to benchmark the different GES dynamical selections. Applying the same observational GES cuts to nine Auriga galaxies with a GES, we find that the Jr-Lz method is best for sample purity and the eccentricity method for completeness. Given the average metallicity of GES (−1.28 < [Fe/H] < −1.18), we use the z = 0 mass–metallicity relationship to find an average of ∼4 × 108 M⊙. We adopt a similar procedure and derive for the GES-like systems in Auriga and find that the eccentricity method overestimates the true by ∼2.6 × while E-Lz underestimates by ∼0.7 ×. Lastly, we estimate the total mass of GES to be using the relationship between the metallicity gradient and the GES-to-in-situ energy ratio. In the end, we cannot just ‘pick and choose’ how we select GES stars, and instead should be motivated by the science question
Can we really pick and choose? Benchmarking various selections of Gaia Enceladus/Sausage stars in observations with simulations
Large spectroscopic surveys plus Gaia astrometry have shown us that the inner
stellar halo of the Galaxy is dominated by the debris of Gaia Enceladus/Sausage
(GES). With the richness of data at hand, there are a myriad of ways these
accreted stars have been selected. We investigate these GES selections and
their effects on the inferred progenitor properties using data constructed from
APOGEE and Gaia. We explore selections made in eccentricity, energy-angular
momentum (E-Lz), radial action-angular momentum (Jr-Lz), action diamond, and
[Mg/Mn]-[Al/Fe] in the observations, selecting between 144 and 1,279 GES stars
with varying contamination from in-situ and other accreted stars. We also use
the Auriga cosmological hydrodynamic simulations to benchmark the different GES
dynamical selections. Applying the same observational GES cuts to nine Auriga
galaxies with a GES, we find that the Jr-Lz method is best for sample purity
and the eccentricity method for completeness. Given the average metallicity of
GES (-1.28 < [Fe/H] < -1.18), we use the mass-metallicity relationship to
find an average of . We
adopt a similar procedure and derive for the GES-like systems
in Auriga and find that the eccentricity method overestimates the true by while E-Lz underestimates by .
Lastly, we estimate the total mass of GES to be using the relationship between the metallicity gradient and
the GES-to-in-situ energy ratio. In the end, we cannot just `pick and choose'
how we select GES stars, and instead should be motivated by the science
question.Comment: 20 pages, 14 figures, submitted to MNRA
The low abundance and insignificance of dark discs in simulated Milky Way galaxies
We investigate the presence and importance of dark matter discs in a sample of 24 simulated Milky Way galaxies in the apostle project, part of the eagle programme of hydrodynamic simulations in ΛCDM cosmology. It has been suggested that a dark disc in the Milky Way may boost the dark matter density and modify the velocity modulus relative to a smooth halo at the position of the Sun, with ramifications for direct detection experiments. From a kinematic decomposition of the dark matter and a real space analysis of all 24 haloes, we find that only one of the simulated Milky Way analogues has a detectable dark disc component. This unique event was caused by a merger at late time with an LMC-mass satellite at very low grazing angle. Considering that even this rare scenario only enhances the dark matter density at the solar radius by 35 per cent and affects the high-energy tail of the dark matter velocity distribution by less than 1 per cent, we conclude that the presence of a dark disc in the Milky Way is unlikely, and is very unlikely to have a significant effect on direct detection experiments
Magnetic field amplification in cosmological zoom simulations from dwarf galaxies to galaxy groups
Magnetic fields are ubiquitous in the Universe. Recently, cosmological
simulations of galaxies have successfully begun to incorporate magnetic fields
and their evolution in galaxies and their haloes. However, so far they have
mostly focused on Milky Way-like galaxies. Here we analyse a sample of high
resolution cosmological zoom simulations of disc galaxies in haloes with mass
from to
, simulated with the Auriga galaxy formation model.
We show that with sufficient numerical resolution the magnetic field
amplification and saturation is converged. The magnetic field strength reaches
equipartition with turbulent energy density for galaxies in haloes with
. For galaxies in less
massive haloes, the magnetic field strength saturates at a fraction of
equipartition that decreases with decreasing halo mass. For our lowest mass
haloes, the magnetic field saturates significantly below of
equipartition. We quantify the resolution we need to obtain converged magnetic
field strengths and discuss our resolution requirements also in the context of
the IllustrisTNG cosmological box simulations. We show that, at ,
rotation-dominated galaxies in our sample exhibit for the most part an ordered
large scale magnetic field, with fewer field reversals in more massive
galaxies. Finally, we compare the magnetic fields in our cosmological galaxies
at with simulations of isolated galaxies in a collapsing halo setup. Our
results pave the way for detailed studies of cosmic rays and other physical
processes in similar cosmological galaxy simulations that crucially depend on
the strength and structure of magnetic fields.Comment: 18 pages, 12 figures, submitted to MNRAS, comments welcom
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