REVIEW
Systematic uncertainties in the determination of the local dark matter density
High Energy Astrophysical Phenomena · Submitted 7 Jun 2010 · 41 references
Pith's verdict: A useful simulation-based estimate of halo-shape systematics on the local dark matter density, but the headline 21% is a midpoint from one plane of one simulated galaxy, and the mapping from dynamical observables to the spherical average is asserted rather than derived.
T0 · AI review
Why Pith says this
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Referee report
The paper
The paper claims
A Milky Way-like simulation indicates that the dark matter density at the Sun's position on the stellar disk is on average 21% higher than the spherical average used in most dynamical determinations.
The reading
What carries the argument
The central machinery is a shape-and-ring analysis of the simulated halo. An inertia tensor built from dark matter particles in ellipsoidal shells yields axis ratios $b/a$ and $c/a$ and the triaxiality parameter $T$; comparing the minor axis with the normal to the stellar disk measures how the halo is oriented relative to the galaxy. The authors then carve the shell $7.5 < R/ < 8.5$ into orthogonal rings of volume $2 $ kpc$^3$ and measure the dark matter density in $ /4$ angular bins along the stellar disk plane and two perpendicular planes. The spherical average $ _0$ from the same shell is the benchmark, while the density on the disk plane is what an observer at the Sun would sample; that comparison is what turns a simulated halo shape into a systematic shift on a dynamical measurement.
Extended reading notes
Core claim
In the simulated Milky Way analog SR6-n01e1ML, the dark halo is flattened along the normal to the stellar disk, so an observer on the disk at $R_0 8$ kpc lies in a local dark matter overdensity relative to the spherical shell average. The measured ratio is $ _0/ _0 = 1.01$-$1.41$; the corresponding dark-matter-only realization gives the wider range $0.39$-$1.94$. The authors apply a mean 21% upward correction to a recent dynamical determination and quote $ _0 = 0.466 0.033( ) 0.077( ) $ for an Einasto profile. They also argue that baryon-induced contraction changes the central profile but not the local density inference, because local observables constrain both the enclosed mass and its slope.
Load-bearing premise
The load-bearing premise is that dynamical measurements return the spherically averaged density $ _0$ rather than a disk-plane density that already samples the flattened halo, with a secondary bet that one simulated Milky Way analog is representative enough to calibrate the correction for the real Galaxy.
Editorial extensions
If this is right
- Direct-detection event rates, which scale linearly with $ _0$, would be about 21% higher for a fixed particle model than estimates using the spherical average.
- Indirect annihilation signals from the solar neighborhood, which scale as $ _0^2$, would rise by about 46% under the same correction.
- Dynamical determinations of the local dark matter density should carry a systematic error of tens of percent from halo triaxiality, comparable to or larger than recent statistical errors.
- The specific benchmark $ _0 = 0.385 0.027 $ becomes $0.466 0.033( ) 0.077( ) $ for an Einasto profile.
- Baryonic contraction mainly affects central annihilation signals and the inner profile, not the local density inferred from local dynamical observables.
Reading between the lines
- The sign of the correction is not guaranteed: in this simulation the halo is flattened along the disk, but a Milky Way with a misaligned or more triaxial halo could place the Sun in an underdensity, so the 21% should be read as a calibration of magnitude and likely sign, not a universal law.
- The 21% figure is drawn from one galaxy; turning it into a probability distribution for the real Milky Way would require a population of simulated analogs spanning merger history, disk orientation, and baryon fraction.
- If a direct-detection signal is ever identified, the halo-shape systematic will be degenerate with the dark matter mass and cross section; independent constraints on the Milky Way's halo shape from stellar or gas kinematics could break that degeneracy.
- The ring method itself is portable: applying it to the real Galaxy's stellar halo or gas tracers could test whether the dark halo is indeed flattened along the stellar disk.
Original abstract
A precise determination of the local dark matter density and an accurate control over the corresponding uncertainties are of paramount importance for Dark Matter (DM) searches. Using very recent high-resolution numerical simulations of a Milky Way like object, we study the systematic uncertainties that affect the determination of the local dark matter density based on dynamical measurements in the Galaxy. In particular, extracting from the simulation with baryons the orientation of the Galactic stellar disk with respect to the DM distribution, we study the DM density for an observer located at $\sim$8 kpc from the Galactic center {\it on the stellar disk}, $\rho_0$. This quantity is found to be always larger than the average density in a spherical shell of same radius $\bar{\rho}_0$, which is the quantity inferred from dynamical measurements in the Galaxy, and to vary in the range $\rho_0/\bar{\rho}_0=1.01-1.41$. This suggests that the actual dark matter density in the solar neighbourhood is on average 21\% larger than the value inferred from most dynamical measurements, and that the associated systematic errors are larger than the statistical errors recently discussed in the literature.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
A precise determination of the local dark matter density and an accurate control over the corresponding uncertainties are of paramount importance for Dark Matter (DM) searches. Using very recent high-resolution numerical simulations of a Milky Way like object, we study the systematic uncertainties that affect the determination of the local dark matter density based on dynamical measurements in the Galaxy. In particular, extracting from the simulation with baryons the orientation of the Galactic stellar disk with respect to the DM distribution, we study the DM density for an observer located at $\sim$8 kpc from the Galactic center {\it on the stellar disk}, $\rho_0$. This quantity is found to be always larger than the average density in a spherical shell of same radius $\bar{\rho}_0$, which is the quantity inferred from dynamical measurements in the Galaxy, and to vary in the range $\rho_0/\bar{\rho}_0=1.01-1.41$. This suggests that the actual dark matter density in the solar neighbourhood is on average 21\% larger than the value inferred from most dynamical measurements, and that the associated systematic errors are larger than the statistical errors recently discussed in the literature.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | A precise determination of the local dark matter density and an accurate control over the corresponding uncertainties are of paramount importance for Dark Matter (DM) searches. | background 1.00 | 0.04 | 0.03 | asserted 0.99 |
| s1 | Using very recent high-resolution numerical simulations of a Milky Way like object, we study the systematic uncertainties that affect the determination of the local dark matter density based on dynamical measurements in the Galaxy. | contribution 0.72 | 0.93 | 0.04 | asserted 0.92 |
| s2 | In particular, extracting from the simulation with baryons the orientation of the Galactic stellar disk with respect to the DM distribution, we study the DM density for an observer located at $\sim$8 kpc from the Galactic center {\it on the stellar disk}, $\rho_0$. | method 0.91 | 0.94 | 0.04 | asserted 0.89 |
| s3 | This quantity is found to be always larger than the average density in a spherical shell of same radius $\bar{\rho}_0$, which is the quantity inferred from dynamical measurements in the Galaxy, and to vary in the range $\rho_0/\bar{\rho}_0=1.01-1.41$. | result 1.00 | 0.95 | 0.30 | asserted 1.00 |
| s4 | This suggests that the actual dark matter density in the solar neighbourhood is on average 21\% larger than the value inferred from most dynamical measurements, and that the associated systematic errors are larger than the statistical errors recently discussed in the literature. | interpretation 0.95 | 0.83 | 0.88 | hedged possibility 0.99 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:1006.1322 by the authors.
References
How the literature uses it
58 arXiv papers cite this in 69 sentences
Papers citing it · 44 sentences without a confident label
Uses its method or tool 1 paper
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Following the standard procedure [ref] the gamma ray flux can be written as [equation] where $n_DM$ is the dark matter number density, $i$ represents all the possible annihilation channels ${ee̅, μμ̅, ττ̅, uu̅, dd̅, cc̅, bb̅, , }$ , $dN_γ^i/dE(E)$ is the expected number of photons in the energy range of $(E,E+dE)$ produced from a given process $i$, and the so-called halo factor $J̅(ΔΩ)$ is defined as [equation] Here we have used the canonical value $ρ_⊙=ρ(r_⊙)=0.3$ GeV for the dark matter density around the Sun[More recent determinations in Ref. [ref] indicate slightly larger values.
Very light right-handed sneutrino dark matter in the NMSSM · Indirect detection · 0.86
Uses its data or values 15 papers
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With the inclusion of baryons, it was concluded that halo asphericity could lead up to $41%$ overdensity at the local distance with respect to the spherically averaged value [ref] .
Systematic uncertainties from halo asphericity in dark matter searches · Introduction · 0.88
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For $Ω_DM$ we adopt the central value of the WMAP determination, while for $ρ_DM$ we adopt, following Ref. [ref] , the value $ρ_χ=0.385 ^-3$ (see also [ref] ).
Global fits of the Minimal Universal Extra Dimensions scenario · Prospects for MUED discovery · 1.00
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We propose two different strategies to specify $ρ_χ$: * Consistency check: we impose that [equation] and we adopt for this quantity the value obtained in a recent paper by Catena and Ullio [ref] , through a careful analysis of dynamical observables in the Galaxy, namely $ρ_χ=0.385 ^-3$ (see also [ref] ).
Identification of Dark Matter particles with LHC and direct detection data · Future direct detection data · 1.00
Compares with it 2 papers
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in rough agreement with Refs. [ref] .
Systematic uncertainties from halo asphericity in dark matter searches · Discussion and Conclusions · 0.94
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For comparison, the latest determination of the local dark matter density in our solar neighborhood, based on data from both dynamical observations and galactic simulations, gave $0.47±0.03±0.08$ [ref] (the first uncertainty is statistical while the second one is systematic).
Effects of Velocity-Dependent Dark Matter Annihilation on the Energy Spectrum of the Extragalactic Gamma-ray Background · Further comments on modeling the dark matter distribution · 0.96
Cites it as support 1 paper
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Combining data from multiple experiments with different targets significantly increases the accuracy with which the WIMP parameters can be measured [ref] .
Astrophysical uncertainties on direct detection experiments · Parameterizing the speed distribution · 0.99
Cites it as background 4 papers
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The effects of triaxiality on the estimates for the local DM density have been studied in Ref. [ref] , using a simulation with only DM, and in Ref. [ref] , with simulations of two Milky-Way-like galaxies including baryons.
Systematic uncertainties from halo asphericity in dark matter searches · Introduction · 0.80
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In the standard minimal WIMP framework, which is applicable to the MSSM dark matter throughout the model parameter space, the results of direct detection searches are presented as upper bounds on (or perhaps, in the future, measurements of) the spin-independent neutralino-proton elastic scattering cross section at zero momentum exchange,[Converting the actually measured event rates into a cross-section bound or measurement requires a number of assumptions, such as local dark matter density (see e.g. [ref] ) and velocity distributions (see e.g. [ref] ), isospin symmetry of WIMP-nucleon couplings (see e.g. [ref] ), etc.
Fine-Tuning Implications of Direct Dark Matter Searches in the MSSM · Motivation and Philosophy · 0.92
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The average density of dark matter at a galactic radius equal to the Sun is reasonably well known based on galactic kinematics (for a recent discussion, see [ref] ).
Interpreting Dark Matter Direct Detection Independently of the Local Velocity and Density Distribution · Introduction · 0.85
What it cites · 32 sentences without a confident label
Uses the method or tool of 3 papers
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In order to measure the shape of the dark halo in the simulations under scrutiny (described in section [ref]), we follow [ref] and compute for a given set of $N_p$ dark matter particles the matrix [equation] where $i,j=1,2,3$ index the coordinates in the reference system.
The Causes of Halo Shape Changes Induced by Cooling Baryons: Disks Versus Substructures · Halo shape · 0.90
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The modelling includes realistic recipes for star formation [ref] , supernova feedback (SNII and SNIa), stellar mass loss, gas cooling/heating and metal enrichment [ref] .
Cosmological MHD simulation of a cooling flow cluster · Baryons dominated Milky Way model · 0.85
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The modelling includes realistic recipes for star formation [ref] , supernova feedback (SNII and SNIa), stellar mass loss, gas cooling/heating and metal enrichment [ref] .
The History of the Baryon Budget: Cosmic Logistics in a Hierarchical Universe · Baryons dominated Milky Way model · 0.85
Uses data or values from 3 papers
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For the Milky Way, we know that the disk can be decomposed in a pseudo-bulge of stellar mass $M_B ≃1-2 ×10^10$ M$_⊙$ and a rather extended disk of mass $M_D ≃5-6 ×10^10$ M$_⊙$ [ref] .
Unified Rotation Curve of the Galaxy -- Decomposition into de Vaucouleurs Bulge, Disk, Dark Halo, and the 9-kpc Rotation Dip -- · Models for the Milky Way halo · 0.95
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For the Milky Way, we know that the disk can be decomposed in a pseudo-bulge of stellar mass $M_B ≃1-2 ×10^10$ M$_⊙$ and a rather extended disk of mass $M_D ≃5-6 ×10^10$ M$_⊙$ [ref] .
LCDM-based models for the Milky Way and M31 I: Dynamical Models · Models for the Milky Way halo · 0.95
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We also know that the Milky Way rotation curve is rather flat, with a maximum measured velocity of 220-250 km/s ( [ref] and references therein).
Mass models of the Milky Way · Models for the Milky Way halo · 0.83
Compares with 4 papers
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Such is not achieved by other recent simulations that present resolutions of a few kpc [ref] .
The impact of baryons on dark matter haloes · Baryons dominated Milky Way model · 0.90
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Such is not achieved by other recent simulations that present resolutions of a few kpc [ref] .
Galaxy-Induced Transformation of Dark Matter Halos · Baryons dominated Milky Way model · 0.90
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Such is not achieved by other recent simulations that present resolutions of a few kpc [ref] .
Dark matter response to galaxy formation · Baryons dominated Milky Way model · 0.90
Cites as support 2 papers
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The dark matter distribution will be therefore only mildly affected by the baryons dynamics and pure N-body simulations can be reliably used to estimate the local DM density at the Sun radius [ref] .
The impact of baryons on dark matter haloes · Models for the Milky Way halo · 0.98
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Notice that we are disregarding fine grained structures (such as microhalos and streams) since their effect on DM searches is likely negligible as shown in [ref] .
Impact of Dark Matter Microhalos on Signatures for Direct and Indirect Detection · Introduction · 0.99
Cites as background 12 papers
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[ref] and [ref] studied the effect of disk formation on the local dark matter density, specifically the formation of a dark disk component.
A dark matter disc in three cosmological simulations of Milky Way mass galaxies · Baryons dominated Milky Way model · 0.88
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[ref] and [ref] studied the effect of disk formation on the local dark matter density, specifically the formation of a dark disk component.
Dark Matter Direct Detection Signals inferred from a Cosmological N-body Simulation with Baryons · Baryons dominated Milky Way model · 0.88
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A number of papers have appeared recently on this subject, where the authors attack the problem of the determination of $ρ_0$ in light of recent observational results [ref] .
The dark matter density at the Sun's location · Introduction · 0.92
Discussion
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