Probing the nature of Dark Matter with the SKA

0.16 MB·English

by  Sergio Colafrancesco

Probing the nature of Dark Matter with the SKA
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1
0
2
SergioColafrancesco∗1, MarcoRegis2, PaoloMarchegiani1,GeoffBeck1,Rainer

n
Beck3, HannesZechlin2, AndreiLobanov3, DieterHorns4
a
J 1SchoolofPhysics,UniversityoftheWitwatersrand,Johannesburg,SouthAfrica

2 2DipartimentodiFisica,UniversitàdegliStudidiTorinoandINFN-SezionediTorino,

viaP.Giuria,1,10125Torino,Italy
]
E 3Max-Planck-InstitutfürRadioastronomie,AufdemHügel69,53121Bonn,Germany
H 4InstitutfürExperimentalphysik,UniversitätHamburg,LuruperChaussee149,22761Hamburg,
h. Germany
p
-
o E-mail: [email protected]
r
t
s Dark Matter (DM) is a fundamentalingredientof our Universe and of structure formation, and
a
[ yetitsnatureiselusivetoastrophysicalprobes.Informationonthenatureandphysicalproperties

1 oftheWIMP(neutralino)DM(theleadingcandidateforacosmologicallyrelevantDM)canbe
v
obtainedbystudyingtheastrophysicalsignalsoftheirannihilation/decay.Amongthevariouse.m.
8
3 signals, secondaryelectronsproducedbyneutralinoannihilationgeneratesynchrotronemission
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inthemagnetizedatmosphereofgalaxyclustersandgalaxieswhichcouldbeobservedasadiffuse
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0 radio emission (halo or haze) centered on the DM halo. A deep search for DM radio emission
.
2 withSKAinlocaldwarfgalaxies,galaxyregionswithlowstarformationandgalaxyclusters(with
0
offsetDM-baryonicdistribution,likee.g. theBulletcluster)canbeveryeffectiveinconstraining
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1 theneutralinomass,compositionandannihilationcross-section. Forthecaseofadwarfgalaxy,
:
v likee.g.Draco,theconstraintsontheDMannihilationcross-sectionobtainablewithSKA1-MID
Xi will be at least a factor ∼103 more stringent than the limits obtained by Fermi-LAT in the g -
r rays. These limits scale with the value of the B field, and the SKA will have the capability to
a
determine simultaneously both the magnetic field in the DM-dominatedstructures and the DM
particleproperties. TheoptimalfrequencybandfordetectingtheDM-inducedradioemissionis
around∼1GHz,withtheSKA1-MIDBand1and4importanttoprobethesynchrotronspectral
curvatureatlow-n (sensitivetoDMcomposition)andathigh-n (sensitivetoDMmass).
AdvancingAstrophysicswiththeSquareKilometreArray,
June8-13,2014
GiardiniNaxos,Sicily,Italy
∗Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
UnveilingDarkMatterwithSKA SergioColafrancesco
1. Unveiling thenature ofDarkMatter
The Square Kilometre Array (SKA) is the most ambitious radio telescope ever planned, and
it is a unique multi-disciplinary experiment. Even though the SKA, in its original conception,
has been dedicated to constrain the fundamental physics aspects on dark energy, gravitation and
magnetism,muchmorescientificinvestigationcouldbedonewithitsconfiguration: theexploration
ofthenatureofDarkMatterisoneofthemostimportantadditional scientificthemes.
Among the viable competitors for having a cosmologically relevant DM species, the leading
candidate is the lightest particle of the minimal supersymmetric extension of the Standard Model
(MSSM, Jungman et al. 1996), plausibly the neutralino c , with a mass Mc in the range between
afewGeVtoseveralTeV.Information onthenatureandphysical properties oftheneutralino DM
can be obtained by studying the astrophysical signals of their interaction/annihilation in the halos
ofcosmicstructures. Thesesignals(seeColafrancesco etal. 2006,2007fordetails)involve,inthe
case of a c DM, emission along a wide range of frequencies, from radio to g -rays (see Fig. 1 for
aDMspectral energydistribution (SED)inatypicaldwarfgalaxy). Neutralpionsproducedin cc
annihilation decay promptly in p 0 →gg and generate most of the continuum photon spectrum at
energies E &1GeV.Secondaryelectronsareproducedthrough variouspromptgeneration mecha-
nisms and by the decay of charged pions, p ± →m ±+n m (n¯m ), with m ± →e±+n¯m (n m )+n e(n¯e).
The different composition of the cc annihilation final state will in general affect the form of the
electron spectrum. The time evolution of the secondary electron spectrum is described by the
transport equation:
¶ n ¶
e =(cid:209) [D(cid:209) n ]+ [b (E)n ]+Q (E,r), (1.1)
¶ t e ¶ E e e e
where Q (E,r) is the e± source spectrum, n (E,r) is the e± equilibrium spectrum (at each fixed
e e
time) and b ≡−dE/dt isthe e± energy loss per unit time, b =b +b +b +b (see
e e ICS synch brem Coul
Colafrancesco et al. 2006 for details). The diffusion coefficient D in eq.(1.1) sets the amount of
spatial diffusion for the secondary electrons: it turns out that diffusion can be neglected in galaxy
clusters while it is relevant on galactic and sub-galactic scales (see discussion inColafrancesco et
al. 2006,2007). Undertheassumptionthatthepopulationofhigh-energy e± canbedescribedbya
quasi-stationary (¶ n /¶ t≈0)transportequation,thesecondaryelectronspectrumn (E,r)reaches
e e
its equilibrium configuration mainly due to synchrotron and ICS losses at energies E &150 MeV
and due to Coulomb losses at lower energies. Secondary electrons eventually produce radiation
by synchrotron emission inthe magnetized atmosphere of cosmic structures, bremsstrahlung with
ambient protons and ions, and ICS of CMB (and other background) photons (and hence an SZ
effect, Colafrancesco 2004). These secondary particles also heat the ambient gas by Coulomb
collisions.
A large amount of efforts have been put in the search for DM indirect signals at g -ray en-
ergies looking predominantly for two key spectral features: the p 0 → gg decay spectral bump ,
and the direct cc →gg annihilation line emission, with results that are not conclusive yet (e.g.,
Daylan et al. 2014, Weniger 2012, Doro etal. 2014). The non-detection of signals related to DM
annihilation/decay from various astrophysical targets (including observations of dwarf spheroidal
galaxies, the Galactic Center, galaxy clusters, the diffuse gamma-ray background emission) is in-
terpreted intermsofconstraints onthe(self-)annihilation cross-section (ordecaytime)oftheDM
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UnveilingDarkMatterwithSKA SergioColafrancesco
-10 -10
10 10
] ]
2-1 s 10-11 Mc = 100 GeV, s v at EGRET level 2-1 s 10-11 Mc = 100 GeV, s v at EGRET level
-m B = 1 m G p 0 -m
rg c10-12 m rg c10-12 B = 4 m G
e e m
) [ 10-13 ) [ 10-13 2
r IC on r
-30 s10-14 synch. CMB -30 s10-14 1
= 1 -15 = 1 -15 0.5
10 10
nDWI(, 10-16 IC on nDWI(, 10-16 0.1
-17 -17
n 10 starlight n 10
-18 -18
10 10
7.5 10 12.5 15 17.5 20 22.5 25 27.5 7.5 10 12.5 15 17.5 20 22.5 25 27.5
log ( n [ Hz ] ) log ( n [ Hz ] )
Figure 1: Left. The Draco dSph multi-wavelength spectrum for a 100 GeV WIMP annihilating into bb¯.
Right. TheeffectofvaryingthemagneticfieldstrengthontheDracomulti-wavelengthspectrumfora100
GeVWIMPannihilatingintobb¯. TheWIMPpairannihilationratehasbeentunedastogiveag -raysignal
attheleveloftheEGRETmeasuredfluxupperlimit(fromColafrancescoetal. 2007).
particle candidate. Assuming, for instance, a canonical WIMP of Mc =100GeV annihilating to
b-quarks, stacked observations of dwarf spheroidal galaxies with Fermi-LAT put a constraint of
hs Vi<2×10−25cm3s−1 (95% c.l.) on the thermally-averaged self-annihilation cross-section of
DM particles (Ackerman et al. 2014). Hopes of discovering annihilating WIMPs in g -rays are
relegated toFermi-LATsuccessors andtheforthcoming CTAexperiment (Doroetal. 2013).
Thereare,however,alsogoodhopestoobtainrelevantinformationonthenatureofDMfromradio
observations ofDMhalosonlargescales, i.e. fromdwarfgalaxiestoclustersofgalaxies.
2. Radio emissionsignalsfrom DarkMatter annihilation
Observations of radio halos produced by DM annihilations are, in principle, very effective
in constraining the neutralino mass and composition (see, e.g., Colafrancesco and Mele 2001,
Colafrancescoetal. 2006,2007),underthehypothesisthatDMannihilationprovidesanobservable
contribution totheradio-halo flux.
Thewiderangeoffrequencies probedbytheSKAandthevarietyofachievable observational
targets (and in turn of magnetic fields) will allow testing the non-thermal electron spectrum from
about 1 GeV to few hundreds of GeV, that is the most relevant range in the WIMP search. Fig-
ure2displays thepredicted spectral differences between variousannihilation channels andWIMP
masses: notethatthesedifferences manifestmainlyinlow-n slopevariationfordifferingannihila-
tion channels and inthehigh-n spectral flattening/steepening forlarger/smaller masses. TheSKA
sensitivity curvesforSKA1-LOWandSKA1-MIDaretakenfromDewdneyetal. (2012).
The surface brigthness produced by DM-induced synchrotron emission is heavily affected by dif-
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UnveilingDarkMatterwithSKA SergioColafrancesco
102 102
101 bb 101 bb60
100 τ+τ− 100 bb500
1100−−21 bb 1015M⊙ 1100−−21 bb60 1015M⊙
10−3 τ+τ− 10−3 bb500
Jy() 1100−−54 1012M⊙ Jy() 1100−−54 1012M⊙
) )
ν( 10−6 ν( 10−6
F 10−7 bb F 10−7 bb60
SKA SKA
1100−−98 τ+τ− 107M⊙ 1100−−98 bb500 107M⊙
10−10 10−10
10−11 10−11
10−12 10−12
101 102 103 104 105 101 102 103 104 105
ν(MHz) ν(MHz)
Figure 2: Flux densitiesfor dwarfgalaxies(M=107 M ), galaxies(M=1012 M ), andgalaxyclusters
⊙ ⊙
(M =1015 M ). The halo profile is NFW, hBi=5 m G, and the annihilation cross section is fixed to the
⊙
valuehs Vi≈3×10−27 cm3 s−1. Black linesarefordwarfgalaxies, redlinesforgalaxiesandgreenfor
clusters. Left: WIMPmassis60GeV,solidlinesareforcompositionbbanddashedlinesfort +t −. Right:
Compositionisbb,solidlinesareforWIMPmass60GeVanddashedlinesfor500GeV.Dash-dottedlines
areSKAsensitivitylimitsforintegrationtimesof30,240and1000hours(Dewdneyetal. 2012). Theflux
iscalculatedwithinthevirialradius(fromColafrancescoetal. 2014).
fusioninsmallscalestructures, e.g.,dwarfandstandardgalaxies,whileitislessimportantinlarge
structures, e.g.,galaxyclusters(seeColafrancesco etal. 2006-2007 foradetailed discussion).
Polarization from DM-induced radio emission isexpected atverylowfractional levelsdueto
thefactthatDMspatial andvelocity distribution isnearlyhomogeneous andthatDMannihilation
is mediated by secondary particle production. Therefore, a low polarization level of detectable
radio signals in the directions of DM halos would be consistent with the DM origin of such radio
emission. Residual high-polarization signals could be hence attributed to astrophysical sources in
the direction or within the DM halos, and one could use these signals to infer properties of the
magneticfieldinthesestructures (seeR.Becketal. 2015,andF.Govonietal. 2015).
2.1 Cosmological evolutionofDarkMatterradioemission
Figure 3 displays the evolution of radio emission from DMhalos of mass 107 M , 1012 M ,
⊙ ⊙
and 1015 M for a constant magnetic field of 5 m G, in accordance with arguments made in Co-
⊙
lafrancesco etal. (2014). Theannihilation channel isbb,themassoftheneutralino is60GeVand
a DM annihilation cross-section hs Vi=3×10−27 cm 3 s−1 was adopted. Emission from dwarf
galaxy halos (M ≈107 M ) is just below the SKA detection threshold for this value of hs Vi but
⊙
would be visible at redshift z≤0.01 for the assumed DM annihilation cross-section. This justify
the search of DM-induced radio signals mainly indwarf galaxies of the local environment, at dis-
tances ∼<3 Mpc (z ∼<0.0007). Emission from galactic DM halos (M ≈1012 M⊙) are detectable
by SKA out to z≈0.8 even with the reference value of hs Vi, and can provide a non-detection
upper-bound onhs Vianorderofmagnitude belowtheassumedvalueevenatsuchhighredshifts.
Emissionformgalaxyclusterhaloscanprovidesimilarconstraintsbutouttohigherredshiftsz∼<3.
These objects thus offer theoption ofdeep-field observations thatcanscan alarger fraction ofthe
DMparameterspacethanthebestcurrentdata.
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UnveilingDarkMatterwithSKA SergioColafrancesco
Figure 3 also shows that the effect of diffusion are far less significant when observing higher-z
objects, againsimplifying themodelling andanalysis oftheSKAobservations.
10−3 102
10−1
FνJy()()111111000000−−−−−−111531975 zzz===310...0001 SKA FνJy()()1111100000−−−−−119753 zzz===310...0001 SKA FνJy()() 111100001−−−−086420 zzz===310...0001 SKA
10−17 10−13 10−10
10−19 z=5.0 10−15 z=5.0 10−12 z=5.0
10−21 10−17 10−14
10−21301 102 103 104 105 10−11901 102 103 104 105 10−11601 102 103 104 105
ν(MHz) ν(MHz) ν(MHz)
Figure 3: Flux densities for various DM halos within the virial radius. Left: dwarf spheroidal galaxies
(M =107 M ); mid panel: galactic halos (M =1012 M ); right panel: galaxy clusters (M =1015 M ).
⊙ ⊙ ⊙
We assumeaNFWhaloprofile,hs Vi=3×10−27 cm3 s−1 andhBi=5 m G.WIMPmassis60GeVand
thecompositionisbb. Solidlinesarewithoutdiffusion,anddottedarewithdiffusion. TheSKAsensitivity
limits(dash-dottedlines,Dewdneyetal. 2012)areshownforintegrationtimesof30,240and1000hours,
respectively(fromColafrancescoetal. 2014).
2.2 OptimalDMlaboratories
In order to identify the optimal DM laboratories for radio observations we scan a parame-
ter space extending from dwarf galaxies to galaxy clusters over a wide redshifts range z≈0−5.
The choice to examine the halos of both large and small structures is crucial, as dwarf spheroidal
galaxiesarewellknowntobehighlyDMdominatedbutproducefaintemissions,whilelargerstruc-
tures, but not immaculate test-beds for DM emissions, provide substantially stronger fluxes. This
indicates that a survey of DM halos with different mass is essential to identify the best detection
prospects forfutureradiotelescopes liketheSKA.
Figure4showstheredshift-mass exclusion plotobtained byusing theSKAsensitivity bound
forSKA1LOWandSKA1MID(at1GHzinBand1). ForeachDMhalo weobtain theDMhalo
mass and redshift combination that produce the minimal SKA-detectable fluxes. DM-dominated
objectslyingabovetheblackandgreencurvescannotbedetectedwiththeSKA1atthegivenconfi-
dencethresholdforhs Vi=3×10−27 cm3s−1. ObjectsbelowacurvearevisibletoSKA1,andthe
further below thecurvetheyliethegreater theregion ofthecross-section parameter space wecan
explore through the observation of the object. For reference, the yellow dash-dotted line displays
thecurvegivenforhs Vi=3×10−30 cm3s−1and1s confidencelevel. Afewrepresentativeknow
objects(irrespectiveoftheirlocationinthesky)withgoodestimatesoftheDMmassareplottedin
theM −zplaneforthesakeofillustration oftheDMsearchpotential withtheSKA.
DM
Dwarfgalaxies, giventheirextremeproximity,provideanexcellenttest-bedforDMradioprobes,
granting access to a parameter space that extends even below the value hs Vi = 3×10−30 cm3
s−1. Additionally their large mass-to-light ratios and absence of strong star formation and diffuse
non-thermal emissionmakethemverycleansources forradioDMsearches.
Galaxies can be probed to significantly larger redshifts than the dwarf galaxies due to their larger
DMmass,andthoselocatedintheredshiftrange0.5∼<z∼<1.0providestrongerconstraints. How-
ever, an optimized DM search should be confined to galaxies with little background radio noise,
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UnveilingDarkMatterwithSKA SergioColafrancesco
101
100
Bullet
10−1 A2163
A2255
Coma
z10−2
NGC3877
M87
NGC3198
10−3 M81
NGC7793
10−4
dSph
10−5
107 108 109 1010 1011 1012 1013 1014 1015 1016
M(M⊙)
Figure4: ExclusionplotinredshiftversushalomassbasedonprojectedSKA(at1GHzinBand1)sensi-
tivitydataforthereferencevalueofhs Vi=3×10−27cm3 s−1 with30hourintegrationtime(blacklines)
and1000hourintegrationtime(greenlines).hBi=5m Gwasadopted.Solidlinesarethe1s sensitivityex-
clusion,dashedlinesthatof2s anddottedlinescorrespondto3s .Theyellowdash-dottedlinecorresponds
to30hoursofintegrationand1s confidencewithhs Vi=3×10−30 cm3 s−1. Anannihilationchannelbb
isassumedwithaneutralinomassof60GeV. RepresentativeobjectswithknownDMmassareshownfor
illustrativepurposesofDMradiosignaldetection. ThedSphgroupcontainsthegalaxies: Draco,Sculptor,
Fornax,CarinaandSextans. UnlabelledGalaxiesare: NGC3917,NGC3949andNGC4010. Forverylocal
objectstheredshiftisestimatedfromtheaveragedistancedata. FromColafrancescoetal. 2014.
making low star-formation-rate galaxies good candidates. High-z galaxies come also with the ad-
vantage ofobserving moreprimitivestructures withfewersources ofbaryonic radioemission.
Clustersofgalaxiesprovideextremelygoodcandidates incases,suchastheBulletcluster, where
thedarkandbaryonic matterarespatially separated. Ourrecent analysis oftheATCAobservation
of the Bullet cluster (Colafrancesco and Marchegiani 2014) indicates that deeper radio observa-
tions (possible with the SKA) will be able indeed to separate the DM-induced signal from the
CR-induced one and hence have the possibly to investigate the nature of DM particles using the
technique hereproposed. Moreingeneral, thelarge predicted radiofluxesduetoDMannihilation
inclusters indicate thatDM-induced radioemissioncanbeobserved inradioouttolargeredshifts
z≈2,againwiththeadvantage offewersourcesofbaryonic radioemission.
2.3 DisentanglingmagneticfieldsandDarkMatter
Studyingthemagneticproperties ofDMhalosarecrucialtodisentangle theDMparticle den-
sityfromthemagneticfieldenergydensitycontributingtotheexpectedsynchrotronradioemission
fromDMannihilation. TheSKAisthemostpromisingexperimenttodeterminethemagneticfield
structure in extragalactic sources (see Johnston-Hollitt et al. 2015), and will have the potential of
measuring RMs toward a large number of sources allowing a detailed description of the strength,
structure, and spatial distribution of magnetic fields in dSph galaxies, galaxies (see Beck et al.
2015)andgalaxyclusters(seeGovonietal. 2015). Westressthatthesemeasurementsofthemag-
netic field can be obtained by the SKA simultaneously, for the first time, with the constraints on
DMnaturefromtheexpectedradioemission.
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UnveilingDarkMatterwithSKA SergioColafrancesco
3. The impactofSKA onthe search forthe DMnature
Deep observations of radio emission in DM halos are not yet available, and this limits the
capabilities of the current radio experiments to set relevant constraints on DM models. We have
already explored a project (Regis et al. 2014a,b,c) dedicated to the WIMP search making use of
radio interferometers, that could be considered as apilot experiment for the next generation high-
sensitivity andhigh-resolution radiotelescope arraysliketheSKA.Fortheparticle DMsearchwe
are interested in, the use of multiple array detectors having synthesized beams of ∼ arcmin size
hasanumberofadvantageswithrespecttosingle-dishobservations. First,thelargecollectingarea
allows for an increase in the sensitivity over that a single-dish telescope. The best beam choice
forthedetection ofadiffuseemissionrequiresalargesynthesized beam(inordertomaximizethe
integratedflux),butstillsmallerthanthesourceitselftobeabletoresolveit. Agoodangularreso-
lutionisalsocrucialinordertodistinguish betweenapossiblenon-thermal astrophysical emission
andtheDM-induced signal, whichclearly becomes veryhardiftheDMhaloisnotwell-resolved.
Thepossibilityofsimultaneouslydetectingsmallscalesourceswiththelong-baselines ofthearray
allows one to overcome the confusion limit. In the case of arcmin beams, the confusion level can
be easily reached withobservations lasting for fewtens of minutes, even bycurrent telescopes. A
source subtraction is thus a mandatory and crucial step of the analysis. Finally, single dish tele-
scopes face the additional complication related to Galactic foreground contaminations, which are
instead subdominant fortheangularscalestypically probedbytelescope arraysatGHzfrequency.
The limits derived from ATCA observations of 6 dSphs (Regis et al. 2014c) on the WIMP
annihilation/decay rate as a function of the mass for different final states of annihilation/decay
are already comparable to the best limits obtained with g -ray observations and are much more
constraining than what obtained in the X-ray band or with previous radio observations (Spekkens
etal. 2013,Natarayanetal. 2013). Inthiscontext,theSKAwillhavethepossibilitytoexploreDM
modelswithcross-section valueswellbelowtheDMrelicabundance one(seeFig.6inRegisetal.
2014c). TheSKA1-MIDBand1(350-1050MHz)willprobablybethemostpromisingfrequency
range for the majority of WIMP models. The full SKA-2 phase will bring another factor ∼10×
increaseinsensitivityandanextendedfrequencyrangeuptoatleast25GHz. Typicalvaluesofthe
SKAsensitivity (A /T =2×104 m2/K)and bandwidth (300 MHzatGHzfrequency) provide
eff sys
rms flux values of ≈30 nJy for 10 hours of integration time. This is about a 103 factor of gain
in sensitivity with respect to the most recent ATCA observations (Regis et al. 2014a). A further
improvement byafactor of2-3canbeconfidently foreseen duetothelarger numberofaccessible
dSphsatellites fromthesouthern hemisphere. TheSKAwillalsohavetheunique advantage tobe
able to determine the dSph magnetic field (via FR measurements and possibly also polarization),
provideditsstrengthisaroundthem Glevel(asexpectedfromstarformationratearguments,Regis
et al. 2014c). This will make the predictions for the expected DM signal much more robust
and obtainable with a single experimental configuration. The prospects of detection/constraints
of the WIMP particle properties with the SKA will therefore progressively close in on the full
parameterspace,eveninapessimisticsensitivitycase,andupto∼TeVWIMPmasses,irrespective
ofastrophysical assumptions.
TheSKAwillalso allowtoinvestigate thepossibility thatpoint-sources detected intheprox-
imity of the dSph optical center might be associated to the emission from a DM cuspy profile.
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UnveilingDarkMatterwithSKA SergioColafrancesco
This possibility is likely only in the "loss at injection" scenario, while spatial diffusion should in
any case flatten the e± distribution, making the source extended rather than point-like. The in-
vestigation ofthese sources with the SKAwilldeserve particular attention, since wehave already
foundthattheWIMPscenariocanfitthepoint-likeemissionwithannihilationratesconsistentwith
existing bounds(Regisetal. 2014c).
105 104
104 103
103 b¯b 102 b¯b 107M⊙
31−ms) 110012 ττ¯ 107M⊙ 31−ms) 110001 ττ¯
c c
27− 100 27− 10−1
1010−1 1010−2
(3×10−2 b¯b (3×10−3 b¯b 1012M⊙
σVi10−3 ττ¯ 1012M⊙ σVi10−4 ττ¯
h10−4 b¯b h10−5 b¯b 1015M⊙
10−5 ττ¯ 1015M⊙ 10−6 ττ¯
10−6 10−7
101 102 103 101 102 103
Mχ(GeV) Mχ(GeV)
Figure5: Thehs Viupperlimitsfrom30hourofSKAintegrationtimeforz=0.01at300MHz(top)and
1 GHz (bottom)as the neutralinomass Mc is varied with annihilationchannelbb in solid lines and t t¯ in
dashedlines. A valuehBi=5 m G was adopted. Black linescorrespondto haloswith mass1015 M , red
⊙
linesto1012M andgreenlinesto107M (fromColafrancescoetal. 2014).
⊙ ⊙
The SKA1-MID Band 1 (350-1050 MHz) to Band 4 (2.8-5.18 GHz) are important to probe
the DM-induced synchrotron spectral curvature at low-n (sensitive to DM composition) and at
high-n (sensitive to DM particle mass), and the implementation of Band 5 (4.6-13.8 GHz) will
bring further potential to assess the DM-induced radio spectrum. As we can see from Fig.3, the
best frequency range to detect these radio emissions is around 1 GHz. So, the upper frequency
regions of SKA1-MID Band 1 provides the strongest spectral candidate for probing the cross-
sectionparameterspaceduetoanoptimalcombinationoftheSKAsensitivitywithinthisbandand
therelativelystrongfluxesatthesefrequencies. Thisfrequencybandwillalsoallowforanoptimal
description ofthemagneticfield(seeJohnston-Hollitt etal. 2014).
TherearetwomaincaveatsintheforecastsforDMdetectionintheradiofrequencyband. The
firststemsfromthefactthat,foranextended radioemission,the confusion issuebecomesstronger
and stronger as one tries to probe fainter and fainter fluxes. Thus, the source subtraction proce-
dure becomes crucial and this can affect the estimated sensitivities. The impact of this effect on
the actual sensitivity ishardly predictable atthepresent time, especially fortheSKA,since itwill
depend on the properties of the detected sources, the efficiency of deconvolution algorithms, and
theaccuracy ofthetelescope beamshape.
The second caveat is that by bringing down the observational threshold, one can possibly start to
probetheverylowlevelsofpossiblenon-thermalemissionassociatedtothetinyrateofstarforma-
tionindSph, oringalaxies andgalaxy clusters. TheDMcontribution shouldbethendisentangled
fromsuchastrophysicalbackground. ThesuperiorangularresolutionoftheSKAwillallowforthe
precise mappingofemissions, putatively eitherDMorbaryonically induced, andwillenable their
correlation withthestellarorDMprofiles(obtained viaopticaland/orkinematic measurements).
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UnveilingDarkMatterwithSKA SergioColafrancesco
Early DM science can be done with a small sample of local dSphs, a small sample of nearby
galaxies with good DM density profile reconstruction, and the Bullet cluster, observing with a
somewhatlargerbeam(≈7′′−10′′). Theseobjectshavebeenalreadystudiedinradiowithsimilar
objectives (e.g., DM limits) and therefore provide the best science cases to prove the capabilities
of the SKA1 for the study of the nature of DM with radio observations. The implementation
of the SKA1-MID Band 5 (4.6-13.8 GHz) will increase the ability to detect the expected high-
frequency spectral curvature of DM-induced radio emissions, and the ability to place constraints
on the DM cross-section and to differentiate between different annihilation channel spectra. The
10× increased sensitivity of SKA2-MID compared to SKA1-MID will allow us to increase the
angular resolution by a factor ≈ 20 or to increase the sensitivity of SKA2-MID to DM-induced
radio signals by afactor ≈10. Thepossibility toextend thefrequency coverage of theSKAinits
Phase-2 realization up to ∼25 GHz, will allow to detect the expected high-n spectral cut-off of
DM-induced radioemissions, andthensetaccurate constraints totheDMparticlemass.
4. Conclusion
The SKA has the potential to unveil the elusive nature of Dark Matter. Its ability to resolve
the intrinsic degeneracy (between magnetic field properties and particle distribution) of the syn-
chrotron emission expected from secondary particles produced in DM annihilation (decay) will
allow such a discovery tobe unbiased and limited only by the sensitivity to the DM particle mass
and annihilation cross-section (decay rate). The unprecedented sensitivity of the SKA to the DM
fundamental properties will bring this instrument in a leading position for unveiling the nature of
thedarksectoroftheuniverse.
Theinformation provided bytheSKAcanbecomplemented withanalogous studiesinotherspec-
tralbands, whichwillbeabletoprovetheICSsignal ofDM-produced secondary electrons (span-
ningfromm wavestohardX-raysandg -rays)andthedistinctivepresenceofthep 0→gg emission
bumpintheg -rays(seeFig. 1). Thenextdecadewillofferexcellentmulti-frequency opportunities
inthis respect withtheadvent ofMillimetron, thelargest space-borne single-dish mm. astronomy
satellite operating in the 102−103 GHz range (optimal to prove the DM-induced SZ effect), the
Astro-Hmissionoperating inthehardX-raysfrequency range(withthehighest expectedsensitiv-
itytoprobethehigh-energy tailoftheDM-induced ICSemission), andtheCTAwithunprecedent
sensitivity intheenergyrangebetweenafewtensGeVtohundreds TeV.
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