Astrometric Microlensing by Local Dark Matter Subhalos

Astrometric Microlensing by Local Dark Matter Subhalos Adrienne Erickcek CITA & Perimeter Institute with Nicholas Law University of Toronto Dunlap Institute arxiv: 17.4228 ApJ in press

Dark Matter Halos are Clumpy! Via Lactea II Aquarius 43 kpc Springel et al. 28 Diemand et al. 28 High-resolution simulations of Galaxy-sized halos with billions of particles Aquarius halo has >2, resolved subhalos with M sub > 4 1 4 M 2

Subhalos in our Neighborhood Locations of Subhalos in Aquarius Simulation dn We are here! M 1.9 sub dm sub normalized number density kpc from galactic center Springel et al. 28 3

Subhalos are Gravitational Lenses When galaxies produce multiple images of a quasar; subhalos can modify the properties of these images. Mao & Schneider 1998; Metcalf & Madau 21; Chiba 22; Dalal & Kochanek 22 Keeton& Moustakas 29; Congdon et al. 21 Koopmans et al. 22; Chen et al. 27; Williams et al. 28; More et al. 29 Yonehara et al. 23; Inoue & Chiba 25; Zackrisson et al. 28; Riehm et al. 29 What about astrometric deflections of stars within the Galaxy? we re looking for a dynamical signature from a local subhalo subhalos are diffuse, so we need high-precision astrometry I I S Obs S V /d L T 4

Astrometric Microlensing Star field over 4 years 15 We need a subhalo center to pass by a star with an impact parameter of ~1 arcseconds. 2µas motion 1 Y/arcsec 5 2 km/s -5-1 -15-5 5 X/arcsec 1 15 Lens virial mass: 5 15 M! Lens distance: 5 pc 5

Lensing with a General Profile 15 1 yr 1 "=2. (SIS) 5 ρ(r) r γ 1 yr!y [µas] 5 "=1.8 5 "=1.5 5 yrs 5 "=1.2 5 yrs 5 "=1 (NFW) 1 yrs -2-1!x [µas] 1 2 The steepness of the density profile determines the shape of the image s path across the sky and the rate of its motion. 6

High Precision Astrometry Gaia is an ESO satellite scheduled to launch in late 212. astrometric precision per epoch: ~35 microarcseconds for its brightest targets (~5 million stars) SIM PlanetQuest was the top space mission recommended by NASA s Exoplanet Task Force. astrometric precision per epoch: 1 microarcsecond for planet-finding, 4 microarcseconds for general high-efficiency astrometry (~1, stars) Ground-based telescopes have great potential. Keck can reach ~1 microarcsecond precision TMT is designed for 5 microarcsecond precision and could reach much higher precision (Cameron et al. 29) 7

Lensing Cross Sections We define a lensing cross-section based on a minimum value for the lensing signal; all stars within this area will produce S > Smin. Motion of subhalo center during detection run S > Smin Smin! SNR 1.5σinst Area [arcsec2 ] 4 3 Smin = 5 µas Smin = 5 µas Smin = 2 µas γ=2. γ=1.8 γ=1.5 γ=1.2 2 1 1 1 1 O 1 1 1 M <.1pc [ M! ] 1 1 1 Lens distance: 5 pc; Lens velocity: 2 km/s; Source Distance: 5 kpc 8

Area [arcsec 2 ] Area [arcsec 2 ] Lensing Cross Sections 4 3 2 S min =5µas S min = 2 µas S min = 5 µas γ=2. γ=1.8 γ=1.5 γ=1.2 1 1 1 1 O 1 1 1 1 1 1 M<.1pc [ M ] 4 3 2 S min =5µas S min = 2 µas S min = 5 µas γ=2. γ=1.8 γ=1.5 1 1 1 1 1 1 1 1 1 1 1 1 1.1 M vir [M ] Lens distance: 5 pc;lens velocity: 2 km/s; Source Distance: 5 kpc 9

Lensing Event Rates We can combine the lensing cross sections with a subhalo mass function to calculate the fraction of the sky that is detectably lensed ( S > S min ) by a subhalo. We derived a local subhalo mass function from the results of the Aquarius simulations. Fraction of Sky Lensed by a Subhalo (1.8 < γ < 2.) 1 11 ( Smin 5 µas ) 1.6 But what if dark matter is clumpier? 1

Lensing probability (f=1) Lensing Event Rates We can combine the lensing cross sections with a subhalo mass function to calculate the fraction of the sky that is detectably lensed ( S > S min ) by a subhalo. All the halo mass is contained within.1 pc of a subhalo center 1-5 1-6 1-7 S min =5µas S min = 2 µas S min = 5 µas 1-8 1 1 1 1 1 1 1 1 1 M<.1pc [ M ] Lens velocity: 2 km/s; Source Distance: 2 kpc γ=2. γ=1.8 γ=1.5 γ=1.2 1

Detection with Targeted Observations Finding a subhalo through astrometric microlensing is unlikely, but what if you know where to look? Kuhlen et al. 29 Fermi may detect emission from dark matter annihilation in subhalos and could localize the center of emission down to a few sq. arcminutes. Area [arcsec 2 ] 4 3 2 1 S min =5µas S min = 2 µas S min = 5 µas 1 1 1 1 1 1 1 1 1 1 1 1.1 M vir [M ] Lens distance: 5 pc; Lens velocity: 2 km/s; Source Distance: 5 kpc γ=2. γ=1.8 γ=1.5 11

Summary Local subhalos deflect the light from background stars, producing a unique astrometric microlensing signature. only the innermost.1 pc of the subhalo can produce a signal the star s apparent motion depends on the subhalo density profile the image deflection is measured in microarcseconds -- we can do that! To see a subhalo lensing event, we d have to get lucky! nearly impossible to find a subhalo through lensing, unless subhalos are more numerous and/or more concentrated than expected if Fermi points the way, high-precision astrometry can follow; we can detect subhalos within 1 pc of us with (stripped) masses > 1 M. For more details see Erickcek & Law 211 (arxiv:17.4228) 12

EXTRA SLIDES

Subhalo Density Profiles Unfortunately, even the best simulations can only probe the density profiles of the largest subhalos ( M sub > 1 8 M ), and the inner 35 pc are unresolved. Via Lactea II: ρ(r) r (γ 1.24) for large subhalos. Aquarius: ρ(r) r (γ<1.7) for large subhalos. Springel et al. 28 Simulations of first halos: Earth-mass halos at a redshift of 26 have ρ(r) r (1.5<γ<2.) extending to within 2 AU of the Diemand et al. 25; Ishiyama et al. 21 center. ρ(r) = We ll assume a generalized NFW profile: ( ρ ) γ ( ) 3 γ r r 1+ r r r is set by the concentration ρ is set by the virial mass Diemand et al. 28 r r r r ρ(r) r γ ρ(r) r 3 α r 2 γ deflection angle α r 1 deflection angle 14

The Signal We re Looking For 15. Predicted PM 1. Detection run Y/µas 5.. -5. -1. Calibration period PM subtracted trajectory -15. -1. -5.. 5. 1. X/µas 15

The Signal We re Looking For 15. Predicted PM 1. Detection run Y/µas 5.. -5. -1. Signal Calibration period PM subtracted trajectory -15. -1. -5.. 5. 1. X/µas 15