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Substructure from Simulations Can the standard reionization scenario explain the current population of satellite galaxies? (How low does galaxy formation go?) James S. Bullock (UC Irvine) Tyler Kelley ( UC Irvine ) ( UC Irvine > Texas

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Substructure from Simulations Can the standard reionization scenario explain the current population of satellite galaxies? James S. Bullock (UC Irvine) (How low does galaxy formation go?)

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Tyler Kelley ( UC Irvine ) ( UC Irvine —> Texas )

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Star-less halos: How low do we go?

mp=2.e4 Msun mp=2.e5 Msun mp=2.e6 Msun mdm~2.e3 Msun

Aquarius

S p r i n g e l e t a l . 8

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Star-less halos: How low do we go?

H2 cooling limit V~2 km/s V~16 km/s H cooling limit

mp=2.e4 Msun mp=2.e5 Msun mp=2.e6 Msun mdm~2.e3 Msun

Aquarius

S p r i n g e l e t a l . 8

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How Many Do We See?

mp=2.e4 Msun mp=2.e5 Msun mp=2.e6 Msun

ELVIS

G a r r i s

i m m e l + 2 1 4

Classical MW satellites V~30 km/s All MW satellites V~16 km/s H cooling limit

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How Many Do We See?

mp=2.e4 Msun mp=2.e5 Msun mp=2.e6 Msun

ELVIS

G a r r i s

i m m e l + 2 1 4

Classical MW satellites V~30 km/s All MW satellites V~15 km/s H cooling limit Dark substructure? V<8 km/s

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1995

“…a photo ionizing background suppresses the formation of galaxies with circular velocities vc<30 km/s …” (Tvir ~ 30,000 K)

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“…a photo ionizing background suppresses the formation of galaxies with circular velocities vc<30 km/s …” (Tvir ~ 30,000 K) Naively, this is the suppression scale you would expect from from the ionized IGM temperature…. Onorbe et al. 2015 TIGM ~ 10,000 - 30,000 K

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Draco SMC LMC Sculptor Fornax Carina Sag Sextans UMaI Milky Way

100,000 light years

Bullock/Geha

Klypin+ 1999

Missing Satellites: 1999

Moore et al. 1999; Klypin et al.1999

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Mismatch sets in V~30 km/s

Klypin+ 1999 30 km/s

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30 km/s Reionization solves the problem. Bullock+2000

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Sawala+2014

8 17 30 4 Vmax (km/s)

Galaxies get dark at Vmax~20-30 km/s because of reionization. (Every halo is dark below 8 km/s.)

Similar results: Gnedin 2000; Hoeft et al. 2006; Okamoto et al. 2008; Ocvirk et al. 2016 [CoDa simulations]

sim particle mass: mbaryon ~ 10,000 Msun

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FIRE simulations of Milky Way

Hopkins+2018 Garrison-Kimmel+2018 Wetzel+2017 ‘Green flash’ is reionization

mbaryon ~ 5000 Msun

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600 kpc

Dark Matter

Garrison-Kimmel et al. 2018

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600 kpc

Stars

Garrison-Kimmel et al. 2018

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10 zoom simulations of Milky Ways

Garrison-Kimmel + 2018

subhalos get ‘dark’ at Vmax<20 km/s

“…do not exhibit the missing satellites problem…” ELVIS on FIRE

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Fitts et al. 2017 FIRE simulations

12 zoom simulations of field dwarfs mbaryon = 500 Msun Reionization suppression: Vmax ~ 20-25 km/s Tvir ~ 20,000 K

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8 17 30 4 Vmax (km/s)

Sawala+2014

If we take this as the canonical expectation, what should we see when counting very low- mass dwarfs?

Similar results: Gnedin 2000; Hoeft et al. 2006; Okamoto et al. 2008; Ocvirk et al. 2016 [CoDa simulations]; Fitts et al. 2017; etc.

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Milky Way

Five-fold increase in last in 14 yrs ~40 ultra-faint satellite galaxies discovered since ‘missing satellites’

Willman et al. 2005; Zucker et al. 2006; Belokurov et al. 2007; Koposov et al. 2015a; Bechtol et al. 2015; Kim et al. 2015

SDSS SDSS DES

“Ultra-faint dwarfs”

M*~103-105 Msun

Roughy half the sky unexplored to this depth. Certainly not complete beyond 50 kpc.

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600 kpc

Stars

Garrison-Kimmel et al. 2018

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600 kpc

Dark Matter

Garrison-Kimmel et al. 2018

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600 kpc

Dark Matter

Garrison-Kimmel et al. 2018

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FIRE Hydrodynamics

100 kpc

(dark matter) Garrison-Kimmel+2017

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FIRE Hydrodynamics

100 kpc

Pure N-Body

100 kpc

(dark matter)

Baryons Matter (A Lot!)

(same halo) Garrison-Kimmel+2017 Also: Brooks & Zolotov 2014, Zhu + 2016,

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FIRE Hydrodynamics

100 kpc

Pure N-Body

100 kpc

(dark matter)

Baryons Matter (A Lot!)

(same halo) Garrison-Kimmel+2017 Also: Brooks & Zolotov 2014, Zhu + 2016,

NO substructure (V > 5 km/s) within 20 kpc

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FIRE Hydrodynamics

100 kpc

Pure N-body

Most important Factor is Central Galaxy Potential

N-body + Gal. Potential

Garrison-Kimmel+2017

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Substructure within radii of relevance for known GC stream heating is destroyed…

Garrison-Kimmel+2017

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Phat ELVIS: Mv = 0.8-2 x 1012 Msun

12 high-resolution zoom simulations with Milky Way potentials evolved to z=0
12 identical simulations run with Dark Matter only

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Factor of ~10 reduction in substructure within ~25 kpc

Kelley et al. 2018

DMO Galaxy

Also: Garrison-Kimmel et al. 2018

R (kpc)

N(<R)

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Kelley et al. 2018

DMO Galaxy

Also: Garrison-Kimmel et al. 2018

R (kpc)

DMO Galaxy

Preferentially destroy halos with percenters <~ 20 kpc

N(<R)

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Radial distribution of Satellites

DMO Galaxy

Vmax > 4.5 km/s

N(<R)

Kelley et al. 2018

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DMO Galaxy

Vpeak > 10 km/s

N(<R)

Kelley et al. 2018

Radial distribution of Satellites

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DMO Galaxy

Vpeak > 10 km/s

Radial distribution of Satellites

DMO Galaxy

Vpeak > 10 km/s

known Milky Way satellites N(<R)

14 ultra-faint dwarfs w/in 50 kpc

Tvir ~ 3,800 K

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N(<R) Median of Galaxy Potential Runs MW dwarf satellites sky correction

Vpeak = 8 km/s Tvir ~ 2,400 K Nominal re-ionization scale Vmax = 20 km/s

LOW! NOT atomic cooling halos! Required to match :

Why are there so many ultra-faint dwarf satellites?

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Vmax > 4.5 km/s subhalos known MW dwarfs

Radial Profile: full scatter of 12 Galaxy simulations

Graus et al. 2018

(no sky coverage correction (M~5.e6Msun)

100% scatter (12 halos)

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Vmax > 4.5 km/s subhalos

Apply fiducial “toy” model of how reionization makes halos dark

20 km/s Tvir ~ 20,000 K

100% scatter (12 halos) Graus et al. 2018

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Vmax > 4.5 km/s subhalos

“Observed” to account for coverage incompleteness

20 km/s

100% scatter (12 halos) Average (<1)

Tvir ~ 20,000 K

Graus et al. 2018

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Vmax > 4.5 km/s subhalos

100% scatter (12 halos)

12 km/s Tvir ~ 5,500 K

Graus et al. 2018 Average

“Observed” to account for coverage incompleteness

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Vmax > 4.5 km/s subhalos

100% scatter (12 halos) Graus et al. 2018 Average

Threshold mass for galaxy formation is apparently very low…

> In order to explain known MW galaxies, we must populate majority subhalos with Vpeak~12km/s (Tvir ~ 5,500 K). > Median halo in the suite needs even lower threshold (V ~ 8 km/s) > Do ultra-faint dwarfs really live in such low-mass halos?

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CONCLUSIONS

1. Central galaxy potential destroys most

substructure in the inner ~25 kpc of cosmological ‘zoom’ MW halos.

DMO galaxy

Must be accounted for when making

predictions for stream heating & other substructure probes.

2. In order to account for the ~14 Milky Way dwarf galaxies within 50 kpc,

we must populate most subhalos with galaxies down to Vpeak~10km/s

This is well below the canonical Vpeak~20 km/s scale where

reionization was thought to start making galaxies go dark!

Also below the atomic cooling scale (which is V~16 km/s)

Graus et al. 2018

Kelley et al. 2018

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Garrison-Kimmel+2014

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Ancient stars (ultra-faint)

Wheeler+15 Classical FIRE-1 Simulations

17 km/s 30 km/s

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