Ultra-Faint dwarfs: The Living Fossils of the First Galaxies

Stefania Salvadori

NOVA fellow

First Stars IV – From Hayashi to the Future – Kyoto, Japan 23 May 2012

HOW MANY ULTRA-FAINT DWARFS ? OBSERVATIONS

SDSS coverage

2003 census data:

Classical dSphs = 11

2009 census data:

Classical dSphs = 11Ultra-faint dSphs = 14

Willman+05, Zucker+06a/b, Belokurov+06/07/08

Milky Way satellite galaxies

Kirby+08Kirby+08

DWARF SPHEROIDAL GALAXIES OBSERVATIONS

Ultra faint dSphs: Ltot ≤ 105L Classical dSphs: Ltot > 105L

Gas-free galaxies (only exception LeoT) hosted by small dark matter haloes M < 109M. They all contain an old stellar population and they are metal-poor.

Kirby+08

N* = 130

25%

SDSSDART

Classical dSphs: Ltot > 105LUltra faint dSphs: Ltot ≤ 105L

N* = 933

N* = 202N* = 364

N* = 513

Battaglia+06; Helmi+06; Starkenburg+10

< 5%

Simple Star Formation History Complex Star Formation Historye.g. Dolphin+02; de Boer+11; Lemasle+12Okamoto+in prep.

Geha+in prep.

METALLICITY DISTRIBUTION FUNCTIONS OBSERVATIONS

CHEMICAL ABUNDANCES OBSERVATIONS Shetrone+01/03, Koch+08a/b, Aoki+09, Cohen+09, Feltzing+09, Tolstoy, Hill & Tosi 09, Frebel+10a/b, Norris+10, Simon+10, Tafelmeyer+10, Lemasle+12, Venn+12

SCLSXT

Ultra-Faint vs classical vs stellar halo

Courtesy of Kim Venn

*

*

*

Venn+12; see also Simon+10

THE MILKY WAY SYSTEM FORMATION

Radiative feedback: minimum mass for SF

Chemical feedback:

Z Zcr PopIII stars M = 200 M

Z > Zcr PopII/I stars Larson IMF

Mechanical feedback Mej εw Nsn /v2esc

MW

GALACTIC M

EDIU

M

z = 20

Tim

e

z = 0

SEMI-ANALYTICAL MODELS THE MW FORMATION Tumlinson06/10; Salvadori+07/08;Komiya+09;DeLucia&Helmi08;Li+09;Font+11

Accounting for the cosmological context and for the evolution of single proto-galaxies

faint SN m* = 25 M

GAlaxyMErgerTree&Evolution

Salvadori,Schneider&Ferrara07

GAMETE

Kobayashi+11

SATELLITES CANDIDATES THE MW FORMATION Haiman+97/00; Gnedin00;Ciardi+00; Nishi&Tashino00; Madau+01; Machacek+01; Ricotti+01/02; Dijkstra+04; Susa&Umemura04; Ahn&Shapiro07;Johnson+07;Wise&Abel08;Okamoto+08..

Dijkstra+04Kitayama+00

f* H2

f* (Tvir /104 K)3

Madau, Ferrara & Rees 01; Ricotti & Gnedin 01

H2-cooling haloes

ERM

LRM

Salvadori&Ferrara09/12

dynamicsDiemand+05; Moore+06

Early/late reionization histories Gallerani+06

Dissociating LW background Ahn+09

Mass threshold Machacek+01

ULTRA-FAINT DWARFS

Formation epochs Dwarf spheroidal galaxies

THE IRON-LUMINOSITY RELATION DWARF GALAXIES Salvadori & Ferrara 09/12; Bovill & Ricotti 09/11; Muñoz+09; Revaz+09; Li+10; Tumlinson 10; Sawala+10; Okamoto+10

Salvadori & Ferrara 09/12

Ultra faint dSphs

Classical dSphs

Lookbacktime [Gyr]

131210 13.28 13.4

L/L

2 4 6 8

0

−1

−3

−4

[Fe/

H]

−2

Ultra-faint dwarfs are the living fossils of star-forming H2-cooling minihaloes (Salvadori&Ferrara09;Bovill&Ricotti09;Muñoz+09) which assembled at z > 8.5

METALLICITY DISTRIBUTION FUNCTIONS DWARF GALAXIES

Helmi+06

Starkenburg+10

Kirby+08

Sculptor

Ultra faint

Sculptor

Ultra faint dSphs

Salvadori & Ferrara 09

The broad MDF of ultra-faint dwarfs reflects the inefficient star-formation of H2-cooling haloes turning into stars < 3% of the potentially available baryons.

F = M* / fc Mh

Madau+08

0

−1

−3

−4[F

e/H

]

−2

L/L

2 4 6 8

Frebel+10

What is the origin of extremely metal-poor stars in classical dSphs ?

Salvadori, Ferrara & Schneider 08Lower pre-enrichment

STAR-FORMATION HISTORIES DWARF GALAXIES

Sculptor

CB

Hercules

Hercules

UMi

ComaBerenice Ursa Minor Sculptor

Salvadori et al. in prep

42 120 6 8 10 14redshift

42 120 6 8 10 14redshift

42 120 6 8 10 14redshift

42 120 6 8 10 14redshift

Lookbacktime [Gyr]

1312 13.28 1312 13.28 131210 13.28 131210 13.28

Lookbacktime [Gyr] Lookbacktime [Gyr] Lookbacktime [Gyr]

Merging processes become important

1010

L/L

2 4 6 8

0

−1

−3

−4

[Fe/

H]

−2

Log

SFR

[M

yr−

1 ]

0

−1

−2

−3

−4

−5

DWARFS-DLAs CONNECTION

VERY METAL-POOR DLAs DWARFS-DLAs CONNECTION

Bensby andFeltzing 06

Fabbian+09

Cooke+11b

Cooke+11a

Galactic halo stars vs Damped Lyα Absorption systems

Q: What’s the origin of very metal-poor DLAs and their connection with dSphs ?

Abundance pattern consistentwith a Z =0 faint SN of 25M

Kobayashi+11

Tvir < 104K Tvir > 104K

Formation epochs

Mh < 108 M Mh = 108-11 M

M* < 104 M M* = 106-9.5 M

Ψ = (0.1-10)M yr−1Ψ = 0

Gas-rich satellites @ z =2.34 :: NHI ≥ 2 ×1020 cm−2

Salvadori&Ferrara12

Cooke+2011b

Cooke+2011a

Prochaska+2007

DLA candidates

log

N H

I

[Fe/H]

THE MILKY WAY @ z = 2.3 DWARFS-DLAs CONNECTION

Formation epochs DLA candidates

Failed ultra-faint dwarfs

M* = 102-4 M

Salvadori&Ferrara12

[O/H][C

/O]

Star-less minihaloes

CHEMICAL ABUNDANCES DWARFS-DLAs CONNECTION

Passive evolution

Becker+2012

C-enhanced DLAs are associated to star-forming minihaloes virialized at z > 8 in metal-free and neutral regions of the MW environment

PopII stars may start to form as soon as Z ≥ Zcr ≈ 10−4Zor [Fe/H] ≈ −5 The PopIII imprint is hidden by these stellar populations

CONCLUSIONS

Ultra-Faint dwarfs

− Ultra-faint dwarfs are fossil relics of H2-cooling minihaloes formed at z > 8.5.

− Their broader MDF reflects the inefficient cooling by H2 molecules.

− The rare [Fe/H] < –3 stars in classical dSphs formed in progenitor minihaloes formed at very high redshifts.

CONCLUSIONS ULTRA-FAINT DWARF GALAXIES

Failed Ultra-Faint dwarfs

− Very metal-poor DLAs are star-less minihaloes imprinted by SNII.

− C-enhanced DLAs are the gas-rich counterpart of Ultra-faint dwarfs.

− They host PopIII stars but their chemical imprints are hidden by normal, low-metallicity PopII stars, which start to form as soon as Z > Zcr .

Accretion Self-enrichment

IMPRINT BY FIRST STARS ? FINAL REMARKS Salvadori, Schneider & Ferrara 2007

Zcr = 10 – 4 Z Zcr = 10 – 6 Z

Zcr = 0

Galactic halo stars: 2nd generation vs all generations

mPopIII = 200 M mPopIII = 200 M mPopIII = (0.1-100) M

Does the unusual chemical compositionof two Hercules stars reflect the

self-enrichment by first stars?

THANKS !