The Masses and Metallicities of the Least Luminous Galaxies

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The Masses and Metallicities of the Least
Luminous Galaxies
Or, Why Segue 1 Is Not a Star Cluster

• Josh Simon
• Carnegie Observatories

Marla Geha (Yale) Beth Willman (Haverford) Joe
Wolf (UC Irvine) Manoj Kaplinghat (UC
Irvine) James Bullock (UC Irvine) Louie Strigari
(Stanford) Erik Tollerud (UC Irvine) Anna
Frebel (Harvard) Evan Kirby (Santa Cruz)
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The Faintest Dwarfs Tell Us About

• Galaxy formation
• Dwarf galaxies are astrophysically simple systems
  
• Dark matter
• Dwarf galaxies are the closest and densest dark
  matter-dominated objects
• (Aaronson 1983 Mateo et al. 1993)
• First stars
• Dwarf galaxies may be the best places to look for
  the most metal-poor stars
• (Kirby et al. 2008 Frebel et al. 2009
  Muñoz et al. 2009)

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The Faintest Dwarfs Tell Us About

• Galaxy formation
• Dwarf galaxies are astrophysically simple systems
  
• Dark matter
• Dwarf galaxies are the closest and densest dark
  matter-dominated objects
• (Aaronson 1983 Mateo et al. 1993)
• First stars
• Dwarf galaxies may be the best places to look for
  the most metal-poor stars
• (Kirby et al. 2008 Frebel et al. 2009
  Muñoz et al. 2009)

not as _at_ complicated as bigger galaxies
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Dwarf Galaxy Scaling Relations
Geha et al. (2009)
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The Least Luminous Galaxies
These dwarfs (1) Control the missing satellite
problem (Tollerud et al. 2008 Koposov et al.
2009 Busha et al. 2009) (2) Will be the
brightest DM annihilation sources (Strigari et
al. 2008a Geha et al. 2009) (3) Are the most
vulnerable to systematics (Simon Geha 2007
Niederste-Ostholt et al. 2009)
Strigari et al. (2008b)
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The Least Luminous Galaxies
Segue 1 (d23 kpc)
Bootes II (d42 kpc)
These dwarfs (1) Control the missing satellite
problem (Tollerud et al. 2008 Koposov et al.
2009 Busha et al. 2009) (2) Will be the
brightest DM annihilation sources (Strigari et
al. 2008a Geha et al. 2009) (3) Are the most
vulnerable to systematics (Simon Geha 2007
Niederste-Ostholt et al. 2009)
ComBer (d44 kpc)
UMa II (d32 kpc)
Strigari et al. (2008b)
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Boo II Kinematics
Boo II
foreground

• 21 members, v -126.2 km/s, ? 7.6 1.5 km/s
• M1/2 2.5 ? 106 M?
• Fe/H -2.49 0.07, metallicity spread 0.50
  dex

Simon et al. (in prep)
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An Experiment

• Search for effects of tidal interaction
• Ideal target would be
• Close to MW (to maximize tidal force)
• Compact (to search out to large relative radii)
• Well-separated from MW velocity (for
  clean member selection)

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An Experiment

• Search for effects of tidal interaction
• Ideal target would be
• Close to MW (d 23 kpc)
• Compact (r 4.4')
• Well-separated from MW velocity (v
  207 km s-1)

Segue 1
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A Complete Survey of Segue 1
almost
?

• Keck/DEIMOS spectroscopy of every photometric
  member candidate in Segue 1 out to r 10' (67
  pc)
• If Segue 1 does not have an extended DM halo, its
  tidal radius should be less than 50 pc

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A Complete Survey of Segue 1

• Velocity and metallicity separate Milky Way
  foreground stars

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A Complete Survey of Segue 1

• 65 members, v 208 km/s, ? 5.4 0.8 km/s
• M1/2 9.0 ? 105 M?
• Fe/H -2.4, metallicity spread large

Simon et al. (in prep)
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A Complete Survey of Segue 1

• Signs of tidal disruption?
• Velocity gradient
• Excess of stars at large radii
• Velocity dispersion increasing with radius

No
No
No
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The Least Evolved Galaxies
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Abundances in the Ultra-Faint Dwarfs

• High-resolution spectroscopy
• Accurate abundances for many elements
• Requires bright targets long integrations

UMa II
Coma

• members
• HIRES targets
• nonmembers

Frebel et al. (2009)
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Metallicities in the SDSS Dwarfs
Is the metal-poor component of the MW halo made
up of destroyed ultra-faint dwarfs?

• MW disk (Venn04)
• MW halo (Venn04)
• dSphs (Venn04, from Shetrone, etc.)
• ultra-faint dSphs

Frebel et al. (2009)
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Metallicities in the SDSS Dwarfs

• Low abundances of neutron-capture species
• Large scatter within individual galaxies
• (? age spread)

• MW disk (Venn04)
• MW halo (Venn04/Barklem05)
• dSphs (Venn04, from Shetrone, etc.)
• ultra-faint dSphs

Frebel et al. (2009)
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Our Conclusions

• New measurements confirm that Bootes II and
  Segue 1 have large velocity dispersions

• No evidence supporting tidal disruption for
  either system

• Segue 1 is therefore a key target for dark
  matter studies

• Ultra-faint dwarfs have halo-like abundances