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Going Bang in the Night Searching for the Progenitors of Type Ia

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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)
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)
The Faintest Dwarfs Tell Us About:
• Galaxy formation
   – Dwarf galaxies are astrophysically simple systems
                        not as #%@#$ complicated as
                        bigger galaxies
• 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)
Dwarf Galaxy Scaling Relations




                          Geha et al. (2009)
     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)
     The Least Luminous Galaxies
                                              Segue 1 (d=23 kpc)
                                                Bootes II (d=42 kpc)
These dwarfs:
                                                   ComBer (d=44 kpc)
(1) Control the missing                             UMa II (d=32 kpc)
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)
              Boo II Kinematics
                                    Boo II       foreground




• 21 members, v = -126.2 km/s,  = 7.6 ± 1.5 km/s
             +1.8
• M1/2 = 2.5 –1.0  106 M
• [Fe/H] = -2.49 ± 0.07, metallicity spread = 0.50
  dex                                    Simon et al. (in prep)
               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)
                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
 A Complete Survey of Segue 1
                                      almost
                                        
• Keck/DEIMOS spectroscopy of every
  photometric member candidate in
  Segue 1 out to r = 10' (67 pc)

                                       29 pc
                                       59 pc
                                       88 pc



  – If Segue 1 does not have an extended DM halo, its
    tidal radius should be less than 50 pc
  A Complete Survey of Segue 1

• Velocity and metallicity separate Milky
  Way foreground stars
  A Complete Survey of Segue 1




• 65 members, v = 208 km/s,  = 5.4 ± 0.8 km/s
             +3.7
• M1/2 = 9.0 –2.8  105 M
• [Fe/H] ~ -2.4, metallicity spread large
                                      Simon et al. (in prep)
  A Complete Survey of Segue 1




• Signs of tidal disruption?
  – Velocity gradient No
  – Excess of stars at large radii No
  – Velocity dispersion increasing with radius   No
The Least Evolved Galaxies
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)
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)
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)
          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

								
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