The Masses and Metallicities of the Least Luminous Galaxies Josh Simon Carnegie Observatories 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) 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) Or, Why Segue 1 Is Not a Star Cluster

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) 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 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) 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 complicated as bigger galaxies The Faintest Dwarfs Tell Us About:

Dwarf Galaxy Scaling Relations Geha et al. (2009)

The Least Luminous Galaxies Strigari et al. (2008b) 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)

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) Segue 1 (d=23 kpc) Bootes II (d=42 kpc) ComBer (d=44 kpc) UMa II (d=32 kpc) Strigari et al. (2008b)

Boo II Kinematics 21 members, v = km/s,  = 7.6 ± 1.5 km/s M 1/2 = 2.5  10 6 M  [Fe/H] = ± 0.07, metallicity spread = 0.50 dex 21 members, v = km/s,  = 7.6 ± 1.5 km/s M 1/2 = 2.5  10 6 M  [Fe/H] = ± 0.07, metallicity spread = 0.50 dex +1.8 –1.0 Simon et al. (in prep) Boo II foreground

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) 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 ) 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 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 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  almost 29 pc 59 pc 88 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 M 1/2 = 9.0  10 5 M  [Fe/H] ~ -2.4, metallicity spread large 65 members, v = 208 km/s,  = 5.4 ± 0.8 km/s M 1/2 = 9.0  10 5 M  [Fe/H] ~ -2.4, metallicity spread large +3.7 –2.8 Simon et al. (in prep)

A Complete Survey of Segue 1 Signs of tidal disruption? –Velocity gradient –Excess of stars at large radii –Velocity dispersion increasing with radius Signs of tidal disruption? –Velocity gradient –Excess of stars at large radii –Velocity dispersion increasing with radius No

The Least Evolved Galaxies

Abundances in the Ultra-Faint Dwarfs members  HIRES targets nonmembers UMa IIComa High-resolution spectroscopy –Accurate abundances for many elements –Requires bright targets + long integrations High-resolution spectroscopy –Accurate abundances for many elements –Requires bright targets + long integrations Frebel et al. (2009)

MW disk (Venn04) MW halo (Venn04)  dSphs (Venn04, from Shetrone, etc.)  ultra-faint dSphs Metallicities in the SDSS Dwarfs Frebel et al. (2009) Is the metal-poor component of the MW halo made up of destroyed ultra-faint dwarfs?

Metallicities in the SDSS Dwarfs Frebel et al. (2009) Low abundances of neutron-capture species Large scatter within individual galaxies (  age spread) 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

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