Ultra-Faint, Ultra-Dark, and Ultra-Handsome The New Satellites of the Milky Way Josh Simon Caltech Collaborators: Marla Geha (Yale) Louie Strigari (Irvine) Evan Kirby (Santa Cruz) Anna Frebel (Texas) Beth Willman (Harvard) James Bullock (Irvine)

Outline I. Intro: CDM has its problems II. Discovery of the ultra-faint dwarfs III. Ultra-faint dwarf kinematics and the missing satellite problem IV. Avenues for future work

The Cold Dark Matter Model CDM model is in concordance with a wide range of astronomical observations CMB, LSS, SNe Ia, BBN, BAO, Ly-a forest, . . . NASA/WMAP Science Team; Spergel et al. (2007) Riess et al. (2007) Eisenstein et al. (2005)

Then What’s the Problem? On small scales, LCDM is not in concordance with observations

Then What’s the Problem? On small scales, LCDM is not in concordance with observations Central density problem Galaxies should have cuspy density profiles Galaxy centers are not dense enough

The Central Density Problem cusp core

Then What’s the Problem? On small scales, LCDM is not in concordance with observations Central density problem Galaxies should have cuspy density profiles Galaxy centers are not dense enough Angular momentum problem Simulated galaxy disks are too small

Angular Momentum Problem Simulations always produce large bulges But many disk galaxies lack significant bulges Abadi et al. (2003) NGC 5907

Then What’s the Problem? On small scales, LCDM is not in concordance with observations Central density problem Galaxies should have cuspy density profiles Galaxy centers are not dense enough Angular momentum problem Simulated galaxy disks are too small Substructure (missing satellite) problem Only 1-10% of the predicted number of dwarf galaxies are actually observed

CDM and the Missing Satellites CDM predicts large numbers of subhalos (~500 for a Milky Way-sized galaxy) Milky Way only has 24 known satellites Are the simulations wrong? Or do these objects actually exist despite the lack of observational evidence? Springel et al. 2001

CDM and the Missing Satellites CDM predicts large numbers of subhalos (~500 for a Milky Way-sized galaxy) Milky Way only has 24 known satellites Are the simulations wrong? Or do these objects actually exist despite the lack of observational evidence? Springel et al. 2001

What Do These Problem Tell Us? Two basic sets of possible solutions: Modifications to CDM What modifications? Power spectrum, DM particle mass/decay/interaction cross-section? Astrophysics comes in on small scales Reionization/feedback/winds prevent stars from forming in most low-mass halos? Baryonic feedback/outflows adjust the mass distribution at galaxy centers?

A Recent Breakthrough Only 11 Milky Way dwarfs known through 2004 Since 2005, SDSS has discovered: 9 new dSphs 1 dIrr 3 ??? dSph/GCs? Willman et al. (2005a), Zucker et al. (2006a,b), Belokurov et al. (2006,2007,2008) Irwin et al. (2007) Willman et al. (2005b), Belokurov et al. (2007), Walsh et al. (2007)

SDSS Search Technique Raw SDSS stellar density SDSS blue stars (g-i < 0.5) MV = -4.1 D = 44 kpc

New SDSS Dwarfs Old dwarfs (Leo II) New dwarfs (Willman 1) SDSS Willman et al. (2005)

Keck Spectroscopic Survey of SDSS Dwarfs Medium-resolution spectra of 889 stars (448 members) across 9 dwarfs (CVn I, CVn II, Coma Berenices, Hercules, Leo IV, Leo T, Segue 1, UMa I, UMa II) Stellar velocities measured from cross-correlation with templates Templates include GKM giants, KM dwarfs, HB stars Stellar metallicities measured from EW of the Ca triplet lines (Rutledge et al. 1997) or spectral synthesis (Kirby et al. 2008a) Velocities can be used to constrain the masses of these extreme systems

Kinematics of the SDSS Dwarfs Incredibly small velocity dispersions to go with their tiny luminosities Previous lower limit for dSphs (Gilmore et al. 2007) Simon & Geha (2007) Martin et al. (2007) Geha et al. (2008)

Are They Dwarfs or Globulars? Simplest possible mass model: equilibrium, spherical, isotropic systems where M follows L Galaxy Mass Luminosity UMa II UMa I Segue 1 Leo T Leo IV Coma CVn II CVn I Herc 5.4 ± 2.4  106 M 1.6 ± 0.5  107 M 4.6 ± 2.8  105 M 8.6 ± 4.1  106 M 1.1 ± 1.1  106 M 1.4 ± 0.5  106 M 1.3 ± 0.6  106 M 2.8 ± 0.3  107 M 7.8 ± 3.1  106 M 4.1  103 L 1.4  104 L 3.4  102 L 5.9  104 L 8.6  103 L 3.7  103 L 7.8  103 L 2.4  105 L 3.7  104 L M/L > 100 M/L!

Are They Dwarfs or Globulars? Simplest possible mass model: equilibrium, spherical, isotropic systems where M follows L Galaxy Mass Luminosity 3 lower limit on mass UMa II UMa I Segue 1 Leo T Leo IV Coma CVn II CVn I Herc 5.4 ± 2.4  106 M 1.6 ± 0.5  107 M 4.6 ± 2.8  105 M 8.6 ± 4.1  106 M 1.1 ± 1.1  106 M 1.4 ± 0.5  106 M 1.3 ± 0.6  106 M 2.8 ± 0.3  107 M 7.8 ± 3.1  106 M 4.1  103 L 1.4  104 L 3.4  102 L 5.9  104 L 8.6  103 L 3.7  103 L 7.8  103 L 2.4  105 L 3.7  104 L M > 3.5  105 M M > 3.3  106 M M > 3.4  103 M M > 3.8  105 M M > 0 M > 1.9  105 M M > 8.3  104 M M > 1.6  107 M M > 7.8  104 M

Are There Still Satellites Missing? Comparison to Via Lactea N-body simulation (Diemand et al. 2007) Including new dwarfs, total MW population projects to 76 dwarf galaxies over the full sky (Simon & Geha 2007; Koposov et al. 2007)

Is Sloan Complete? If SDSS is complete: If SDSS is not complete: Need another ~200 low-mass (starless?) halos If SDSS is not complete: Assume radial subhalo distribution from Via Lactea simulation

Are There Still Satellites Missing? Comparison to Via Lactea N-body simulation (Diemand et al. 2007) What if dwarfs only populate the most massive halos? (Stoehr et al. 2002) Simon & Geha (2007)

Are There Still Satellites Missing? Comparison to Via Lactea N-body simulation (Diemand et al. 2007) How about the subhalos that were most massive before being accreted by the Milky Way? (Kravtsov et al. 2004) Simon & Geha (2007)

Are There Still Satellites Missing? Comparison to Via Lactea N-body simulation (Diemand et al. 2007) Halos that reached vcirc ~ 8 km/s before reionization . . . (Bullock et al. 2000; Ricotti & Gnedin 2005; Moore et al. 2006) Simon & Geha (2007)

Is Sloan Complete? If SDSS is complete: If SDSS is not complete: Need another ~200 low-mass (starless?) halos If SDSS is not complete: Assume radial subhalo distribution from Via Lactea simulation

If Sloan Is Still Missing Satellites Observed dwarfs (corrected for sky coverage and incompleteness) Observed dwarfs (corrected for sky coverage) Observed dwarfs (not corrected for sky coverage) Tollerud et al. (2008) 300-600 dwarfs with L > 1000 L around Milky Way!

Dwarf Galaxy Scaling Relations Geha et al. (2008)

Better Mass Measurements Drop mass follows light assumption CDM prior, marginalize over unknown parameters (density profile, velocity anisotropy) Mass is best constrained within a fixed physical radius of ~0.3 kpc M0.3 instead of M”total” Use maximum-likelihood technique to calculate PDF for mass given the data Strigari et al. (2007)

Better Mass Measurements Mass follows light Maximum likelihood with CDM prior Strigari et al. (2008)

A Lower Limit for Galaxy Formation Is galaxy formation impossible below Mvir~108 M (M0.3~107 M)? Milky Way dwarfs; Strigari et al. (2008) Lower mass limit? Massive galaxies from CLF; Eke et al. (2004) van den Bosch et al. (2007) CLF Strigari et al. (2008)

The Most Metal-Poor Galaxies Dwarf galaxies (including the ultra-faint dwarfs) obey a metallicity-luminosity relation Kirby et al. (2008b)

The Most Metal-Poor Galaxies Metallicity distribution in the ultra-faint dSphs is similar to the Milky Way halo . . . Helmi et al. 2006 Kirby et al. 2008b

The Most Metal-Poor Galaxies . . . As is the abundance pattern MW disk (Venn04) MW halo (Venn04) dSphs (Venn04) ultra-faint dSphs Could the MW halo be made up of destroyed ultra-faint dwarfs?

Conclusions Ultra-faint dwarfs should solve the missing satellite problem If SDSS is complete, observed mass function tells us that reionization suppressed star formation in most halos If SDSS is not complete, observed luminosity function plus Via Lactea suggests >300 luminous dwarfs We have plenty to learn about galaxy formation . . . ALL MW dwarf spheroidals appear to have the same mass Abundances in the ultra-faint dwarfs are consistent with those of stars in the Milky Way halo