Hypervelocity Stars Ejected from the Galactic Center STScI Colloquium Oct 3, 2007 Warren R. Brown Smithsonian Astrophysical Observatory Collaborators: Margaret Geller, Scott Kenyon, Michael Kurtz

Radial Velocities from the MMT

The first “Hypervelocity Star”

Predictions Hills, 1988, Nature: prediction Hills, 1991, AJ: orbits Yu & Tremaine, 2003, ApJ: rates NY Times 2/22/2005 “It’s high time someone found it.” - Jack Hills SF Chronicle, 2/11/2005

The Milky Way Kaufmann

The Galactic Center

Eisenhauer et al. 2003

Three-body exchange Bromley 2005

An Unexpected Star B9 main sequence star. Solar metallicity. g=19.8 thus d=110 kpc. Travel time ~160 Myr. Brown et al. (2005)

Our Search for more Hypervelocity Stars Fukugita et al (1996) Brown et al. (2006a, 2006b, 2007a, 2007b)

Lowest Mass White Dwarf Kilic et al (2007a,b) Extremely Metal Poor Galaxy Kewley et al. (2007) Brown et al. (2007c) log(O/H)+12 = 7.44 Spectroscopic Observations of an Unusual Parameter Space

Radial Velocities Brown et al (2007b)

HVS: Ejection Model Bromley et al (2006); Brown et al. (2007a)

HVS: Main Sequence Stars Brown et al. (2007b) Blue HB MS Kaufmann Horizontal Branch HVSs

HVS: Locations and Travel Times Brown et al. (2007b)

km/s HVS: Sky Distribution Brown et al. (2007a)

HVS: Space Density Brown et al. (2007b)

Theoretical Applications Dark Matter Potential: Gnedin et al (2005), Yu & Madau (2007) Binary Black Hole / origin: Baumgardt et al, Gualandris et al, Merritt, Levin, O’Leary & Loeb, Perets et al, Sesana et al., Lu et al., Svensson et al. Stars orbiting the BH: Ginsburg & Loeb Stellar Populations: Demarque & Virani, Kollmeier & Gould Ginsburg & Loeb (2006) LISA

Future Work Discovery survey: MMT, Whipple 1.5m. Spectroscopic identifications: VLT (Heber), WHT (Keenan). Space velocities: HST (Gnedin). Variability: MDM (Stanek). Numerical simulations: (Bromley). Unusual objects: more to come!

Conclusions MBH = hypervelocity stars. First HVS: B star +850 km/s. Now 10 known HVSs. HVSs  unique window on the Galactic Center: Mass function of stars In-fall history Massive black hole (binary?) NY Times

The Hypervelocity Stars IDTypeg’ (mag) V minRF (km/s) d (kpc) t GC (Myr) Citation HVS1B Brown et al. (2005) HVS2sdO Hirsch et al. (2005) HVS3B ? Edelmann et al. (2005) HVS4B Brown et al. (2006a) HVS5B Brown et al. (2006a) HVS6B Brown et al. (2006b) HVS7B Brown et al. (2006b) HVS8B new! HVS9B new! HVS10B new!

Mass Function of Stars Arches Predicted: 2000 HVSs (Yu & Tremaine) We Observe: 7 HVS in 6000 deg 2 ~ M sun HVSs M sun stars ~ M sun stars NASA HST Salpeter

Halo Structure: Sgr Stream