Dark Matter in Galaxies

Transcription

1 Dark Matter in Galaxies Garry W. Angus VUB FWO 3rd COSPA Meeting Université de Liège

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3 Ellipticals. Old stars. Gas poor. Low star formation rate.

4 Spiral (disk) galaxies. Often gas rich => star formation. Ellipticals. Old stars. Gas poor. Low star formation rate. Bar strength Bulge strength

5 Spiral (disk) galaxies. Often gas rich => star formation. Ellipticals. Old stars. Gas poor. Low star formation rate. Kinematically useful Weak bars & bulges Bar strength Bulge strength

6 Dwarf spheroidal galaxies. Difficult to identify against the stellar background. Low Mass. Typically no gas. Found orbiting large host galaxies. Dark matter dominated.

7 The inclination is important Face-on : i=0 deg Rotation into the screen Edge-on : i=90 deg Rotation out of the screen Left handed rotation We want to measure rotation speed of galaxy as function of radius, V(R)

8 V(R) -> M(R) Take a nearly edge on galaxy and measure its rotation using neutral hydrogen emission (radio - 21cm) Known mass from stellar and gas distribution Dark Matter (DM) 30kpc M31/Andromeda, i~77 deg 1 light year ~ 0.3 parsec

9 Planck Collaboration Temperature fluctuations in photons of cosmic microwave background as a function of angular scale 2 colliding clusters of galaxies: the bullet cluster Clowe et al. (2006), Bradac et al. (2007) CMB and clusters of galaxies demonstrate the need for DM. How do we expect this DM to affect galaxies?

10 N-body simulations of DM from high redshift to present day

11 N-body simulations of DM from high redshift to present day NFW profile provides excellent fits to simulated DM halos. defined by scale radius and scale density or virial mass (m200) and concentration (c)

12 N-body simulations of DM from high redshift to present day NFW profile provides excellent fits to simulated DM halos. defined by scale radius and scale density or virial mass (m200) and concentration (c)

13 N-body simulations of DM from high redshift to present day NFW profile provides excellent fits to simulated DM halos. DM Density Rotation velocity defined by scale radius and scale density or virial mass (m200) and concentration (c) Radius

14 N-body simulations of DM from high redshift to present day Cusp/Core Controversy Navarro, Frenk & White (1996,1997; NFW) Moore (1999); Gentile et al. (2004); de Blok (2010) DM Density Rotation velocity pseudo-isothermal sphere profile provides excellent fits to the rotation curves of most observed disk galaxies. Radius NFW profile provides excellent fits to simulated DM halos. defined by scale radius and scale density or virial mass (m200) and concentration (c)

15 N-body simulations of DM from high redshift to present day NFW profile provides excellent fits to simulated DM halos. defined by scale radius and scale density or virial mass (m200) and concentration (c) Cusp/Core Controversy Navarro, Frenk & White (1996,1997; NFW) Moore (1999); Gentile et al. (2004); de Blok (2010) DM Density Rotation velocity pseudo-isothermal sphere profile provides excellent fits to the rotation curves of most observed disk galaxies. Radius Supernova feedback? Or something more mundane.

16 Rotation curve for large spiral galaxy. total measured rotation speed Possible contribution from DM and stars NFW goodness of fit depends on mass of stars (M/LK). Numerical stellar evolution models say M/LK~1. Can we measure M/LK directly?

17 Bahcall (1984) What if we had a 2nd (independent) measure of the dynamics? Rotation Speed Vertical Velocity Dispersion M/LK V (R) V 2 (R) M/LK :-> red and light blue dotted lines Stellar vertical VelDisp gives σz 2 =>Σtotal => Σ => M/LK Explained later Need galaxies with i~20-40 deg to simultaneously measure V(R) and σz

18 The DiskMass Survey Bershady et al Surface brightness Rotation speed stellar velocity dispersion

19 Galaxy disks have been dieting Martinsson et al. 2013ab k is a parameter fixed by the vertical stellar distribution. G is Newton s constant. hz is the exponential scale-height of the galaxy disk. Σ is the stellar disk surface density. Σtotal is the total disk surface density. Σatom is the atomic gas (HI, He) surface density. Σmol is the molecular gas (H2) surface density. Azimuthally averaged luminosity density of stars This gives Σ M/LK Azimuthally averaged vertical velocity dispersion of stars Σtotal=σz 2 /π G k hz This gives Σtotal Σtotal Σ Σmol Σatom Values from old models Radius Σ =Σtotal-Σatom-Σmol-ΣDM Combining all this info gives Result is that galaxy disks weigh 2-3x less than previously thought. More room for DM in central parts.

20 Missing satellites problem Klypin et al. (1999) Bullock (2010) Simulated Milky Way DM halo Expect 100s of dwarf or satellite galaxies Observed Milky Way satellites Nsats~25 Problem is a little overstated since low mass satellites have many roadblocks to formation: reionisation, supernova feedback etc. The real problem is at the high mass end.

21 Masses of brightest dwarf galaxies around Milky Way Strigari et al Walker et al Proxy for Mass(R) Compare this with most massive DM sub-halos in simulated Milky Way DM halo Radius [kpc]

22 Using many different Milky Way analogues (different masses etc), typically 8 massive halos with no dwarf galaxy counterpart Boylan-Kolchin et al. (2011,2012) Proxy for Mass(R) Radius [kpc]

23 Solutions? Governato et al. (2011) Supernova feedback can erase cusps in large galaxies, but ineffective for dwarf galaxies (low stellar density). Related to formation of satellite galaxies Tidal forces over lifetime and gas removal due to ram pressure. Arraki et al. (2013) Evidence for more exotic DM like self-interacting DM? Could erase cusps and reduce mass of dwarf galaxies. Spergel & Steinhardt (2000), Loeb & Weiner (2011), Vogelsperger et al. (2012), Roch et al. (2013) Still no strong evidence that vanilla CDM is lacking.