Ursa Major II A Dark Matter Free System? R.Smith, M. Fellhauer, P. Assman
Perseus dwarf Sagitarius dwarf
A Blaze of Stars in the Core of Dwarf Galaxy NGC 1705 All images courtesy of Hubble Heritage UdeC Ursa Major II A Dark Matter Free System? R.Smith, M. Fellhauer, P. Assman
Ursa Major II dwarf, Zucker et al. 2006
Introduction: The Missing Satellite Problem
Simulation predicts 100's to 1000's of low mass halos for every Milky Way mass halo - we see ~30
Springel et al., 2001
Introduction: A solution to the problem?
Low mass galaxies preferentially effected by physical mechanisms that reduce their gas content/halt star formation
Supernovae feedback: Reionisation:
Supernovae feedback and winds from The UV radiation from the first first generation of stars ejects gas from low mass halos Efstathiou 1992) Prohibits cooling of primordial hydrogen Dekel & Silk 1986) & helium Substantial mass loss for Vcirc < 100 kms-1 Thoul & Weinberg 1996) Gas heating before collapse Mac Low & Ferrara 1998) more important: SN make holes in gas allowing winds to ● Total suppression for Vcirc < 30 km s-1 escape harmlessly ● Little effect for Vcirc > 70 kms-1 ● Max. Mass for blow-away 106 Msol ● Too low for dwarf galaxy suppression
Introduction: A good solution to the problem?
Comparison to Via Lactae simulations Simon et al. 2007, (Diemand et al., 2007), Simon et al. 2007 Halos that reach Vcirc < 8 kms-1 before (All MW dwarfs includes extrapolation to areas reionisation of sky not covered by SDSS)
Some authors (e.g. Koposov et al. 2008, Maccio et al. 2009) claim that the missing satellite problem is now solved Introduction: A problem with the solution to the problem
Local group dwarf SFHs:
Grebel & Gallagher, 2004, Greyscale indicates intensity of star formation There is no indication of a halt in star formation during the re-ionisation era.
Introduction: Numerous faint & ultra faint dSphs discovered (Willman et al. (2005a), Zucker et al. (2006a,b), Belokurov et al. (2006,2007))
Belokurov et al, 2006
13 new dSphs discovered since 2005 through the SDSS
Introduction: Numerous dSphs appear highly dark matter dominated
Simon & Geha, 2007
...the missing satellites are uncovered? Ursa Major II First observations (Zucker 2006)
SDSS:
Δα 0.0 g-i 1.0
Subaru First properties:
● Ultra-faint: M =-3.9 ± 0.5 V (fainter than some individual MW stars)
● Ellipticity: 0.5 g-i Ursa Major II Later observations: Dynamics (Simon et al. 2007)
● Radial velocity: -116.5 ± 1.9 km s-1
● Radial velocity gradient: 8.4 ± 1.4 km s-1 (East to West) (hint of increase from South to North too)
● Velocity dispersion: 6.7 ± 1.4 km s-1
● Mass-to-light ratio (assuming virial equilibrium): →2000 ! (at the time, apprarently the most dark matter dominated galaxy discovered) Ursa Major II Deep photometry (Munoz 2010)
Properties: ● Misalignment with Orphan Stream
● Irregular morphology
● Ellipticity: 0.5 ± 0.2
● Central surface brightness: 2 ●~29 mag arcsec V
Distorted morphology suggests undergoing tidal disruption So mass-to-light ratio UMaII Isodensity map could be highly inflated (not a true surface brightness map) Simulations Inflated mass-to-light ratios through tidal disruption
Simulation code: SUPERBOX (Fellhauer 2000) Particle-mesh code, with high resolution sub-grids Fast and efficient – million particle simulations on desktop computers Highly suited for modeling dwarf galaxy tidal disruption UMaII progenitor: DARK MATTER FREE Plummer sphere of stars: 2 parameters (Mass, concentration)
Simulations Orbits
Three components MW static potential:
1) An logarithmic dark matter halo potential 2) A Miyamoto-Nagai disk potential 3) A Hernquist bulge potential
Orbits: Progenitor model orbits within MW-like potential for 10 Gyrs until at current RA & Dec
Simulations After 10 Gyrs......
Plane of MW Sun
35 kpc
Simulations Matching theAfter observations 10 Gyrs...... – best match
● Progenitor loses 86% of it's stars. ● Final magnitude (assuming stellar M/L=2.2): M =-4.2 (observed: -3.9 ± 0.5) V
Comparison with observations Surface density (units: mag arcsec2) V
29.1
29.2 ● Close agreement with observed central surface brightness (model: 29.1, UMaII: 29.2)
● Ellipticity in good agreement 32.6 (model: 0.45-0.55, UmaII: 0.5 ± 0.2)
32.6 ● Irregular morphology to centre
Comparison with observations Surface density (units: mag arcsec2) V
Bootstap realisations of observed data:
●Exact morphology not well defined
Munoz et al. 2010 Comparison with observations Dynamics (radial velocity gradient, velocity dispersion)
● Excellent agreement with observed velocity dispersion (Model=6.6 km s-1,UMaII= 6.7 km s-1± 1.4 km s-1)
● Radial velocity well reproduced (Model=-118.0 km s-1, UMaII=-116.5 ± 1.9 km s-1)
● Radial velocity difference (from East to West) just outside error bars (Model=9.9 km s-1, UMaII=8.4 ± 1.4 km s-1)
The radial velocity gradient
Plane of MW Sun
35 kpc
● Radial velocity difference (between East and West) reproduced by acceleration along orbit as UMaII crosses our line of sight
● Boosted velocity dispersion not due to looking down a stellar tidal tail The velocity dispersion
Time
Stars of different velocities group together at apocentre
● Boosted velocity dispersion at apocentre
● Occurs for half of the time period of the orbit Predictions from model
-1 ● Current proper motions: (μ ,μ )=-0.3,-1.4 mas yr α β ● A weaker North-South gradient: 7 km s-1 ● Progenitor properties: Total mass: 75700 Msol, Half-light radius: 11.0 pc …. consistent with extended MW globular clusters
So would a proper motion prove/disprove a dark matter free UMaII? No.... ➔ If in agreement, can easily reproduce observations with dark matter halo too. Many more free parameters: dark matter mass & concentration, stellar mass & concentration, rotation. Degenerate.
● If in disagreement, there are many other orbits that could potentially reproduce the observations with a different dark matter free progenitor. AND... with dark matter it is even easier
Reproducing the observations can be done in many ways (with or without dark matter)
Conclusions
● The assumption of virial equilibrium in tidally disrupting stellar systems is heavily broken - this can result in highly boosted mass-to-light ratios, if assuming virial equilibrium.
● A dark matter free model of UMaII can reproduce almost all the observations reasonably well for an appropriate choice of progenitor and orbit.
● It is highly likely a different choice of orbit and progenitor could lead to the same match to observations
● So it is unclear how many 'ultra-faint dSphs' may actually be tidally disrupting star clusters.
Table comparing observed UMaII properties to the model