the ultra-faint milky way satellites
DESCRIPTION
A Guide to the New Dwarfs = (same shape) UMa I = Leo T (blue eyes) Coma Berenices (you try saying it!) = Hercules (the odd one out) = CVn I (the leader) = UMa II Leo IV (no followup observations) = = CVn II (roundest) = (faintest) The Globular Cluster - Dwarf Galaxy ConnectionTRANSCRIPT
The Ultra-Faint Milky Way Satellites
Josh Simon (Caltech) Marla
Geha (HIA)
Implications for the Missing Satellite Problem
The Globular Cluster - Dwarf Galaxy Connection
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A Guide to the New Dwarfs
=
Leo T
(blue eyes)
=(same shape)
UMa I
=CVn II
(roundest)
UMa II
(faintest)
=
Leo IV
(no followup observations)
=
Coma Berenices
(you try saying it!)
=
CVn I
(the leader)
=
Hercules
(the odd one out)
=
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CDM Has a Missing Satellite Problem
Springel et al. 2001
CDM predicts large numbers of subhalos (~100-1000 for a Milky Way-sized galaxy)Milky Way only has 23 known satellites
What happened to the rest of them?
The Globular Cluster - Dwarf Galaxy Connection
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CDM predicts large numbers of subhalos (~100-1000 for a Milky Way-sized galaxy)Milky Way only has 23 known satellites
What happened to the rest of them?
Springel et al. 2001
CDM Has a Missing Satellite Problem
The Globular Cluster - Dwarf Galaxy Connection
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• Two basic sets of possible solutions: Modifications to CDM
-What modifications? Power spectrum, DM particle mass/decay/interaction cross-section?
Astrophysics prevents stars from forming in most low-mass halos-What astrophysics? Reionization, feedback, winds?
What Does This Problem Tell Us?
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A Recent Breakthrough• Only 11 Milky Way dwarfs known through 2004
• Since 2005, SDSS has discovered: 8 new dSphs 1 dIrr 3 ??? dSph/GCs?
Belokurov et al. (2006)
Willman et al. (2005a), Zucker et al. (2006a,b), Belokurov et al. (2006,2007)
Irwin et al. (2007)
Willman et al. (2005b), Belokurov et al. (2007), Walsh et al. (2007)
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Keck Spectroscopic Survey of SDSS Dwarfs
• Medium-resolution spectra of 841 stars (424 members) across 8 dwarfs(CVn I, CVn II, Coma Berenices, Hercules, Leo IV, Leo T, UMa
I, UMa II)
• Measured stellar velocities + metallicities
-Velocities can be used to constrain the masses and densities of these extreme systems
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Keck Spectroscopic Survey of SDSS Dwarfs
• Medium-resolution spectra of 841 stars (424 members) across 8 dwarfs(CVn I, CVn II, Coma Berenices, Hercules, Leo IV, Leo T, UMa
I, UMa II)
• Measured stellar velocities + metallicities
• Simon & Geha (2007) UMa II (tidally disrupting; SG07)• Martin et al. (2007)
Previous lower limit for dSphs (Gilmore et al. 2007)
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Velocity Dispersion Profiles
• (r) declines with r in globular clusters• (r) = const is typical for dSphs (see Walker et
al. 2007b)
• Evidence for extended dark matter halos around the ultra-faint dwarfs
Simon & Geha (in prep)
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Velocity Dispersion Profiles
Simon & Geha (in prep)
• (r) declines with r in globular clusters• (r) = const is typical for dSphs (see Walker et
al. 2007b)
• Evidence for extended dark matter halos around the ultra-faint dwarfs
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Masses of the Ultra-Faint Dwarfs
• Simplest possible model: equilibrium, spherical, isotropic systems where M follows L (although see L. Strigari’s talk, Strigari et al. in prep)
• Simon & Geha (2007) UMa II (tidally disrupting; SG07)• Martin et al. (2007)
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Mateo Plot• Do all dwarfs live in similar halos?• Is there a minimum mass for dwarfs?
Simon & Geha (2007)
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Moore et al. (1999), also see Klypin et al. (1999)
Does the correction from the observed number of satellites to the true number go in the horizontal or vertical direction??
?
The Missing Satellites
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Are There Still Satellites Missing?
Comparison to Via Lactea N-body simulation (Diemand et al. 2007)
Including new dwarfs, total MW population projects to 51 dwarf galaxies over the full sky(Simon & Geha 2007; Koposov et al. 2007)
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Are There Still Satellites Missing?
Simon & Geha (2007)
Comparison to Via Lactea N-body simulation (Diemand et al. 2007)
What if dwarfs only populate the most massive halos?(Stoehr et al. 2002)
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Are There Still Satellites Missing?
Simon & Geha (2007)
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)
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Are There Still Satellites Missing?
Simon & Geha (2007)
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)
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Summary1) Ultra-faint dwarfs are extreme
systems in every sense Lowest luminosities (2600 L < L < 124000 L),
lowest metallicities ([Fe/H] < -2) Smallest velocity dispersions (3 km/s < s < 8
km/s), lowest masses (106 - 107 M), largest mass-to-light ratios (M/L = 100 - 1000 M/L)
• Is there still a missing satellite problem? Ultra-faint dwarfs alleviate but don’t remove the
substructure problem . . . UNLESS dwarf galaxy formation was strongly
suppressed by reionization at z 12