How cold is Cold Dark Matter dSph galaxies as a probe

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How cold is Cold Dark MatterdSph galaxies as a
probe

• Gerry Gilmore
• IoA Cambridge
• Dynamics, abundances with Mark Wilkinson, Rosie
  Wyse,
• Jan Kleyna, Andreas Koch, Wyn Evans, Eva Grebel
• Discovery work with Vasily Belokurov, Dan Zucker,
  
• Sergey Koposov, et al

ApJ 663 948 2007 (july10), arXiv
0706.2687 Also Wilman, Walker Mateo, Kamaya,
Grillmair, Simons Geha
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The smallest galaxies are the places one must see
thenature of dark matter, galaxy formation
astrophysics mssm has 120 free parameters
lots to learn
Inner DM mass density depends on the type(s) of
DM
Dwarf galaxy mass function depends on DM type
Figs Ostriker Steinhardt 2003
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Satellite population problem Fewer expected in
LG? reaching the cooling limit? NB predictions
running out just where the data are today.
Ishiyama etal 0708.1987. dashed line from Moore
etal
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Real satellite luminosity function
Koposov et al 07 arXiv0706.2687
Open symbols Volume-corrected satellite LF from
DR5
Filled symbols all Local Group dSph
Coloured curves Semi-analytic theory (Benson et
al 02, red Somerville 02, blue) --severe
surface- brightness discrepancies
Grey curve power- law fit to data Slope 1.1
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Sgr Field of Streams (and dots)outer halo is
lumpy but is a tiny mass fraction
Streams ? accretion history
Several new satellites ? CDM test
Two wraps? ?Halo DM shape
Disk accretion? Warp?
Belokurov et al (2006b)
SDSS data, 19lt rlt 22, g-r lt 0.4 colour-coded by
mag (distance), blue (10kpc), green, red
(30kpc) Sgr discovered 1994 Ibata, Gilmore,
Irwin Nat 370
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Walcher et al 2005
Dotted line is virial theorem for stars, no DM

• There is a discontinuity
• in (stellar) phase-space
• density between small
• galaxies and star clusters.
• Main difference is in
• surface brightness
• Why?
• Dark Matter?
• Size differences

dSph
Phase space density ( ?/s3) 1/(s2 rh)
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New systems extend overlap between galaxies and
star clusters in luminosity
Belokurov et al. 2006
Analyses of kinematic follow-up underway
? 103 L?
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New photometric and kinematic studies of UCDs,
nuclear clusters, etc ? ALL the small things are
purely stellar systems, M/L1-4
Seth etal astro-ph 0609302
Virgo Fornax UCDs have stellar M/L Hilker
etal, AA 463 119 2007
MWG nuclear cluster has size 5pc,
mass 106Msun Schodel etal AA 469 125
N5128 GC study by Rejkuba et al 2007
faint fluffies
MWG GCs extend down to M-2
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Slightly different perspective
(updated data)
M31 MWG Other
Nuclear clusters, UCDs, M/L 3
Pure stars
Dark Matter haloes
boundary
Tidal tails
dSph galaxies
star clusters
Gilmore, Wilkinson, Wyse et al 2007 ApJ 663 948
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Conclusion one

• Galaxy scaling relations work well, and indicate
  a systematic star-cluster vs small galaxy
  distinction in phase-space density
• There is a well-established half-light size
  bi-modality
• all systems with size lt 40pc are purely stellar
• -16lt Mv lt 0 (!!) M/L 3 e.g. UCDs, Hilker
  et al 07 Rejkuba et al 07
• all systems with size greater than 120pc have a
  dark-matter halo
• There are no known (virial equilibrium) galaxies
  with half-light radius r lt 120pc
• So now look at the dSph galaxies masses

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Note different scales information at small and
large r poor.
Mateo, walker etal
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Derived mass density profiles
Jeans equation with assumed isotropic velocity
dispersion All consistent with cores (similar
results from other analyses) Not conclusive yet.
CDM predicts slope of -1.3 at 1 of virial
radius and asymptotes to -1 (Diemand et al. 04)
Need different technique at large radii, e.g.
full velocity distribution function modelling..
And understand tides.
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Conclusion two

• High-quality kinematic data exist
• Jeans analyses ? prefers cored mass profiles
• Mass-anisotropy degeneracy allows cusps
• Substructure, dynamical friction ? prefers cores
• Equilibrium assumption is valid inside optical
  radius
• More sophisticated DF analyses underway
• Cores always preferred, but not always required
• Central densities always similar and low
• Consistent results from available DF analyses
• Extending analysis to lower luminosity systems
  difficult due to small number of stars
• Integrate mass profile to enclosed mass

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2007 extension of dynamic range UMa, Boo,
AndIX, new kinematic studies Mateo
plot improves.
Mass enclosed within stellar extent 4 x 107M?
Now a factor of 300 in luminosity, 1000 in
M/L
Scl Walker etal
If NFW assumed, virial masses are 100x larger,
Draco is the most massive Satellite (8.109M?)
(old data)
?Globular star clusters, no DM?
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Summary

• A minimum physical scale for galaxies, 100pc,
  max size for star clusters 30pc
• Galaxy mass size scale somewhat larger (?)
• Galaxy nuclei are just massive star clusters?
• Cored mass profiles, with similar mean mass
  densities 0.1M?/pc3, 5GeV/cc
• Phase space densities fairly constant, maximum
  for galaxies (cf Walcher et al 2005)
• An apparent characteristic (minimum) mass dark
  halo in all dSph, mass 4 x 107M? ???
• This is just a consistency check, not new info
• dSph debris not yet found cannot be (much of)
  the MWG halo, thick disk, or thin disk
• How did everything get pre-enriched?
• context substructure issue, old disks, one
  thick disk, too few dead bodies, old red gals

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Consistency?

• A minimum half-light size for galaxies, 100pc
• ? mass scale similar, or a little larger
• Probably cored mass profiles, with similar mean
  mass densities 0.1M?/pc3, 5GeV/cc
• An apparent characteristic (minimum) mass dark
  halo at dSph,
• mass 4 x 107M?
• characteristic mass profile convolved
  with
• characteristic normalisation must imply
• characteristic mass
• ? internal consistency
  only
• Minimum size natural with cored potential?

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Implications for Dark Matter

• Characteristic Density 10GeV/c²/cm³
• If DM is very massive particles, they must be
  extremely dilute (Higgs 100GeV)
• Characteristic Scale above 100pc, several 107M?
• Cooling? power-spectrum scale break?
• This would (perhaps!) naturally solve the
  substructure and cusp problems
• Number counts low relative to CDM
• lots of similar challenges on galaxy scales
• Need to consider seriously non-C DM candidates

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Properties of Dark-Matter dominated dSph
galaxies
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Does the Mateo plot extend to the lowest
luminosities? Data still limited, lowest surface
brightness gals may have lowest sigma.
Simon Geha these are central values
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Central velocity dispersion masses are really
dispersions, and are only just resolved by the
RV errors eg Simon/Geha here, our outer Draco
cold, etc. Independent confirmation is desirable
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No parameters are VERY accurate. CVnI (top) has
s13.9 (Ibata 2006) or 8.1 (Simon Geha, here),
from the same instrument. LeoIV has s3.3-1.7
derived here 2bins?
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Implications from Astrophysics Can one plausibly
build a dSph as observed without disturbing the
DM?

• Star formation histories and IMF are easily
  determined ? survival history, energy input
• Chemical element distributions define gas flows,
  accretion/wind rates,
• debris from destruction makes part of the field
  stellar halo well-studied, must also be
  understood
• Feedback processes are not free parameters

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What are we really measuring with simple,
non-Distribution Function, analyses?

• Dispersion profile close to flat, so sigma cst,
  and range of sigma is small (data lt2)
• derivative term is (log) luminosity profile
  light, NOT mass, and this is similar in scale for
  all the dSph (factor of few)
• So the derived mass really is a measure of the
  radial extent of the data, and only a weak
  function of anything else.
• Increase in M in Mateo plot is a measure of
  increase in data range

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Hernandez, Gilmore Valls-Gabaud 2000
Carina dSph
Leo I
UMi dSph
Atypical SFH
Intermediate-age population dominates in
typical dSph satellite galaxies with very low
average SFR over long periods (5M?/105yr),
until recently
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Comparing globular cluster structures,
abundances, orbits, ages and likely
survival Implies 5 ltSgr-like mergers in total,
forming 20 of the outer halo (Mackey Gilmore
MNRAS 355 504 2004) This is consistent with
SDSS-observed halo lumpiness, and older (eg
Unavane, GG, RW 1996 MN) age-metallicity limits
Globular cluster view of halo accretion
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Constant mass scale of dSph?
Based on central velocity dispersions only? low
M/L line corresponds to dark halo mass of 107M?
Mateo 98 ARAA
dSph filled symbols
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2007 extension of dynamic range UMa, Boo,
AndIX, new kinematic studies Mateo
plot improves.
Mass enclosed within stellar extent 4 x 107M?
(old data)
?Globular star clusters, no DM?
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It isnt only gas-poor galaxies all small
galaxies are similar
Mass to light ratios for local dSph The star
is LeoA, a gas-rich dwarf with recent star
formation, the arrow shows how it will fade with
age. The square is Phoenix. This is from Brown,
Geller etal arXiv0705.1093
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Dynamics three regimes

• Body of galaxy, out to break/r_lim recent vast
  increase in good data (Camb group,
  Ibata/Chapman/Martin at keck, Simon/geha at keck,
  Walker/mateo at Magellan/MMT, good agreement,
  real progress, now pushing limits of known
  systems
• Outer limits tidal tails, etc data very
  limited, agreement only fair, rather open
  analyses, fair outcome no strong effects in
  distant objects, Sgr a model for the nearby.
• Cores just starting now.

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Survival of cold subsystem in UMi dSph
implies shallow mass density profile (Kleyna et
al 03)
Breaking the degeneracy first steps

• Dynamical friction limits on Fornax dSph
• Globular Clusters also favour cores to extend
• timescales Goerdt etal 2006

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Main Focus Dwarf Spheroidals

• Low luminosity, low surface-brightness satellite
  galaxies, classical L 106L?, ?V 24 mag/sq?
• Extremely gas-poor
• Apparently dark-matter dominated
• ? 10km/s, 10 lt M/L lt 100
• Metal-poor, mean stellar metallicity lt 1.5 dex
• All contain old stars extended star-formation
  histories typical, intermediate-age stars
  dominate
• Most common galaxy nearby
• Crucial tests for CDM and other models

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Walker etal arxiv0708.0010
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Omega Cen Reijns etal AA 445 503
Mass does not follow light
Leo II Koch etal
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Other lumps exists too, and are not understood at
all.
astro-ph/0701790
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CDM predicts many more satellite galaxies than
observed, at all masses (Moore et al 1999)
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• new large datasets of stellar line of sight
  kinematics, now covering spatial extent,
  photometry for dSph satellite galaxies
• ? new discoveries SDSS mostly original key
  project (also Willman et al 05 Grillmair 06
  Grillmair Dionatos 06 Sakamoto Hasegawa 06
  Jerjen 07..)
• ? confirm and extend scaling relations
• ? Dark matter properties

G. Gilmore, M. Wilkinson, R.F.G.Wyse, J.
Kleyna, A. Koch, N. Evans E. Grebel 2007, ApJ
v663 p948 astro-ph/0703308
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M31 and MWG GC size-lum, from Federici etal,
0706.2337, Stars From Mackey etal (M31),
triangles nuclei of Virgo dEs asterisk Virgo
UCDs,
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Core properties adding anisotropy
Koch et al 07 AJ 134 566 07
Fixed ß
Radially varying ß
Leo II
Core and/or mild tangential anisotropy slightly
favoured
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• Mass anisotropy degeneracy prevents robust
  cusp/core distinction, but core provides better
  fit (see also Wu 2007 astro-ph/0702233)
• Break degeneracy by complementary information
• Ursa Minor has a cold subsystem, requiring
  shallow gradients for survival (Kleyna et al 2003
  ApJL 588 L21)
• Fornax globular clusters should have spiralled in
  through dynamical friction unless core (e.g.
  Goerdt et al 2006)
• Simplicity argues that cores favoured for all?
• New data and df-models underway to test (GG
  etal, VLT high-resolution core/cusp project)

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NFW fits require very high mass, and a very wide
range of mass Draco 8.109Msun and M/L100,000
MWG vs M31 offset no simple mass-luminosity
link astro-ph/0701780 Strigari etal (in
prep) central mass fits no simple rank
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Thick and thin disk element ratio data The
thick/thin distinction is evident. The thick
disk occupies an empty part of the halo-dSph-Sgr
plot, suggesting its parent was different
again This fig from AA 465 271 Ramirez etal
2007 Sgr and the thick disk are 2 good
accretions, But both seem unique
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Thick disk
Galaxy halo (green), dSph (blue), LMC (cyan), Sgr
(red) and dIrr (yellow) element ratios The
systematic difference is apparent (from Geisler,
Wallerstein, etal 0708.0570) NB Sgr is very
distinctive it must be the first such event.
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CDM predicts many more satellite galaxies than
observed, at all masses

• Solutions warm DM self-interacting DM star
  formation suppression by re-ionization
  self-regulated star formation very high M/L plus
  some other variant predictions wrong, count
  different things predictions host-dependent .
• Conclude
• very many proposed solutions suggests there is
  still much to learn, both in models and data

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Satellite population depends on environment?
Fewer expected in LG? NB predictions running out
just where the data are today. What should be
believe from the simulations?
Ishiyama etal 0708.1987. dashed line from Moore
etal
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Where are the bodies? Little debris in
inner MWG?
Field halo
Thick disk
Thin disk
dSph members
NB the pre-enrichment problem has become
more extreme no very metal poor stars are found
in dSph or GC
SMC LMC dIrr
Sgr
including thick disk (red) and thin disk (blue)
stars Chemically the local halo is much more
similar to the thick disk (progenitor?) than
anything else, but has very different orbital
angular momentum. Sgr and its clusters are shown
from Sbordone etal AA 465 815 2007
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Very many attempts to model feedback on CDM
structure.

• Some of our examples
• Read et al 2006 MN 367 387, MN 366 429, 2005 MN
  356 107 Fellhauer etal in prep
• Conclusion

DM halos certainly respond to tides and
mass-loss, but secularly If various histories
leave similar mass profiles, history cannot be
dominant
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Stellar kinematic data across faces of dSph now
quite extensive e.g. Gilmore et al 2007 MOND
problem
dSph
Seitzer 1983 van de Ven et al 06
Globular cluster M/LV 2.5
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dSph only one part of the challenge

• Among the first systems to collapse, form stars
• Star formation history and chemical enrichment
  are sensitive probes of stellar feedback,
  galactic winds, ram pressure stripping,
  re-ionization effects..
• BUT all seem pre-enriched
• Most extreme (apparently) dark-matter dominated
  systems trends contain constraints on its nature
  (Dekel Silk 1986 Kormendy Freeman 04
  Zaritsky et al 06)
• What are mass profiles within dSph? CDM predicts
  a cusp in central regions
• Accessible through current observations
• Luminosity and mass functions critical tests

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The MWG cdm challenge is not rare large disk
galaxies with no bulge are common, and are
a very serious challenge for CDM. They should
not exist. In fact large old disks are a REALLY
big challenge cf Kormendy Kennicutt ARAA
2004 and arXiv0708.2104 The small pseudo-bulge
here is a disk bar.
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Kormendy Freeman 04
Disk galaxy scaling relations are
well- established Do the dSph (green)
fit? Yes, KF
No, they tend to lie OFF the disk galaxy
extrapolations.. Limits instead
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Bulk of stellar halo is OLD, as is bulge Did
not form from typical satellites disrupted
later than a redshift of 2
Unavane, Wyse Gilmore 1996
Scatter plot of Fe/H vs B-V for local
high-velocity halo stars (Carney) few stars
bluer (younger) than old turnoffs (5Gyr, 10Gyr,
15Gyr Yale)
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Mass measurements the early context

• The standard value for local DM at the Sun
  is 0.3GeV/c²/cm³, all in a halo component
• (cf pdg.lbl.gov Eidelman etal 2004)
• the original work, and origin of this value, is
  the first analysis to include a full 3-D
  gravitational potential, parametric modelling,
  and a direct determination of both the relevant
  density scale length and kinematic (pressure)
  gradients from data, allowing full DF modelling
  for the first time
• Kuijken Gilmore 1989 (MN 239 571, 605,
  651), 1991 (ApJ 367 L9)
• Gilmore, Wyse Kuijken (ARAA 27 555 1989)
  
• cf Bienayme etal 2006 AA 446 933
  for a recent study
• Dark halos are predicted down to sub-earth
  masses but
• Neither the local disk, nor star clusters, nor
  spiral arms, nor GMC, nor the solar system, have
  associated DM Given the absence of a very local
  enhancement, what is the smallest scale on which
  DM is concentrated? How can sub-halos in dSph
  galaxies with star formation over 10Gyr avoid
  collecting any baryons?

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From kinematics to dynamics Jeans equations,
simple and robust method

• Relates spatial distribution of stars and their
  velocity dispersion tensor to underlying mass
  profile
• Either (i) determine mass profile from projected
  dispersion profile, with assumed isotropy, and
  smooth functional fit to the light profile
• Or (ii) assume a parameterised mass model M(r)
  and velocity dispersion anisotropy ß(r) and fit
  dispersion profile to find best forms of these
  (for fixed light profile)
• Full distribution function modelling, as opposed
  to velocity moments, also underway needs very
  large data sets. Where available, DF and Jeans
  models agree.
• King models are not appropriate for dSph ? too
  few stars

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Main sequence luminosity functions of UMi dSph
and of globular clusters are indistinguishable.
? normal stellar M/L
HST star counts
Wyse et al 2002
Massive-star IMF constrained by
elemental abundances also normal
M92
?
M15
?
0.3M?
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From Walcher etal 2006 apj 649 692
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Challenges for small-scale DM

• On large gtMpc scales LCDM is an astonishingly
  good description of data, but n-1 (and maybe
  w-1.) so not much physics made clear lots
  still to learn
• On galaxy scales there is an opportunity to learn
  some physics everything should happen late. But.
• 0 big old galaxies, big old disks, SFR peaks
  zgt1,
• 1 the MWG has a thick disk, just one of them,
  and it is old. This seems common.
• 2 massive old pure-thin-disk galaxies exist.
  None should
• 3 Sgr proves late minor merging happens, but is
  clearly a rare event
• 4 the substructure problem where are the
  bodies?
• 5 the feedback problem what is it
• 6 the early enrichment problem what did it? When?