little things in context

a note on the unknown

Gerry GilmoreIoA Cambridge

Dynamics with Mark Wilkinson, Rosie Wyse, Jan Kleyna, Andreas Koch, Wyn Evans, Eva Grebel

Discovery work with Vasily Belokurov, Dan Zucker, Sergey Koposov, et al

Globular cluster properties with Dougal Mackey, Becky Elson

ApJ 663 948 2007 (july10), arXiv 0706.2687

Figs: Ostriker & Steinhardt 2003

Dwarf galaxy mass function depends on DM type

Inner DM mass density depends on the type(s) of DM

The smallest galaxies are the places one must see thenature of dark matter, & galaxy formation astrophysics

Challenges for small-scale DM

On large [>Mpc] 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 z>1, 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?

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 differentorbital angular momentum.Sgr and its clusters are shown from Sbordone etal A+A 465 815 2007

Where are the bodies? No debris in inner MWG

NB: the pre-enrichment problem has become moreextreme: no very metal poor starsare found in dSph or GC

SmcLmcdIrr

Comparing globular cluster structures, abundances, orbits, ages and likely survivalImplies ~5 [<Sgr-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, RW1996 MN) age-metallicity limits

Globular cluster view of halo accretion

Walcher et al 2005

dSph

There is a discontinuityin (stellar) phase-space density between small galaxies and star clusters. Why?

Dark Matter?

Phase space density (~ ρ/σ3) ~ 1/(σ2 rh)

Dotted line is virial theorem for stars, no DM

From Walcher etal 2006 apj 649 692

Belokurov et al. 2006

New systems extend overlap between galaxies and star clusters in luminosity

~103 L

Analyses of kinematic follow-up underway

Seth etal astro-ph 0609302

New photometric and kinematic studies of UCDs, nuclear clusters, etc ALL the small things are purely stellar systems, M/L~1-4

MWG GCs extend down to M~-2

Virgo & Fornax UCDshave stellar M/L –

Hilker etal,A&A 463 119 2007

N5128 GC study by Rejkuba et al 2007

faintfluffies

MWG nuclearcluster has size~5pc, mass10^6MsunSchodel etalA+A 469 125

Gilmore, Wilkinson, Wyse et al 2007

M31; MWG; Other

dSph galaxiesstar clusters

boundary

Slightly different perspective…

Nuclear clusters, UCDs, M/L ~ 3

(updated data)

Tidal tails

Pure stars Dark Matterhaloes

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 < 30pc are purely stellar

−16< Mv < 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 < 120pc

So now look at the dSph galaxies’ masses

~

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

Dispersion profile close to flat, so sigma ~ cst, and range of sigma is small (data <2)

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

Note different scales: information at small and large r poor.

Mateo, w

alker etal

Jeans’ equation with assumed isotropic velocity dispersion:All consistent with cores (similar resultsfrom other analyses)

Need different technique at large radii, e.g. full velocity distribution function modelling.. And understand tides.

Derived mass density profiles:

CDM predicts slope of -1.3 at 1% of virial radiusand asymptotes to -1(Diemand et al. 04)

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:

Constant mass scale of dSph?

Mateo 98ARAA

Based on central velocity dispersions only low M/Lline corresponds to dark halo mass of 107M

dSph filledsymbols

2007: extension of dynamic range [UMa, Boo, AndIX], new kinematic studies: Mateo plot improves.

Globular star clusters, no DM

Now a factor of 300+ in luminosity, 1000+ in M/L

(old data)

Scl – Walker etalIf NFW assumed, virial massesare 100x larger, Draco is the most massive Satellite (8.109M)

Mass enclosed within stellar extent ~ 4 x 107M

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 implycharacteristic mass internal consistency only

The information in the Mateo plot is the same as in the size and mass profile relations

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?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 An apparent characteristic (minimum) mass dark halo in all dSph, mass ~4 x 10in all dSph, mass ~4 x 1077

MM ?????? 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…

Does the Mateo plot extend to the lowest luminosities?Data still limited, lowest surface brightness gals mayhave lowest sigma.

Simon & Geha: these are central values

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

2007: extension of dynamic range [UMa, Boo, AndIX], new kinematic studies: Mateo plot improves.

Globular star clusters, no DM (old data)

Mass enclosed within stellar extent ~ 4 x 107M

Carina dSph Leo I

Hernandez, Gilmore & Valls-Gabaud 2000Hernandez, Gilmore & Valls-Gabaud 2000

Intermediate-age population dominates in typicaldSph satellite galaxies – with very low average SFR over long periods (~5M/105yr), until recently

UMi dSph

AtypicalSFH

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 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

Properties of Dark-Matter dominated dSph galaxies:

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 arXiv:0705.1093

It isn’t only gas-poor galaxies: all small galaxies are similar

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.

Survival of cold subsystem in UMi dSph implies shallow mass density profile (Kleyna et al 03)

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

Breaking the degeneracy – first steps

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 < M/L < 100 Metal-poor, mean stellar metallicity < –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

~

~

~

Walker etal arxiv:0708.0010

Omega Cen: Reijns etal A&A 445 503

Leo II: Koch etal

Mass does not follow light

astro-ph/0701790

Other lumps exists too, and are not understood at all.

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

Derived satellite luminosity function

Open symbols:Volume-correctedsatellite LF from DR5

Filled symbols:‘all Local Group dSph’

Coloured curves:Semi-analytic theory(Benson et al 02, redSomerville 02, blue)Ignores surface-brightness discrepanciesetc.

Koposov et al 07 arXiv:0706.2687

Grey curve: power-law ‘fit’ to dataSlope 1.1

No parameters are *VERY* accurate. CVnI (top) has σ=13.9 (Ibata 2006) or 8.1 (Simon

+Geha, here), from the same instrument. LeoIV has σ=3.3+-1.7 derived here – 2bins?

CDM predicts many more satellite galaxies than observed, at all masses (Moore et al 1999)

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

M31 and MWG GC size-lum, from Federici etal, 0706.2337,Stars From Mackey etal (M31), triangles: nuclei of Virgo dEsasterisk Virgo UCDs,

Core properties: adding anisotropy

Leo II

Fixed β Radially varying βKoch et al 07AJ 134 566 ‘07

Core and/or mild tangential anisotropy slightly favoured

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)

NFW fits require very high mass, and a very wide range of massDraco = 8.10^9Msun and M/L=100,000 MWG vs M31 offsetno simple mass-luminosity link astro-ph/0701780Strigari etal (in prep) central mass fits – no simple rank

Thick and thin diskelement ratio data:

The thick/thin distinctionis evident.

The thick disk occupiesan empty part of the halo-dSph-Sgr plot, suggesting its parentwas different again…

This fig from A+A 465 271Ramirez etal 2007

Sgr and the thick disk are2 good `accretions’,But both seem unique…

Galaxy halo (green), dSph (blue), LMC (cyan), Sgr (red) and dIrr (yellow) element ratiosThe systematic difference is apparent (from Geisler, Wallerstein, etal 0708.0570) NB Sgr is *very* distinctive: it must be the first such event.

Thick disk

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

Ishiyama etal 0708.1987. dashed line from Moore etal

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?

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 certainlyrespond to tides and mass-loss, but secularly

If various histories leavesimilar mass profiles,history cannot be dominant

Stellar kinematic data across faces of dSph now quite extensive e.g. Gilmore et al 2007 MOND problem…

dSph

GlobularclusterM/LV ~ 2.5

Seitzer 1983;van de Ven et al 06

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

dSph: only one part of the challenge

The MWG cdm challenge is not rare: large disk galaxies with no bulge are common, and are avery serious challenge for CDM.

They should not exist.

In fact large old disks are a REALLY bigchallenge…

cf Kormendy & Kennicutt ARAA 2004and arXiv:0708.2104

The small pseudo-bulge here is a disk bar.

Disk galaxy scaling relations are well-established

Do the dSph (green) fit?Yes, KFNo, they tend to lie OFF the disk galaxy extrapolations.. Limits instead

Kormendy & Freeman 04

Sgr & Field of Streams (and dots)outer halo is lumpy: but is a tiny mass fraction

SDSS data, 19< r< 22, g-r < 0.4 colour-coded by mag (distance), blue (~10kpc), green, red (~30kpc)Sgr discovered 1994 Ibata, Gilmore, Irwin Nat 370

Belokurov et al (2006b)

Two wraps?

Disk accretion?Warp?

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)

Bulk of stellar halo is OLD, as is bulge:Did not form from typical satellites disrupted later than a redshift of 2

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 A&A 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?

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 β(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

M92 M15

Main sequence luminosity functions of UMi dSph and of globular clusters are indistinguishable. normal stellar M/L

Wyse et al 2002

HST star counts

Massive-starIMF constrainedby elementalabundances – also normal

0.3M