Stéphane Courteau

Nearly Normal Galaxies 

in a ΛCDM Universe 

UC Santa Cruz 2005 

Global Properties of Disk Galaxies 

Stéphane Courteau (Queen’s) 

With contributions from: 

Aaron Dutton, Frank van den Bosch (ETH) 

Avishai Dekel (Hebrew University), Lauren MacArthur (UBC/Caltech) 

Jon Holtzman (NMSU), Eric Bell, Hans-Walter Rix (MPIE) 

Roelof de Jong (STScI), Guy Worthey (WSU), Mike McDonald (Queen’s) 

Claude Carignan (Montréal), Ken Freeman* (ANU) 

* From whom some of the Ascona/Terschelling slides/figures were borrowed

Nearly Normal Galaxies 

in a ΛCDM Universe 

UC Santa Cruz 2005 

Many workshops on Disk Galaxies 

this summer, including: 

• The Formation of Disk Galaxies 

Ascona, Switzerland (June 26-July 1) 

• Island Universes: Structure and 

Evolution of Disk Galaxies 

Terschelling, Netherlands (July 3-8)

Ascona/Terschelling Highlights 

• Many new (and some old) results on disk galaxies 

– The disk is the end product of the dissipation of most 

of the baryons, and contains almost all of the baryonic 

angular momentum 

– Understanding its formation is the most important goal 

of galaxy formation theory. 

• Brief summary restricted to some highlights 

– Structure and properties of extended stellar disks 

– Thick disks (bulges, bars, peanuts) & streams 

– Expanding horizons: panchromatic surveys 

– CDM problems: bulge-less exponential disks, 

solid-body rotation curves, …


• Heroic star counts in M31 

(Ferguson/Chapman/Guhathakurta/Worthey) 

– Very extended structure 

(disk>50 kpc; halo to 150 kpc?) 

– Disk or metal-poor halo? 

– kinematics, [Fe/H] 

• UV & Hα evidence for faint outer disks 

(Bland-Hawthorne, Zaritsky) 

• Origin of these components? 

– Star formation threshold? 

– Is there a link with presence of HI warp or flare?

The outer regions of M31 

Worthey, Espana, MacArthur & Courteau 2005 

HST photometry in the outermost 

regions of M31 yields stellar 

metallicity distribution 

The metallicity gradient bottoms 

out at R=15kpc, at [Fe/H] = -0.5 

The high [Fe/H] in the outer 

parts of M31 suggests that most 

of the stars in the outer “halo” 

might be stars of the outer disk.


Galactic open clusters 

• abundance gradient 

bottoms out at 

R G = 10-12 kpc, 

[Fe/H] = -0.5 

for clusters with 

ages from 1 to 5 Gyr 

(no age-abundance 

relation) 

• The abundance gradient 

in the disk has flattened 

with time, tending 

towards solar values. 

• outer disk is α-enhanced 

Yong & Carney 2005


• New evidence that the disk of M31 goes out to > 50 kpc 

= 10 scale lengths (maybe to 150 kpc: Guhathakurta et 

al. 2005 

• Kinematics of red giants confirms that the outer disk is 

rotating almost as rapidly as the inner disk (Ibata et al 

2005) and has a velocity dispersion of ~ 30 km s -1 ) 

• The CMD of M31 suggests that the outer disk is 

probably fairly old (several Gyr), as in M33 and the MW 

• Outer disk of M31, like the MW, is α-enhanced, 

with [α/Fe] = + 0.2 (also Eu-enhanced): indicates 

fairly rapid star formation history in the outer disk, 

unlike the solar neighborhood.


Summary of outer disks 

• The disks of some spirals (NGC 300, M31) extend out 

beyond 10 scale lengths. 

• The outer disks of M31, M33 and the MW 

include a component that is at least several Gyr old. 

• The abundance gradients in the outer disks of M31 

and the MW bottom out at [Fe/H] = - 0.5 

• The older stars of the outer galactic disk are 

α-enhanced, indicating that they formed rapidly. 

This α-enhancement is less for the younger stars 

of the outer disk 

• Outer disk stars formed from reservoir of gas that 

had a different star formation history from the solar 

neighborhood. Star formation may be triggered by 

a merger event in the outer disk.


• Most disk galaxies have additional, thick, disk 

(Yoachim/Dalcanton) 

– Modest mass fraction, 

but increases below 

v c~120 km s -1 

– Some counter-rotate: 

⇒ external origin 

– Heavier counter-rotating 

disks seen in S0s 

(Kannappan) 

• [α/Fe] vs [Fe/H] in MW thick disk ⇒ it cannot have 

formed by accretion of small stellar lumps (Venn) 

• Origin in gas-rich merger (Sommer-Larsen)

NGC 4244 

Hint of a thick disk 

Two pure disk galaxies 

vdK&S 

NGC 5907

NGC 4656: small bulge 

and prominent thick disk 

vdK&S


What is the origin of this disk truncation - common and seen 

more easily in edge-on galaxies than in face-on galaxies 

Kregel etal (2002): 

R max/h R = 3.6 ± 0.6 

for 34 edge-on disk 

galaxies 

Perez (2004) finds 

similar results at 

0.6


Disk truncations 

M33 surface brightness 

profile 

(Ferguson etal 2003) 

Sharp decrease in SB 

beyond 5 disk scale 

lengths. 

Star 

counts 

Light profile 

(reaches V~31 mag arcsec -2 !)


Disk truncations 

NGC 300* surface 

brightness profile 

(Bland-Hawthorne 

etal 2005) 

Exponential profile 

extends to 10 disk scale 

lengths without a 

truncation! 

* Similar to M33 

Light profile 

NGC 300 M33 

Light profile 

Star 

counts 

Star counts

Interpretations of disk truncations 

• The radius associated with the maximum angular momentum 

of the disk baryons in the proto-galaxy -- unlikely -- many 

disks have HI extending far beyond the truncation radius 

Star 

counts 

Light profile 

• The radius to which the disk has grown today -- unlikely -- 

The outer disks may be younger but still typically many 

Gyr old (eg Bell & de Jong 2000; Ferguson et al 2003). 

In some galaxies (eg M83, Milky Way), star formation 

continues in the outer disk but there is an underlying old 

component. 

• The radius where the gas density goes below the 

critical value for star formation (Kennicutt 1989) - 

star formation regulated by disk stability -- likely.


• Disk truncation is not understood yet: remains an 

interesting problem 

• Beautiful multi-wavelength high-resolution imaging 

– From UV (GALEX) via HST to Spitzer and HI/CO 

– Kennicutt, Regan, Murphy/Braun et al. 

• Systematic representative surveys (e.g. SINGS) 

– Separation of stars and different types of dust 

– Get full SED’s, star formation rates, resolved structures 

• Key challenge is to model all this in detail (Dopita)

0.15 µm 0.4-0.8 µm 1.2-2.2 µm 3.6 µm 

8.0 µm 24 µm 70 µm 160 µm

ΛCDM Structure Formation Models 

Small scales, big problems? 

• Scaling relations normalization 

– Cannot simultaneously predict LF and TF zero-points 

• Central regions too dense 

– Predicted dark matter halos have more mass near center 

(too cuspy) than allowed by observed rotation curves 

• Angular momentum crisis 

– Disk galaxies are too small for given rotation speed 

– Predicted distribution of specific angular momentum 

different from observed 

• Too much substructure 

– Galaxy size dark matter halos have hundreds of sub-halos, 

but Milky Way has only ~10 satellites 

• Backwards Colour-Luminosity relation 

– Lack of very luminous galaxies 

– Colour-magnitude relation of spiral galaxies (Bell et al 2003)

ΛCDM Formation Models 

• Failures: 

– LF normalization & TF zero-point simultaneously 

Log V rot (km s -1 ) 

M I – 5 log(h) 

Navarro & Steinmetz (2000) 

Abadi etal (2003)


• Failures: 

– LF normalization & TF zero-point simultaneously 

– Disk size (angular momentum “problem”) 

Log j disk (km s -1 h -1 kpc) 

Log V rot (km s -1 ) 

Navarro & Steinmetz (2000) 

Abadi etal 2003)


• predictions: 

– Hierarchical formation: small structures 

collapse first (youngest / massive galaxies 

form in clusters today) 

– Galaxies evolve inside-out 

– “Universal” (NFW) halo profiles 

– Sub-maximal disks: DM dominates at all radii 

(for R > 1 disk scale length) 

mass distribution on all scales is an 

important constraint of cosmological models

Mass Modeling: Dutton etal (2005) 

• Ingredients: 

– Hα + HI rotation curves (Blais-Ouellette 2000) 

– R-band surface brightness profile 

– gas density profile 

– adiabatic contraction 

Blumenthal et al. (1986) 

(verified by Jesseit etal 2002; Wilson 2003; Gnedin etal 2004; 

Sellwood & McGaugh 2005; Kazantzidis etal 2005) 

– (disk thickness, oblate halos) 

€ 

• Assume disk M/L and invert light profile ⇒ V disk 

2 2 2 2 

Vobs = Vdisk + Vgas + Vhalo 

rM(r) = const 

• Free parameters: disk M/L, halo (α, c, V 200)

Effect of Adiabatic Contraction 

(same disk ϒ d) 

Final 

Initial (NFW) 

Without adiabatic contraction, χ 2 = 1.7, v 200 = 91, c = 14.8 

Dutton et al (2005)

Mass Modeling Degeneracies 

fixed M/L α = 1 (NFW) fixed c 

Dutton, Courteau, de Jong, & Carignan (2005)

Moving Forward in Mass Modeling 

Requires: 

• disk M/L from accurate stellar population models 

• OPT/IR light profiles (no extinction; trace older 

underlying population) 

• 2D Hα/HI/CO velocity fields (SparsePak, FaNTOM) 

• halo constraints from N-body simulations 

(e.g. c anti-correlated with V 200) 

• proper error bars 

• oblate/prolate halos 

• model response of the halo profile to a cooling 

disk that grows by accretion

Future work: Stellar Velocity 

NGC 3198 

Dispersions 

€ 

2 

vz 1/ 2 

R= 0 

∝ µ 0z 0(M /L)

An argument for pure stellar disks 

• At a given mass, a ∆R exp of 20% yields 

– 10% change in V disk 

– 30% offset in luminosity from mean TFR 

– Such an effect should be detectable 

– DM halo (especially if cuspy) will reduce 

this effect

SB Independence of TFR 

1700 spirals: Mathewson (1992), Dale/Giovanelli etal (1999), Courteau etal (2000)

Residual Correlations 

Courteau et al. 2005

TFR as a Tracer of DM 

Pure self-gravitating exponential disks should have 

∂logV( 

M ) 

∂logR 

( M 

exp 

r = − 

0. 

5 

but observed spiral disks have 

∂logV 2.2 

∂logR exp 

r 

) 

= −0.08 ± 0.04

Comparison With Models 

• Simple exponential disk embedded in a DM halo 

• Use density profile for collisionless CDM 

simulations of halo formation (NFW) 

• Assume adiabatic invariance (Blumenthal etal) 

• Use stellar disks of various M/L ratios and 

R exp = 3 kpc, and compute the disk-halo 

contributions to the rotation curve. 

– Get ∂ logV 2.2 / ∂ logR exp for each value of V disk/ V tot 

• Test with bulge and isothermal halos

Comparison With Models 

→ V disk / V tot = 0.55 ± 0.05 at R = 2.2R exp 

Courteau & Rix (1999); Courteau et al. 2005

Evidence for Sub-Maximal Disks 

• Bottema (1997): stellar kinematics of galactic disks 

• Predicted by analytical models of galaxy formation 

(e.g. Mo, Mao, & White 1998). Assumes AC. 

• Courteau & Rix (1999), Courteau etal (2005): TF residuals 

Assumes AC. 

• Kregel et al. (2002): disk flattening of edge-on galaxies 

• Trott & Webster (2002): lensing + rotation curve 

constraints 

• Dutton et al. (2005): mass modeling with stellar population 

constraints, including barred galaxies. Assumes AC. 

V disk /V tot ≤ 0.6 M DM /M tot ≥ 0.7 

(on average at 2.2 disk scale lengths)

Evidence for Maximal 

(Bright and Barred) Disks 

• Weiner (2002), Perez (2003): fluid models 

applied to streaming motions in barred galaxies 

• Slyz, Kranz & Rix (2003): gas kinematics and 

structure of spiral arms -- range of dark matter 

content with SB 

• N-body simulations: resonance issues related 

to bar-halo friction 

• What can we do to move forward? HSB galaxies 

could be light dominated while LSB galaxies are 

DM-dominated at 2 disk scale lengths.

SparsePak Study of Barred 

Galaxies

TFR of Barred Galaxies 

Courteau et al. 2004

€ 

Scaling Relations of Galaxies 

• For a virialized DM halo: 

• At early times, with: 

MVir 3 

RVir ∝ a−3 

• Constant M/L : 

€ 

• N-body simul : 

Observed: 

€ 

€ 

€ 

V obs ∝ a −1/ 2 L 0.33 

€ 

Vobs ∝ LI 0.31±0.02 

€ 

€ 

L ∝ MVir 2 MVir VVir ∝ 

RVir a = (1+ z) −1 

VVir ∝ a −1/ 2 1/ 3 

MVir € 

R disk ≅ λR Vir 

R disk ∝ aL 0.33 

€ 

R disk ∝ L 0.28±0.02 

1/ 3 

RVir ∝ aMVir V obs ≅ V Vir

Galaxy scaling relations 

1700 spirals: Mathewson (1992), Dale/Giovanelli etal (1999), Courteau etal (2000)

λ = halo spin 

c = halo conc. 

m d = disk mass 

(following Mo, 

Mao, & White 

1998) 

Assumes 

σ ln=0.3 for each 

variable. 

Scaling Relations of Disk Galaxies 

Dutton etal 2005

Major issues for NNG 2005 

• Surface brightness independence of TF relation 

why should the luminous/dark ratio change so as to keep the TF relation 

independent of surface brightness? 

• (Slope), scatter (colour), normalization of scaling 

relations 

• Mass models of barred and unbarred galaxies 

(need to use a common language and same 

dynamical tests) 

• What is the final halo shape: Is it NFW + 

(Blumenthal etal) adiabatic contraction? 

• Why can’t we reproduce bulgeless exponential 

disks?

Stéphane Courteau

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