Island universes structure and evolution of disk galaxies
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Island Universes: Structure and Evolution of Disk Galaxies. Early career: radio continuum work Late 70’s: focus on optical data Key contributions to our understanding of disk galaxies: Truncation at ~4 scale-lengths Constant vertical scale-height

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In the footsteps of kapteyn

Early career: radio continuum work

Late 70’s: focus on optical data

Key contributions to our understanding of disk galaxies:

Truncation at ~4 scale-lengths

Constant vertical scale-height

Pioneering stellar kinematics

Supervision of number of high-quality PhD theses

Major influence on development of astronomy on (inter)national level

In the footsteps of Kapteyn

This conference
This Conference

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

    • Disk galaxies are quite complex ‘Island Universes’

    • They are key to galaxy formation process

    • Significant challenge to theory

  • Brief summary restricted to some highlights

    • Structure and properties of stellar disks

    • Thick disks, bulges, bars, peanuts & other components

    • Expanding horizons: panchromatic surveys to high z

  • Outlook

Stellar disks
Stellar Disks

  • Gospel by Freeman (1970) and by van der Kruit & Searle (1981-2) re-addressed and extended

  • Three types of surface brightness profile (Pohlen)

    • Pure exponential, sometimes to 9 disk scale-lengths

    • Double exponential, like vdK+S truncation (Freeman II)

    • Idem, but outer slope shallower than inner one

  • Ratio of scale lengths constant out to z~1(Perez)

  • Studies of ~face-on and edge-on disks disagree?

    • Fractions of three different types need to be checked

    • Try to do this in much larger samples (SDSS/MGC)

Outer regions
Outer Regions

  • Heroic star counts in M31 (Ferguson/Guhathakurta)

    • Very extended structure (>50 kpc)

    • Disk or metal-poor halo?

    • Spectroscopy: kinematics, [Fe/H]

  • UV & H evidence for faint outer disks (Zaritsky)

  • Origin of these components?

    • Star formation threshold?

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

  • Origin of warps most likely connected to torque caused by local cosmic inflow pattern (Binney)

Dark halos kinematics and m l
Dark halos, kinematics, and M/L

  • HI studies of dark halo now ‘mature’

    • Some work seems stalled in ‘cusp wars’: time to refocus

  • Key development: determine (M/L) of disk

    • Measure vertical velocity dispersion

    • Breaks the degeneracy between stellar vs halo mass

    • Builds on Piet’s early work, followed by Bottema

  • IFU spectroscopy can constrain shape of velocity ellipsoid and value of (M/L) (Verheyen/Bershady)

  • Compare with (M/L) from colors/linestrengths

    • Sets limits on IMF (de Jong/Bell)

    • Specific angular momentum content of halo (Kassin)

Other components
Other Components

  • Classical picture of thin disk + R1/4-bulge embedded in dark halo is now more complicated

  • Thick disks are ubiquitous

  • Majority of disk galaxies are barred (~70%)

  • Much confusion about the central regions:

    • Classical and ‘pseudo’-bulges, bars and peanuts

  • Neutral and ionized gas in halos

  • Not discussed: central black holes in disks

Thick disks
Thick disks

  • Most disk galaxies have additional, thick, disk (Yoachim/Dalcanton)

    • Modest mass fraction, but increases below vc~120 km/s

    • Some counter-rotate:  external origin

    • Heavier counter-rotating disks seen in S0s

  • [/Fe] vs [Fe/H] in MW thick disk  it cannot have formed by accretion of small stellar lumps (Venn)

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


  • Bar influences dynamics and star formation activity

    • They drive gas to the center (and so do interactions)

    • This can trigger starburst, sometimes in ring

  • Bars slow down and change shape due to friction by halo (Sellwood/Athanassoula)

    • Consistent with ~constant bar fraction since z~1? (Bell)

  • Bars do not transform into bulges today (Elmegreen)

    • End-on peanut looks like bulge (Bureau)

Impostor bulges
Impostor Bulges?

  • Traditional bulges may be mix of structures

    • Some classical bulges similar to E/S0s

    • Nearly 50% of bulges have exponential profiles, and may be disks; these all have central star clusters

    • In many cases the central bar may be the bulge; it may have an embedded central disk (cf Milky Way)

    • Not clear at all which of these have central black hole

  • Some classification schemes tied to prejudices about formation scenario

    • Definition of ‘pseudo-bulge’ needs further clarification

Baryonic halo

Extended extraplanar gas

HI (Fraternali)

H and X-rays (Dettmar)

Properties consistent with theory of fountains/winds

Not evident that in situ star formation is needed in halo

Effect of run-away stars to be investigated

Baryonic Halo

HI in NGC 891

Oosterloo, Fraternali, Sancisi 2005

The milky way
The Milky Way

  • Piet measured scale-length of MW disk

    • Based on Pioneer 10 data

    • This is Paper V in the legendary series (but vdK only)

    • Generated follow-up work to measure run of  with R

  • Milky Way received relatively little attention here

    • Kinematics of disk stars (Binney)

    • Stellar halo not formed by dissolving present-day like dSph’s, dIrr, of LMC-clones (Venn)

    • Comparison of MW properties with simulations: key test of formation paradigm (Bullock/Sommer-Larsen)

Panchromatic surveys
Panchromatic Surveys

  • 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

    • Still to be linked with NIR extinction method (Alves)

    • Get full SED’s, star formation rates, CR structure

    • Images constrain lifetime of embedded phase of massive star formation

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

0.15 mm 0.4-0.8 mm 1.2-2.2 mm 3.6 mm

8.0 mm 24 mm 70 mm 160 mm

Integral field spectroscopy


Integral-field Spectroscopy

NGC 7742


Studies to high redshift
Studies to high redshift

  • Much work on properties of galaxies to high z

    • Be careful with selection biases (Vogt)

    • Most objects found: progenitors of present-day spheroids (Pettini)

    • Disks detected to z~2 (Abraham/Kassin)

    • DLA hosts resemble local galaxies with HI (Zwaan)

    • Inside-out formation (Bell/Trujillo)

    • Evolution of population  evolution of objects (Bell)

  • Metallicity determinations to z~3

    • Much progress for disks (Kewley)

    • Also for the ISM, via DLA’s (Fall)

    • Beware of selection effects

The future
The Future

  • Studies of resolved stellar populations in MW and Local Group galaxies hold much promise

  • Panchromatic surveys complemented with IFU spectroscopy in optical and IR, to high z

  • Theoretical models & simulations need to fit observations in detail; this is within reach

  • Herschel, ALMA, GAIA and JWST to come

  • Much scope for more pioneering work by Piet