Dynamical Phenomena for Bulge and Disk Formation

Françoise CombesRingberg, 21 May 2010

Outline: Disk formation scenarios

AM transfers, radial migrations

Bars and gas flows

Bulge formation scenarios

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Disk formation1- Fall & Efstathiou (1980), van der Kruit (1987)AM conservation during the collapse of a uniformly rotating uniform sphere (rm=0.2h)produces an exponential disk, with a cut-off at 4.5hHI extensions accreted later??

Van der Kruit 1987

The baryons cannotlose AM, since it isbarely sufficient Disks too concentrated

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Possible scenarios 2- Star formation threshold, for the stellar exp disk (Kennicutt 1989)But extended UV disks: 2 SF prescriptions?

3- Maximum AM of the cooled gas (van den Bosch 2001)But again, large central concentration

Zf begin to accrete with a speed ~aM= 5 E11 Mo, =0.129

Problem:AM acquired by tidal torques before virialisationIn mergers, baryons lose AM against DMToo small disksGas-dominated mergers at high z?(Robertson et al 2006)

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Surface density breaks Debattista et al 2006

Face-on Edge-on

Consequence of AM exchangeOuter and inner disks breaks Rbreak ~ ROLR

Could be a stellar processus(Pfenniger & Friedli 1991)

Or a gas process, with SFthreshold(Roskar et al 2008)

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Break moves radially outwards

Roskar et al 2008

TreeSPH simulation of collapse: break moves due to gas breakand Toomre Q increase

Stars

Gas

SFR

Age

Spiral and bartransfer inner starsto the outer parts

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Age gradient reverses after 8kpc

M33: CMD diagrams fromHST/ACS (Williams et al 2009)

--- Mo et al 1998, … all radii _____ inside 8kpc

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Migrations of stars and gas Resonant scattering at resonances

Sellwood & Binney 2002

All stars with small epicycles

LCR

ILR OLR

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L exchange without heating

Sellwood & Binney 2002

Invariant: the JacobianEJ= E- p L E = p L

JR = (p-)/ L

If steady spiral, exchange at resonance only

In fact, spiral waves are transient

The orbits which are almost circular will be preferentially scattered

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Chemical evolution with migration

Shoenrich & Binney 2009

O/H, and O/FeThick disk is both -enriched and low Z

Churning = Change in L, without heatingBlurring= Increase of epicyclic amplitude, through heating

Gas contributes to churning, and is also radially driven inwards

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Transfert of L, and migrations Bars and spirals can tranfer L at Corotation

Transfer multiplied if several patterns with resonances in common

Much accelerated migrations

Minchev & Famaey 2010

Bar Spiral

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Effect of coupled patterns Time evolution of the L transfer with bar and 4-arm spiral, in the MW

Top: spiral CR at the Sun

Bottom: near 4:1 ILR

Minchev et al 2010

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Migration extentTime evolution of the L transfer with bar and 4-arm spiral

Explains absence of AMR Age Metallicity Relation+AVR relation

Initial positionof stars ending in thegreen intervalafter 15 and 30 Rotations

Black: almost circular = 5km/s

Minchev et al 2010

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Bar+spiral migrationsOverlap of resonances

Minchev et al 2010

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Bars formation andre-formation

Self-regulated cycle:Formation of a bar in a cold diskBar produces gas inflow, and Gas inflow destroys the bar +gas accretion

Gas accretes by intermittenceFirst it is confined outside OLR until the bar weakens,

then it can replenish the disk, to make it unstable again to bar formation

2% of gas infall is enough to transform a bar in a lens(Friedli 1994, Berentzen et al 1998, Bournaud & Combes 02, 04)

Time (Myr)

Bar

Str

engt

h

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No gas accretion

gSb-dmx no

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With gas accretion

gSb-dmx ac

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gSb models

--- Purely stellar--- With gas and SF+feedback--- With gas accretion (5Mo/yr)--- Double accretion rate (10Mo/yr)

Mhalo=Mbar

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Scenarios of bulge formation

In major mergers, the tidal trigger first forms strong bars in the partner galaxies, which drive the gas inwardFormation first of a pseudobulge

Then the merger of the two galaxies could provide a classical bulgeWhich will then co-exist with the pseudo ones

Alternatively, after a classical bulge has formedsubsequent gas accretion, could re-form a bar, and a diskand drive the gas towards the center, pseudobulge

Clumpy galaxies at high z bulge formationAgain problem for the bulgeless galaxies

Major mergersMinor mergersBarsClumps

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Major merger, N4550 prototype bulge formation

gas stars gas stars

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2 CR stellar disks, but only one gas rotation

stars gas

Red= prograde galaxy, Blue= retrograde galaxy, Black=total

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Angular momentum exchange

Crocker et al 2008

Formation of a bulge with low n ~1-2Special geometry, of aligned or anti-aligned spins

Red= prograde galaxy, Blue= retrograde galaxy, Black=totalFull lines= orbital AM, Dash lines= individual spins

The gas settles in corotation with the thicker, more perturbed, disk

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Pseudo-bulge formation

Pseudobulges have characteristics intermediate between a classical bulge (or Elliptical) and normal disks (Kormendy & Kennicutt 2004)

Sersic index ~ r1/n, with n =1-2 (disks: n=1, E: n=4 or larger)

Flattening similar to disks, box/peanut shapes Bluer colors

Kinematics: more rotation support than classical bulges

Combes & Sanders 1981Combes et al 1990

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Multiple minor mergers

Bournaud, Jog, Combes 2007

50 mergersof 50:1 mass ratio

Even more frequent Than 1:1

The issue is not the mass ratio ofindividual mergers But the total mass accretedIf 30-40% of initial mass Formation of an elliptical

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Formation in clumpy galaxies

Rapid formation of exponential diskand bulge, through dynamical frictionNoguchi 1999, Bournaud et al 2007

Chain galaxies, when edge-on

Evolution slightly quicker than with spirals/bars?

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Frequency of bulge-less galaxies

Locally, about 2/3 or the bright spirals are bulgeless, or low-bulgeKormendy & Fisher 2008, Weinzirl et al 2008Some of the rest have both a classical bulge and a pseudo-bulgePlus nuclear clusters (Böker et al 2002)

Frequency of edge-on superthin galaxies (Kautsch et al 2006)1/3 of galaxies are completely bulgeless

SDSS sample : 20% of bright spirals are bulgeless until z=0.03 (Barazza et al 2008)Disk-dominated galaxies are more barred than bulge-dominated ones

How can this be reconciled with the hierarchical scenario?

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BRAVA kinematical results: Milky Way

The bulge of the Milky Way is compatible with only pseudo

Old stars (99% > 5Gyr)+a large variety of O/H, Fe/H

Fe/H decreases with z Not only a bar?(Zoccali et al 2008)

Shen et al 2010

BRAVA+ Rangwala et al 2009

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Age (Gyr)

bulge

halo

Metallicity indicators

Bulge stars are oldand -enrichedBut inner disk starsare not known

Zoccali et al 2009

bulge

thick

thin

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No possible classical bulgeEven a classical bulge of 8% Mdisk worsens the fit to the data

Shen et al 2010

Older, low Fe/H stars have been scattered at high z, for a longer timeSeveral bars?

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NGC 4565: SBb, No classical bulge

In addition to the bar, a pseudo-bulge of 6% in mass

Pseudo, since flattened, andSersic index 1.3-1.5

HST 1.6m

Kormendy & Barentine 2010

Spizer-3.6

Spizer-8

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Thick disk formationAt least 4 scenarios:1) Accretion and disruption of satellites (like in the stellar halo)2) Disk heating due to minor merger3) Radial migration, via resonant scattering4) In-situ formation from thick gas disk (mergers, or clumpy galaxies)

Orbit excentricity of starscould help to disentangle

Sales et al 2009

SB09

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CONCLUSION

Disk formation poses problems to the standard model: too concentrated, too small, loss of AM

Radial migration, resonant scattering by spirals

Bars efficient to AM exchange, gas radial flows

Fraction of bulgeless galaxies: challenge for hierarchical scenario?

Thick disk formation: several scenarios

Bulge formation: coexistence of many processesSome major mergers could keep a diskMultiple minor mergers lead to spheroids Clumpy galaxies at high-z