Gas Accretion and Secular Processes
How much mass assembled in mergers?
How much through gas accretion and secular evolution?
Keres et al 2005, Dekel & Birnboim 2006, Ceverino et al 2010
1- Star formation efficiency, history
2- Size of disks and evolution
3- Metallicity gradients
4- Bulges: how to avoid formation
5- Thick disks
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Françoise Combes 14 December 2011
Merger Fraction from GEMS
< 10% of SF
in z=0.6 massive
galaxies is triggered
by major interactions
(Robaina et al 2009)
Starburst mode at z=2
Only 10% of the SF
Rodighiero et al 2011
Herschel-GOODS
Jogee et al 2009
2
Merger fraction in the EGS
The decrease in SFR in this z-range comes from gas fraction
or SF efficiency, but not from the decrease of mergers
Lotz et al 2008
3
Relative role of gas accretion and mergers
Analysis of results from
a cosmological simulation
with hydro: most of the
SF is due to smooth flows
Dekel et al (2009)
Fraction of mass acquired from accretion 77% (mergers 23%)
until z=0 (Lhuillier et al 2011)
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AEGIS galaxies
Molecular gas at IRAM,
at z~2.3 and at z~1.2
High detection rate >75%, in these « normal » massive Star
Forming Galaxies (SFG)
Quiescent SF, in the main sequence
Gas content ~34% and 44% in average at z=1.2 and 2.3 resp.
SFR proportional to M*0.8 (1+z)2.7
Tacconi et al 2010, Daddi et al 2010
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Mergers and SSFR
Genzel et al 2010
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SSFR history
3
5
Dutton, van den Bosch, Dekel 2010
Accretion rate for a given M in (1+z)2.25
Stop sSFR at high z: metallicity?
Krumholz & Dekel 2011
Mergers dominate at high z?
Khochfar & Silk 2011
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9
Disk size evolution
Bars and spirals re-distribute
angular-momentum
Stars
Log S
Gas
SFR
Age
Log R
Roskar et al 2008
8
Bar+spiral: radial migrations
Overlap of resonances
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Minchev et al 2010
Size evolution with redshift
102 SF galaxies at z=1.5-3 , about half the radius of local galaxies
Nagy et al 2011, z=2-3 Weinzirl et al 2011
re ~(1+z)-a
a=1.4 Nagy et al 2011
a=1.3 van Dokkum et al 2010
a=1.1 Mosleh et al 2011
Stellar radii at a given mass are
~half lower, at z=2-3
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Minor mergers to increase galaxy radius?
Candels: search for companions around quiescent red galaxies
~15%
Possible if te < 1Gyr (te merging time)
But possible only for z=1,
At z=2 other processes are
required
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Newman et al 2011
Fundamental metallicity relation
Requires slow gas infall, chemical time-scale long wrt dynamical
Mannuci et al 2010
12
Gas dilution due to flyby: triggered bar
Bar drives low-Z gas to the center, and triggers SF
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Montuori et al 2010
Relation between SFR and Z
F= merger
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Montuori et al 2010
Low Bulge Mass in spiral galaxies
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Weinzirl et al 2009
Constraints of bulge formation
Major mergers or a large number of minor mergers
form a massive spheroid  classical bulge
Secular evolution:
bars and vertical resonance elevate stars in the center
into a pseudo-bulge: intermediate between a spheroid and a disk
Frequent for late-type galaxies
Clumpy galaxies at high z can also form a bulge,
through dynamical friction
Solution : most clumps should be disrupted
before reaching the center?
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Thick disk formation
At least 4 scenarios:
1) Accretion and disruption of satellites (like in the stellar halo)
2) Disk heating due to minor merger
3) Radial migration, via resonant scattering
4) In-situ formation from thick gas disk (mergers, or clumpy galaxies)
Loebman et al 2011
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Gas accretion
May mimick
mergers
Mastropietro et al 2011
Gas accretion may explain
-- asymmetries, lopsidedness
-- clumpiness
-- maintained SFR
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CONCLUSION
 Importance of mergers: only 10% in the second half of the universe
<10% of SF is due to mergers
 Size of disks: non-axisymmetries redistribute matter
Exponential disks + radial migration
 Metallicity dilution due to gas accretion, and mergers
Bulge formation: too massive with mergers
Pseudo-bulge with bars, secular evolution
Clumpy galaxies: how to avoid a too massive bulge?
 Thick disk formation: mergers, or secular evolution?
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Transient Ring
formation
Hoag object (HST)
The ring may disappear
If the accretion continues
Mastropietro et al 2011
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