Download Formation of elliptical galaxies

Nearby mergers:
ellipticals in formation?
Thorsten Naab
University Observatory, Munich
October 4th, 2006
From the Local Universe to the Red Sequence
Space Telescope Science Institute, Baltimore
Making ellipticals by mergers
“Merger hypothesis” : ellipticals form by major mergers of … but what
merged when?
Do present day disk mergers evolve into elliptical galaxies?
Making elliptcals by mergers of disk galaxies
mergers
of disks
EncounterCollisionless
survey, statistical
properties
of 96 remnants
Naab & Burkert 2003, ApJ, 893
Collisionless mergers of disks
•Formation of tidal tails and bridges, similar to local interacting
galaxies : formation of ellipticals?! (Toomre & Toomre 1972,1977)
•Equal-mass remnants are spherical, slowly rotating, show
shells, loops etc. similar to elliptical galaxies (Barnes 1988,1992;
Hernquist 1992,1993)
•Phase-space constraints and kinematics rule out pure
exponential disks as progenitors for ellipticals - bulges
and/or dissipation needed (Hernquist 1993, Jesseit et al. 2005, Naab &
Trujillo 2006, Cox et al. 2006)
•Mass ratio determines the kinematics, orbital content and
shape of the remnants (Bekki 1998, Barnes 1998; Naab, Burkert & Hernquist
1999; Bendo & Barnes 2000, Naab & Burkert 2003; Gonzalez-Garcia, Cesar & Balcells
2005; Jesseit, Naab & Burkert 2005)
•Shape of the LOSVD in general not in agreement with observed
rotating ellipticals (Bendo & Barnes 2000; Naab & Burkert 2001; Naab, Jesseit & Burkert 2006;
Jesseit, Naab & Burkert 2006, astro-ph)
Surface density profiles of collisionless mergers
no gas
with gas, stars analyzed
Sersic-function: =0*exp(-r 1/n),
Distribution peaks at
nser ≤ 4
for all mass ratios!
Naab & Trujillo 2006, MNRAS
Collisionless mergers of disks
Unequal-mass mergers
• rotate faster
• are more elongated
• are less supported
by random motions
• are more dominated by
tube orbits
Naab & Burkert 2003
Orbital content of merger remnants: The stellar backbone
Minor-axis
tubes
Box orbits
Box shape of tubes only in triaxial potentials!!!
Outer
major-axis
tubes
Inner
major-axis
tubes
Boxlets
Jesseit, Naab & Burkert 2005, MNRAS, 1185
Orbital content of merger remnants: The stellar backbone
•1:1 mergers are dominated by
box orbits scattered from
the progenitor disks and bulges
•3:1 mergers are dominated by
tube orbits from the more
massive progenitor disks
All collisionless merger remnants are
dominated by box orbits at their
centers (Barnes 1998; Bendo & Barnes 2000) and
orbital content correlates with shape
and kinematics (Jesseit, Naab & Burkert 2005)
Mergers with gas…
Zur Anzeige wird der QuickTime™
Dekompressor „YUV420 codec“
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3:1 disk merger, 10% gas
Naab et al., 2006
Mergers of disks: Gasdynamics
•Galaxy merger drive large gas fractions to the centres of the
merger remnants (Negroponte & White 1983; Hernquist 1989, 1991;
Barnes & Hernquist 1991, 1996)
•Central gas concentration makes the potential more axisymmetric
and favours the population of tube orbits (Barnes & Hernquist 1996;
Barnes 1998, Naab, Jesseit & Burkert 2006)
•The stellar body of the remnants becomes more axisymmetric
•Global stellar kinematics is only weakly affected, isophotal shape
and shape of the LOSVD changes (Naab et al. 2006)
•Structure of the progenitors influences gas flow: Central bulges
suppress inflow after first encounter (Mihos & Hernquist 1994,1996)
•Left over gas settles in large-scale disks (Mihos & Hernquist 1996;
Naab & Burkert 2001; Barnes 2002; Naab et al. 2006; Jesseit et al. 2006)
Merger dynamics
•Disks are heated after first
encounter and during the final
merger
• Apparent mass ratios
12reff,1/22 reff,2
trace real mass ratios and
agree with observed separated
nearby ULIRGS which are in the
range 1:1 - 3:1 (Dasyra et al. 2006)
•Galaxies might follow the MBH-
relation after first encounter
Naab et al. 2006; Dasyra at al. 2006 a,b
Influence of gas on the stellar component
•Fraction of box orbits
is reduced
•Remnants are more
axisymmetric
see Bendo & Barnes 2000
•Global kinematic
properties only weakly
affected
•Isophotal shape strongly
affected
•Shape (asymmetry)
of the LOSVD changes
Naab et al. 2006; Data Rothberg & Joseph 2006; Dasyra et al. 2006
Line-of-sight velocity distribution
a4 < 0
boxy
a4 > 0
disky
LOSVD analysis using Gauss-Hermite polynomials – vlos, los, h3, h4
Bender, Saglia & Gerhard 1994
Influence of gas on the LOSVD: statistics
All “dry” remnants: star particles analysed
Influence of gas on the LOSVD: statistics
All “wet” remnants: only star particles analysed
Influence of gas on the LOSVD: statistics
All “wet” remnants: star and gas particles analysed
2D kinematical
analysisproperties
of merger
remnants
Encounter
survey, statistical
of 96
remnants
2D analysis and comparison to observations helps to figure
out formation mechanisms (see Bendo & Barnes 2002)
Jesseit, Naab & Burkert 2006, submitted
A model for disk galaxy evolution
• Analytical model for the formation of galactic disks
Milky Way (vc, sol, rdisk)
• During initial merger phase follow most common halo
bulge formation
• vhalo= const., 0 = const.,
rdisk rhalo  1/H(z), cooling gas forms exponential disk rbulge 
0.25 rdisk
• Infallrate known:
star formation,
chemical evolution,
photometric evolution
• Radial distribution of gas and stars is known at any
• Full evolution of a galaxy, the Galaxy
time
Naab & Ostriker, MNRAS, 2006
Assembly of an early-type disk galaxy
Disk formation
Bulge
formation
Assembly of an early type disk
galaxy
• Outer young “disk” with a scale
length of 3kpc
• Inner old “bulge” with a scale
length of 600 pc
• B/T ≈ 0.2
The ideal disk merger
• Take stellar populations of two equal
mass disks
• Convert gas inside 2 rd into stars at
given metallicity
• Stop infall and further star formation
to make the galaxies “red & dead”
• Check present day properties for
different merger redshifts
Naab & Ostriker 2006, in prep.
Conclusions
•Kinematic and photometric properties of nearby ongoing mergers
and merger remnants are in agreement with disk merger simulations
with mass ratios in the range 1:1 - 3:1
•Good agreement of both with intermediate mass giant elliptical
galaxies (see also Cox et al. 2006)
•Disk merger remnants, as well as ULIRGS, can follow the
Fundamental Plane and the MBH-relation (Robertson et al. 2005; Springel et al. 2005)
•Mergers trigger strong central and extended starbursts and AGN
activity (Barnes 2002, di Matteo et al 2006)
•So far most dynamical, kinematical and photometric constraints
indicate that gaseous 1:1 to 3:1 disk mergers can have evolved
into intermediate mass M* elliptical galaxies and if merging now
be ellipticals within the next 4-6 Gyrs
•Present day formation rate is low, was higher in the past but then the disks
were smaller and less massive
Ellipticals more massive than M* ellipticals are not
and have not formed by binary disk mergers!