Building Massive Galaxies via
Major Mergers
T.J. Cox
Harvard/CfA
Lars Hernquist, Phil Hopkins, Brant Robertson
Sukanya Chakrabarti (CfA), Matt Covington (UCSC), Avishai Dekel (HU), Tiziana Di Matteo (CMU),
Suvendra Dutta (CfA), Patrik Jonsson (UCSC), Elisabeth Krause (CfA/Bonn), Yuexing Li (CfA),
Jennifer Lotz (NOAO), Desika Narayanan (Arizona), Greg Novak (UCSC), Joel Primack (UCSC),
Rachel Somerville (MPIA), Volker Springel (MPA)
Galaxy Mergers and the
Formation of Elliptical Galaxies
“Merger Hypothesis,” as originally posed by Toomre(s),
states that elliptical galaxies are produced by the merger of
spiral disk galaxies
What type(s) of mergers produce ellipticals?
When did these mergers occur?
Galaxy Mergers and the
Formation of Elliptical Galaxies
“Merger Hypothesis,” as originally posed by Toomre(s),
states that elliptical galaxies are produced by the merger of
spiral disk galaxies
What type(s) of mergers produce ellipticals?
When did these mergers occur?
What type(s) of mergers produce
elliptical galaxies?
Possible merger events:
•
major gas-free (“dry”) merger
•
major gas-rich (“wet”) merger
•
minor mergers
•
multiple mergers
•
…… combination(s) of the above, something else?
Who type of merger produces an
elliptical galaxy?
Possible merger events:
•
major gas-free (“dry”) merger
•
major gas-rich (“wet”) merger
•
minor mergers
•
multiple mergers
•
…… combination(s) of the above, something else?
Methods
•
Simulations: 3-D, time-dependence
(GADGET2, Springel 2005)
•
Consider: star formation, supernova
feedback (sub-resolution), black hole
growth, feedback (sub-resolution)
•
~10^6 particles
•
Binary mergers of disk galaxies on a
variety of orbits, with different disk
orientations
•
(new/in progress ….) Re-mergers
A Single Binary Gas-rich (>30%) Merger
can produce remnants that…
1. appear morphologically similar to ellipticals
2. have fine structure (Hernquist & Spergel 1994)
3. are kinematically akin to low-luminosity elliptical galaxies, i.e., they are
typically oblate isotropic rotators (Novak et al., 2006, Cox et al. 2006)
4. have ~r1/4 surface brightness profiles with possible central light/mass
excesses, i.e., they have sufficient phase space density (Hernquist, Spergel & Heyl 1993)
5. are red if star formation is quickly quenched (Springel et al. 2005)
6. have hot gas halos (Cox et al. 2006)
7. may have kinematic subsystems (Hernquist & Barnes 1992, Cox et al. 2007)
8. satisfy observed scaling relations, MBH-, size-mass, FP (Di Matteo et al. 2005,
Robertson et al. 2006)
** This single event can also self-consistently explain quasar,
merging and red galaxy populations (Hopkins et al. 2005/6/7)
Problems at High-mass
Gas-rich mergers have trouble
producing
•
uniformly boxy, slowly-rotating
ellipticals (see also Naab & Burkert 2003)
•
enough X-ray luminosity (likely
results from not including stellar massloss and cosmological infall)
Cox et al. (2005) - data from Kormendy & Bender (1996)
Possible Solutions ….
additional processes/mechanisms involved.
•
•
•
•
major gas-free (“dry”) disk-galaxy
merger
minor mergers
Phase space constraints; also
appear to produce low-luminosity
ellipticals (Hernquist 1992, Bendo &
Barnes 2000, Naab & Burkert 2003
)
spheroid-spheroid merger
Have long been speculated to be
relevant for the most massive E’s
multiple mergers
(e.g.,MBK’s talk, Nieto et al. 1991, Bender et al.
1992, Khochfar, Naab, & Burkert 2006)
Formation of cD galaxies (e.g.,
Dubinski 1998, Weil & Hernquist 1996, Bekki
2001
)
Possible Solutions ….
additional processes/mechanisms involved.
•
•
•
•
major gas-free (“dry”) disk-galaxy
merger
minor mergers
Phase space constraints; also
appear to produce low-luminosity
ellipticals (Hernquist 1992, Bendo &
Barnes 2000, Naab & Burkert 2003
)
spheroid-spheroid merger
Have long been speculated to be
relevant for the most massive E’s
multiple mergers
(e.g.,MBK’s talk, Nieto et al. 1991, Bender et al.
1992, Khochfar, Naab, & Burkert 2006)
Formation of cD galaxies (e.g.,
Dubinski 1998, Weil & Hernquist 1996, Bekki
2001
)
Evidence for Spheroid-Spheroid Mergers
Observed red mergers in the ~local Universe
* however, which are gas-poor disk mergers and
which are truly spheroid-spheroid mergers?
van Dokkum (2005)
What role do spheroid-spheroid
mergers play in the formation of
elliptical galaxies?
Proposed Test:
Use the existence/properties of kinematically
decoupled components and fine structure in present
day elliptical galaxies to probe their assembly
history.
Kinematic Subsystems in Elliptical Galaxies
NGC 4362, Davies et al. (2001)
Jedrzejewski &
Schechter (1988)
The Zoo of (simulated) Remnant Kinematics
Rotating remnants
(not all are flattened)
Velocity along slit
Non-rotating remnants
(not many of these)
Stellar Surface Density
Stellar Velocity Field
The Zoo of (simulated) Remnant Kinematics
Decoupled central region
Decoupled central region
The Zoo of (simulated) Remnant Kinematics
Decoupled central region
Multiple components
The Zoo of (simulated) Remnant Kinematics
40% gas merger
KDC is mainly newly
formed stars
Similar merger that is
entirely dissipationless
Kinematic Subsystems
From a subset of mergers we find 8/15 dissipational remnants have kinematic
subsystems, which can be easily observed (from 2D kinematics) from ~2/3 of the
projections: ~36%
Observations:
• SAURON: 7/48 (~15%) E/S0’s and 52% of Sa bulges are observed to have
kinematic subsystems which are younger than the bulk of the stellar population
(McDermid et al. 2006, Falcon-Barroso et al. 2006)
(old) studies found ~10-30% of ellipticals (Jedrzejewski & Schechter 1988,
Franx et al. 1989, Zeeuw & Franx 1991, Schweizer 1998)
• Previous
Caveat: there may be other mechanisms to produce kinematic subsystems (Bendo &
Barnes 2000)
Fine Structure in Elliptical Galaxies
• Shells in ellipticals (Malin &
Carter 1983): phase-wrapping
of “cold” stellar material
(Quinn 1984; Quinn & Hernquist 1986)
• NOT signature of major
mergers of spheroids, just
the opposite!
NGC 3923
Fine Structure in Elliptical Galaxies
• Produced at all radii in
major mergers (Hernquist &
Spergel 1992)
• Produced at large radii
during satellite accretion
(Quinn 1984, Quinn & Hernquist
1986, Hernquist & Quinn 1989)
What happens to KDC and fine
structure after a spheroid-spheroid
merger?
Re-merger Remnant Kinematics
Original
KDC is absent in
remnant after
subsequent merger
Re-merger Fine Structure
before
after
Proposed Test:
Use the existence/properties of kinematically
decoupled components and fine structure in present
day elliptical galaxies to probe their assembly
history.
Proposed Test:
Use the existence/properties of kinematically
decoupled components and fine structure in present
day elliptical galaxies to probe their assembly
history.
Kinematically Decoupled Components
Yes: gas-rich major merger
* abundantly produced by gas-rich major mergers
* destroyed by subsequent merging.
Fine Structure
* present at all radii in remnants of spiral major mergers
* at large radii after satellite accretion
* destroyed at small radii, diluted at large radii by subsequent
merging
Yes:
(all radii) spiral major merger
(only outer) sat. accretion