Galaxy Mergers: Simulations, Observations, and Active Galactic Nuclei (AGN)

96 Pages·2006·7.06 MB·English

by  Joel Primack

ACS Picture of “The Mice” by Ford et al.
UC Berkeley Lunch Seminar - 5-2-06
GALAXY MERGERS:
Simulations, Observations,
and Active Galactic Nuclei
Joel R. Primack (UCSC) Avishai Dekel (Hebrew U)
Thomas J. Cox (UCSC CfA) Matt Covington (UCSC)
Patrik Jonsson (UCSC) Greg Novak (UCSC)
Jennifer Lotz (UCSCArizona) Christy Pierce (UCSC)
Goals of Galaxy Interaction Simulations
Understand the amount of star formation due to
galaxy mergers TJ Cox PhD thesis Sept 2004
Study properties of merger remnants TJ Cox+05,06
! ! DM/stellar/gas distributions Matt Covington
! ! Angular momenta and kinematics Greg Novak
Predict appearance of interacting galaxies
throughout merger, including dust scattering,
absorption, and reradiation Patrik Jonsson PhD
thesis Sept 2004, MN in press, MN submitted 2006
Statistically compare to observations (DEEP2 and
GOODS: ACS, Chandra, Spitzer, etc.) Jennifer
Lotz, Piero Madau, and Primack 2004, AJ; Lotz et
al. 05, 06; Pierce et al. 2006, Nandra et al. 2006
Galaxy Collision
T.J. Cox
PhD Dissertation
UC Santa Cruz
http://physics.ucsc.edu/~tj/work/thesis/
Cox et al. ApJL 2004, astro-ph this week
Numerical Simulations of Star Formation
in Colliding Disk Galaxies: Earlier Work
• Major mergers (Mihos & Hernquist 1996,
Springel 2000) (original disks are identical)
generate significant bursts of star formation
Star consuming ~80% of the original gas mass.
Formation
• Internal structure of progenitor disk galaxy
(i.e. the presence of a bulge or not) dictates
when the gas is funneled to the center and
turned into stars.
Time
• Minor mergers (Mihos & Hernquist
1994) (satellite galaxy is 10% of the
original disk mass) generate significant
bursts of star formation only when Star
Formation
there is no bulge in progenitor disk
galaxy.
NOTE: These simulations used a version of SPH which has been
shown not to conserve entropy (Springel & Hernquist 2002).
Parameterizing Starbursts
Based upon the results of
Mihos & Hernquist (the 3
‘data’ points), Somerville,
Primack & Faber (2001, N o b u l g e
SPF01) estimated the burst
efficiency (amount of gas
e
g
ul
b
converted to stars due to
the galaxy merger) as a
function of the merger mass
ratio. A motivation of the
present work is to improve
the statistics and
Mass Satellite/Mass Progenitor
understanding of mergers.
Major
= MH 'data' point Merger
Cosmological Semi-Analytic Models
(SAMs) Bell et al 2005 also find that this
model also agrees with GEMS
Feeding the parameterized and Spitzer data at z ~ 0.7
starbursts into semi-
The bursting
analytic models for galaxy
mode of star
formation, SPF01 found
formation
this model (as opposed to dominates at
models without collisional high redshift
starbursts) better fits data
for:
1) Comoving number density
of galaxies at z > 2
2) Luminosity function at z =
Quiescent star
3 (and more recently the
formation dominates at
star formation rate to z = 6)
low redshift
The majority of stars were
generated by star formation
Note: this is redshift, not time!
induced by galaxy minor mergers
Our New Work
In order to investigate galaxy mergers (and interactions) we
build observationally motivated N-body realizations of
compound galaxies and simulate their merger using the SPH
code GADGET (Springel, Yoshida & White 2000, Springel
2005). These simulations include:
• An improved version of smooth particle hydrodynamics
(SPH) which explicitly conserves both energy and entropy
(Springel & Hernquist 2002).
• The radiative cooling of gas
• Star formation: ρ ~ ρ /τ for (ρ > ρ )
sfr gas dyn gas threshold
• Metal Enrichment
• Stellar Feedback
* Our simulations contain ≥ 170,000 particles per galaxy and the
resolution is typically ~100 pc. (Tested up to 1.7x106 particles.)
Selecting Parameters
Kennicutt (1998) determined that the surface density of star formation was very tightly
correlated with the surface density of gas over a remarkable wide range of gas densities
and in a wide variety of galactic states. We use this ‘law’ to calibrate our star formation (c )

and feedback (β) parameters by requiring an isolated disk to follow the Kennicutt law.
Kennicutt (1998)
Merger
Isolated spiral
IR luminous galaxies
(some are mergers)
Nuclear region of Previous work was
same spirals
normalized low
Spiral galaxies
Galaxies tend to fall off the law once
gas is depleted.
Initial Conditions
The orbits and initial conditions for our galaxy merger
simulations are motivated by cosmological simulations and
observational data on galaxy properties.
Galaxies NFW Dark Matter Halo (M , c, l)
vir
Exponential disk (m , gas fraction f, R )
d d
Bulge (m , r )
b b
Orbits Galaxies are placed on an orbit defined by the initial
separation R , their impact parameter b, the
start
eccentricity e, and disks may be inclined with respect to
the orbital plane
Feedback
Supernova feedback (pressurizes star forming regions)
similar to Springel (2000) and Robertson et al. (2004)
Disk Galaxy Models
• The Milky Way + Mass Excursions (40+ Major Mergers)
! A large, low gas fraction galaxy has been the starting point for the
! majority of all merger simulations to-date (MH94-96, Springel 2000, and
! our early work). The mass excursions have a higher gas fraction (50%).
• Sbc/Sc models (50+ Major Mergers)
Built to model the observed properties (Roberts & Haynes 1994) of local
Sbc/Sc galaxies. While (roughly) the same size as the Milky way these
models have a large amount of extended gas. Model Sc has no bulge and
Model Sbc has a small bulge.
• G models (13 Major Mergers, 88+ Minor Mergers)
There are 4 G galaxies (G3,G2,G1,G0, ordered by mass) which are
statistically average galaxies whose properties are extracted from SDSS
plus other local, early-type galaxy surveys. The dark mass and
concentration are constrained to match the baryonic TF relation.