Starbursts In Interacting Galaxies

1
Starbursts In Interacting Galaxies

• Interactions and Starbursts in Dwarfs
• The Remnants (brief)
• Tidal Dwarfs

• T.J. Cox
• Harvard-Smithsonian Center for Astrophysics
• Suvendra Dutta, Tiziana Di Matteo (CMU), Lars
  Hernquist, Phil Hopkins, Brant Robertson, Volker
  Springel (MPA)
• and
• Patrik Jonsson (UCSC), Joel Primack (UCSC),
  Rachel Somerville (MPIA), Avishai Dekel (HU)

2
(No Transcript)
3
Star Formation during a Major Merger
4
Merger Induced Starbursts Basics
Starbursts are produced by large perturbations

• The two galaxies must be nearly equal in mass
  (gt15) - however, there is no requirement
  about what this mass is composed of
• the passage must be close
• a bulge in one of the progenitor disks provides
  stability during any initial encounters (Mihos
  Hernquist 94,96, Springel 00)
• disks co-planar with the orbit feel stronger
  tidal forces (Mihos Hernquist 96)

11 13 15 120
NOTE there is no statement about the absolute
mass of the interacting systems
5
Star Formation as a function of progenitor mass
5x1010 M?
1013 M?
1012 M?
(MW)
Differences as a function of mass

• absolute SFR increases
• burst at first passage (sm, med, lg)
• duration of final burst (long-gtshort)
• burst ratio, first / second (small-gt large)
• burst SFR /quiescent SFR (4,10,4)

• gas consumption (70 -gt 95)
• BH feedback becomes more significant for
• large mass (sharp drop after the merger, for
  larger systems)

6
Mass-dependent SF Gas consumption timescale
Robertson et al. (2006)
Small galaxies only have a short consumption
timescale during the height (or maximum)
interactions
Large galaxies ALWAYS have a short consumption
timescale
our SF-law, SFR ?
?3/2 includes an threshold density for star to
occur
7
Mass-dependent SF The cooling of hot gas
Galaxy collisions produce hot gas (Tvir) through
shocks and feedback. Because the cooling
efficiency (for Z0 gas) is much higher in small
mass halo, this gas provides a continued fuel
source for SF.
Contours show the X-ray emission from from hot
gas.
8
The size of the merger remnants the effects of
differential dissipation
Robertson et al. (2006)
The large galaxies have very little shrinkage
for various gas fractions, I.e., their sizes are
all within 50 of each other.
Small galaxies have a large change in radius
(gtfactor of 2) depending on the gas fraction.
9
Remnant Kinematics
Not much change in the remnant kinematics as a
function of size, except for one orientation that
was strongly altered in the low-mass systems.

• small mass remnants also tend to have disky
  isophotal shapes
• there does not appear to be any trends in shape
  or velocity anisotropy
• all remnants (that weve looked at) show tidal
  features such as shells, faint tidal features and
  kinematic subsystems
• were currently looking at the profiles

10
Remnant Metallicity

• all remnants are nearly solar
• slight trend for more massive mergers to
  produce more metal rich remnants
• however, with the varying SF histories, we
  might expect varying alpha/Fe
• strong metallicity gradients

11
Tidal Material Dwarfs?
Low Resolution
High Resolution
12
Tidal Material Dwarfs?
Low Resolution
High Resolution
13
Tidal Material Dwarfs?
Low Resolution
High Resolution
14
Tidal Dwarfs Composition, Example 1
Gas
Dark Matter
Old Stars
New Stars
15
Tidal Dwarfs Mass Function

• primary is several 1010 M?
• individual particle masses are 105 M? (high),
  and 106 M? (low)
• simply FOF, so there is no check for bound
  structures (TDGs should be)

16
Tidal Dwarfs Stellar Ages

• Burst at first passage is near T1.0 Gyr
• Merger and largest burst are at T2.5 Gyr
• large spread in ages, with a concentration near
  the first passage and also after the final merger

17
Tidal Dwarfs Where were they before?

• tidal features from extended gas distribution,
  and hence their production depends sensitively on
  the disk-orbit orientation (Duc et al., Wetzstein
  et al.)

• Questions
• what determines the mass function?
• Kinematics (Bournaud et al.)
• Do zoom-in simulation
• Feedback
• What is their long-term evolution? Will they
  survive a subsequent merger event?

18
Results (In case I forget - or for further
discussion )
Mergers/Interactions produce bursts of star
formation.
DWARFS TOO!
Smaller mass (dwarf) major mergers produce

• starbursts that are prolonged, compared to
  larger-mass interactions.
• bursts only at the final passage, and moderate
  compared to the quiescent star formation .
• significantly less feedback from accreting BHs
  than larger systems.
• (in the absence of including a prescription
  for starburst-driven winds - were looking into
  this)
• remnants that experience more dissipation, hence
  theyre relatively smaller and denser.
• remnants that, in general, slowly rotate, are
  disky, and lower metallicity.

Smaller mass (dwarf) galaxies and minor mergers

• only a small range of mass ratios produce
  starbursts
• the perturbing member of a minor interaction
  experiences a starburst

Large-mass gas-rich major mergers produce tidal
dwarfs.

• They form from the extended gas distribution.
• They consist entirely of gas and young stars,
  there are no old stars or dark matter.
• Their mass is 106-9 M? , and there is a small
  range of stellar ages.

19
Whats Next To do list

• Run more small mass major mergers.
• What do the starbursting dwarfs and merger
  remnants look like? Anything like the real
  things? We have the ability to make this
  comparison in the UV/optical, X-ray, and
  molecular CO (and soon in the infrared).
• In order to realistically compare to the dwarf
  starbursts well need to add starburst-driven
  winds (this is currently underway for the more
  massive galaxy mergers).
• Add more advanced methods to track the metal
  enrichment.
• Should we worry about the star formation and
  feedback recipes currently used when the particle
  masses become 102-3 M??
• If so, we could use the current simulations as
  boundary conditions for more detailed zoom-in
  simulations.

20
Fancy Movie - Optical Image
21
Remnant Kinematics
Not much change in the remnant kinematics as a
function of size, except for one orientation that
was strongly altered in the low-mass systems.
22
Merger Simulations

• GADGET2 (Springel 2005, Springel Hernquist
  version of SPH)
• SFSpringel-Hernquist (2003) multiphase-feedback
  model, SF set to match Kennicutt (1998),
  including a black hole (although, this doesnt
  significantly affect the results shown here)
• 200,000 particles, resolution 100 pc
• MW-like galaxy models, x gas fraction, no bulge
• Parabolic orbits
• 15 different galaxy orientations (7 idealized
  pro-pro, retro-pro, polar, etc. 8 sample orbits
  to sample unit sphere)

Nearly all major merger simulations performed
to-date has involved Spiral galaxies with masses
equal to the Milky Way
23
Starbursts in Dwarfs triggered by Interactions
T. J. Cox What causes dwarf starbursts?
Feedback and hyrdodynamics of minor mergers
and tidal dwarfs
Lets Drink Beer!
24
M/L Profiles increased dissipation in low-mass
remnants
Total Mass / Stellar Mass M/L
25
Star Formation

• over 3 orders of magnitude initial SFR reflects
  range of mass
• remnants have nearly equivalent SFR
• smallest mass systems has the most prolonged
  burst

26
Star Formation Normalized

• star formation efficiency scales with mass
• smaller mass mergers produce less hot gas, lower
  cooling time