ben moore, institute for theoretical physics, university of zurich +oscar agertz, roman teyssier+...
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Ben Moore, Institute for Theoretical Physics, University of Zurich
+Oscar Agertz, Roman Teyssier+
Galaxy formation: is the end in sight?
Obergurgl, December 2009
GHALO: A billion particle simulation of the dark matter distribution surrounding a galaxy. 3 million cpu hours with the parallel gravity code pkdgrav (Stadel et al 2008)
50 parsec, 1000Mo resolution, 100,000 substructures
GHALO: A billion particle simulation of the dark matter distribution surrounding a galaxy. 3 million cpu hours with the parallel gravity code pkdgrav (Stadel et al 2008)
50 parsec, 1000Mo resolution, 100,000 substructures
What is the origin of morphology?
There is a large diversity in the galaxy population. Most stars are in disk galaxies, most galaxies are dSph/dE.
What about S0, E, dIrr, LSB, Polar Rings, Bars……
Many of the observed galaxies start off as disks and undergo transformation between classes
Malin1 Malin2
200 kpc
Moore & Parker 2007
Malin1 has hardly had time to rotate once! How can stars form ~100kpc from the center?
The Cartwheel Ring The Cartwheel Ring GalaxyGalaxy
200kpc
T=100Myr=CartwheelT=100Myr=Cartwheel
T=1Gyr=Malin1T=1Gyr=Malin1
Mapelli, Moore et al 2008
MNRAS, 383, 1223
Very low surface brightness galaxies could be the evolved state of ring galaxies
Here are some of the key baryonic components of galaxies that we need to simulate correctly
Evrard, Summers & Davis 1994
A few hundred SPH particles per ‘glob’
Steinmetz & Navarro 1998
1000-10000 SPH particles
Abadi et al 2002Most of stars in a massive spheroid50,000 SPH particlesBaryons far too concentrated
Governato et al 200610^5 SPH particles in total, 500pc resolutionBetter match with T-F. Huge spheroid, disk is unresolved single phase cold gas
No thin disk dominated system, no morphological detail, no spiral patterns, bulge/spheroid dominated systems.
What is resolved and what is put in by hand?
The dark matter distribution is resolved to about 0.5% of the virial radius with 1 million particles – that’s about ~1kpc for the MW.
The shock heating of baryons and cooling processes are well resolved.
Multiphase flows are not followed correctly with SPH (Agertz et al 2008) – almost all galaxy formation studies have used SPH.
SPH
AMR
SPH
GRID
T=0.2 T=0.5 T=0.8
Hydro/star formation simulations have reached 10^5Mo per gas particle and gravitational force resolution of 0.5kpc, little else is resolved, not even the disk scale height and certainly not the 10pc molecular disk.
SPH disks are smooth featureless blobs of single phase gas – no molecular clouds, no spiral patterns – but lets see the next talk?!
Star particles put in places where the gas density is high (sets a maximum radius) – but stars form in molecular clouds and the molecular cloud mass function depends on gravitational instability which varies with radius.
Simulating star formation in disk galaxies:the present status - 2009
• The ISM is under-resolved
• To radially resolve a galactic disk such as the Milky Way can be done using ∆x∼0.5 kpc. At this resolution the whole scale height is within one resolution element. The true disk stability is not captured as both the density and velocity structure (gas and stellar dispersion) is numerically affected. This still allows for the disk to have the correct global characteristics such as gas and stellar mass compositions, thin and thick disk, and at high resolution, realistic spiral structure. In this case a statistical star formation recipe based on the local gas density and free fall time is well motivated both theoretically and observationally
• Satellites remain unresolved (the gas doesn’t reach the density threshold)
This is possible but the relevant variables (star formation threshold and efficiency) must be understood robustly!
Simulating star formation in disk galaxies:the future - 2015
• The ISM is fully resolved
• This means that the scale height of all ISM components are resolved using at least 10 resolution elements (Romeo 1994). This translates into at least ∆x∼1 pc. If this is not satisfied the true disk stability will not be modelled accurately. At such a resolution, star formation occurs in their natural sites i.e. massive clouds such as GMCs. This treatment is the goal of most simulations but is due to their computational load beyond the capabilities of modern simulations attempting to study the assembly and evolution of large spiral galaxies to z = 0. In addition, as the star formation sites become resolved the codes need to incorporate the radiative feedback in order to accurately treat the life-times of the GMC structures (Murray et al. 2009). Pandora’s box (new small scale physics must be treated)!
Computationally impossible in a cosmological context today
Agertz et al 2008:
AMR simulations of isolated disks at high resolution can resolve the formation and evolution of molecular clouds.
Gravitational instability and MC interactions sets a minimum floor to the disk velocity dispersion.
A 1 kpc region of the disk
Agertz et al ( 2008)
“The observed dispersion velocity is generated through molecular cloud formation and subsequent gravitational encounters”
What is the state of the art in cosmological simulations of galaxy formation?
We can resolve the ISM to ~50-200pc, enough to follow giant molecular cloud complexes.
AMR simulations – Agertz, Teyssier & Moore
Feedback
• SNII: 10^51 ergs dumped thermally after ~30 Myr. These are the M>8 Msun stars. Given a typical IMF, this occurs for 10 % of the mass of a given population and 10 % of that mass is returned as enriched material. We keep track of the young stars that form and turn off cooling in these regions allowing for the hot pockets of gas to develop into a blastwave. If this is not done the under-resolved medium unphysically radiates away that energy (McKee & Ostriker, Thacker & Couchman)
• SNI: As a star of mass 1<M<8 Msun exits the main sequence we calculate the fraction of them being in a binary system and hence is eligible for a type I event. We enriched the ISM with the outcoming metals and Sn energy.
• Winds: A population of stars loose 30-40% of its mass in winds. We return this mass to the gas component at the end of a stars life.
SN Feedback Thermal Dump z=10
SN Feedback Thermal Dump and Delayed Cooling z=10
The complex gas flows into a dark matter halo with a forming disk galaxy at a redshift z=3. R=temperature, G=metals and B=density. (Agertz, Teyssier & Moore 2009). One can clearly distinguish the cold pristine gas streams in blue connecting directly onto the edge of the disk, the shock heated gas in red surrounding the disk and metal rich gas in green being stripped from smaller galaxies interacting with the hot halo and cold streams of gas. The disk and the interacting satellites stand out since they are cold, dense and metal rich.
The complex gas flows into a dark matter halo forming a disk galaxy
R=temperature, G=metals and B=density
Agertz et al (2009)
Elmegreen et al 2009: Clumpy high redshift galaxies – chains, clusters etc.
Clumpy (10^8Mo), high star-formation rates, extended over ~10kpc radii
ACS images (Elmegreen et al)
NGC 4650A
No direct evidence for cold infalling gas…very hard to detect.
Maccio’, Moore, Stadel (2005) serendipitous detection of a polar ring galaxy in a cosmological hydro-simulation.Evidence for cold accretion on sub-L* scales.
We managed to make a nice looking disk at z=3, but what about the evolution to z=0?
Parameter study
• For realistic choices we always get nice disk galaxies.
• Due to regulation (balance of star formation and destructive feedback), a resolved disk is less sensitive to the actual choice of parameters.
• If the disk is under-resolved, the efficiency sets the bulge to disk ration. The threshold for star formation must be set low enough to avoid “missed star formation events” in the outer disk.
• Analytical derivations of required resolution given a star formation recipe will be provided!
• Resolution• Star formation threshold• Star formation efficiency• Inclusion of SN 1 events and wind
mass-loss
The disk, SR5n1e1MLGas densityGas density
StarsStars
Gas temperatureGas temperature
Gas metallicityGas metallicity
Gas densityGas density
StarsStars
Gas temperatureGas temperature
Gas metallicityGas metallicity
• Thin extended disk of stars and gas• Thick stellar disk and bulge• Hot pockets of gas from supernovae• Massive hot gaseous halo• Gravitational instability: we resolve spiral structure• Stars preferentially form in the arms• Galactic fountain• Realistic metallicity gradient
Regardless of parameter choice, all disks get roughly the same global properties. Disks with low B/D ratios are obtained when star formation is resolved at large disk radii and when the efficiency per free fall time is kept below 2%.Detailed properties are unconstrained.
The Kennicutt-Schmidt law
Bigiel (2009), fig. 15. Compilation using THINGS
data + other
Our AMR simulations – different colours are different
star formation efficiencies
Gas densityGas density
StarsStars
Gas temperatureGas temperature
Gas metallicityGas metallicity
The disk, SR6n01e1ML
Gas densityGas density
StarsStars
Gas temperatureGas temperature
Gas metallicityGas metallicity
No dominating bulge!Sb galaxy where the bulge forms from a
buckled bar.
The rotation curve of the simulated galaxy with low star formation efficiency matches well that of the Milky Way.
Compare SDSS stellar mass function with DM halo mass function
M⊙12105.2 MWM
Forero et al 2009
Gao et al 2009
Halo gas, SR5n1e1ML
TemperatureTemperature MetallicityMetallicity
• Complex large scale structure in temperature and metallicity• Satellite stripping a la Magellanic Stream
Observation
Simulation
A diffuse gaseous halo is robustly predicted with a density and temperature profile in good agreement with the Milky Way non-detection. It can explain the observed HVC head-tail features as well as a ram-pressure origin of the Magellanic Stream. Using the obtained values (n~10^-4 cm-3 and T~ few 10^6 K) we can explain e.g. Smith’s Cloud: The cloud of 10^6 Msun of HI gas is moving towards the disk of the Milky Way at 73 km/s. Smith's Cloud is expected to merge with the Milky Way in 27 million years at a point in the Perseus arm.
The interacting Magellanic Clouds – MW system
Our goal is to resolve molecular cloud formation within a cosmological context: i.e. <10 parsec resolution.
Aim is to study formation and evolution of galaxies across the Hubble sequence, in different environments and within the current cosmological context
Star-formation will still be sub-grid, but will be spatially, kinematically and physically more believable
This is just a few years away with planned 10-50 x code improvements.
Agertz et al (2008)
ISM kinematics using the RAMSES AMR code +/- star formation +/- SN feedback
GasGas
StarsStars
Lots of predictions, no evidence for CDM yet, no strong counter evidence. Complicated by galaxy formation
Direct or indirect detection of dark matter may happen in the next 5 years, but it may also never happen since the cross section may be so small
A new complex model of galaxy formation is emerging in which baryons accrete to the halo center in three ways – cold streams, cooling flow, metal enriched stripped debris.
At z=2 we can match the clumpy morphology of HST galaxies.
At z=0 we can make thin disks with small bulges and nice spiral patterns. The parameters have to be tuned since molecular cloud (star) formation is unresolved. The global properties are correct but details such as disk scale lengths are uncertain.
Is the end in sight? Yes, but still not very predictive.
By 2015 we will reach the 1 parsec resolution required to resolve the molecular disks and spatially resolved star formation.