Two Phase Formation of Massive

Galaxies

T.Naab, P. Johansson, R. Cen, K. Nagamine, R. Joung and J.P.O.

PPPL:, 19 Dec 2012

ApJ.L.,658,710 (2007) ApJ.,697, 38 (2009)

ApJ.L.,699,L178 (2009)ApJ.,725,2312 (2010)

ApJ.,744,63(2012)

Focus: High Resolution Cosmological Zoom Simulation

of Massive Galaxies

But first, what have we learned from 50 years of observations?

(archeology)

• Most stars in giant ellipticals are old- 10->13by –with good evidence for late repeated bursts of star-formation totaling a few percent of stellar mass.

• Metallicity in central regions correlated with luminosity indicating that stars formed in situ from inflowing gas.

• Outer parts are very old and metal poor and heterogeneous possibly indicating accretion of low mass systems (which tend to be old and metal poor).

• Two systems of globular clusters – inner red, and outer bluer, again indicating two phase formation.

• Inner orbits isotropic, outer ones more radial.

But first, what have we learned from 50 years of observations?

(observational look back and local evidence)

• Giant elliptical galaxies form early and grow in size and mass without much late star-formation (van Dokkum et al).

• Major mergers are real but rare at late times (or else disk galaxies would have been destroyed) but real, and typical (mass weighted) merger ratio is 1:6.

• Dark matter does not dominate the inner parts of elliptical galaxies.

• Half of all metals are ejected from massive systems (cf winds and cluster metals) and seen in the surrounding gas.

• All massive galaxies are embedded in hot gas bubbles emitting thermal X-rays (ROSAT) at 1040 -> 1042 erg/s.

Now Consider the Standard Cosmological Model: LCDM

• Small, gaussian adiabatic perturbations of a homogeneous universe of gas, radiation and dark matter at a redshift of z = 1000. Fluctuation level is one part per 100,000.

• Overdense regions expand slower and underdense regions faster – ie gravitational growth of instabilities.

• Ultimately, massive lumps with density greater than the critical density re-collapse to form the seeds from which grow galaxies.

• Gas and dark matter falls into these potential wells, the gas turning to stars at some rate.

• “Feedback” - energy output from stars and black holes limits the fraction of gas turned into stars to ~ 10%.

• Ultimately dark energy takes over, the universe accelerates and gas infall and star formation cease.

Cosmological Simulation:Start with WMAP CBR Sky

NB, fluctuation level only 10-5 at high redshift

• tot = 1, [=1.010 +-0.016]

• cdm = 0.23 ± 0.01• baryon= 0.046 ± 0.002• lambda = 0.73 ± 0.02• n = 0.963 ±

0.015• H0 = 70.4 ±

1.3km/s/Mpc• 8 = 0.81 ± 0.02• scat =0.087 ± 0.001

(WMAP 7)

fast forward to structure growth computed in dark matter component ->

Second Step: hydrodynamic “Zoom Method” .

• Select region of interest.• Put down finer grid.• Add hydrodynamic equations.• Add atomic physics: adiabatic, + cooling,

+heating, + non-equilibrium ionization.• Radiative transfer: global average, +shielding

of sinks, +distribution of sources.• Heuristic treatment of star-formation.• Repeat calculation using tidal forces from

larger region and do details of smaller region.

Star Formaton Algorithm• Heuristic treatment of star-formation

– For gas that is dense, cooling and collapsing make stellar particle:

dM* = const x DMgas x dt/Max(Tcool,Tdyn).(const ~ 0.025)

– Label particle with position, mass, metallicity and epoch.– Give particle velocity of gas and follow dynamics as if it were

a dark matter particle.– Allow output of mass, energy and radiation from each particle

consistent with a star-cluster of same mass and age – via standard stellar evolution theory: supernova

What have we learned?

• The onset of massive galaxy formation is early and follows re-ionization at z = 6.

High sigma peaks rapidly form stars from merging streams to initiate formation of cores of most galaxies. Disks and massive envelopes are formed later.

Phase In situ star formation Accretion of stars

Epoch 6>z>2 3>z>0

Baryonic mass source Cold gas inflows Minor and major

mergers

Size of region < ~ 1kpc ~ 10 kpc

Stellar metallicity Super-solar Sub-solar

Energetics Dissipational Conservative

Overall Picture of Two-Phase Growth

What is the observational* evidence (M. Kriek; ‘09)

z ~ 2.5

*Chart color represents specific star formation rate: high rate = blue.

Convergence to low and to a flat rotation curve at high resolution:

Detailed Hydro Simulations (N,J,O&E : 2007, ApJ, 658,710)

In Situ Star Formation

Convergence to stellar system formed very early which quickly becomes “red and dead”.

Questions • 1) Convergence: how do results change with resolution

improvement; and why was high resolution needed?• 2) Why does gas temperature increase though cooling time

is short and no feedback was included?• 3) Why is there a dramatic evolution of size?• 4) Why is galaxy “red and dead” early but continues to

grow in luminosity?• ANSWER: TWO PHASE GROWTH WITH LARGE

GRAVITATIONAL HEATING IN THE SECOND PHASE.

Gas Properties

Gas, at all radii, becomes hotter with time despite fact that the “cooling time”< the Hubble time! Why?

Physics - why does gas not cool?

• Gas is steadily being heated by in-falling new gas ( -PdV and Tds).

• “Dynamical Friction” due to in-falling stellar lumps is very important for evolution of the stellar and DM components.

• Of course “feedback” from central black holes and from supernovae also exists and must be complementary to effects listed above (and this is now being added to the codes – thesis projects).

Minor Mergers Dominate the Accretion

Dark Matter Evolution - density declines in second phase

Binding Energy ~ 1060 erg from both in-situ and accreted stars - “gravitational heating”:- In-situ energy is radiated, - - Accreted energy heats gas and pushes out DM

First attempt at showing data from a set of

100^3 simulations(L.Oser, Naab…)

Where & when are stellar particles made?

Size evolution - substantial growth (observed and computed); what is the cause?

More Massive Systems are Older

Fit to observations is good

“Faber Jackson”

What have we learned? Old news.

• For massive systems the 1977 work of Binney, Silk and Rees & Ostriker appears to be correct :

Cooling time of gas becomes longer than the dynamical time and star formation ceases. Systems live in hot bubbles and then grow by accretion of smaller stellar systems.

Or about 1012 Msolar

Or about 75 kpcr

3) Why is there a dramatic evolution of size?4) Why is galaxy “red and dead” early but continues to grow in luminosity?

• Evolution of size is apparent, not real. Both components keep roughly constant in size, but mean size grows as accreted material dominates.

• During the second phase, the luminosity and stellar mass may double but very few stars are formed.

Simplest Physical Modeling - via Virial Theorem

• Make initial, stellar system dissipatively from cold gas with gr radius RI, Mass MI, velocity dispersion < VI

2> & energy EI :– EI = - 0.5 G MI

2 / RI = -0.5 MI < VI2>

• Add stellar systems conserving energy with total Mass MA = MI, velocity dispersion < VA

2> = < VI2> & energy EA :

– EA = -0.5 MI < VI2>

• To make combined stellar system with grav radius RF, Mass MF = MI(1 + ), velocity dispersion < VF

2> & energy EF :– EF = - 0.5 G MF

2 / RF = -0.5 MI < VF2>(1+ )

• Then, equating total initial and final states– EF = EI + EA, gives for the ratios of the in-situto the ultimate state as follows:

– (< VF2>/ < VI

2>) = [ (1 + ) / ( 1 + ) ]

– (RF /RI ) = [ (1 + )2 / ( 1 + ) ]

– (F/ I) = [ (1 + )2 / ( 1 + )3 ]

“major mergers”

-> = 1formulae reduce to the classic result,

BUTIf minor then

-> << 1,

velocity dispersion declines, the surface density declining dramatically, as in the numerical simulations.

Primary cause of mass growth

• Early times and low mass galaxies:–Gas inflows.

• Late times and high mass galaxies:–Accretion of satellites.

[In neither period is it major mergers.

Conclusions: High Mass Systems• High resolution SPH simulations without feedback

produce normal, massive but small elliptical galaxies at early epochs from in-situ stars made from cold gas.

• Accreted smaller systems add, over long times, a lower metallicity stellar envelope of debris (obvious test exists).

• The physical basis for the cutoff of star-formation is gravitational energy release of in-falling matter acting through -PdV and +Tds energy input to the gas.

• This simple two phase process explains the decline in velocity dispersion and surface brightness at later times.

• Feedback from SN and AGN are real phenomena - but secondary and mainly important for clearing out gas at late times and reducing stellar mass as compared to the simulations.

Right and Wrong• Get right the gross properties – the what, when, where, how

of massive galaxy formation.• Get wrong the mass, galaxies are too massive in relation to

the dark matter halos in which they live• Get wrong the interaction with the environment, modelled

galaxies do not produce observed outflows.• Get wrong properties of lower mass galaxies, eg normal

spirals like our own Milky Way.• ----------------------------------------------• Expectation is that better treatment of feedback from

exploding stars and central black holes will help to solve all of these problems.

• An exciting time to be making these calculations, finding out how the building blocks of the universe are constructed.

To Be Done• Do many more cases at high resolution; and

repeat with different (eg AMR w R. Joung) codes.

• Check two population GC predictions (w C. Conroy).

• Look at X-ray properties.• Improve gas cooling and radiative transfer.☐• Repeat with SNI&II and AGN feedback.☐• Add recycled gas.☐

(and make more mpgs!)