SURVEYS: THE MASS ASSEMBLY AND STAR FORMATION HISTORY · SURVEYS: THE MASS ASSEMBLY AND STAR...
-
Upload
trannguyet -
Category
Documents
-
view
212 -
download
0
Transcript of SURVEYS: THE MASS ASSEMBLY AND STAR FORMATION HISTORY · SURVEYS: THE MASS ASSEMBLY AND STAR...
SURVEYS: THE MASS ASSEMBLY AND STAR FORMATION HISTORY
Lecture #4
Observational facts
Olivier Le Fèvre – ON Rio de Janeiro School 2014
Putting it all together
Clear survey strategies Instrumentation and observing procedures Selection function estimates
Let’s measure galaxy evolution !
Lecture plan
1. What are the main contenders to drive galaxy SFR and mass growth ?
2. The luminosity function and its evolution 3. The star formation history: luminosity
density and SFRD 4. The mass function and the stellar mass
density evolution 5. Mass assembly from merging 6. A scenario for galaxy evolution ?
What may drive galaxy evolution ?
A rich theory/simulation literature… Identify key physical processes When ? On which timescales ?
Beware: fashion of the day (e.g. from simulations) may fade quickly…
…Stick to facts !
Main physical processes driving evolution
Hierarchical assembly by merging Increases mass “catastrophically”
Gaz accretion Cold / Hot Fuels star formation Increases mass continuously along the cosmic web
Feedback: sends matter back to the IGM AGN (jets, …) Supernovae (explosion)
Star formation and stellar evolution Luminosity / color, lifetime Star formation quenching
Environnement, f(density) Quenching, Harassement, Stripping,…
5
Hierarchical merging
6
• The basics: hierarchical growth of structures
• Merging of DM halos • Galaxies in DM halos merge by
dynamical friction • Major mergers can produce
spheroids from disks • Merging increases star
formation (but maybe short lived)
• Increases mass (minor, major) • Merger Rate ∝ (1+z)m
Stellar mass growth from star formation and evolution of stellar populations
In-situ gas at halo collapse transforms into stars
Accreted gas along lifetime transforms into stars
Stars evolve (HR diagram) Luminosity evolution Color evolution
Stellar population synthesis models: (Bruzual&Charlot, Maraston,…)
7
Along the filaments of the cosmic web
Steady flow for some billion years can accumulate a lot of gas
Gas transforms into stars
Produces important mass growth
From Press-Schechter theory
8
Simulations
Dekel et al., 2009 At z~2
Cold gas accretion
Feedback Takes material out of a galaxy
back to DM halo
May quench star formation ?
AGN feedback
εf=0.05 (thermal coupling efficiency) εr=0.1 (radiative efficiency)
SNe feedback ψ: instantaneous SFR
feedback efficiency
Vhot=485km/s and αhot=3.2
9
A lot of “definitive” theories and simulations
Hopkins et al., 2006
White and Rees, 1978
White & Frenk, 1991
Cool simulations, but… need to measure galaxy evolution !
A short summary of previous lectures…
With deep galaxy surveys Imaging & Spectroscopy
In large volumes Minimize cosmic variance
For large numbers Statistical accuracy
Measure properties at different epochs to trace evolution
Use these measurements to derive a physical scenario
13
Main evolution indicators
Luminosity function, luminosity density Star formation rate density Stellar mass function Stellar mass density Merging Accretion …
Evolution ! Canada-France Redshift Survey back in 1995
600 zspec
First evidence of evolution over ~7 Gyr
M* brightens by ~1 magnitude
Global LF Lilly et al., 1995
Le Fèvre et al., 1995
1 mag
CFRS: LF evolution per type to z~1
The LF of red galaxies evolves very little since z~1 Red early-type galaxies are
already in place at z~1 Consistent with passive
evolution (no new star formation)
Strong evolution of the LF for blue star-forming galaxies Luminosity or number
evolution ?
Little evolution
Strong evolution
A jump to z~2-4: UV LF from LBG samples
Using the LBG samples of Steidel et al. ~700 galaxies with redshifts
Continued evolution in luminosity L*
Steeper faint end slope α
From Reddy et al., 2008
Probing the LF to z~4 with the magnitude-selected VVDS
Steep slope for z>1 Continuous evolution
in luminosity Evolution in density
before z~2
Cucciati et al. 2012
1 mag
2.5 mag
Downsizing
The most massive / luminous galaxies form first, followed by gradually lower mass galaxies
The most massive galaxies stop forming stars first, with lower mass galaxies becoming quiescent later
This is ‘anti-hierarchical’ !
SFR(z) vs. Halo mass
De Lucia et al., 2006
Quenching
Star formation is stopped
But what produces quenching ? Merging Mass-related (feedback ?) Environment
Peng et al., 2010
The Star Formation Rate Evolution: the ‘Madau diagram’ back in 1996
Putting together several measurement: the strong evolution in
luminosity density observed by the CFRS from z~0 to z~1
Lower limits on SFRD from LBG samples at z~3
Lower limits on SFRD from HST LBG samples 2.7<z<4
A peak in SFRD at z~1-2 ?
From CFRS
From Steidel et al.
Let’s call it the “et al. diagram”…
From HST Hubble Deep Field
SFRD from the UV
Direct observation of UV photons produced by young stars
But absorbed by dust: need to estimate dust absorption
SFRD from the IR
UV photons produced by young stars are warming-up dust
Dust properties: calibration of UV photons to IR flux
Star formation rate evolution: today
Cucciati et al., 2012 • SFRD rise to z~2, then flat, then decreases • Considerable uncertainties at z>3
Stellar mass function evolution
Get stellar mass of galaxies from SED fitting Uncertainties ~x2 (Initial
Mass Function, Star formation history, number of photometric points on the SED, …)
Compute the number of galaxies at a given mass per unit volume
Stellar mass function evolution
Use double Schechter function Because of the different
shape of the MF for different galaxy types (next slide)
Massive galaxies are in place at z~1.5
Strong evolution of the low-mass slope
Evolution in number density
Redshift
MF evolution per type Star-forming galaxies Strong evolution in M* Strong evolution of α
Quiescent galaxies Strong evolution in M*
to z~1.5, then no-evolution
Strong evolution in number density
Ilbert et al., 2013
The mass growth of galaxies: stellar mass density ρ* evolution
Integrate the MF Global and per type
Smooth increase of the global ρ*
z=1-3: the epoch of formation of quiescent/early-type galaxies Almost x100 from z~3 to z~1
Galaxy mass assembly: Cold gas accretion or merging ?
Cold gas accretion: The main mode of gas/mass assembly ? « This stream-driven scenario for the formation of disks and spheroids is an alternative to the merger picture » (Dekel et al., 2010)
Merging major merging ? minor merging ? Occasional but large mass increase
Over time mergers can accumulate a lot of mass
Need to measure the GMRH since the formation of galaxies Mergers more/less frequent in the past Integral mass accrued from mergers
36
?
Method 1, A priori: pairs of galaxies
Method 2, A posteriori: merger remnants, shapes
Both methods require a timescale Timescale for the pair to merge
(vs. mass and separation) Timescale for features visibility
(vs. redshift, type of feature…)
At high redshifts z>1: pairs Faint tails/wisps lost to (1+z)4
surface brightness dimming
37
Measuring the evolution of the galaxy merger rate
Merging rate from pair fraction
38
Merging rate Pair count Number density
Merger probability in Tmg
Merging Timescale
Tmg depends on separation rp and stellar mass Kitzbichler & White 2008 computed timescales ~x2 larger than previously assumed ~1Gy vs. 500My
39
z=0.35
z=0.63
z=0.93
Spectroscopy enables to identify real pairs
Both galaxies have a spectroscopic redshift No contamination issue
Mergers at z~1.5 from MASSIV survey
80 galaxies selected from VVDS
Observed with SINFONI: 3D velocity fields
Straightforward classification: 1/3 galaxies are mergers
10kpc
Mergers at z~1.5
40 Lopez-SanJuan, 2013
What about merging at early epochs ? Merging pairs at higher z from VUDS
41
Merging pair at z~2.96
HST/ACS VIMOS spectra
Tasca et al, 2014
Galaxy Merger Rate History since z~3 from spectroscopic pairs
Peak in major merger rate at z~1.5-2 ?
Integrate the merger rate: >40% of the mass in
galaxies has been assembled from merging with >1/10 mass ratio since z~1
Doubling of the mass since z~3
Merging is an important contributor to mass growth
42
Building a galaxy evolution scenario ?
Several key processes have been identified, Direct: mergers, stellar evolution Indirect: accretion, feedback, environment
Properties have been quantified over >12Gyr Observationnal references exist to confront models
Semi-analytical models Take the DM halo evolution Plug-in the physical description of processes Get simulated galaxy populations
Semi-successful… some lethal failures Over-production of low-mass/low-z and under-production of
high-mass/high-z galaxies Reproducing low-z LF/MF AND high-z LF/MF
More to be done ! 44