Lecture #4 SURVEYS: THE MASS ASSEMBLY AND STAR FORMATION HISTORY 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 Hierarchical assembly by merging Increases mass catastrophically Gaz accretion Cold / Hot Fuels star formation Increases mass continuously along the cosmic web evolution 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 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 6
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
Cold gas accretion 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 Simulations At z~2 Dekel et al., 2009 8
Feedback Takes material out of a galaxy back to DM halo May quench star formation? AGN feedback ε f =0.05 (thermal coupling efficiency) ε r =0.1 (radiative efficiency) SNe feedback ψ: instantaneous SFR feedback efficiency V hot =485km/s and α hot =3.2 9
Example: combined effect of feedback and cooling on mass function 10
A lot of definitive theories and simulations White and Rees, 1978 White & Frenk, 1991 Hopkins et al., 2006
Dekel, 2013
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
The luminosity function From lecture #1
The reference at z~0.1: SDSS Blanton, 2001 10000 galaxies Blanton, 2003 150000 galaxies
Galaxy types vs. color
Evolution! Canada-France Redshift Survey back in 1995 600 z spec 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 1 mag 2.5 mag Cucciati et al. 2012
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. From HST Hubble Deep Field Let s call it the et al. diagram
SFRD from the UV SFRD from the IR Direct observation of UV photons produced by young stars But absorbed by dust: need to estimate dust absorption UV photons produced by young stars are warming-up dust Dust properties: calibration of UV photons to IR flux
Comparing Luminosity density from UV and IR Same shape: transformation is extinction E(B-V)
Star formation rate evolution: today SFRD rise to z~2, then flat, then decreases Considerable uncertainties at z>3 Cucciati et al., 2012
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 noevolution 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
Specific star formation rate evolution: SFR/M* Tasca et al. 2014
Galaxy mass assembly: Cold gas accretion or merging? Cold gas accretion: The main mode of gas/mass assembly? «This streamdriven 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
Measuring the evolution of the galaxy merger rate 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
Merging rate from pair fraction Merging rate Pair count Number density Merger probability in T mg Merging Timescale T mg depends on separation r p and stellar mass Kitzbichler & White 2008 computed timescales ~x2 larger than previously assumed ~1Gy vs. 500My 38
z=0.35 Spectroscopy enables to identify real pairs Both galaxies have a spectroscopic redshift No contamination issue z=0.93 z=0.63 39
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 Mergers at z~1.5 10kpc Lopez-SanJuan, 2013 40
What about merging at early epochs? Merging pairs at higher z from VUDS Merging pair at z~2.96 Tasca et al, 2014 HST/ACS VIMOS spectra 41
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
Cold gas accretion? First evidence in 2013?
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
Circa 2002
Hopkins et al., 2008