STAR FORMATION HISTORIES FROM Z=0 TO Z=8 Peter Behroozi, Stanford - PowerPoint PPT Presentation

STAR FORMATION HISTORIES FROM Z=0 TO Z=8 Peter Behroozi, Stanford University Risa W echsler, Stanford University Charlie Conroy, Harvard / UCSC Santa Cruz Galaxy W orkshop 2011 Wednesday, August 10, 11

Major Questions When did galaxies form their stars, as a function of their mass today? ( Or their mass at an earlier redshift? ) LMC; Hubble Heritage Team Wednesday, August 10, 11

Major Questions What’s the solution to the discrepancy between the cosmic SFR and the cosmic SM density? 0 1 2 3 4 z Wilkins et al. 2008 Wednesday, August 10, 11

Approach Parametrize the evolution of the stellar mass function; Include nuisance parameters for all conceivable systematic e ff ects.* Constrain using ALL available data, currently including observations of the stellar mass function, specific star formation rates, and the cosmic star formation rate ( all from z=0 - 8 ) . *see Behroozi, Conroy, & W echsler 2010 Wednesday, August 10, 11

Approach In equations: ( ∆ t ) · SFR m d ( t now ) = ( new stars ) SM m d ( t now ) ( expected stellar mass ) � � MMP m p ,m d + SUBS m p ,m d · (1 − ICL ( m p , m d )) − ( number of contributing progenitors, corrected for ICL losses ) × SFH m p ,t ( t now )(1 − SL t ( t now )) ( stellar population of progenitors, corrected for stellar death ) Talk to me about details! Wednesday, August 10, 11

Approach Include parametrized models for all conceivable systematic e ff ects: Many have suggested that high - redshift galaxies have bursty/dusty star formation,* but no - one’s done a self - consistent check. Let’s try out a simple model! * Stark et al. 2009, K.S. Lee et al. 2009, Schaerer & Barros 2010 Wednesday, August 10, 11

SFR / SM Tension A λ ( z ) = Incompleteness/burstiness: 1 + exp( B − z ) 0.9 0.8 Completeness 0.7 0.6 0.5 0.4 1 2 3 4 5 z Wednesday, August 10, 11

SFR / SM Tension W e can immediately better match the cosmic SFR. 0.1 -3 ] -1 Mpc • yr Cosmic SFR [M O 0.01 Without including CSFR constraint Face Value Bursty/Dust Obscured Bursty/Dust Obscured w/o SSFR scaling 0.001 0 1 2 3 4 5 6 Redshift Wednesday, August 10, 11

SFR / SM Tension And we can constrain the incompleteness ( in this model ) : 1 0.9 0.8 Completeness 0.7 0.6 0.5 0.4 1 2 3 4 5 z Wednesday, August 10, 11

SFR / SM Tension Result: W e have an extremely powerful framework for testing models to resolve the cosmic SFR / SM tension. Burstiness / dust obscuration helps, but does not completely explain the cosmic SFR / SM tension at z>4. Wednesday, August 10, 11

Star Formation Rates W e’ve run our models to z=0, and found the same trends: strongly decreasing SFRs at fixed halo mass with redshift: Preliminary Wednesday, August 10, 11

Star Formation Histories But, if we ask the complementary question about galaxy star formation histories, there’s no single trend! Preliminary Wednesday, August 10, 11

Star Formation Histories The rate at which the stellar population grew in low - mass galaxies has always been increasing; whereas for high - mass galaxies, the rate peaked at an early redshift and then declined. 80 1 11.0 M O -1 13.0 M O -1 70 M h = 10 • h M h = 10 • h 60 -1 ] 0.8 -1 ] • yr • yr 50 SF History [M O SF History [M O 0.6 40 30 0.4 20 0.2 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Scale Factor Scale Factor Preliminary Wednesday, August 10, 11

Star Formation Histories Best - fit model: SFH ( a ) = Aa B exp[ C (1 − a )] Even better in detail: SFH ( a ) = Aa B exp[ C (1 − √ a )] 3 80 1 11.0 M O -1 13.0 M O -1 70 M h = 10 • h M h = 10 • h 60 -1 ] 0.8 -1 ] • yr • yr 50 SF History [M O SF History [M O 0.6 40 30 0.4 20 0.2 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Scale Factor Scale Factor V alid across a wide range of redshifts ( 0<z<4 at least ) . Wednesday, August 10, 11

Final Words Still preliminary work, but: Our approach combines constraints from the observed stellar mass function at all times, as well as the observed clustering of galaxies ( through the galaxy - halo connection ) , as well as the cosmic SFR and specific SFRs. Wednesday, August 10, 11

Final Words Still preliminary work, but: Our approach combines constraints from the observed stellar mass function at all times, as well as the observed clustering of galaxies ( through the galaxy - halo connection ) , as well as the cosmic SFR and specific SFRs. W e can also start to constrain models for resolving the discrepancy between cosmic SFRs and SMs. Wednesday, August 10, 11

Final Words Still preliminary work, but: Our approach combines constraints from the observed stellar mass function at all times, as well as the observed clustering of galaxies ( through the galaxy - halo connection ) , as well as the cosmic SFR and specific SFRs. W e can also start to constrain models for resolving the discrepancy between cosmic SFRs and SMs. W e’ve already obtained constraints on the functional form of both the star formation history and the star formation rate for galaxies. Wednesday, August 10, 11

Image Credits http://www.williamsclass.com/ http://spacefellowship.com/news/ EighthScienceW ork/ art15504/picture - of - the - day - the - ImagesEighth/ birth - of - stars.html SuperNovaReminant.jpg Wednesday, August 10, 11