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Galaxy Formation, Reionization, the First Stars and Quasars

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Galaxy Formation, Reionization, the First Stars and Quasars Ay 127 Galaxy Formation, Reionization, the First Stars and Quasars Coral Wheeler Galaxy Formation • The early stages of galaxy evolution — no clear-cut boundary — has two principal aspects: assembly of the mass, and conversion of gas into stars ! • Related to large-scale (hierarchical) structure formation, but very messy process, much more complicated than LSS formation and growth ! • Probably closely related to the formation of the massive central black holes as well ! • Star formation peaks in massive galaxies at z ~ 3, lower mass peak later. Cosmic SFR peaks near z ~2 ! • Observations have found thousands of galaxy candidates at z > 6, with spectroscopically confirmed galaxies out to nearly z = 9 ! • The frontier is now at z ~ 6 - 10, the so-called Era of Reionization (EoR) A General Outline • The smallest scale density fluctuations keep collapsing, with baryons falling into the potential wells dominated by the dark matter, achieving high densities through cooling – This process starts right after the recombination at z ~ 1100 ! • Once the gas densities are high enough, star formation ignites – This probably happens around z ~ 20 – By z ~ 6, UV radiation from young galaxies reionizes the universe ! • These protogalactic fragments keep merging, forming larger objects in a hierarchical fashion ever since then ! • Star formation enriches the gas, and some of it is expelled in the intergalactic medium, while more gas keeps falling in – SNe feedback can dramatically affect dwarf galaxy morphology and star formation ! • If a central massive black hole forms, the energy release from accretion can also create a considerable feedback on the young host galaxy An Outline of the Early Cosmic History (illustration from Avi Loeb) ! ! ! ! Recombination: Dark Ages: Reionization Era: Galaxy Release of the CMBR Collapse of Density The Cosmic evolution Fluctuations Renaissance begins Toy model of galaxy formation 1. Primordial abundance gas (hydrogen + helium) is dragged from into potential wells set by dark matter 2. The gas shocks and is heated to the halo virial temperature. The shock expands and fills the dark matter halo with hot gas that roughly traces the dark matter potential. 3. The hot gas is supported by pressure in quasi-static equilibrium, and cools radiatively. The cold gas falls in to form a disk. Classical Galaxy Formation Picture If the gas cools and contracts without angular momentum loss to form a thin, angular momentum supported exponential disc, the disc scale radius is given by Mo, Mao & White (1998): Rhalo = min(Rcool, Rvir) Simulated CDM haloes typically have The function f (∼1) in equation (2) contains information on the halo profile shape and contraction from baryonic infall, and depends on the initial halo concentration c ≡ Rhalo/Rs and the disc mass, md, in units of the total mass within Rhalo. Physical Processes of Galaxy Formation Galaxy formation is actually a much messier problem than structure formation. Must also include: ! • The evolution of the resulting stellar population • Feedback processes generated by the ejection of mass and energy from evolving stars • Production and mixing of heavy elements (chemical evolution) • Effects of dust obscuration • Formation of black holes at galaxy centers and effects of AGN emission, jets, etc. • … etc., etc., etc. • Also, may not be the same for all galaxy masses! Hydrostatic Force Balance For a gas in hydrostatic equilibrium, the gas profile, ρg(r), will be set by a competition between the isothermal gas pressure, , and the gravitational potential. If we assume that the gravitational force is dominated by the NFW halo potential, the hydrostatic force balance equation yields a pressure-support radius Pressure support radius Angular momentum support radius Kaufmann, Wheeler & Bullock 2007 Oñorbe et al. 2015 The Evolving Dark Halo Mass Function (Press-Schechter) Perturbations on all scales imprinted on the universe by quantum fluctuations during an inflationary era. Later, as radiation redshifts away, become mass perturbations, grow linearly. From small to large mass scales, perturbations collapse to z = 0 form galaxies/clusters 5 10 20 z = 30 Barkana & Loeb The Evolving Dark Halo Mass Function (Press-Schechter) Perturbations on all scales imprinted on the universe by quantum fluctuations during Cosmological (CDM) an inflationary era. Later, as radiation redshifts away, become mass perturbations, grow linearly. From small to large mass scales, perturbations collapse to z = 0 form galaxies/clusters 5 10 20 z = 30 Barkana & Loeb Read & Trentham 2005 CDM simulations Galaxy luminosity fct Read & Trentham 2005 CDM simulations Galaxy luminosity fct Read & Trentham 2005 CDM simulations Galaxy luminosity fct Cooling & AGN Feedback (QSOs) Read & Trentham 2005 CDM simulations Galaxy luminosity fct SNe Feedback Cooling & AGN Feedback (QSOs) LCDM predicts 1000s of subhalos around the Milky Way Stadel et al. 2009 Currently ~30 known MW satellites Drlica-Wagner et al. 2015 Koposov 2015, Leavens 2015, Martin 2015, Kim 2015, Kim & Jerjen 2015b UFDs have ancient stellar pops 30 6 3 2 Redshift 0.5 0.15 Hercules 1 Leo IV Hercules CVn II UMa I Hercules Cumulative SFH Leo IV Boo I Brown et al. 2014 CVn II Com Ber (STMAG) UMa I Magnitude 814 Bootes I m Epoch of reionization Coma Beren Fraction of stars formed Fraction of stars -0.4 -0.3 14 13 12 11 10 9 8 7 6 5 4 3 02 m606-m814 (STMAG) Age (Gyr) Stopped forming stars ~10 Gyr ago Simulated UFDs have ancient stellar pops 5 3 2 Redshift 0.5 0.25 UFD Simulated UFDs 106 ) sun 105 Simulated (M * Classical Dwarfs M Centrals 104 Cumulative SFH Satellites 103 109 1010 12 10 8 6 4 2 Mhalo (Msun) Age (Gyr) Wheeler et al. 2015 Both satellites & centrals Simulated UFDs have ancient stellar pops Infall times 5 3 2 Redshift 0.5 0.25 UFD Simulated UFDs 106 ) sun 105 Simulated (M * Classical Dwarfs M Centrals 104 Cumulative SFH Satellites 103 109 1010 12 10 8 6 4 2 Mhalo (Msun) Age (Gyr) Wheeler et al. 2015 Quenching unrelated to infall, likely reionization SimulatedMocking UFD continues the to Universe form stars when no reionizing background present 1.0 105 ] 0.75 0.5 104 0.25 Cumulative Stellar Mass [M 3 Cumulative Star Formation History Normal Reionization 10 Normal Reionization No Reionization No Reionization 0 1 2 3 4 0 1 2 3 4 Time [Gyr] Time [Gyr] Elbert et al. in prep Garrison-Kimmel et al. 2014 Lyman Alpha Forest Gunn & Peterson, 1965 QSO Observations Suggest the End of the Reionization at z ~ 6 Songaila (2004) Evolution of transmitted flux from observations of 50 high signal-to-noise quasars (2 < z < 6.3) Strong evolution of Lyα absorption at z > 5 • Transmitted flux quickly approaches zero at z > 5.5 • Complete absorption troughs begin to appear at z > 6 • Gunn-Peterson optical depths >> 1, indicating a rapid increase in the neutral fraction QSO Observations Suggest the End of the Reionization at z ~ 6 Ionized Neutral Songaila (2004) Evolution of transmitted flux from observations of 50 high signal-to-noise quasars (2 < z < 6.3) Strong evolution of Lyα absorption at z > 5 • Transmitted flux quickly approaches zero at z > 5.5 • Complete absorption troughs begin to appear at z > 6 • Gunn-Peterson optical depths >> 1, indicating a rapid increase in the neutral fraction Reionization Tricky process to determine! Depends on a number of poorly understood parameters: ! • Abundance and spectral shapes of early galaxies • Fraction of ionizing photons produced that are able to escape galactic environment contribute to reionization, fesc • IGM gas density distribution ! fesc in local Universe extremely low, but may increase at higher redshift High resolution simulations show fesc = 0.01 - 0.20, with possible boosts if one considers older stellar populations and binaries Substantial Diversity of IGM Absorption Seen IGM transmission over same redshift window along 4 different QSO lines of sight A Considerable Cosmic Variance Exists in the transmission of the Lyα forest at z > 5 Also possibly due to “patchy” reionization Cube = 200 million light-years Time = first billion years of the universe Blue = heated & ionized regions around galaxies Visualization credit: Marcelo Alvarez, Ralf Kaehler (Stanford), Tom Abel When did reionization start? Use the CMB: • Light from the CMB is Thompson scattered by free electrons, polarizing the signal • Estimate the column density via this polarization • Planck measured the scattering optical depth, found best “instantaneous reionization” model at z ~ 8 CMB Constraints on Reionization Planck Collaboration 2016: τ = 0.058 ± 0.012 for instantaneous reionization model, z = 8.2 ± 1.1 favored Upper limit to the width of reionization period: ∆z < 2.8 Data points from QSOs, Ly-alpha emitters and Ly- alpha absorbers Looking Even Deeper: The 21cm Line We can in principle image HI condensations in the still neutral, pre-reionization universe using the 21cm line. Several experiments are now being constructed or planned to do this, e.g., the Mileura Wide-Field Array in Australia, or the Square Kilometer Array (SKA) (Simulations of z = 8.5 H I, from P. Madau) Reionization Simulations Pawlik et al. 2016 fesc ~ 0.6 fesc ~ 0.3 Escape fraction and SNe feedback efficiency tuned to match cosmic SFH Simulation with the most realistic escape fraction matches observations best Reionization in Simulations Sims using common models for reionization backgrounds did not reproduce heating claimed by those same models!!! This has important considerations for many aspects of galaxy formation, such as the minimum halo mass that can form a galaxy Onorbe et al. 2016 What Reionized the Universe? • First stars in minihalos? - initially thought to start reionization, now likely form too early • First galaxies? - How low mass are needed? What happens to the L-fct at high redshift? What happens to those galaxies today? • Active Galactic Nuclei / QSOs? - Are there enough at high z? Minihalos First stars expected to form in dark matter (DM) minihalos of typical mass ∼ 106 M⊙ at redshifts z ∼ 20.
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