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Home , Reionization, Structure formation

Ay 127 Formation, , the First and

Coral Wheeler Galaxy Formation • The early stages of galaxy evolution — no clear-cut boundary — has two principal aspects: assembly of the , and conversion of into stars ! • Related to large-scale (hierarchical) , 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 ! • formation peaks in massive 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 falling into the potential wells dominated by the dark , achieving high densities through cooling – This process starts right after the recombination at z ~ 1100 ! • Once the gas densities are high enough, ignites – This probably happens around z ~ 20 – By z ~ 6, UV from young galaxies reionizes the ! • 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 morphology and star formation ! • If a central massive forms, the release from accretion can also create a considerable feedback on the young host galaxy An Outline of the Early Cosmic History (illustration from )

! ! ! ! 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 ( + ) is dragged from into potential wells set by 2. The gas shocks and is heated to the halo virial . The shock expands and fills the 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 • 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 ! 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 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 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

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 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 stars of -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 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 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 , polarizing the signal

• Estimate the column density via this

• 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.

• Primordial density field randomly enhanced over the surrounding matter; perturbation amplified by until it decouples from the Hubble flow, turns around, and collapses into virial equilibrium.

• Typical gas in minihalos are below ∼ 104 K (efficient cooling due to atomic hydrogen).

• H2 required for SF (no metal line cooling): fH2 ∝ Tvir^(1.5)

• Deeper potential well —> larger the Tvir —> higher molecular abundance —> stronger cooling effect due to molecules. The First Stars Gas infall into the potential wells of the dark matter fluctuations leads to increased density, formation of H2, molecular line cooling, further condensation and cloud fragmentation, leading to the formation of the first stars

Krumholz, M. R., Klein, R. I., & McKee, C. F. 2007 Pop III Stars

Minimum mass at onset of collapse determined by Jeans mass: ! ! ! But this is only a rough estimate. Mass also set by feedback processes:

• Photodissociation of H2 in accreting gas reduces cooling rate

• Ly-alpha pressure can reverse infall in polar regions at 20-30 Msun

• Expansion of HII regions can reduce accretion rate at 50-100 Msun • Photoevaporation-driven mass loss from the disk stops accretion and fixes the mass of the star (60-300 Msun) • Possibly also must account for: B-fields, cosmic rays, or other ionizing sources in early Universe, etc. etc. … Difficult process to understand because Pop III stars different from contemporary stars (hotter, lack dust, weaker B-fields …) Primordial Star Formation: a Top-Heavy IMF? Expected in all modern models of Pop III star formation,

characteristic M ~ few x 10 M!

Bromm 2013 How to detect high redshift galaxy

Finkelstein 2016

Spectroscopy Galaxies at different z show Ly-alpha break at Spectroscopic redshifts different redshifts highly accurate, but blind surveys only reach out to Where they “drop-out” in z < 0.5 each photometric color band gives redshift The Lyman-Break Method Absorption by the interstellar and intergalactic hydrogen of the UV flux blueward of the Ly alpha line, and especially the Lyman limit, creates a continuum break which is easily detectable by multicolor imaging, confirmed later with spectroscopy Halo mass — Galaxy L-fct discrepancy in place at early times

Finkelstein 2015 QSO / / AGN "quasi-stellar radio sources," or "quasars,"

• QSOs are thought to be powered by supermassive black holes at the center of galaxies, accreting material (AGN) • z ∼ 2 − 3 has been often designated as the “quasar epoch”, i.e. the period where QSOs were most active • An accurate determination of high-z QSO LFs might enable us to set more stringent limits on the ionizing production by these sources during Reionization Properties of high-z Quasar Population • UV through X-ray spectra generally have properties indistinguishable across cosmic time — most distant quasars establish their main emission characteristics by the time Universe only 7% of current age • However, recent detection of high-z quasars with no emission from hot dust (likely associated with the accretion disk) seen in lower redshift quasar spectra may change this • Possible decrease in the radio-loud fraction at the highest redshifts could indicate a change in the relative accretion modes and spin for the first quasars • Most distant QSO (ULAS J112001.48+064124.3), at z = 7.085 has no radio continuum emission — consistent with very massive black hole (2 − 3 × 109 M⊙) with lower spin due to isotropic accretion The Nature of the BH “Seeds” Some plausible choices include: • Primordial, i.e., created by localized gravitational collapses in the early universe, e.g., during phase transitions – No evidence and no compelling reasons for them, but would be very interesting if they did exist; could have “any” mass… – See many good reviews by B. Carr

• Remnants of Pop. III massive stars: Mseed ~ 10 - 100 M◉ – Could be detectable as high-z GRBs • Gravitational collapse of dense star clusters, or runaway mergers of 2 4 stars: Mseed ~ 10 - 10 M◉ • Direct gravitational collapse of dense protogalactic cores: continues directly as a SMBH growth More than one of these processes may be operating … QSO Luminosity Fct Flattens at high z

Richards 2006 What Reionized the Universe?

Yan & Windhorst 2004 Bouwens et al. 2015 But is this the whole picture?

Boylan-Kolchin, Bullock & Garrison-Kimmel 2014

Faint galaxy counts required to sustain reionization over-predict the number of dwarf galaxy satellites around the MW at z = 0

** for “reasonable escape fractions” **