Galaxy Formation, Reionization, the First Stars and Quasars

Transcription

1 Ay 127 Galaxy Formation, Reionization, the First Stars and Quasars Coral Wheeler

2 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)

3 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

4 An Outline of the Early Cosmic History (illustration from Avi Loeb)! Recombination: Release of the CMBR! Dark Ages: Collapse of Density Fluctuations! Reionization Era: The Cosmic Renaissance! Galaxy evolution begins

5 Toy model of galaxy formation

6 1. Primordial abundance gas (hydrogen + helium) is dragged from into potential wells set by dark matter

7 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.

8 3. The hot gas is supported by pressure in quasi-static equilibrium, and cools radiatively. The cold gas falls in to form a disk.

9 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.

10 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!

11 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

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15 Oñorbe et al. 2015

16 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 form galaxies/clusters z = z = 0 Barkana & Loeb

17 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 form galaxies/clusters z = 30 Cosmological (CDM) z = Barkana & Loeb

18 Read & Trentham 2005 CDM simulations Galaxy luminosity fct

19 Read & Trentham 2005 CDM simulations Galaxy luminosity fct

20 Read & Trentham 2005 CDM simulations Galaxy luminosity fct Cooling & AGN Feedback (QSOs)

21 Read & Trentham 2005 SNe Feedback CDM simulations Galaxy luminosity fct Cooling & AGN Feedback (QSOs)

22 LCDM predicts 1000s of subhalos around the Milky Way Stadel et al. 2009

23 Currently ~30 known MW satellites Drlica-Wagner et al Koposov 2015, Leavens 2015, Martin 2015, Kim 2015, Kim & Jerjen 2015b

24 UFDs have ancient stellar pops Redshift Magnitude m814 (STMAG) Hercules Leo IV CVn II UMa I Bootes I Coma Beren Hercules Leo IV CVn II UMa I Boo I Com Ber Fraction of stars formed Hercules Cumulative SFH Brown et al Epoch of reionization m606-m814 (STMAG) Age (Gyr) Stopped forming stars ~10 Gyr ago

25 Simulated UFDs have ancient stellar pops Redshift UFD Simulated UFDs M* (Msun) Satellites Centrals Mhalo (Msun) Simulated Classical Dwarfs Cumulative SFH Age (Gyr) Wheeler et al Both satellites & centrals

26 Simulated UFDs have ancient stellar pops Infall times Redshift UFD Simulated UFDs M* (Msun) Satellites Centrals Mhalo (Msun) Simulated Classical Dwarfs Cumulative SFH Age (Gyr) Wheeler et al Quenching unrelated to infall, likely reionization

27 Simulated UFD continues form stars when no Mocking thetouniverse reionizing background present 105 Cumulative Stellar Mass [M ] Cumulative Star Formation History Normal Reionization No Reionization Time [Gyr] 1 2 Time [Gyr] Elbert et al. in prep Garrison-Kimmel et al Normal Reionization No Reionization 3 4

28 Lyman Alpha Forest Gunn & Peterson, 1965

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30 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

31 QSO Observations Suggest the End of the Reionization at z ~ 6 Ionized Songaila (2004) Neutral 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

32 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 = , with possible boosts if one considers older stellar populations and binaries

33 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

34 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

35 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

36 CMB Constraints on Reionization Planck Collaboration 2016: τ = ± 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 Lyalpha absorbers

37 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)

38 Reionization Simulations Pawlik et al 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

39 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

40 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?

41 Minihalos First stars expected to form in dark matter (DM) minihalos of typical mass 10 6 M at redshifts z 20. Primordial density field randomly enhanced over the surrounding matter; perturbation amplified by gravity until it decouples from the Hubble flow, turns around, and collapses into virial equilibrium. Typical gas temperatures in minihalos are below 10 4 K (efficient cooling due to atomic hydrogen). H 2 required for SF (no metal line cooling): f H2 T vir^(1.5) Deeper potential well > larger the T vir > higher molecular abundance > stronger cooling effect due to molecules.

42 The First Stars Gas infall into the potential wells of the dark matter fluctuations leads to increased density, formation of H 2, 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

43 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 Msun Expansion of HII regions can reduce accretion rate at Msun Photoevaporation-driven mass loss from the disk stops accretion and fixes the mass of the star ( 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 )

44 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

45 How to detect high redshift galaxy Finkelstein 2016 Spectroscopy Spectroscopic redshifts highly accurate, but blind surveys only reach out to z < 0.5 The Lyman-Break Method Galaxies at different z show Ly-alpha break at different redshifts Where they drop-out in each photometric color band gives redshift 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

46 Halo mass Galaxy L-fct discrepancy in place at early times Finkelstein 2015

47 QSO / Quasar / 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 photon production by these sources during Reionization

48 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 J ), at z = has no radio continuum emission consistent with very massive black hole ( M ) with lower spin due to isotropic accretion

49 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: M seed ~ M Could be detectable as high-z GRBs Gravitational collapse of dense star clusters, or runaway mergers of stars: M seed ~ M Direct gravitational collapse of dense protogalactic cores: continues directly as a SMBH growth More than one of these processes may be operating

50 QSO Luminosity Fct Flattens at high z Richards 2006

51 What Reionized the Universe? Yan & Windhorst 2004 Bouwens et al. 2015

52 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 **