Large Scale Structure with the Lyman-α Forest

Large Scale Structure with the Lyman-α Forest Your Name and Collaborators Lecture 1 - The Lyman-α Forest Andreu Font-Ribera - University College London Graphic: Anze Slozar 1

Large scale structure The distribution of matter in the Universe tells us about: Accelerated expansion of the Universe / dark energy Tests of general relativity on cosmological scales Initial conditions of the Universe / inflation Particle physical properties of dark matter Mass and number of neutrino species However, most of the matter in the Universe is in the form of dark matter and we need indirect tracers to study it Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 2

Redshift Surveys Underdensity Overdensity BOSS Lyα forest 160k spectra 2.0 < z < 3.5 BOSS galaxies 1.3M spectra 0.2 < z < 0.7 Look back time (billion years) Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 3

Motivation I: expanding universe at high-z Acceleration Planck prediction Expansion rate Deceleration Redshift Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 4

Motivation II: small scale clustering Large scales Small scales Late time Early time CMB CMB Lensing Photometric Galaxies Spectroscopic Galaxies Lyman-α Forest 21cm? Weak Lensing 0 1 2 Redshift 5 1100 Lyman-α forest offers a unique window to study small scale clustering Combined with CMB, it allows us to study: dark matter properties neutrino mass shape of primordial P(k) 1000 100 Mpc 10 1 Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 5

Motivation III: 3D tomography We can map the cosmic web at high redshift (z > 2) CLAMATO (Lee et al. 2016): high-density survey over tiny area Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 6

Outline Lecture I Introduction Physics of the Lyman-α forest Lyman-α forest as a biased tracer Lecture 2 Large spectroscopic surveys (BOSS / DESI) Measuring Lyman-α BAO Lecture 3 Other Lyman-α data sets Small scale clustering Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 7

Outline Introduction Physics of the Lyman-α forest Lyman-α forest as a biased tracer Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 8

Introducing the Lyman-α forest z=2.95 z=3.56 Figure from William C. Keel Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 9

Introducing the Lyman-α forest Lya Simulation by Andrew Pontzen https://www.youtube.com/watch?v=6bn7ka0tjjw Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 10

Introducing the Lyman-α forest CLAMATO (Lee et al. 2016): high-density survey over tiny area, allows 3D view of the cosmic web https://www.youtube.com/watch?v=tvhlgdxylqk Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 11

Outline Introduction Physics of the Lyman-α forest Lyman-α forest as a biased tracer Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 12

Physics of the Lyman-α forest Figure by Casey Stark (LBNL) Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 13

Physics of the Lyman-α forest Let us ignore Helium today Rauch 1998, Meiksin 2009, McQuinn 2015 Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 14

Physics of the Lyman-α forest Let us simplify the model to have a better intuition We can describe the gas/baryons as a fluctuating field around its mean density Temperature - density relation (good approximation at low densities) equation of state of gas McDonald 2003 Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 15

Physics of the Lyman-α forest Peculiar velocities of the gas change absorption features Coherent velocities change absorption position Random velocities caused by gas temperature broaden lines McDonald 2003 Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 16

Physics of the Lyman-α forest We can simulate it very well using hydrodynamic simulations Figures from Casey Stark (UC Berkeley) Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 17

Outline Introduction Physics of the Lyman-α forest Lyman-α forest as a biased tracer Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 18

Lyman-α forest as a biased tracer What do we mean by bias? It can get confusing Consider an observable τ tracing density fluctuations If you like configuration space If you prefer Fourier space Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 19

Threshold bias (Kaiser 1984) Kaiser 1984 Gaussian density fluctuations Galaxies populate high densities Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 20

Threshold bias Kaiser 1984 Joint probability at two points Conditional probability given galaxy at point 2 Modified threshold Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 21

Threshold bias Kaiser 1984 Galaxy correlation function - Joint probability of having two galaxies On large scales (small correlations) Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 24

Threshold bias Kaiser 1984 Play the same game for voids Can also get void bias Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 26

Threshold bias Kaiser 1984 Play the same game for voids Can also get void bias Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 27

Taylor expansion bias McDonald 2006 Taylor expansion on powers of density fluctuations Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 28

Taylor expansion bias McDonald 2006 EXAM: What is the bias of the following tracers? Lyman-α forest has negative bias Remember: Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 29

Taylor expansion bias McDonald 2006 Is this the right answer? Mean Fluctuations Higher order bias Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 30

Taylor expansion bias McDonald 2006 Now expand to third order Cross-correlate with density at large separation Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 31

Redshift space distortions Padmanabhan et al. (2012) Hamilton (1997) Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 32

Redshift space distortions Linear redshift space distortions (RSD) in galaxy clustering The volume is deformed in redshift space Density of galaxies is modified by velocity gradient In Fourier space, and assuming linear perturbation theory Kaiser 1987, Hamilton 1997 Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 33

Redshift space distortions Same applies for linear RSD in optical depth How about the Lyman-α forest? And in Fourier space McDonald 2003, Seljak 2012 Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 34

Summary Lyman-α forest opens new window Lyman-α forest as a negative biased tracer RSD are non-trivial in the forest Biasing can be confusing (but also fun) Andreu Font-Ribera - Large Scale Structure with the Lyman-α Forest - Lecture I Punta Mita - Dec 12th 2017 35