The History of Light: How Stars Formed in Galaxies Kai Noeske European Space Agency/ Space Telescope Science Institute Hubble Science Briefing, 1 Mar 2012

What is a Galaxy? 2

The Milky Way 100 Billion Stars like our sun 3

The Milky Way 4

Meet the Neighbors. M51 (“Whirlpool Galaxy”) M104 (“Sombrero Galaxy”) M31 (“Andromeda Galaxy”), our close neighbor and similar to the Milky Way 5

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Stars are not evenly distributed in the universe. Stars are born and live in galaxies. Most galaxies have billions of stars. There are billions of galaxies in the known universe. Did they always look the same? 7

A long time ago in galaxies far, far away: The HST Ultra Deep Field 8

A long time ago in galaxies far, far away: The HST Ultra Deep Field 9

A long time ago in galaxies far, far away: The HST Ultra Deep Field 10

Two immediate results: I. Galaxies formed at some point in the distant past II. Galaxies evolved with time Where do the Stars and Galaxies come from? 11

Timeline (very rough) ● Most galaxies have very old stars ● Most galaxies started forming their stars some Billion years ago, shortly after the beginning of the Universe 12

22% 74% 3.2% 13

The Cosmic Microwave Background: a baby photo of the Universe when it was just 300,000 years old It reveals tiny irregularities; the density of matter varied by parts in a million 14

Dark Matter is more abundant, and dominates gravity. To understand how gravity created structure (galaxies) from the early homogeneous Universe, we need to simulate Dark Matter. Outcome depends strongly on the structure/geometry of the Universe and the content of Dark Matter 15

Supercomputer simulations of Dark Matter: gravity grows the initial density perturbations, structure forms 16 From choose a “rotating box” version such as

Gravity grows a “Cosmic Web” of Dark Matter - voids, filaments, clusters of clumps that host galaxies Simulation: A.Kravtsov 17

Gravity grows a “Cosmic Web” of Dark Matter - voids, filaments, clusters of clumps that host galaxies Simulation: A.Kravtsov Galaxies form from overdense regions 18

Luminous matter, formation of gas disk and stars: Luminous matter (gas!) is viscous, and heated as it falls into dark matter halos; then heat is radiated away - gas cools - contracts angular momentum is conserved - >spin-up of rotation (“figure skater”) - fast rotating disk energy in turbulent/random motions (perpendicular to disk) is dissipated (viscosity->friction->heating ->heat is radiated away) -> motions perpendicular to ordered rotation disappear ->cold, dense gas disk -> STARS 19

Recap: From Dark Matter to Stars 1)The Universe contains mostly Dark Matter 2) Tiny irregularities in the Dark Matter density in the early Universe grew rapidly through gravity 3) Gas fell into the resulting Dark Matter clumps/”halos” (galaxies) and formed cold, dense gas disks 4) Stars are born and live in galaxies because they need cold, dense gas to form 20

Hierarchical galaxy formation; disks merge to disk bulges and Ellipticals Blue: Dark matter Halo; yellow: gas; red: stars In a “hierarchical” scenario, smaller structures form first, and later merge into bigger ones: -Galaxies merge to form larger ones -Mergers of roughly equal-sized galaxies often (not always) turn Spirals into Ellipticals Bertola et al. 21

Galaxy interactions/mergers: Observations and Numerical simulations 22 Credits: Patrik Jonsson, Greg Novak & Joel Primack, University of California, Santa Cruz

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II. How did we learn about galaxy formation? 27

New Sky Surveys at many Wavelengths 28

Multiwavelength surveys: combined efforts to get the whole picture. A new era of astronomy: big collaborations, huge databases 29

HST (visual, near infrared) GALEX (UV) star formation XMM (X-ray) Dust, star form., black holes... SPITZER (infrared) Chandra (X-ray) VLA (radio) (gas, mass, black holes, star formation) Redshift, dynamics,... DEEP2 (KECK,DEIMOS) Multiwavelength surveys: combined efforts to get the whole picture. A new era of astronomy: big collaborations, huge databases 30

time light travels to reach us Short (millions of years) Long (billions of years) Text even more distant galaxy nearby galaxy distant galaxy 31 Astronomers can look back in time: light from very distant galaxies took billions of years to reach us. Looking far is looking back

time light travels to reach us Short (millions of years) Long (billions of years) Text even more distant galaxy nearby galaxy distant galaxy 32 Astronomers can look back in time: light from very distant galaxies took billions of years to reach us. Looking far is looking back

time light travels to reach us Short (millions of years) Long (billions of years) Text even more distant galaxy nearby galaxy distant galaxy 33 Astronomers can look back in time: light from very distant galaxies took billions of years to reach us. Looking far is looking back

Astronomers can look back in time: light from very distant galaxies took billions of years to reach us. Looking far is looking back time light travels to reach us Short (millions of years) Long (billions of years) Text even more distant galaxy nearby galaxy distant galaxy 34

Large telescopes on the ground: Spectroscopy gives each galaxy a “time stamp” 35

DEIMOS spectrograph on the Keck II telescope Built by Sandra Faber & team, UC Santa Cruz Can observe spectra of hundreds of distant galaxies at a time 36

Overlapping slitmask layout 37

120 spectra of distant galaxies wavelength emission lines of ionized gas The emission lines are at longer wavelengths than measured in the lab: They are “redshifted”. This is because distant galaxies move away from us (“Doppler effect”, expansion of the Universe). The redshift (=velocity) measures the distance and how far we look back in time wavelength 38

For galaxies in the early universe, the infrared matters: 39

For distant galaxies, light from young stars (UV) and older stars (visible) is redshifted to long wavelengths (Infrared) wavelength spectral flux young stars (starbirth) older stars young stars (starbirth) older stars UV Visible Light Infrared Nearby Galaxy (not redshifted) Distant Galaxy (redshifted) 40

Spitzer Extended Deep Survey Reduction: M. Ashby The Spitzer Space Telescope provided infrared data: pierce through the dust, measure star formation rates 41

Hubble & JWST Probe the Early Universe HST: currently the most sensitive telescope in the short-wavelength infrared (near- infrared): Can observe redshifted UV (star formation) from the most distant galaxies JWST (launch: 2018) will be more sensitive, and reach longer infrared wavelengths: will reach even further back in time, and observe redshifted visible & infrared light in earliest galaxies 42

HST Ultra Deep Field JWST Ultra Deep Field Simulation JWST will have much improved sensitivity to faint distant galaxies: First Stars & Galaxies Small galaxies across cosmic time... 43

Star formation in galaxies over the last 10 billion years Heavens et al Hopkins & Beacom 2006 now 10 Billion yrs ago Space Density of Star Formation now 10 Billion yrs ago Big Galaxies Small Galaxies 44

Co-moving star formation rate (SFR) density declined by ~x10 Galaxy star formation histories are mass- dependent: massive galaxies formed bulk of stars quickly and early, less massive galaxies formed on longer timescales (“Downsizing”) Star formation in galaxies over the last 10 billion years Heavens et al Hopkins & Beacom 2006 now 10 Billion yrs ago Space Density of Star Formation now 10 Billion yrs ago Big Galaxies Small Galaxies 45

Co-moving star formation rate (SFR) density declined by ~x10 Galaxy star formation histories are mass- dependent: massive galaxies formed bulk of stars quickly and early, less massive galaxies formed on longer timescales (“Downsizing”) Star formation in galaxies over the last 10 billion years Heavens et al Hopkins & Beacom 2006 now 10 Billion yrs ago Space Density of Star Formation now 10 Billion yrs ago Big Galaxies Small Galaxies Reason for declining star formation: Galaxies run out of gas! 46

billions of years ago today (image: Driver 1998) big galaxiessmall galaxies rapid star birth & gas consumption 47

billions of years ago today (image: Driver 1998) big galaxiessmall galaxies rapid star birth & gas consumption slow star birth & gas consumption 48

billions of years ago today (image: Driver 1998) big galaxiessmall galaxies rapid star birth & gas consumption slow star birth & gas consumption 49

billions of years ago today (image: Driver 1998) big galaxiessmall galaxies rapid star birth & gas consumption slow star birth & gas consumption 50

Questions? 52