1. The Formation and Evolution of Galaxies Michael Balogh University of Waterloo

2. Outline Introduction: the scale of the Universe Observations  The local Universe (Sloan Digital Sky Survey)  The distant Universe: what can distant galaxies tell us? Theory:  How do galaxies form and evolve? Summary

3. The scale of the Universe The Milky Way ~20 kpc = 65,000 light years

4. 600 kpc = 2 million light years The Local Group

5. Galaxy clusters: a different world The Coma cluster: 90 Mpc (300 million light years) away 1 Mpc = 3 million light years

6. The Universe Home: the Local Group 13.7 billion years after the Big Bang The Coma cluster: 300 million light years away Typical stellar lifetime is a few billion years So there has been little evolution between the epoch of the Coma cluster and today The Hubble Ultra-deep field most distant galaxy, z=6 13 billion light years away <1 billion years after the Big Bang The Cosmic Microwave background relic radiation from the Big Bang z~1000 < 1 million years after the Big Bang

7. Questions? How did the Milky Way form?  Different components:  Rotating disk of young stars  Central bulge of intermediate-age stars  Large halo of very old stars How did the Local Group form?  ~30 nearby galaxies with different morphologies, sizes, luminosities, ages  Need to explain the diverse properties of galaxies in the Local group. Why do cluster galaxies look so different?  These are mostly old, elliptical galaxies. Why? Two approaches: 1.The local Universe Try to reconstruct the history of galaxy evolution from the appearance of galaxies around us 2.High-redshift galaxies Distant galaxies are seen at an earlier epoch, so can trace evolution directly.

8. Star formation in the Milky Way In our own Galaxy we can measure the ages of individual stars (in some cases) The Milky Way has been constantly growing: the star formation rate has been approximately constant for the last ~10 billion years.

9. Local Group galaxies For some galaxies in the Local Group, it is possible to measure the colours and magnitudes of individual stars  Stellar evolution models can then be applied to determine the star formation history of these galaxies

10. Local group Leo I: intermediate aged galaxy, with most stars formed ~3 billion years ago Galaxies in the Local Group have a wide range of ages. Some show only old stars, others have only very young stars. Leo A: young galaxy, with most stars formed less than 2 billion years ago Ursa Minor: the only galaxy in the Local Group with no young stars.

11. Elliptical galaxies For more distant galaxies, we can’t resolve individual stars: have to model the integrated colours, spectrum. These models show elliptical galaxies tend to be old  Have formed most of their stars at least ~10 billion years ago Star formation histories for spiral galaxies tend to be more complicated  Can measure their current rates of star formation (from UV flux, emission lines, or dust emission)

12. The SDSS Sloan Digital Sky Survey: a great look at the local Universe Will measure accurate photometry for 100 million objects Redshifts (and star formation rates) for 1 million galaxies and 100,000 quasars.  Out to z=0.2  About 1 Gpc, or 2.7 billion light years  The age of the Universe is ~13.7 billion years, so these galaxies are still relatively nearby www.sdss.org

36. Colour-magnitude relation Bright and red: Old galaxies with little or no new star formation Elliptical morphology Sloan DSS data Faint and blue Galaxies with active star formation Spiral morphology

37. Higher redshifts HST Ultradeep field  very deep, but small area and hard to get redshifts λ [nm] % COMBO-17  ¾ square degree  5 broad + 12 medium  ~ 25000 galaxies Keck DEEP2  3.5 sq. degrees, R~24.5, z>0.75  55,000 redshifts CFHT-LS  very wide, wide, and deep fields.  Wide is 170 deg 2 to r=26

38. The epoch of galaxy evolution By measuring the star formation rate per unit volume at different distances, we can reconstruct the star formation history of the Universe. Galaxy formation appears to have occurred mostly about 10 billion years ago. The global star formation rate has been steadily declining since. These results are sensitive to the effects of dust extinction, and assumptions about the shape of the initial mass function.

39. Why Does Star Formation Stop? Fossil history of local group

40. Galaxy formation: Theory

41. In the beginning …

42. z ~ 1000

43. Perturbation Growth Growth of fluctuations is due to gravity, and depends on the energy and matter content of the Universe WMAP

44. The Composition of the Universe Total matter and energy density is equal to the critical density needed to “close” the Universe. The matter density is 30% of this. The remaining 70% is an unknown energy component (“dark energy”). Of all the matter in the Universe, only 16% of it is ordinary matter (protons, electrons, neutrinos etc.). The rest is due to dark matter, an unknown component that interacts only through gravity. Only 5% of ordinary matter is visible!  i.e. the stars and gas we see only make up <1% of all the matter and energy in the Universe.

45. The baryon budget: stars Coma: XMM-Newton ObservatoryComa cluster The baryon budget: hot gas

46. The easy part: Dark matter

47. 150 Mpc/h

48. 3 Mpc/h

50. The hard part: baryons

51. Baryonic Physics Radiative cooling Radiative cooling Invisible baryons: ~10 6 K

52. Mergers

53. Baryonic Physics Radiative cooling Radiative cooling

54. Why so few stars? Dense gas cools very effectively as free electrons interact with each other. This is especially true at early times, when the Universe was much denser. Overcooling leads to the formation of hundreds more small galaxies than are observed. Predicted number of galaxies Observed number of galaxies Simulation: dark matter in the Local Group

55. Supernova feedback? M82 in H  : Subaru Telescope M82 in Xrays: Chandra Observatory Explosive supernova events may be strong enough to eject matter and energy into the gas surrounding galaxies.

56. 200kpc Bubble-like structure Blob1 200kpc Conical structure Evidence for Superwinds at high redshift These winds may be even more effective at early times, when they are most needed. Still, there is probably not enough energy in supernova alone to keep the gas sufficiently reheated Ly  at high redshift: Subaru

57. What about active galaxies? Perseus Cluster & 3C 84 Perseus Cluster / Chandra 10 kpc There is ~100 times more energy available in Active Galactic Nuclei (accreting black holes) than in supernovae Details of how this energy is coupled to the surrounding gas are still uncertain

58. Bubbles in the Intracluster Medium Simulations of bubbles in cluster gas Effective at disrupting cooling in the core, over the ~50 Myr lifetime of the bubble

59. Summary Some things we know:  Galaxies formed most of their stars about 5-10 billion years ago  Growth of galaxies is dominated by dark matter  Birth of stars is regulated by very energetic events: supernovae and black hole accretion Some things we don’t know:  Why has star formation all but stopped by today?  Why do galaxies in rich clusters look different from the Milky Way?  How does nature maintain the precise balance between the rapid cooling of gas and the injection of energy through explosive events? Future progress:  Larger, more specialized telescopes to observe smaller galaxies, at earlier times  E.g. JWST, Thirty-Metre-Telescope  Higher resolution simulations with better input physics to understand the interplay between feedback and cooling.