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Galaxy Formation and Evolution. Chris Brook Modulo 15 Room 509 email: cbabrook@gmail.com. Lecture 3: Feedback and Outflows. How to make a galaxy. Create Hydrogen, Helium and dark matter in a Big Bang. Allow quantum fluctuations to cause some regions to be denser than others.
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Galaxy Formation and Evolution Chris Brook Modulo 15 Room 509 email: cbabrook@gmail.com
How to make a galaxy Create Hydrogen, Helium and dark matter in a Big Bang Allow quantum fluctuations to cause some regions to be denser than others Add Dark Energy so the Universe expands at correct rate Ensure a large amount of dark matter, so there is enough mass to ensure the dense regions collapse due to gravity THESE ARE THE BASIC INGREDIENTS OF COSMOLOGY THE PILLARS ARE: Abundances of Light Elements The Cosmic Microwave Background The Large Scale Structure in the Distribution of Galaxies The Expanding Universe
How to make a galaxy Create Hydrogen, Helium and dark matter in a Big Bang Quantum fluctuations to cause some regions to be denser than others Add Dark Energy so the Universe expands at correct rate Ensure a large amount of dark matter, so there is enough mass to ensure the dense regions collapse due to gravity Fragmenting gas within the collapsed regions forms stars Energy from Massive Stars, Supernovae, and Active Galactic Nuclei (AGN) heats and expels gas
Problems of Galaxy Formation in CDM Universe The shape of luminosity function- too few dwarf galaxies (missing satellite problem) and too few high, mass galaxies The ``cooling flow” problem- why don’t cold gas flows result in bright blue galaxies at the centre of clusters? “Downsizing”- why are the most massive galaxies no longer forming stars (“red and dead”)? while low mass galaxies are forming stars “Missing baryons”- less than 10% of baryons form stars, and we cannot account for more than 50% of baryons. “cusp/core” problem- dark matter halos have steep inner density profiles (“cusps”) while observed galaxies have flat inner density profiles (“cores”) “Angular Momentum” problem- dark matter halos have large amount of low angular momentum material. Disc galaxies do not. Is Energy Feedback the answer to all our woes?
Outflows observed Low redshift: we observe outflows in Massive starburst galaxies
How to make a galaxy Create Hydrogen, Helium and dark matter in a Big Bang Quantum fluctuations to cause some regions to be denser than others Add Dark Energy so the Universe expands at correct rate Ensure a large amount of dark matter, so there is enough mass to ensure the dense regions collapse due to gravity Fragmenting gas within the collapsed regions forms stars Energy from Massive Stars, Supernovae, and Active Galactic Nuclei (AGNS) heats and expels the gas Heavier elements, Carbon, Oxygen Iron etc are formed in the stars and supernova explosions
Outflows observed: absorption lines Absorption lines contain information on the elemental abundances and physical properties (temperature, density, ionization state) of the absorbing gas. The strongest absorbers (damped Lyman-alpha systems) trace gas in and around galaxy disks and proto-galaxies. The strength and velocity spread of the high-ion lines and their correlation with other DLA properties give important observational constraints on the kinematics and structure of the galaxies. The figure shows that the C IV velocity widths in DLAs are broader than the neutral velocity widths. These high ion kinematics indicate outflows. Fox et al. 2007, A&A, 473, 791
Outflows inferred: enriched IGM Outflows observed The IGM is metal-enriched: metal lines are detected at the same redshift as Ly-alpha forest lines at low H I column densities (low-density regions). How homogeneous is the metal enrichment? How were the metals transported to the IGM? At very high redshift in an early generation of Population III stars? or are they dispersed later? High redshift
Outflows observed High redshift
Indirect Evidence of Outflows Springel & Hernquist 2003
Indirect Evidence of Outflows Mass-Metallicity relation
Indirect Evidence of Outflows Consistency with CDM Cosmology
Re-ionisation effects The evolution of the Universe: three distinct phases following the Big Bang: 1. the `recombination era’: the first few hundred thousand years. Ionized hydrogen and helium plasma generated by the Big Bang gives way to cooler, neutral atomic gas; 2. the `dark ages', lasting ~500,000 years. The Universe is populated by neutral gas; 3. `cosmic reionization', where the first episodes of star and black hole formation lead to photoionisation of neutral hydrogen. A key question is the identity of the cosmic reionizers - were quasars (i.e. massive black holes) primarily responsible, or were more conventional star clusters, or Population III objects, the key ingredient? Simulation shows that around star forming regions (red) the neutral hydrogen (green) is re-ionized.
Re-ionization and the Jeans Mass (one theory to explain the “missing satellites”) When a volume of the IGM is ionized by stars, the gas is heated to a temperature TIGM ~ 104 K (IGM = inter-galactic medium). The result is that after reionization, gas infall is suppressed in halos of Mass < 109 M* (this mass cut off may be as high as 1010 M*) ~ e.g. Efstathiou 1992, Somerville 2002
Massive Star feedback Specific Luminosity from Radiation Stellar Winds Supernovae Type II But how do these energies affect their surroundings? Ongoing theoretical and observational research!
Radiation Pressure from Massive Stars Radiation pressure: Light (mostly from the youngest stars) scatters off gas and dust in the galaxy. Each time a photon scatters or is absorbed, it imparts some momentum to that gas, “pushing” away the gas and dust. This does not “heat up” the gas, but can impart an enormous amount of momentum Photo-ionization Photo-Ionization: The light from the stars also ionizes gas, heating it up to ~10^4 K. These ionized ‘bubbles’ can push on the gas significantly in very low-mass galaxies (where the corresponding velocities of the gas are comparable to the disk orbital velocities). It can also destroy molecules, a critical ingredient for the next generation of star formation. These are most effective in dense gas, so may be important in massive galaxies and at high redshift
Stellar Winds Stellar winds: Young stars blow winds off their surface that can have velocities as large as ~1000 km/s. This shocks and provides a large amount of thermal energy to heat the gas. Older stars blow “slow” winds at just ~10 km/s, but the total mass recycled into the ISM can be very large, ~30% of the original mass in stars.
Stellar Winds: Properties Strickland & Stevens 2000
Supernova feedback Supernovae: After a few million years, massive stars begin to explode as supernovae. Each such event imparts energy to the nearby ISM. Many “overlapping” events build up huge hot bubbles of gas that generate pressures sufficient to “blow out” of the disk and vent material into the intergalactic medium.
Supernova feedback GMC= Giant Molecular Cloud So, the question is.. How does the energy from SN “add up” ?
Supernova feedback on Galaxy Scales There remains a lot of uncertainty in how effective this feedback is (how efficiently it couples to the ISM), and in how it is best modeled
Veilleux et al. 1994; Cecil et al. 2001; Cecil, Bland-Hawthorn & Veilleux 2002
Supernova feedback Modellers make various assumptions about the strength of supernova feedback, and how it is dependent on SFR, the mass of the halos
overcooling Benson et al 2003
