How Black Holes Rule Hubble’s Classification

◼ Hubble classification is thought to reflect the evolution of galaxies. Evolution

◼ Ellipticals are called early type, and spirals are called late type galaxies. ◼ The bulges of spiral galaxies look exactly like elliptical galaxies. So, structurally, a spiral galaxy is a small with a disk. ◼ Which way does the evolution go? ◼ A: Elliptical galaxies evolve from spiral galaxies by losing their disks. ◼ B: Spiral galaxies evolve from elliptical galaxies by growing a disk. Galaxy Evolution Galaxy Evolution Summary

◼ Galaxies start small and grow by mergers and fresh gas accretion. ◼ Major mergers destroy disks and turn spiral galaxies into ellipticals. ◼ Elliptical galaxies accreting new gas turn back into spiral galaxies, with the elliptical at the center becoming a bulge. ◼ New form only in the disk, consuming the gas. ◼ As the universe expands, mergers become less frequent and fresh gas accretion slows down. Galaxies and Black Holes

◼ We now know the AGN are powered by black holes. ◼ When there is gas around, black holes swallow it, making it shine on its way to the horizon. ◼ What do these BH do when there is no gas around? Galaxies and Black Holes

◼ We now know the AGN are powered by black holes. ◼ When there is gas around, black holes swallow it, making it shine on its way to the horizon. ◼ What do these BHs do when there is no gas around? ◼ Hence, some galaxies must have “dormant”, inactive black holes in their centers (and the Milky Way does). ◼ AGN are rare, but there could be a lot of galaxies with these dormant black holes. Galaxies and Black Holes

◼ How to find these dormant black holes? A. Look for a small black circle in the photograph of a galaxy. B. Look for its effect on the surrounding gas. C. Look for its effect on the surrounding stars. D. Check the Wikipedia article on that galaxy. Search for Dormant BHs

◼ With only stars around, one has to try to deduce the existence of a BH from the density of stars at the center and from their velocities. ◼ Example: Holmberg 15A. ◼ A giant elliptical galaxy at the center of a large (50 times more stars than the Milky Way). Holmberg 15A

◼ Stellar “kinematics” – how stars move. Holmberg 15A

◼ There is no obvious sign of a BH at the center, one needs to do careful modeling of stellar orbits. ◼ The conclusion: there must be a BH at the center with the mass of 10 (4.0±0.8)×10 M8 . ◼ This evidence is indirect, but it is strong. Holmberg 15A

10 ◼ This is what a 1.7×10 M8 BH looks like. Galaxies and Black Holes

◼ How many galaxies have big BHs at their centers? Could be all. ◼ In every galaxy that was looked at carefully enough a Super-Massive Black Hole (SMBH) was found. ◼ There are no galaxies for which good observations exist and in which no SMBH was found.

https://en.wikipedia.org/wiki/List_of_most_massive_black_holes M-sigma Relation

◼ The bigger the galaxy, the bigger is the BH.

◼ That would not be surprising, if they had any way of knowing about each other. M-sigma Relation

◼ By size the big BHs are 10-100 million times smaller than their parent galaxies. ◼ By mass they are < 1%. M-sigma Relation

◼ The best correlation is between the BH mass and the velocity dispersion of stars in the bulge, called the “M-sigma relation”. Galaxies and Black Holes

◼ The existence of the M-sigma relation implies that galaxies and BHs co-evolve, i.e. they grow in mass in such a proportion to each other as to maintain the M-sigma relation. ◼ Who controls whom then? A. Galaxies control how their black holes grow. B. Black holes control how their galaxies grow. Galaxies and Black Holes

◼ The existence of the M-sigma relation implies that galaxies and BHs co-evolve, i.e. they grow in mass in such a proportion to each other as to maintain the M-sigma relation. ◼ Who control whom then? A. Galaxies control how their black holes grow. B. Black holes control how their galaxies grow.

◼ Recall, that BH become active (i.e. grow) when their parent galaxy is disturbed by some other galaxy, essentially a random process. ◼ Galaxies are unable to control their BHs! Galaxies and Black Holes

◼ The galaxy blow-up movie again. Galaxies and Black Holes

◼ The galaxy blow-up movie again. ◼ The movie is an exaggeration – we don’t see galaxies blown into pieces by their black holes, but it is not completely wrong. ◼ The line driven wind from an AGN is powerful enough to heat up all the gas from its host galaxy to millions of degrees Kelvin. Why Galaxies Have Disks

◼ Gas in galaxies always forms a disk. Why? ◼ Galaxies live for many billions of years, so they are close to the equilibrium (forces balance). ◼ There are only 2 equilibrium shapes in space: balls and disks. ◼ Gas in galaxies forms a disk because A. it glows in the dark. B. it pushes equally into all directions. C. the disk is the most appropriate shape for the gas. D. galactic disks are made out of gas. Why Galaxies Have Disks

◼ Gas in galaxies always forms a disk because gas always cools, i.e. loses its energy (“glows in the dark”) and is unable to maintain its pressure without an energy source (recall stars). ◼ Stars only form from very cold gas (< 100 K); hence all young stars are located in the galactic disks. ◼ If a galaxy has a super-massive black hole, the black hole becomes active from time to time and heats some of the gas in the disk up to 106K. Why Galaxies Have Disks

◼ If you assign a human trait to BHs, do you think they are A. Kind B. Selfish C. Stupid D. Elegant E. Abhorrent

Black holes are selfish! They prevent gas in galaxies from forming stars because they want to keep that gas for themselves! Quenching

◼ The world is fair – in the long run it never pays to be selfish. ◼ Hot gas remains hot for long time, its pressure prevents it from falling onto the BH. ◼ The central BH starves as the result.

◼ This process is called “quenching” – BHs quench formation in their host galaxies but also deprive themselves of the fresh fuel supply, and the larger the BH, the more efficient it is in doing that. Quenching

◼ How exactly this happens, and whether it is the line driven wind or the jet that does most damage is yet not understood. Where Do They Come From?

◼ Option #1: take a stellar mass black hole and make it grow. ◼ Pros: ➢ We know how the stellar mass black holes form. ➢ Massive stars have short lives and are known to exist 400 Myr since the Big Bang, and perhaps as early as 200 Myr (but hopefully not). ◼ Cons: ➢ Super-massive black holes found too soon after the Big Bang. Where Do They Come From?

◼ There are 10-20 new early quasars discovered every year.

2014 2016

Wang+ 2018 Where Do They Come From?

◼ Recall the Eddington limit: BHs cannot grow too fast, or their own radiation stops the accretion. Where Do They Come From?

◼ Come direct collapse black holes: more massive than BHs (>100 M8) but smaller than 5 super-massive BHs (<10 M8). Direct Collapse BHs

◼ Pros: ➢ Explain why there are so many bright quasars less than 1Gyr since the Big Bang. ◼ Cons: ➢ No one ever saw one (after many careful searches). Science or Fantasy?

◼ When scientists unleash their imagination, things get wild. ◼ So, where do the big black holes really come from? I wish I knew…