Galaxy collisions galaxy formation n Collisions of galaxies

Galaxy collisions & galaxy formation n Collisions of galaxies Formation of galaxies Dark Matter

NGC 4622

Collisions of galaxies n Galaxy collisions are comparatively common (and spectacular!) n Major collision l l n collision of 2 big galaxies Quite rare Minor collision l l Collision of a large galaxy with a small “dwarf” galaxy Very common!

M 51 Credit : Tony and Daphne Hallas

The Antennae Galaxy

Merger of two Spiral Galaxies Chris Mihos & Sean Maxwell

Merger of a Spiral and an Elliptical Galaxy

Merger of two Elliptical Galaxies

When Spirals Collide

The Antennae Galaxy

Collisions of galaxies n Galaxy collisions are comparatively common (and spectacular!) n Major collision l l n collision of 2 big galaxies Quite rare Minor collision l l Collision of a large galaxy with a small “dwarf” galaxy Very common!

Big Galaxies Tear up Small Ones

Spiral Galaxy dining on a Dwarf Spheroidal (side view)

Spiral Galaxy dining on a Dwarf Spheroidal (top view)

The Cartwheel Galaxy

Simulation of the Cartwheel Galaxy

Internal evolution n Galaxy collision can drive “internal evolution” of galaxies… n Rapid star formation l l n Galactic collisions makes gas clouds collapse and turn into stars Makes galaxy look blue (since there can be many young, hot stars) Quasar activity l l l Galactic collision drives gas into center of galaxy Gas can rain onto central massive black hole and produce tremendous amounts of energy… More about this possibility in next class

III : Galaxy formation n How did galaxies form? l l l n Believed that universe started off very uniform/smooth… just small ripples Gravity caused ripples to grow… These eventually collapsed to become galaxies and clusters of galaxies! Nowadays, can study this process using computer simulations

Zoom in on a forming galaxy cluster (Virgo consortium) This movie zooms in on one patch of a larger simulation where we know that a galaxy cluster is about to form.

Las Campanas Redshift survey

How do Galaxies Form? n “Bottom-up” formation scenario… l l n All driven by gravitational collapse Some small things form first Collisions/mergers cause bigger things to grow… Dwarf galaxies galaxy clusters superclusters and so on. “Bottom-up” formation scenario…

III : The mass of galaxies and the need for dark matter n First think about stars… l l l n we want mass, but see light Construct the “massto-light” ratio Msun=2 1030 kg Lsun=4 1026 W Msun/Lsun=5000 kg/W From now on, we will use Msun/Lsun as a standard reference.

Other stars n Let’s use star-light to weigh a whole galaxy… have to average M/L over all stars. n Different types of stars have different mass-tolight ratios l l l n Massive stars have small M/L. Low-mass stars have large M/L. Neutron stars and black hole hardly shine at all (very high M/L) Averaging stars near to the Sun, get l M/L 10 Msun/Lsun

Measuring a Galaxy’s Mass n n n Typically measure L=1010 Lsun So, mass of stars is M=1011 Msun But, there’s another way to measure mass…

Kepler’s Third Law n n n Use same laws of motion as for planets going around a star… Remember Kepler’s Third Law for Planets. We can use this as an approximate formula for a star’s motion around the Galactic Center.

Velocity dependence on radius for a planet orbiting a star…

Measuring a Galaxy’s Mass n Apply same arguments to a galaxy…

Measuring a Galaxy’s Mass l Consider a star in the galaxy at distance D from center at speed V l Then, mass of the galaxy within distance D, Msun(inside D)

What do we see? Galactic Rotation Curves.

Real measurements - Strange “Rotation” Curves

How Can this Be? n Orbital velocity of stars/gas stays flat as far out as we can track it l l l n n Means that enclosed mass increases linearly with distance… even beyond point where starlight stops So, in these outer regions of galaxies, the mass isn’t luminous… This is DARK MATTER. All galaxies seem to be embedded in giant dark matter balls (called halos) At least 10 time more dark matter than visible stuff.

Called a dark matter “halo”

What is Dark Matter? n Is most dark matter normal Dust/Gas? What about Black Holes, Neutron Stars, Planets? l l l n No!! No enough of this stuff! Solid arguments from cosmology limit the amount of “normal” matter to less than that needed for dark matter halos. So, this is something new… non-baryonic matter. (matter not based on protons and neutrons). 80 -90% of matter in universe is non-baryonic dark matter!! Neutrinos? l l They are part of the “standard model” of particle physics… they have been detected and studied. No… each neutrino has very small mass, and there are not enough of them to explain dark matter.

What is Dark Matter? n WIMPs (Weakly Interacting Massive Particles)? l Generic name for any particle that has a lot of mass, but interacts weakly with normal matter § Must be massive, to give required mass § Must be weakly interacting, in order to have avoided detection l Various possibilities suggested by Particle Physics Theory… § Super-symmetric particles § Gauge bosons l Many experiments currently on-going

Supermassive Black Holes Monsters in the Closet

II : Evidence for supermassive black holes – three case studies n Case I : M 87 l l l Large elliptical galaxy Black Hole suspected due to presence of prominent jet Target of early study by Hubble Space Telescope

l l l HST found… Rotating gas disk at galactic center Measured rotation implied a central object of 3 billion solar masses! Mass cannot be due to normal stars at center… not enough light is seen. Good evidence for 3 billion solar mass black hole.

n Case II : M 106 l l l Contains central gas disk Disk produces naturally occurring MASER emission Radio telescopes can measure position & velocity of MASERs to great accuracy. Velocity changes with radius precisely as expected if all mass is concentrated at center! 30 million solar mass black hole

MCG-6 -30 -15

n Case III : MCG-6 -30 -15 l l l “Active galactic nucleus” Bright X-ray source Find signature of a gas disk in X-ray spectrum This disk is orbiting something at 30% speed of light! Also see strong “gravitational redshifts” Strong evidence for a very massive black hole in this object.

III : The Center of our Galaxy

There’s something strange at the center of our galaxy… n Modern large telescopes can track individual stars at Galactic Center l l n Need infra-red (to penetrate dust? ) Need very good resolution. We have been observing for past 10 years…

n The central object is l l l n Very dark Very massive (3 million solar masses) Must be very compact (Star S 2 gets within 125 AU of the center) Currently the best case for any supermassive black hole

IV : A Supermassive Black Hole in Every Galaxy? n n Black holes exist in centers of some galaxies… But how widespread are they? Does every galaxy have a supermassive central black hole? Several teams set out to answer that question… l l Use best resources (HST, large telescopes on ground etc. ) to gather lots of data on many nearby galaxies. Systematic search for black holes They found them, and discovered interesting patterns… Correlation between size of black hole and the brightness of the galaxy’s bulge (but not the disk)…

n But, even better correlation with stellar velocity in bulge…

n Correlations crucially important! l l Argues for a connection between the formation of the galaxy and the supermassive black hole. Currently forefront of research…