Astronomy 1143 – Spring 2014 Lecture 22 The Nature of Dark Matter: MACHOs and WIMPs

Key Ideas: Dark matter makes up ~85% of matter content of Universe. Dark Matter Candidates – Exist and give off very little (if any light) Stellar Remnants -- White dwarfs, neutron stars, black holes Failed Stars -- Brown dwarfs and free-floating planets Ruled out as the source of (most) DM because of results of Gravitational Microlensing

Possible candidates Stellar remnants Black holes Neutron stars White dwarfs Brown Dwarfs & Planets Particle Neutrino New Particle – Weakly Interacting Massive Particles (WIMPs) MACHOs -- MAssive COmpact Halo Objects

Life Cycle of Stars

The Iron Catastrophe

Collapse of the Iron Core In the last seconds of the life of a massive star (M > 8 solar masses?), all Si has fused to iron in the core No new source of thermal energy to exert outward pressure Core collapses under gravity. Degeneracy pressure not enough to stop collapse Outer layers ejected, at least sometimes Forms a core-collapse SN

Stellar Remnants White dwarfs Stars with masses < ~8 M sun have their cores end up as white dwarfs (M up to 1.4 M sun ) Densities of 1x10 9 kg/m 3 Neutron stars Stars with masses between ~8M Sun and ~25 M Sun have their cores end up as neutron stars (M between 1.4 and ~3 M sun Densities of 4x10 17 kg/m 3 Black Holes Stars with masses > 25 M sun have their cores end up as neutron stars (M > 3M Sun )

Do White Dwarfs Exist? Sirius B Know Luminosity Know Temperature Therefore know radius Know Mass Earth-sized object with mass of a star! Very low luminosity Lots of mass, not a lot of light

Do Neutron Stars Exist? Neutron stars can be identified by Small size (~10 km) Very high temperatures Masses measured if they are in binary systems Also identified as pulsars Spinning every second (or many times every second!) Radio beam can cross Earth’s path

Do Black Holes Exist? To make a black hole, you need to have a case where nothing can stop the gravitational collapse. Most of the time, something counteracts gravity Thermal pressure Electromagnetic pressure Degeneracy pressure Collapse of the iron-core of a massive star!

Seeing what cannot be seen… Q: If black hole are black, how can we see them? A: By the effects of their gravity on their surroundings: A star orbiting around an unseen massive object. X-rays emitted by gas superheated as it falls into the black hole.

X-Ray Binaries Bright, variable X-ray sources identified by X- ray observatory satellites: Spectroscopic binary with only one set of spectral lines  the companion is invisible. Gas from the visible star is dumped on the companion, heats up, and emits X-rays. Estimate the mass of the unseen companion from the orbit. Black hole candidates will have M  3 M sun

Artist’s Conception of an X-Ray Binary

Black Hole Candidates X-ray binaries with unseen companions of mass > 3 M sun, too big for a Neutron Star. Currently 20 confirmed black hole candidates: First was Cygnus X-1: 7 – 13 M sun Largest is GRS : 10 – 18 M sun Most are in the range of 4 – 10 M sun Estimated to be ~1 billion stellar-mass black holes in our Galaxy alone.

Brown Dwarfs & Planets Another object with (some) mass and not a lot of light is a brown dwarf A ball of gas with M < 0.08 M Sun will not get hot enough in the center to turn H into He Therefore, only glows with the energy of gravitational collapse and is rather pathetic compared to stars Planets around stars we can count up (more later!). Free-floating planets are tougher

Do Brown Dwarfs Exist? Binary brown dwarf system 6.5 light years away Need very high resolution to separate the two stars, as they are 3 astronomical units apart Much too faint to be seen by the naked eye

Detection of Stellar Remnants/Failed Stars Other methods can’t find Single black holes Distant white dwarfs and neutron stars Not so distant brown dwarfs and free- floating planets Need something that is not sensitive to light, but is sensitive to gravity

Big Lenses

Microlensing For objects that aren’t as massive as whole galaxies, the images aren’t separated by enough to see. However, the increase in the brightness from the multiple images is noticeable But we can’t know that a star is “brighter than it should be” So we need a situation where the brightness increases when a stellar remnant passes in front and then decreases at the end of the alignment

Motions of Stars & Remnants in Galaxies Give Us Opportunity

Gravitational Microlensing

Microlensing Events Very Rare Very, very good alignment of lens and source star is needed for a microlensing event to be bright enough to notice. From Earth’s perspective, a star will be microlensed about every million years Solution: Look at millions of stars, highly concentrated in sky Magellanic Clouds Bulge (central region) of our Galaxy

In the 1990s, extensive surveys were done, looking for microlensing from MACHOs

Surveys MACHO: 12 million stars monitored for ~ 6 years EROS: 7 million stars monitored for ~6 ½ years Difficult observational problem Night-to-night variations because of weather Other kinds of variable stars could be mistaken for microlensing events

Microlensing Event In many cases the microlensing is caused by normal stars with planets! Not by stellar remnants.

Stellar Remnants are not most of the Dark Matter Number of events detected not enough to explain the amount of dark matter EROS found 1 event, when 39 events would be expected MACHO and EROS results showed < 20% of the dark matter is in the form of dim objects with about a stellar mass Exact amount depends on issues such as whether stars in the MC are “self-lensing”

Investigation of Dark Matter Stellar remnants/Failed stars seemed like excellent dark matter candidates They have mass, but not (a lot of) light They are known to exist! Microlensing surveys Not enough of MACHOs to explain the speeds of stars in the outskirts of galaxies. Not only reason to exclude them (more to come!) Need a different kind of candidate. Something that doesn’t come in stellar-sized lumps, but much, much smaller…

WIMPs We need a particle that is massive (for a particle) (evidence coming up) interacts very weakly or not at all has a high density in the Universe is stable for a long time or forever Weakly Interacting Massive Particles are predicted by particle physics models