Astronomy 1143 – Spring 2014

Lecture 22The 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 RemnantsWhite dwarfs

• Stars with masses < ~8 Msun have their cores end up as white dwarfs (M up to 1.4 Msun)

• Densities of 1x109 kg/m3

Neutron stars

• Stars with masses between ~8MSun and ~25 MSun have their cores end up as neutron stars (M between 1.4 and ~3 Msun

• Densities of 4x1017 kg/m3

Black Holes

• Stars with masses > 25 Msun have their cores end up as neutron stars (M > 3MSun)

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 Msun

Artist’s Conception of an X-Ray Binary

Black Hole Candidates

X-ray binaries with unseen companions of mass > 3 Msun, too big for a Neutron Star.

Currently 20 confirmed black hole candidates:• First was Cygnus X-1: 7 – 13 Msun

• Largest is GRS1915+105: 10 – 18 Msun

• Most are in the range of 4 – 10 Msun

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 MSun 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

Big Lenses

MicrolensingFor 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