Objects in the Rear-view Mirror May Appear Closer

Than They Are

! When we look back to larger and larger distances do we typically see older or younger galaxies? On average, are more distant galaxies spirals, ellipticals or irregulars?

! How do mergers cause galaxies to change? What is the order of change (e.g. is it spiral elliptical irregular)?

! Where are elliptical galaxies now typically found (e.g., by themselves, in clusters)? What about spiral galaxies?

! Do mergers affect galaxy shapes? How do mergers affect gas and so the age of the stellar populations in galaxies?

! Why are irregulars blue & ellipticals red? Are spirals blue? Do they have disks? Do they undergo major mergers?

! Our galaxy is a spiral. Will it ever have a major merger?

Learning Objectives

The farther away we look the further back we look in time

Destination Light travel timeMoon 1 secondSun 8.3 minutesSirius 8.6 yearsOrion Nebula 1,340 yearsAndromeda Galaxy

2.5 million years

We see the Orion Nebula as it looked 1,340 years ago

We can see what the Universe was like in the past!

At great distances, we see objects as they were when the Universe was much younger

Old Light from Young Galaxies!At great distances (and thus young ages)!There were more

spirals and fewer elliptical galaxies

!Galaxies then were closer together!About one-third of

distant galaxies are in close pairs

!Only about one-tenth of nearby (and thus older) galaxies are in pairs

Deep observations show us very distant galaxies as they

were much earlier in time

! Many galaxies we see at great distances (thus at early times) look violently disturbed!Not like spirals

or ellipticals!Irregular galaxies

! Mergers between galaxies were more common earlier in the Universe - when galaxies were closer together

! Observational evidence suggests that galaxies have evolved by merging together

Galactic Histories are Violent!

Images of “disturbed” (or irregular) galaxies in the early Universe

!Their gravity distorts each other’s disks and kicks out stars from the disk!this causes long tails of stars called “tidal” tails

!Mergers trigger gasclouds to collapse, causing star formation

!The stars, however, almost never collide!The typical separation

between stars is about 107 times their diameter

What happens when large galaxies merge?

Pairs of merging galaxies often exhibit long “tails” of

stars ejected by the merger

Starburst Galaxies! Galaxies with

enhanced rates of star formation

! Appear very blue, lots of young stars

! Usually forming massive stars for a short period of time (a few million years)

! Gas is agitated and compressed by interactions with close galaxies (or in a merger), triggering new star formation throughout a galaxy

The Cartwheel Galaxy

Intruder that caused starburst?

Hydrogen gas linking Cartwheel and “intruder”

Some Nearby galaxies show ongoing (multiple) mergers

M81 Group (one of the nearest galaxy groups to our own)

Optical (stars) Radio (cool hydrogen gas)

Mergers may explain why elliptical galaxies tend to be found where galaxies are closer together— in large clusters of galaxies

!Gravitational forces would destroy the disks of spirals in a “major” merger with another large galaxy

!Also, spirals retain gas and dust and continue to steadily make new stars!Dust would be

kicked out in a major merger and new star formation would “use up” the gas

! Most spirals exist alone in space, or in small groups that only contain 2 or 3 large spiral galaxies

Spirals seem never to have suffered a major merger

Spirals may merge with small galaxies. Such ”minor” mergers just add mass

to a large spiral galaxy’s bulge

Why do galaxies’ stars differ?! Spiral galaxies have both blue and red stars! Spirals may undergo “minor” mergers with smaller

galaxies, but are mostly pristine (spirals’ stars form from the collapse of their protogalactic gas cloud)

! Irregulars contain mainly blue stars! Mergers trigger star formation

! Ellipticals contain mainly red and yellow stars! They’re old galaxies that have undergone many

“major” mergers with other large galaxies. Bursts of star formation have used up all of their gas, so new stars don’t form. They are “red and dead”

Small Magellanic Cloud

Large Magellanic Cloud! Is our Galaxy undergoing galactic interactions?

! Yes! It is “consuming” some of its smallercompanion galaxies

! Both the Large and the Small Magellanic Clouds look irregular, and are probablydisrupted spirals

Mergers and Our Milky Way

Mergers and Our Milky Way! Also, we are

on a collision course with the Andromeda Galaxy

! Andromeda is the other large galaxy (with our Milky Way galaxy) in our local group of galaxies

! Our Galaxy will start to merge with the Andromeda galaxy in about 4 billion years

The Andromeda galaxy (M31)

Next Time

Quasars: Back to the Infant Universe