ASTR 100 Lecture 28: Galaxy classification and lookback time
Galaxies in general: Ch. 15, [16.2], in Essential Tuesday: Hubble s law
Today: Types of galaxies The Hubble tuning fork Galaxy classification Distance ladder and lookback time
Structure of our galaxy: Most stars in 1000ly disk
What can we ask about galaxies? Mass? Composition (chemical/star/dust)? Types? Ages? Do they evolve? Do they die? Do they dance? Is there an equivalent of the HR diagram for galaxies or Cluster HR diagrams for galaxies?
Method 1) use how fast are stars going to find how much mass is pulling them Look at little slices of an edge-on galaxy and use Doppler shift
Surprise: Method 1 and Method 2 do not agree. White line is the velocities measured by method 1 Red line is the velocities predicted by method 2
This is what the mass density of the galaxy looks like. All data indicate that for every 1 kg of mass we can see in the cosmos, there is up to 5 kg of mass that we can t Dark Matter
Dark Matter fun facts: Dark Matter has mass Dark Matter doesn t seem to emit light Dark Matter doesn t seem to absorb light Dark Matter doesn t seem to concentrate in galactic disks
Idea #1) Brown dwarfs, sub-brown dwarfs, rogue planets, and so forth. Not exotic, compact objects in the halo <-- Keep this going: Thousands/Millions of little un-stars MAssive Compact Halo Objects: MACHOS
Idea #2) An exotic type of massive particle which does not interact with light. Massive neutrinos, axions, et cetera Weakly Interacting Massive Particles: WIMPS X X
Idea #3) We just don t know how gravity works on galaxy sized distances. It takes light and therefore, gravity 10 5 years to cross the disk of a Milky Way sized galaxy
What can we ask about galaxies? X Mass? Composition (chemical/star/dust)? Types? Ages? Do they evolve? Do they die? X Do they dance?
Hubble Deep Field: every single on a galaxy
More than just spiral galaxies out there
Galaxy classification S is for spiral -arms -bulge -disk
Galaxy classification SB is for barred spiral -arms -bulge elongated into bar -disk
Galaxy classification E is for elliptical -no disk or arms -can be sphere or elongated into cigar -can be dwarf ellipticals only 2500ly across or giant ellipticals 120,000ly across
Galaxy classification Irr is for irregular -does not fit into either category
Edwin Hubble s tuning fork
Hubble s (incorrect) idea was that galaxies evolved along tuning fork Younger ----------------------> Older
How do galaxies interact, if at all Hickson Compact Group 87: All bound by each other s gravity
We re in one of these. Our so-called local group
Abell 1689: A huge cluster of galaxies
Galactic evolution. Can t really just sit and look at a galaxy evolve 1) From looking at clusters 2) Look at far away galaxies, looking into the past
Color of ellipticals? Why are they more common in clusters?
Galaxies collide all the time: Ellipticals could be formed by encounters with other galaxies Computer simulations support this: https://www.youtube.com/watch?v=d-0gabq494e
We re interacting too.
<- Veteran of many mergers? Hubble s evolution was backwards
Every image has closer and farther objects.
A lot of different distances are in the Hubble deep field http://www.youtube.com/watch?v=ykgragyeg78
Lookback time : Because of the finite speed of light, this is a slice of space and time
Type? Distance? Lookback time?
Once you know distances, you know how out-of-date the light is.
Redness is from dust:
Galaxies seem to have all formed during the first 2 billions years. Young galaxies are irregular, then evolve into other shapes. Young, forming galaxies have
In these really young galaxies, we see JETS!
This solves a long-standing mystery Quasi-Stellar Objects or quasars Point sources, like stars, but really far away and really bright Now think they re the jet at the center of a young galaxy
Key terms: Irregular, Spiral, Barred Spiral and Elliptical galaxies; Lookback time, Groups and clusters of galaxies, Quasars Key Ideas: What are the types used to classify galaxies and what are the basic properties of each type? How are these types organized along the Hubble tuning fork? How does the finite speed of light allow us to look back in time? How do we study galactic evolution? What do we know about how galaxies evolve?