galaxy classification - columbia university
TRANSCRIPT
![Page 1: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/1.jpg)
Galaxy classification
![Page 2: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/2.jpg)
Questions of the Day
• What are elliptical, spiral, lenticular and dwarf galaxies?
• What is the Hubble sequence?• What determines the colors of
galaxies?
![Page 3: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/3.jpg)
The MW is a “spiral” galaxy, or a “late type” galaxy.
The different components have different colors, motions, and chemical compositions different origins!
Top View of the Milky Way
![Page 4: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/4.jpg)
• More disk than bulge (if any!).• High current star formation.
Other Late Type Spiral Galaxies
![Page 5: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/5.jpg)
These are also “late-type” galaxies. Apparent shape depends on orientation
![Page 6: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/6.jpg)
• More bulge than disk.
• Low current star formation.
“Sombrero Galaxy”
Other Types: “Early type galaxies”
![Page 7: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/7.jpg)
• Elliptical Galaxies (or just “ellipticals”)– No disk! All bulge!– Very little gas– Probably old!
Have evolved to the point where no gas is left for
making new stars!
“spheroidals”
And even earlier type galaxies:
![Page 8: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/8.jpg)
• Just a hint of a disk.• Low current star formation.
“S0” galaxies: Like ellipticals, but usually a bit flatter.
And in between, “lenticulars”
![Page 9: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/9.jpg)
Many galaxies have “bars” – linear arrangements of stars (The Milky Way has a bar!)
![Page 10: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/10.jpg)
Ellipticals
Lenticulars
Unbarred and Barred Spirals
All of these different types of galaxy fit nicely into a sequence.
![Page 11: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/11.jpg)
![Page 12: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/12.jpg)
Number indicates how flat the elliptical is
Lowercase “a”, “b”, “c” indicates how unlike the spiral is to an elliptical
![Page 13: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/13.jpg)
![Page 14: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/14.jpg)
1. “Bulge-to-Disk Ratio”2. Lumpiness of the spiral arms3. How tightly the spiral arms are
wound
E Sa Sb Sc“early type” “late type”
Things that vary along the Hubble Sequence:
![Page 15: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/15.jpg)
1. “Bulge-to-Disk Ratio”2. Lumpiness of the spiral arms3. How tightly the spiral arms are
wound
E Sa Sb ScNote: These are not exact trends! Galaxies
are much more complex than stars!
Things that vary along the Hubble Sequence:
![Page 16: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/16.jpg)
1. “Bulge-to-Disk Ratio”2. Lumpiness of the spiral arms3. How tightly the spiral arms are wound
Early Types Late Types
![Page 17: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/17.jpg)
Varying amounts of bulge & disk components suggests different formation & evolution history!
Which has a higher star formation rate?A: Early-Type EllipticalsB: Late-Type Spirals
![Page 18: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/18.jpg)
Varying amounts of bulge & disk components suggests different formation & evolution history!
Which do you think has more gas?A: Early-Type EllipticalsB: Late-Type Spirals
![Page 19: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/19.jpg)
Varying amounts of bulge & disk components suggests different formation & evolution history
• Older Stars• Gas Poor• More Massive
• On-going Star Formation• Gas Rich• Less Massive
On average…
![Page 20: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/20.jpg)
Late-type galaxies tend to have a much larger fraction of their normal matter in the form of gas.
They’ve used up less of their “fuel” for star formation.
The “star formation history” varies along the Hubble sequence:
![Page 21: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/21.jpg)
The colors of late-type galaxies tend to be bluer.
The “star formation history” varies along the Hubble sequence:
![Page 22: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/22.jpg)
• First, what sets the colors of galaxies?
STARS!
Reddening from dust, and light emitted by HII regions have only a secondary effect on color
Why do the colors of galaxies suggest that the stars in galaxies
have different typical ages?
![Page 23: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/23.jpg)
The color of the brightest stars pretty much sets the color of the galaxy.
These stars tend to be young massive main sequence stars, or red giants.
![Page 24: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/24.jpg)
Star clusters of different ages have different fractions of these bright red and blue stars
![Page 25: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/25.jpg)
Galaxies have more complex stellar populations than single clusters.
They have a mix of stars formed at different times.
YoungOld
![Page 26: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/26.jpg)
Galaxies with more recent star formation have a larger fraction of young main sequence stars.
Galaxies with no young stars have red giants as their brightest stars.
YoungOld
![Page 27: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/27.jpg)
Young Old
These stars are bluer and brighter than these stars
Galaxies that are forming stars are BLUER and BRIGHTER
![Page 28: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/28.jpg)
Early-Type Galaxies from the Sloan Digital Sky Survey (SDSS)
![Page 29: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/29.jpg)
(red because of dust)Late-Type Galaxies From SDSS
![Page 30: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/30.jpg)
• “Early-Type”• More Massive• Gas Poor• Older Stars
• “Late-Type”• Less Massive• Gas Rich• On-going Star Formation
Early Types tend to be redder than Late Types
![Page 31: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/31.jpg)
– Early-type galaxies turned almost all of their gas into stars, very quickly, very early in their lives!
– Late-type galaxies turn gas into stars slowly, and have lots of gas left today. They’re forming lots of stars at the present day, at late times.
![Page 32: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/32.jpg)
– What did an elliptical look like billions of years ago?
– What will a late-type spiral look like in billions of years from now?
![Page 33: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/33.jpg)
Stars preserve motion, so
motion reveals history
• Disk ROTATION– Made from gas with high angular momentum
• Stellar halo, bulge RANDOM ORBITS– Made from many different blobs of gas & stars
What do the shapes of galaxies tell us?
![Page 34: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/34.jpg)
Smooth collapse of rotating gas= DISKS
Messy merging of large blobs= SPHEROIDS
(bulges, ellipticals, stellar & dark matter halos)
THESE PROCESSES ARE ONGOING!
![Page 35: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/35.jpg)
Galaxies that do not fit intothe extendedHubble system- ’Peculiar’galaxies
![Page 36: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/36.jpg)
Malin I
Low surface brightness galaxy - LSBG
![Page 37: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/37.jpg)
???
“Dwarf” or “Irregular” Galaxies
There are galaxies beyond the Hubble Sequence that continue this trend.
![Page 38: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/38.jpg)
M31
![Page 39: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/39.jpg)
M32
NGC 205
![Page 40: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/40.jpg)
• Low mass (107-109 stars, vs 1010 for spirals)• High star formation rates (usually)• No obvious bulge or spiral patterns.• Most numerous type of galaxy in the Universe!
“Dwarf” or “Irregular” galaxies tend to have more chaotic appearances…
![Page 41: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/41.jpg)
Dwarf galaxies from the Sloan Digital Sky Survey.
![Page 42: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/42.jpg)
Dwarf Spheroidal galaxy (dSph) in Antila
![Page 43: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/43.jpg)
• Dwarf galaxies are the most numerous type of galaxy in the universe.
• Most giant galaxies are probably made up of merged dwarf galaxies!
Andromeda galaxy
Map of its individual
stars
![Page 44: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/44.jpg)
They’re faint and low contrast against the night sky
A Good Dwarf Galaxy is Hard to Find
![Page 45: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/45.jpg)
The standard broadband photometric system
![Page 46: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/46.jpg)
The morphology isdepending on the frequency windowused at theobservations
![Page 47: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/47.jpg)
M82
Starburst galaxy
![Page 48: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/48.jpg)
The Large Magellanic Cloud (LMC)
![Page 49: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/49.jpg)
The distribution of gas in the LMC
![Page 50: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/50.jpg)
The magellenic stream (gas distribution)
![Page 51: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/51.jpg)
![Page 52: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/52.jpg)
The Local Group• 36 known and probable galaxy members• Radius ~1200 kpc• 3 spirals: M31, Milky Way, and M33.
– 90% of light.• Two more massive galaxies:
– the irregular Large Magellanic Cloud– the small elliptical galaxy M32
• All other galaxies in the Local Group are dwarf galaxies
![Page 53: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/53.jpg)
![Page 54: Galaxy classification - Columbia University](https://reader031.vdocuments.us/reader031/viewer/2022022519/6217cfa72288df0d0f1d377d/html5/thumbnails/54.jpg)