Transcript Diapositiva 1

GALAXIAS
Optico
Infrarojo
The Galactic Disk
The disk of our Galaxy is made up of
three main components:
 Stars
 Gas
 Dust
The gas in between stars is the
“Interstellar Medium” (or “ISM”)
Like everywhere, most gas in the disk is hydrogen.
Molecular (H2):
Cold, dense, tightly clumped.
Stars form within molecular clouds.
“Neutral” or Atomic (HI):
Cool, less dense, less tightly clumped.
Most common phase; the reservoir for forming the next
generation of stars.
“Ionized” (HII):
Hot, more diffuse.
By-product of forming young stars.
Young massive
stars die out,
and electrons
and nuclei
recombine
Ionized
HII
Atomic
HI
Gas is
compressed,
and cools
Molecular
H2
Young massive O-stars
form, and ionize the gas
Fountains and
Blowouts
SNe from newly
formed massive
stars can make
holes in the disk,
driving material out
of the Galaxy or
redistributing it
Blowout in M82 “starburst” galaxy…
Red is “false
color”, showing
location of
extremely hot gas
detected in
X-rays.
Hot because the
gas is mostly the
“ejecta” from
supernovae!
Los brazos de la galaxia
son regiones de alta
densidad donde ocurre la
mayor parte del proceso
de formación de nuevas
estrellas.
Por otro lado, las ondas de
choque generadas por
explosiones de estrellas
evitan que los brazos se
dispersen.
Los brazos no son
rígidos; más bien son
ciertas zonas que las
estrellas atraviesan en
su movimiento de
rotación alrededor del
centro galáctico.
Se calcula que el Sol ya
dio
varias
vueltas
completas.
En
este
momento, está en una
zona "tranquila", al borde
de un brazo secundario.
The Bulge:
A Very Crowded Neighborhood
The density of stars in the
bulge is about
50,000 per cubic parsec
By comparison, the
nearest star to the Sun is
1.3 pc away!
Milky Way is believed to have a
bar at its center…
The Galactic Halo: Ancient Stars
No gas, so no star formation…
…just a bunch of old stars, either by
themselves or in globular clusters.
Stuff in Galaxies moves in two basic ways…
• Ordered Rotation
– Nearly circular
orbits (like planets
around the Sun)
• Randomly
– Highly elliptical
orbits, plunging in
and out of the
center of the
galaxy, at random
orientations.
Side View of the Milky Way
The “halo” is really the “stellar halo” – turns out there’s
actually a larger halo we can’t even see!
Globular clusters
live in the halo
Turns out
there’s a
lot of this
invisible
matter.
Overall Properties of the
Galactic Disk, Halo, and Bulge
Property
Disk
Halo
Bulge
Shape
highly flattened like a
frisbee
roughly spherical; mildly
flattened
M&M shaped
Star Types
young and old
only old stars
young and old stars;
more old stars at greater
distances from center
ISM
gas and dust
no gas or dust
gas and dust, especially
in inner regions
New Stars?
ongoing star formation
no star formation for
past 10 billion years
ongoing star formation in
inner regions
Dynamics
contents move in
circular orbits in the
Galactic plane
stars have random orbits
in three dimensions
largely random orbits
with some net rotation
about the center
Substructure
spiral arms
no obvious substructure
nucleus; ring of gas and
dust near center; bar
Color
overall white color with
blue spiral arms
stars reddish in color
due to old age and cool
temperatures
yellow-white due to mix
of stars
Observational tracers of magnetic fields
• Polarization of starlight: perpendicular field in 2 or 3 kpc
orientation // B⊥ ------------- 9000 stars
• Zeeman splitting: parallel field, in situ (masers, clouds)
△ ∝ B//
------ 30 masers
• Polarization at infrared, mm: perpendicular field
orientation // B⊥ ------ clouds & star formation regions
• Synchrotron radiation: vertical field structures (added)
total intensity S ∝ B⊥2/7,
p% ∝ B⊥u2 / B⊥t2
• Faraday rotation: parallel field, integrated (the halo & disk)
RM ∝ ∫ ne B// ds ------ 500 pulsars + >1000 EGSes
Large-scale magnetic field in the Galactic disk
The largest coherent field structrue detected in the Universe!
Poloidal & Toroidal fields near GC
(from Novak et al. 2003)
Predicted
B-direction
GC
Toroidal fields
(Novak et al. 2003, 2000)
• permeated in the
central molecular zone
(400pc*50pc)
• sub-mm obs of p%
• toroidal field directions
determined by averaged
RMs of plumes or SNR!
Poloidal field
filaments Unique to GC
--- dipolar geometry!
150pc
(Morris 1994; Lang et al.1999)
The Milky Way:
A Barred Spiral Galaxy
The Milky Way is a
“spiral” galaxy,
sometimes also called
a “late type” galaxy.
Side View
“edge-on”
Top View
“face-on”
“Sombrero Galaxy”
• Elliptical Galaxies (or just “ellipticals”)
– No disk! old!
“spheroidals”
“S0” galaxies: Like ellipticals, but usually a
bit flatter.
Early Types
Ellipticals
Late Types
Unbarred and Barred Spirals
Lenticulars
Varying amounts of bulge & disk components
suggests different formation & evolution history
On average…
• Older Stars
• Gas Poor
• More Massive
• On-going Star Formation
• Gas Rich
• Less Massive
Early-Type Galaxies
from the Sloan Digital Sky Survey (SDSS)
Late-Type Galaxies From SDSS
(red because of dust)
There are galaxies beyond the Hubble
Sequence that continue this trend.
???
“Dwarf” or “Irregular” Galaxies
“Dwarf” or “Irregular” galaxies tend to
have more chaotic appearances…
•
•
•
•
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 galaxies from the Sloan
Digital Sky Survey.
“Gas Infall”
• Galaxies continue to form stars.
– Just enough gas in galaxy disks today
to form stars for <109 years.
– Fresh gas must keep it going.
• Fraction of metals (non-H, He) in
stars is lower than expected.
– Fresh Hydrogen must be flowing in.
“Merging” or “Galaxy Interactions”
• Gravity
pulls
galaxies
together!
• They can
orbit each
other &
eventually
merge!
“The Antennae”
(Hubble Image @ Right)
Multiple cores in some
ellipticals in clusters of
galaxies
“Minor
Mergers”
We know this is currently
happening…
Zoom-in
M16 (Eagle)
M17 (Horseshoe)
Milky Way
M8 (Lagoon)
Hale-Bopp
Jupiter
Picture credit: W. Keel
Eagle
Nebula
(M16)
Eagle
Nebula
(M16)
Eagle
Nebula
(M16)
size of our solar system
Eagle
Nebula
(M16)