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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 (H 2 ): Cold, dense, tightly clumped.

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the galactic disk
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
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

Gas is compressed, and cools

Atomic HI

Molecular H2

Ionized HII

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


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 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!

the galactic halo ancient stars
The Galactic Halo: Ancient Stars

No gas, so no star formation…

…just a bunch of old stars, either by themselves or in globular clusters.


Ordered Rotation

    • Nearly circular orbits (like planets around the Sun)
  • Randomly
    • Highly elliptical orbits, plunging in and out of the center of the galaxy, atrandom orientations.

Stuff in Galaxies moves in two basic ways…


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

observational tracers of magnetic fields
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
Large-scale magnetic field in the Galactic disk

The largest coherent field structrue detected in the Universe!

poloidal toroidal fields near gc

Poloidal & Toroidal fields near GC

(from Novak et al. 2003)

Predicted B-direction


Toroidal fields

(Novak et al. 2003, 2000)

permeated in the

central molecular zone


sub-mm obs of p%

toroidal field directions

determined by averaged

RMs of plumes or SNR!

Poloidal field

filaments Unique to GC

--- dipolar geometry!

(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


Top View



Early Types

Late Types

Unbarred and Barred Spirals




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
late type galaxies from sdss
Late-TypeGalaxies From SDSS

(red because of dust)

there are galaxies beyond the hubble sequence that continue this trend
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
“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!

“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
“Merging” or “Galaxy Interactions”
  • Gravity pulls galaxies together!
  • They can orbit each other & eventually merge!

“Minor Mergers”

We know this is currently happening…



M16 (Eagle)

M17 (Horseshoe)

Milky Way

M8 (Lagoon)



Picture credit: W. Keel


size of our solar system

Eagle Nebula