GALAXIAS

Optico

Infrarojo

The Galactic DiskThe 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.

Atomic HI

Molecular H2

Ionized HII

Gas is compresse

d, and cools

Young massive O-stars form, and ionize

the gas

Young massive stars die out, and electrons and nuclei recombine

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 StarsNo 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…

• Randomly– Highly elliptical

orbits, plunging in and out of the center of the galaxy, at random orientations.

• Ordered Rotation– Nearly circular

orbits (like planets around the Sun)

Side View of the Milky WayThe “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 BulgeShape highly flattened like a

frisbeeroughly 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 fieldsObservational tracers of magnetic fields• Polarization of starlight: perpendicular field in 2 or 3 kpcperpendicular field in 2 or 3 kpc

orientation // B⊥ ------------- 9000 stars

• Zeeman splitting: parallel field, parallel field, in situin situ (masers, clouds) (masers, clouds) △ ∝ B// ------ 30 masers

• Polarization at infrared, mm: perpendicular field perpendicular field orientation // B⊥ ------ clouds & star formation regions

• Synchrotron radiation: vertical field structures (added)vertical field structures (added)

total intensity S ∝ B⊥2/7, p% B∝ ⊥u

2 / B t⊥2

• Faraday rotation: parallel field, integrated (the halo & disk)parallel field, integrated (the halo & disk)

RM ∝ ∫ ne B// ds ------ 500 pulsars + >1000 EGSes

Large-scale magnetic field in the Galactic disk

The largest The largest coherentcoherent field structrue detected in the Universe! field structrue detected in the Universe!

Poloidal & Toroidal fields near GCPoloidal & Toroidal fields near GCToroidal fieldsToroidal fields

(Novak et al. 2003, 2000)• permeated in the central molecular zonecentral molecular zone (400pc*50pc)(400pc*50pc)

• sub-mm obs of p%• toroidal field directions determined by averaged RMs of plumes or SNR!

Poloidal fieldPoloidal field filaments Unique to GC --- dipolar geometry! (Morris 1994; Lang et al.1999)

(from Novak et al. 2003)

150pc

GC

Predicted B-direction

The Milky Way is a “spiral” galaxy, sometimes also called a “late type” galaxy.

The Milky Way: A Barred Spiral Galaxy

Side View

“edge-on”Top View

“face-on”

““Sombrero Galaxy”Sombrero Galaxy”

• Elliptical GalaxiesElliptical Galaxies (or just “ellipticals”)– No diskNo disk! old!

“spheroidals”

“S0” galaxies: Like ellipticals, but usually a bit flatter.

Ellipticals

Lenticulars

Unbarred and Barred Spirals

Early Types

Late Types

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…

Early-Type Galaxies from the Sloan Digital Sky Survey (SDSS)

(red because of dust)Late-Type Galaxies From SDSS

???

“Dwarf” or “Irregular” Galaxies

There are galaxies beyond the Hubble Sequence that continue this trend.

• 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!

“DwarfDwarf” or “IrregularIrregular” galaxies tend to have more chaotic appearances…

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.

• Gravity pulls galaxies together!

• They can orbit each other & eventually merge!

“Merging” or “Galaxy Interactions”

“The Antennae” (Hubble Image @ Right)

Multiple cores in some ellipticals in clusters of

galaxies

“Minor Mergers”

We know this is currently happening…

Jupiter

M16 (Eagle)

M17 (Horseshoe)

M8 (Lagoon)

Milky Way

Hale-Bopp

Zoom-in

Picture credit: W. Keel

Eagle Nebula(M16)

Eagle Nebula(M16)

Eagle Nebula(M16)

Eagle Nebula(M16)

size of our solar system