Some spiral galaxies...



So, let us begin our journey with the spiral galaxies.

Spiral galaxies are made up of a central bulge and a dusty disk, inside which curving arms are found. They are classified according to the shape of their central nucleus and to the tightness of their arms.
If the nucleus is large and the arms are tights, the galaxy will be classified as class Sa ; in the opposite case, if the arms are enormous in relation to the nucleus, it will be a class Sc. galaxy.

M104, galaxy type Sa

M104, the Sombrero galaxy, of class Sa, lying 30 million light years away, in the constellation of Virgo.
Notice the dark dust disk and the significant halo around the galaxy, which stands out especially since the galaxy is edge-on.
Source ESO / VLT

Messier66, Sb spiral ESO 269-57, Sc galaxy
Messier 66, a class Sb galaxy in Leo.
Source : AAO
ESO 269-57 , a class Sc galaxy in the constellation of Centaurus, 150 million light years away.
Source ESO / VLT

NGC 4526, type S0
Some galaxies do not have arms in the disk : they are called lenticular galaxies, in the class S0.
Left, NGC 4526 a galaxy lying 26 Mal away in Virgo.

Note the type Ia supernova SN1994D on the bottom left of the galaxy.

Image : Hubble space telescope


Spiral galaxies are often very bright and can harbour hundred billion stars. The high density of gas and dust in the disk allows a continuous formation of new stars, and hence the presence of a lot of young, hot stars. This formation, revealed by an intense ultra-violet radiation, takes place along the spiral arms.

Composite image of M74
A composite image of M74, a class Sc galaxy which is nearly the same size than the Milky Way, in the constellation of Pisces.
In red and yellow colors, the image of the galaxy in visible wavelenghts, and in blue and white colors in the ultra-violet part of the spectrum, characteristic of young stars forming along the spiral arms.
Source NASA/UIT.


structure of a spiral galaxy
Structure of a spiral galaxy : as well as the gas and dust disk, notice the presence of a large halo around it, where we can find globular clusters, which are like miniature elliptical galaxies. Compare this diagram with the image of M104


These galaxies exhibit a global rotating motion, but this motion is neither the same as a solid body, nor a planetary system with a central mass.

Speeds repartition in a spiral galaxy
A comparison of the rotation speeds of a solid body, a planetary system and a spiral galaxy, according to the distance to the center.

The speed curve in the arms of a galaxy implies the presence of a large mass outside its nucleus, spread throughout the galactic halo.
At the end of a spiral arm, the speed of the stars is typically about a few hundred km/s.


The arms do not rotate as a solid body, hence we conclude that the stars are not linked together inside an arm : they just cross it in a process known as density waves.


The phenomenon of density waves is similar to a traffic jam when a car brakes suddenly in the stream and speeds up again.

In the same way as the cars go in and out the traffic jam, the stars are just crossing the spiral arms.

Density waves

The density waves are always on the "trailing" side of the rotation. So they can discharge the galaxy's rotational energy, by transfering some of the rotation towards the outer edges of the galaxy. With this transfer, the galaxy becomes more compact.


Sometimes, there is a bar between the arms of the disk. In the past, the barred galaxies were considered oddities. With the latest measurements in the infra-red, about 3/4 of the galaxies can be considered to be barred spiral galaxies.

NGC1365, barred spiral SBb M83, barred spiral SBc
M83, of class SBc in Hydra

Source : Anglo-Australian Observatory

NGC1365, a class SBb galaxy in Fornax.
Lying 60 million light years away, it is a giant galaxy, 200 000 light years wide.
Its diameter is 2 times larger that our Milky Way's.

Source : ESO


The bar of a spiral galaxy is a mechanism that allows mass transfers towards the nucleus, and it is comparable to a stationary wave. The purpose of these transfers is, like density waves, to minimize the rotational energy of the galaxy.
What's more, this transfer mechanism is self-regulating : resonance phenomena arise to limit the mass which falls onto the central nucleus. These phenomena can stop the growth of the bar, and even make it disappear.
On the other hand, we can sometimes find, in the same galaxy, two bars attached to each other. This can accelerate the mass transfer.

During the life of a spiral galaxy, several bar/spiral episodes can happen.


Two main characteristics :


References :
Gas accretion on spiral galaxies: bar formation and renewal (F. Bournaud, F. Combes )