Starburst Galaxies

Look at the (relatively) nearby galaxy M82:

It is a small galaxy with a system of filaments emanating from the center. These filaments are most prominant when observed in Halpha, meaning they are filaments of ionized gas:

Halpha image


Broad-band Optical Image

What is happening in this galaxy? At first, people though some sort of titanic explosions were ripping the galaxy apart. This is partly right -- there are titanic explosions occuring in the center of M82, but they are not destroying the galaxy. They are supernovae, and we now know M82 to be a starburst galaxy.

A starburst galaxy is a galaxy which is experiencing a brief (107-108 year) burst of intense star formating activity. During these bursts, the star formation rate per unit area can be 10s or 100s of times greater than the star formation rate in normal spiral galaxies. These starbursts are most typically found in the inner kiloparsec of the starburst galaxy, and are sometimes called nuclear starbursts.

Realize that the star formation rate of M81 is similar to that of the luminous (non-starbursting) Sc spiral M101, yet the sizes are very different:


In a starburst galaxy, many massive young stars are formed, making the galaxy very bright and very blue. These massive stars also play havoc on the interstellar gas, heating it through stellar winds and supernovae, and can heat the gas to millions of degrees. This hot gas can flow out of the galaxy as a starburst wind. We can see this gas as the ionized filaments as well as in X-rays.

computer model of starburst wind
left: gas density (bright=dense)
right: gas temperature (bright=hot)


X-ray image of M82

Question: why would the starburst phase be short?

Starburst galaxies are not only bright and blue, but they are typically very dusty. Lots of gas, lots of dust, right? And what does dust do?

Could there be more starburst galaxies out there we don't know about? If dust is absorbing the light from the young stars and blocking us from seeing in the optical, that dust must heat up and emit infrared radiation. Starburst galaxies should be infrared bright.

In 1983, an infrared observing satellite was launched (IRAS). One of its jobs was to search for infrared bright starburst galaxies -- and it found lots.

Ultraluminous IRAS Galaxies. Bright in the infrared, with luminosities > 1012 Lsun. So luminous, in fact, that it is being argued whether or not star formation alone can power these galaxies -- maybe black holes are accreting material and providing luminosity as well.

What do these galaxies look like?

Verrrrry peculiar. As we shall see, these all show signs that they are or have been involved in interactions (and even mergers) with other galaxies. Interactions somehow cause starburst activity!

What about M82 -- is it interacting? You betcha!

Optical Image (stars)
HI image (neutral gas)