Friday Galaxy : Nearby Starburst Galaxy M82

M82 is a galaxy that's relatively near to the Milky Way. It's not in our own local group, but it's in a nearby group of galaxies (the "M81 group"), and is only about 12 million light-years away (which is close for a galaxy). It's notable because it's a "starburst" galaxy— it's undergoing a burst of rapid star-formation, producing large numbers of stars in big clusters in a relatively short period of time. A lot of this activity is near the nucleus of the galaxy.

M82 is a favorite target for infrared astronomers. My cohort in graduate school, James Larkin, wrote his first grad school paper on the galaxy; both of us were part of what was known as the IRA, or "InfraRed Army," at Caltech.

The image below was taken with the Hubble Space Telescope. You can see the disk of the galaxy, but it's clear that there's some action going on at the center, buried behind thick dust. The plumes coming out of the center are mostly Hydrogen gas which has been blown out of the galaxy by shocks from supernova explosions; such explosions are much more common in M81 than in our own galaxy (where we only have about one every hundred years or so) because of the rate at which stars (including the massive, short-lived stars that eventually supernova) are being made.


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Should be 12,000,000 light-years, right? Just 12,000 LY would be close for a galaxy.

By Alan Lund (not verified) on 23 Mar 2007 #permalink

Tell us something about the fascinating theories regarding the regulation of start formation rates. I think high rates lead to the gas being expelled for the disk turning off new star formation -then perhaps
the gas recollects in the disk... Plus I think something similar may be going on with the central supermassive black holes. But my understanding is way too superficial.

Oops! Yes, 12,000,000 light-years. I'll fix the text. Sorry about that.

bigTom -- yes, what you're talking about is called "starburst feedback". If you get too many supernovae, it can blow away the gas and quench the star formation.

The other thing you mention is "AGN feedback", where AGN=Active Galactic Nucleus. The AGN is fueleld by the black hole. If you bring a lot of gas into a galaxy (e.g. through a merger), it can start rapid star formation (which needs a lot of gas in a relatively small volume), and it can also feed the black hole turning on an AGN. The AGN will include a lot of high-energy radiation, outflows, and relativistic jets that can blow away the remaining gas in the system, thereby removing the fuel for and quenching star formation.

I will write a bit more about that at some point when I finally get to writing the post I've been planning for months about supermassive black holes at the cores of galaxies....


Is the gas primordial hydrogen that's been floating around the galaxy unconsolidated, or is it recycled former star material? What is the mass balance in a typical galaxy, in terms of gas/ dust/ various star types ratios?

The hydrogen is both. Most gas clouds in galaxies are a mix of gas that has been through various different generations of stars. Very little gas in at least bright galaxies is primordial any more; there are some very low surface-brightness galaxies that have near-primordial gas.

Almost all of the atoms that make up your body and that make up the Earth have been through more than one star in the past.

In the Milky Way, the baryonic mass is about 90% stars, 10% gas. (The overall mass is about 90% dark matter, 10% baryons.) However, galaxies vary a lot. Some galaxies have almost no gas, or at least no cool gas. (Elliptical galaxies have some very hot ionized plasma, but generally no cool gas.) Some galaxies, particularly smaller dwarf galaxies, can be 90% or more gas. The bigger ones tend to be 10%ish or thereabouts, or less.

When it comes to stars: the vast majority of the stellar mass of a galaxy is made up from the low-mass stars (stars less than 1 solar mass). However, the vast majority of the light you see comes from, typically, heavier stars. If it's a galaxy that's actively forming a lot of stars and has been for some time, it will tend to be pretty blue, and a fair amount of the light you see is from short-lived high-mass stars. Even still in those galaxies, most of the stellar mass is in the low-mass stars whose integrated light doesn't add up to much. In galaxies that aren't forming a lot of stars, the light you see comes from the red giants, which must have been stars that started at at least 0.8 solar masse (for more massive ones live longer than the age of the Universe).

In other words, the starlight we're looking at is not coming from the stars that make up most of the stellar mass of the galaxy.... If the "Initial Mass Function" (IMF) is near-universal, then the ratio of the high-mass to low-mass stars should be similar for all galaxies making stars right now, and the ratio of red giants to low-mass stars can be fairly well predicted as a function of how long its been since stars were last made in bulk. The IMF is probably not truly universal, but is probably approximately so, at least in recent epochs, so generally the light that traces starlight (red light and near-infrared light) is thought to be a good measure of the total stellar mass.


Just to clarify, are black holes and neutron stars a negligible portion of the total mass of galaxies?

For a sort-of related question, would you expect an active, big-star-forming galaxy like M-81 to have a higher ratio of n-process/s-process isotopes than what we have here? Are there any galaxies in which the opposite would be true? e.g. lots of S process, little or no N.

In the last paragraph, by 'plume' do you mean the 'red' materials in the image? Is it really red and how can hydrogen gas emit in that band? Thanks.

By bicycledaave (not verified) on 03 Apr 2007 #permalink

There are 4 quasars with identical redshifts within 8 arc minutes of M82 and having small angular separations. Could they be physical associated with each other and possibly M82?

Could they have been ejected from M82, as they lie along a
small cone on the southeast side of M82...

Also there's a radio source originating at the origin of the ejection cone, presumably as a consequence of the ejection process ?