This odd-looking galaxy has recently become famous in the media, not for what it has but for what is missing!
IC 335 (credit: Hubble)
A recent Hubble image of this galaxy shows it to be a star-filled galaxy with a flat shape not unlike our own Milky Way. But whereas the Milky Way contains vast collections of nebulae and dust clouds, IC 335 seems to have none of this "interstellar medium." A look behind the curtain gives us clues to how two similar galaxies like IC 335 and the Milky Way could turn out so differently.
Nature vs. Nurture: IC 335 Didn't Start Out Looking This Way
This galaxy is a member of a large cluster of galaxies located in the constellation Fornax, at a distance of about 60 million light-years from the Milky Way. The Fornax Cluster contains about 100 galaxies within a volume of space only 10 million light-years across, making it a very tight family of galaxies traveling through space at several million miles per hour. The vast majority of the galaxies in this cluster are only 30,000 to 50,000 light-years across, including IC 335, but NGC 1316 and NGC 1365 are as large as our Milky Way and over 200,000 light-years in diameter. These are the two linchpin galaxies in the Fornax Cluster whose enormous gravity bends the motions of all other galaxies in the cluster so that they orbit these two mass centers. These two galaxies are also interacting with each other and, over billions of years, will probably fall together to create a ginormous "super galaxy."
Fornax Cluster (credit: Hubble)
Astronomers have also found over the years that the space between these member galaxies is filled by a hot X-ray-emitting gas, as well as by individual intergalactic stars that are not bound to any one of the member galaxies. In fact, 10 percent of all stars in the Fornax Cluster are found between the galaxies and not within the galaxies! This is a sure sign that the Fornax Cluster has been the site of many galaxy smashups over the last few billions of years. In fact, NGC 1316 is a complex, massive galaxy that is also the strongest radio-emitting object in this cluster and was called Fornax A in the early days of radio astronomy. It is the fourth-brightest radio source in the sky! The shape of this galaxy shows the signs of many collisions and cannibalism events, but Fornax A is particularly famous as one of the nearest supermassive black holes, weighing in at a gargantuan 150 million times the mass of our Sun.
When galaxies collide, things get very messy. Whatever gas may have been present in the colliding galaxies can be compressed to trigger new rounds of star-forming activity, or the gas can become superheated and get ejected from the galaxy "train wreck." Even individual stars that were minding their own business can get slingshot out of their host galaxy and cast adrift in intergalactic space. All these things have been observed in the Fornax Cluster.
Galaxy collisions are a gift that keeps on giving. Although the tenuous hot gas between the galaxies seems pretty dilute, it can act like slow-acting sandpaper on the gas and dust within other member galaxies. Over time the heated gas can cause the gas within smaller galaxies to be evaporated away, leaving behind only the stars in the galaxy.
The most common types of galaxies you find in dense clusters of galaxies are the so-called lenticular or elliptical galaxies. These are round or flat-looking galaxies that show little or no signs of having an interstellar medium. For some of these galaxies, the gas and dust they had was ejected during their collisions with other nearby galaxies. The gas and dust can also be consumed during the star-building process that is triggered by the collisions. This is probably what is happening in the giant galaxy NGC 1316. You can even see its companion galaxy NGC 1317 diving in for its last encounter!
The other massive galaxy in Fornax, NGC 1365, looks like a beautiful spiral galaxy and is called the Great Barred Spiral Galaxy in Fornax. We think that spiral galaxies probably form from ancient galaxy collisions where the galaxies grazed each other rather than from head-on events. What is left behind is often a beautiful pinwheel form, with gas and dust clouds sluggishly producing new stars over the course of billions of years. Our Milky Way is like that. We know it has experienced collisions in its past, but these were mostly cannibalism events involving much smaller galaxies.
Finally, near the center of the cluster, we have NGC 1399, which is a giant elliptical galaxy about 250,000 light-years across, with a 500-million-solar-mass black hole in its center. The galaxy itself contains about 4 trillion times the mass of our Sun in stars. It also has a hot halo of X-ray-emitting gas that surrounds it out to a distance of 400,000 light-years.
But What About IC 335?
The flattened shape of this galaxy suggests that it was once a typical dusty spiral galaxy like the Milky Way, but over time its encounter with the hot intergalactic medium and the relentless star-forming activity probably depleted its primordial stores of gas and dust clouds. Now these have all gone, leaving behind just an old and dying population of ancient stars with few young stars to replace them. In a dense cluster like Fornax, its destiny will probably involve being cannibalized by one of the larger galaxies, or it may be ejected from the cluster entirely.
Now You See It, Now You Don't
It is interesting to compare IC 335 with another edge-on galaxy called NGC 4565.
NGC 4565 in Coma Berenices (credit: Ken Crawford/Wikimedia Commons)
Here you can see the dust and gas clouds as they obscure the light from the stars in the galactic disk. This is a massive galaxy at least 10 times bigger than IC 335, but thanks to our lucky perspective we can use it to see what IC 335 may have looked like before it lost its gas and dust.
With just a tad more dust and gas, it might look like this beautiful image of NGC 2787!
NGC 2787 (credit: Hubble)
Thanks to the Hubble Space Telescope and other ground-based systems, astronomers can view many different kinds of galaxies in enough detail to identify "missing links" between some of the common forms that we see at lower resolution. The pictures also make for breathtaking calendar illustrations and screensavers.
I love this universe!