The (Not So) Curious Case of Galaxy IC 335

This odd-looking galaxy has recently become famous in the media, not for what it has but for what is missing!

Excerpt from huffingtonpost.com

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!

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The (Not So) Curious Case of Galaxy IC 335