The Other White Meat

In Galaxies by Brian Koberlein5 Comments

When you picture a galaxy, you probably imagine a graceful spiral galaxy. While spiral galaxies are quite common, there are other types. When Edwin Hubble first categorized galaxies, he labeled them as spiral, irregular or lenticular. While spirals are the most common type of galaxy, the second most common is lenticular.

Hubble's tuning fork. Credit: GalaxyZoo

Hubble’s tuning fork. Credit: GalaxyZoo

In Hubble’s day, some presumed that galaxies began as irregular galaxies, when then collapsed into flat spiral galaxies over time. We now know that isn’t the case. It turns out ellipticals begin as small spiral galaxies, in early galactic clusters, but frequent collisions stirred them up. As they ran out of gas and dust to fuel star production and maintain their spirals, further collisions led to their irregular shape. Lenticular galaxies share features of both spirals and irregulars. Like spirals they tend to have a central bulge and a flat disk shape. Like irregulars they tend to be populated by old stars. So how did these galaxies form?

The Sombrero galaxy is a fascinating lenticular. Credit: NASA/ESA

The Sombrero galaxy is a fascinating lenticular.
Credit: NASA/ESA

If they were irregular galaxies on the way to becoming spirals, one would expect them to have active regions of star production and plenty of gas and dust. But lenticulars don’t have much star production, and while they have dust, there’s not as much as in spiral galaxies. Given that lenticulars are populated by older stars, it seems that they are aging spiral galaxies. As spiral galaxies age, they gradually consume the gas available for star production. These regions of gas help maintain the spiral shape of the galaxy. But after the gas is consumed, the spiral arms begin to smear out, and the remaining dust tends to follow circular arcs.

So what once looked like a transition between spiral and irregular galaxies seems to be the shape of things to come for many spiral galaxies.

 

Comments

  1. Spiral galaxies are the most common? What about dwarf galaxies? Ignoring dwarf galaxies, is it not more appropriate to say at what time spiral galaxies outnumber elliptical galaxies?

  2. I really need to know what the latest numbers on galaxy classification are. I can’t seem to find one good place for it. I thought Galaxy Zoo would have something but I can’t seem to find a definitive answer for what the most common type of galaxy is after dwarfs. I have even heard it said that elliptical galaxies are more common than spirals. I understand that it’s different depending on z but I don’t understand what the overall observable universe is thought to contain.

  3. The entire second paragraph before “Lenticular galaxies share features” is extremely confusing.

  4. Ugh, ok, one more post. Sorry for the spam. I am just very interested and confused about this subject at the same time. How does one properly distinguish lenticular galaxies from spiral and elliptical galaxies? I’ve seen all different sizes of dust disks at the centers of both lenticular and elliptical galaxies. Sometimes an edge-on lenticular may look like a spiral. I also wonder if any elliptical galaxies could actually be smooth, face-on disks, which I suppose would be the last vestige of a once-spiral galaxy.

    The Sombrero seems like an exceptionally dusty lenticular. This is more what I think of when I think of “typical” (is there such a thing?) lenticulars. https://www.flickr.com/photos/geckzilla/7508292564/

  5. First line “irregular galaxies, when then collapsed” sbe “which then collapsed”; but

    “It turns out ellipticals begin as small spiral galaxies, in early galactic clusters, but frequent collisions stirred them up. As they ran out of gas and dust to fuel star production and maintain their spirals, further collisions led to their irregular shape. ”

    is extremely confusing. You start talking about ellipticals and end with irregulars with no clear separation.

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