Lenticular Galaxies are the Quiet, Dusty Stellar Cities of the Cosmos

lenticular galaxy
Galaxy NGC 5010 -- a lenticular galaxy which has features of both spirals and ellipticals. NASA/ESA/STScI

There are many types of galaxies "out there" in the universe. Astronomers tend to classify them first by their shapes: spiral, elliptical, lenticular, and irregular. We live in a spiral galaxy, and we can see others from our vantage point on Earth. A survey of galaxies in clusters such as the Virgo cluster shows an amazing array of different shapes of galaxies. The big questions that astronomers who study these objects ask is: how do they form and what is in their evolution that influences their shapes?

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A dusty spiral galaxy as seen by NASA's Hubble Space Telescope. NASA, ESA, and D. Maoz (Tel-Aviv University and Columbia University)

Lenticular galaxies are rather poorly-understood members of the galaxy zoo. They are similar in some ways to both spiral galaxies and elliptical galaxies but are really thought to be a sort of transitional galactic form. 

For instance, lenticular galaxies appear to be like a fading spiral galaxy. However, some of their other characteristics, like their composition, are more in line with elliptical galaxies. So, it's very possible that they are their own, unique galaxy type. 

lenticular galaxy
Galaxy NGC 5010 is a lenticular galaxy which has features of both spirals and ellipticals. NASA/ESA/STScI

Structure of Lenticular Galaxies

Lenticular galaxies generally have flat, disk-like shapes. However, unlike spiral galaxies, they lack the distinctive arms that usually wrap themselves around the central bulge. (Though, like both spiral and elliptical galaxies, they can have a bar structure passing through their cores.)

For this reason, lenticular galaxies can be difficult to tell apart from elliptical ones if they are viewed face-on. It's only when at least a small part of the edge is apparent can astronomers tell that a lenticular is distinguishable from other spirals. Even though a lenticular does have a central bulge similar to that of spiral galaxies, it can be much larger.

Judging by the stars and gas content of a lenticular galaxy, it is far more similar to an elliptical galaxy. That's because both types have mostly old, red stars with very few hot blue stars. This is an indication that star formation has slowed significantly, or is non-existent in both lenticulars and ellipticals. Lenticulars usually have more dust content than ellipticals, however.

Lenticular Galaxies and the Hubble Sequence

In the 20th century, astronomer Edwin Hubble set about trying to understand how galaxies form and evolve. He created what is known as the "Hubble Sequence" - or graphically, the Hubble Tuning For diagram, which placed galaxies on a sort of tuning-fork shape based on their shapes. He imagined that galaxies began as ellipticals, perfectly circular or nearly so.

Then, over time, he thought their rotation would cause them to flatten out. Eventually, this would lead to the creation of spiral galaxies (one arm of the tuning fork) or barred Spiral galaxies (the other arm of the tuning fork).

Hubble classification diagram.
Lenticular galaxies are likely a transiition between elliptical and spiral on the standard Hubble tuning fork diagram that classifies galaxies by their shapes.  NASA

At the transition, where the three arms of the tuning fork would meet, there were the lenticular galaxies; not quite ellipticals not quite spirals or barred Spirals. Officially, they are classified as S0 galaxies on the Hubble Sequence. It turned out that Hubble's original sequence didn't quite match the data we have about galaxies today, but the diagram is still very useful in classifying galaxies by their shapes.

Formation of Lenticular Galaxies

Hubble's groundbreaking work on galaxies may have influenced at least one of the formation theories of lenticulars. Essentially, he proposed that lenticular galaxies evolved out of elliptical galaxies as a transition to a spiral (or barred spiral) galaxy, but one current theory suggests that it could be the other way around.

Since lenticular galaxies have disk-like shapes with central bulges but have no distinctive arms, it is possible that they are simply old, faded spiral galaxies. The presence of a lot of dust, but not a lot of gas suggests that they are old, which would seem to confirm this suspicion.

But there is one significant problem: lenticular galaxies are, on average, much brighter than spiral galaxies. If they were truly faded spiral galaxies, you would expect them to be dimmer, not brighter.

So, as an alternative, some astronomers now suggest that lenticular galaxies are the result of mergers between two old, spiral galaxies. This would explain the disk structure and the lack of free gas. Also, with the combined mass of two galaxies, the higher surface brightness would be explained.

This theory still needs some work to solve some issues. For example, computer simulations based on observations of galaxies throughout their lives suggest that the rotational motions of the galaxies would be similar to those of normal spiral galaxies. However, that is generally not what is observed in lenticular galaxies. So, astronomers are working to understand why there is a difference in rotational motions between types of galaxies. That finding actually lends support to the fading spiral theory. So, the current understanding of lenticulars is still a work in progress. As astronomers observe more of these galaxies, the additional data will help solve the questions about where they lie in the hierarchy of galaxy forms.

Key Takeaways about Lenticulars

  • Lenticular galaxies are a distinct shape that seems to be somewhere between spiral and elliptical.
  • Most lenticulars have central bulges and seem to have differences in their rotational actions from other galaxies.
  • Lenticulars could be forming when spiral galaxies merge. That action would form the disks seen in lenticulars and also the central bulges.

Sources

  • “How to Make Lenticular Galaxies.” Nature News, Nature Publishing Group, 27 Aug. 2017, www.nature.com/articles/d41586-017-02855-1.
  • Information@eso.org. “The Hubble Tuning Fork - Classification of Galaxies.” Www.spacetelescope.org, www.spacetelescope.org/images/heic9902o/.
  • "Lenticular Galaxies and Their Environments." The Astrophysical Journal, 2009, Vol 702, No. 2, http://iopscience.iop.org/article/10.1088/0004-637X/702/2/1502/meta