Lenticular Galaxies – Breathtaking Celestial Hybrids

Lenticular galaxies are celestial hybrids. Their name means “lens-shaped,” and they blend characteristics from other galaxies. For example, lenticulars have a flat, disk-like structure similar to spirals but without the signature spiral arms. Like ellipticals, lenticular galaxies have older star populations with few stellar nurseries.

Scottish astronomer James Dunlop discovered NGC 6861 in 1826. Image: ESA, Hubble, NASA, and J. Barrington

Introduction to Lenticular Galaxies

This glimpse into lenticular galaxies will explore how they form. We’ll look at astronomers’ questions, like the makeup of their stellar populations. Then, we’ll delve into the influence of their cosmic neighborhoods.

Lenticular Galaxies: Structure

Astronomers have some theories about the evolution of lenticular galaxies. One version says they are ancient spirals with faded arms that have run out of gas. But another idea believes their structure comes from spiral galaxy mergers.

NGC 6861, above, is part of the Telescopium Group. It’s the second most bright of at least a dozen galaxies. And more closely neighbored galaxies means more chances for mergers. So, that could be why NGC 6861 has a lenticular structure.

Lenticular galaxies have central disks and bulges but don’t have significant amounts of interstellar material. That’s the star-birthing matter necessary for stellar nurseries. Instead, they mostly contain old Population II stars. In the 1940s, Walter Baade divided stars in galaxies into two groups. And while astronomers classify stellar populations in a more refined system today, the coarse classifications still work.

The location of stars within a galaxy helps determine today’s star classifications.

  • Thick Disk
  • Thin Disk
  • Bulge
  • Halo

However, since many of the stars within lenticular galaxies are Population II stars, astronomers often initially misclassify the galaxies as elliptical. That’s especially true when viewed face-on.

Lenticular Galaxies: Structure
Note the ghostly haze in this lenticular galaxy, NGC 6684. There are no distinct arms around the central bulge. Image: ESA, Hubble, NASA, and R. Tully

The Hubble Sequence and Lenticular Galaxies

In 1926, Edwin Hubble developed a way to classify galaxy evolution. Just as the stellar population groups are outdated, so is the Hubble Sequence or Tuning Fork. However, it’s an excellent basic introduction to how galaxies evolve.

The diagram shows elliptical and spiral galaxies. Hubble assigned the numbers zero to seven to elliptical galaxies. An EO is almost perfectly circular or round, while an E7 is drawn out and very elliptical.

Spiral galaxies get the letters A, B, or C to describe the compactness or tightness of their spiral arms. For example, an “Sa” is tightly wound, while an “Sc” has loosely formed arms. In addition, Hubble noted that a galaxy’s central bulge size increases when the arms are compact.

Spirals are further divided into two groups: normal and barred. The most significant difference is a bar of stars running through the central disk in barred spirals. Their arms start at the end of the bar rather than from the central bulge. So barred spirals get a “B” in their class name. Therefore, an “SBa” is a tightly wound-barred spiral galaxy.

YouTube Hubble Sequence Link

But What About Lenticulars on the Hubble Sequence?

Lenticular galaxies fall under the classification of “S0” on the Hubble Tuning Fork. You can see it right at the mid-point before the fork of the two spiral arms. Scientists see many similarities in S0, Sa, Sb, and elliptical galaxies.

Lenticular galaxies bridge the other two types with a structure pulling from both elliptical and spiral galaxies.

But What About Lenticulars on the Hubble Sequence?
Image: NASA and ESA

How Lenticular Galaxies Form

Early astronomers took Hubble’s classification as an evolutionary sequence. However, today, we know that is an oversimplification. Galaxies form and evolve in far more complex processes than the tuning fork let on.

So, even though scientists still use some of the same terminology, they know that galaxies form in many ways. They initially collapse, some collide with other galaxies, and they experience a natural progression of internal star birth and death.

Scientists aren’t positive about how lenticular galaxies form. They could be old fading spirals or come from merging two galaxies. But in general, galaxies are like grand cities of stars. Stellar nurseries birth stars from swirling cosmic dust. It clumps together through a process called accretion. More and more dust swirls together to form a proto-star. Galaxy “cities” are so visible in the night skies because of the light from all their stars, just like the shining lights from any city on Earth.

Look at the spiral-like motion of lenticular galaxy NGC 524 below. The Hubble Space Telescope snapped this galaxy view about 90 million light-years away in the Pisces constellation. Scientists think this bright disk of old, red stars only held onto small amounts of dust and gas.

They believe the galaxy likely formed from an aging spiral galaxy. At one time, it had pinwheel arms that held dense clouds of dust and gas. So thick that star birth was a regular occurrence. But now, NGC 524 maintains only some of its spiral motion.

How Lenticular Galaxies Form
Image: ESA, Hubble, NASA, and Judy Schmidt

Stellar Populations in Lenticular Galaxies

As we’ve mentioned, lenticular galaxies tend to hold Population II stars. They’re older stars, mainly in galaxies’ bulge or halo regions. They are also low-metal content stars, which means they form at the beginning of a galaxy’s existence. So, many of the stars in lenticular galaxies are billions of years old.

Spiral galaxies often contain enormous stellar nurseries along their dense and bright arms. Check out this Very Large Telescope view of NGC 1232, a grand spiral galaxy. You can see millions of the brightest stars along the spiral arms. The dark interstellar dust lanes hold billions more dimmer normal stars. Can you see the differences between this vital and lively galaxy and the hazier lenticular galaxies?

Stellar Populations in Lenticular Galaxies
Image: NASA, FORS, 8.2-meter VLT Antu, ESO

Environmental Influence on Lenticular Galaxies

Scientists believe that an S0 galaxy’s environment influences its evolution. For example, one study suggests that lenticulars in the field likely formed through a merger. But those in clusters probably developed through rapidly consuming its gases. Perhaps it started as a spiral that starved or succumbed to ram pressure stripping.

The aging galaxy below is NGC 5010, and scientists think it’s in a transition stage. It’s moving from a spiral to an elliptical galaxy but is currently a lenticular. It has features of both types, as seen in this Hubble Telescope view. There are dark spiral arm remnants, but the galaxy is developing a bulge in its disk as it becomes more rounded.

You can also see that the stars are less bright as they age. Few younger blue stars are present since most of the required interstellar dust and gas are gone.

Environmental Influence on Lenticular Galaxies
Image: ESA, Hubble, and NASA

Ongoing Lenticular Galaxy Research

As recently as September 15, 2023, NASA released new images of a lenticular galaxy. The Hubble Space Telescope spotted NGC 3156 about 73 million light-years away in the Sextans constellation.

Notice the dark red-brown dust that crosses the galaxy’s disk. You can see its central bulge of stars surrounded by a vast disk like a spiral galaxy. But there are no arms. And like an elliptical, NGC 3156 has mostly older stars.

Scientists use Hubble data to study the stars in this lenticular galaxy compared to galaxies with black holes of a similar size.

  • Some are globular clusters bound together by gravitational attraction.
  • Others are gobbled up and destroyed by the galaxy’s supermassive central black hole at a higher rate than in other galaxies.

Continued studies of lenticular galaxies help scientists continually improve the classification process that Edwin Hubble began.