Hubble Portrait of a Beautifully Odd Spiral Galaxy

Bad Astronomy
The entire universe in blog form
Nov. 26 2013 8:00 AM

Beauty Is Only Universe Deep

Galaxies come in many shapes and sizes. You can make broad categories for them — spiral, elliptical, irregular, peculiar — but even within those bins there’s a lot of variation. Even more confusingly, they look very different depending on how you look at them: what telescope you use, what colors of light you use, and how you assemble the various observations into a single image.

NGC 4921 is a good, if spectacular, example of this. It’s a face-on spiral located about 300 million light years away, deep in the huge Coma Cluster, a vast collection of galaxies with over a thousand citizens.

NGC 4921
Hubble's grand view of the magnificent face-on spiral NGC 4921. Click to galactinate, and you want to.

Photo by NASA / ESA / Roberto Colombari / Hubble Legacy Archive


That Hubble image is breathtaking! It was put together by Roberto Colombari using observations from 2006 and 2007. It’s not really true color; it’s a combination of observations using two filters; one that lets through light in the red to the near-infrared (shown as red in the image), and the other that lets through light from the blue-green to red (shown as blue in the image). He also created a “pseudo-green” color by combining the blue and red images. This doesn’t add any real information, but helps produce a balanced three-color image that’s prettier.

Still, there’s a lot to see here. The most obvious feature is the spiral; the arms are clear though weaker than in most such galaxies. This is common with spiral galaxies in dense clusters; as they move through the cluster environment, gas between the galaxies can strip a galaxy of its own internal gas, much like you can roll down a car window to flush out the inside air as you drive down a highway. This ram stripping, as it’s called, can suppress the formation of stars, which in turn dampen the strength of the spiral arms themselves. Observations using different telescopes confirms that NGC 4921 is undergoing this sort of process. There is some ongoing star formation — you can see that as clumps of blue, where hot, massive stars are being born — but most spiral galaxies have a much stronger signal of this.

The next obvious bit is the complex structure of dust within the galaxy. This material blocks the light from stars behind it, so we see it as dark filaments twisted around the galaxy. I was surprised to see it popping out in this image, and all at roughly the same distance from the center. Rings of material like this sometimes happen after a galactic collision, or a near pass from another decent-sized galaxy. Again, that’s common in clusters.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

Also, you can see a lot of distance background galaxies as well. Most of these are much farther away, though some are also Coma Cluster members. NGC 4921 looks translucent in this image, actually, but again that’s because it’s so low in dust and gas. You really can see right through it!

Hubble has observed this galaxy many times, because it has a special kind of star in it called a Cepheid variable. These stars pulse at a fairly predictable rate, and that rate is related to their actual, physical luminosity: how much energy they emit. That means that we can measure how bright they appear to be, and use that to determine their distance! This makes them benchmarks in space, allowing us to figure out how far away galaxies are.

NGC 4921
A slightly different view of NGC 4921. Click to embiggen.

Photo by NASA, ESA and K. Cook (Lawrence Livermore National Laboratory)

This image is a combination of red/infrared observations and some taken using a blue filter (and again, a pseudo-green channel made by combining the two). It looks very similar to the new image, but as you can see the colors are a bit more washed out. The dust isn’t as obvious, nor are the star-formation regions. That’s all more an artifact of the filters used, but still, it shows that a galaxy can appear quite different just due to how you look at it.

These images of NGC 4921 are a good — and very beautiful — reminder that what you see is not what you get. There are many ways to see things, and they can all tell you something different and important. You have to be aware of that, and to know how to interpret it. Otherwise, how will you ever hope to understand the magnificent Universe around you?



Smash and Grab

Will competitive Senate contests in Kansas and South Dakota lead to more late-breaking races in future elections?

Stop Panicking. America Is Now in Very Good Shape to Respond to the Ebola Crisis.

The 2014 Kansas City Royals Show the Value of Building a Mediocre Baseball Team

The GOP Won’t Win Any Black Votes With Its New “Willie Horton” Ad

Sleater-Kinney Was Once America’s Best Rock Band

Can it be again?


Forget Oculus Rift

This $25 cardboard box turns your phone into an incredibly fun virtual reality experience.

One of Putin’s Favorite Oligarchs Wants to Start an Orthodox Christian Fox News

These Companies in Japan Are More Than 1,000 Years Old

Trending News Channel
Oct. 20 2014 6:17 PM Watch Flashes of Lightning Created in a Lab  
  News & Politics
Oct. 20 2014 8:14 PM You Should Be Optimistic About Ebola Don’t panic. Here are all the signs that the U.S. is containing the disease.
Oct. 20 2014 7:23 PM Chipotle’s Magical Burrito Empire Keeps Growing, Might Be Slowing
Oct. 20 2014 3:16 PM The Catholic Church Is Changing, and Celibate Gays Are Leading the Way
  Double X
The XX Factor
Oct. 20 2014 6:17 PM I Am 25. I Don't Work at Facebook. My Doctors Want Me to Freeze My Eggs.
  Slate Plus
Tv Club
Oct. 20 2014 7:15 AM The Slate Doctor Who Podcast: Episode 9 A spoiler-filled discussion of "Flatline."
Brow Beat
Oct. 20 2014 9:13 PM The Smart, Talented, and Utterly Hilarious Leslie Jones Is SNL’s Newest Cast Member
Oct. 20 2014 11:36 PM Forget Oculus Rift This $25 cardboard box turns your phone into an incredibly fun virtual-reality experience.
  Health & Science
Medical Examiner
Oct. 20 2014 11:46 AM Is Anybody Watching My Do-Gooding? The difference between being a hero and being an altruist.
Sports Nut
Oct. 20 2014 5:09 PM Keepaway, on Three. Ready—Break! On his record-breaking touchdown pass, Peyton Manning couldn’t even leave the celebration to chance.