Category Archives: evolution of galaxies

Show Us Your Lobes!

Looking at Centaurus A

Along the Electromagnetic Spectrum

Centaurus A, courtesy European Southern Observatory. Click to embiggen

There’s an active galaxy out there called Centaurus A.  It’s the fifth brightest one out there, and if you look at it in various wavelengths, you find out that it’s fairly buzzing with activity – everything from starbirth to stardeath as well as big jets emanating from a radio-loud core.

We see Centaurus A “edge on” and it seems to be a lenticular or elliptical galaxy with a dust lane. That right there tells astronomers that something’s going on with this thing — most particularly that it merged with a spiral galaxy that was once its companion about 100  million years ago.  Mergers do things to galaxies — like warp their shapes and set off waves of star formation.

The core of Centaurus A is bright with thousands and thousands of massive stars, and if you look closely you can see blue regions where starburst activity has been creating batches of bright, massive blue stars.

The galactic heartland has been flaunting its pretty jets at us in x-ray and radio wavelengths for a long time. If you look at Centaurus A in infrared wavelengths (below, right), as the Spitzer Space Telescope did, this thing looks even weirder than it does in the ESO image (above).  First,t here’s this warped shape (often referred to as its “morphology”),  Plus, there’s this  bright spot at the center.  That tells us there’s something going on at the heart of this ginormous stellar city.

Centaurus A as seen by Spitzer Space Telescope. Click to embiggen.
Centaurus A as seen by Spitzer Space Telescope. Click to embiggen.

As astronomers get new instruments that are ever more sensitive to different wavelengths of the electromagnetic spectrum, they turn that equipment toward Centaurus A.

Recently, a group used the Atacama Pathfinder Experiment (APEX) telescope in Chile to look at the heart of Centaurus A in submillimeter wavelengths. What they found is pretty remarkable — details in the jets that have never been seen before, particularly in the submillimeter range.

The new data have been combined with visible and x-ray wavelengths to produce a striking new image (below) that really shows the extent of the jets and lobes emanating out from the center of Centaurus A.  That region — which is bright across the spectrum — is home to a supermassive black hole that has about 10 MILLION times the mass of the Sun.  (For comparison, the black hole at the heart of the Milky Way Galaxy “only” has around 4 million solar masses.) The jets and lobes emanating from the core of Centaurus A are the “smoking gun” evidence pointing right back toward the black hole!

Centaurus A in x-ray, submillimeter and visible light. Courtesy ESO and Chandra. Click to embiggen.
Centaurus A in x-ray, submillimeter and visible light. Courtesy ESO and Chandra. Click to embiggen.

In the new image, which is a composite of data from three instruments, you can make out a dust ring that circles the entire galaxy. In submillimeter wavelengths, we see not only the heat glow from the central dust disk, but also the emission from the central radio source. What’s really cool about this image is that this is the first time that a pair of inner lobes are seen in submillimeter wavelengths of light.

In the x-ray emission you can trace the jets as they emerge from the center of the galaxy. If you look to the lower right part of the image, you can also see a sort of bluish shock front. This is where the expanding lobe of fast-moving material is colliding with the surrounding gas.

Astronomers measured the emissions from the region of the black hole and calculated that the material in the jets is moving at ab0ut half the speed of light. That implies a LOT of energy being generated by activity around the black hole.

I am goggling at all this because it’s a GALAXY that we’re looking at here and for the first time we can see minute details of the magnetically charged chaos around the central black hole as well as details in the jets and lobes. And we can calculate the speed at which jet material is moving. And, we’re seeing this all from a distance of 13 million light-years!

It’s Classified, Part II

Sorting out Some of those Fuzzy Things

Back when people started studying the sky with telescopes, it didn’t take long for them to run across fuzzy-looking things out there. Charles Messier, an 18th-century French astronomer took to the skies each night for many years, searching out comets (which are fuzzy-looking things). Along the way he found other things that didn’t quite resolve into starlike points of light. His list of “fuzzy things in the sky” is the root of a deep-sky list that began with the so-called “M” (Messier) objects, but is extended out to the New General Catalog (NGC, created in the 1880s) and other surveys. Not everything those surveys contain are nebulae. Some are clusters of stars (which can look fuzzy, especially through an not-quite-powerful-enough telescope), and some are galaxies.

Galaxies beyond the Milky Way (our home galaxy) were (at first) just classified as “nebulae.”  They were difficult to resolve through low-power telescopes (just as they are today), but as telescopes improved, so did the view. Eventually, galaxies got separated out from things like the Orion Nebula (M42 in Messier’s list) and astronomers started sorting these “nebulae” by their shapes. The Large and Small Magellanic clouds were the first to be observed from Earth, and they were called what they looked like: clouds. After the telescope was invented and put to use by Galileo in the 1600s, later astronomers (like Messier) found these nebulae to devolve into two forms: elliptical and spiral.

As astronomers got better and better telescopes, they started seeing different “forms” of elliptical and spiral. In fact, it pretty quickly became clear that while no two galaxies were exactly alike, there were some characteristics that could be used to sort them into useful bins for study.  A lot of work (and argument) got done by Edwin Hubble, who formulated the sequence we use today to classify galaxies by their shapes.

Image:Hubble sequence photo.pngThis is the basic Hubble Sequence for galaxy shapes (often called “galaxy morphology” in the astronomy community).

You can pretty clearly see how different the shapes are. As with stars, you can think of the classification types of galaxies as shorthand for a longer story about how each one formed and evolved over time.  There’s also a side story for each type about the families of stars that inhabit them, and there’s an evolving story that is yet to be unraveled about the influence of dark matter on these galaxy shapes. Galaxy studies today are as hot and heavy as ever, and they play heavily in explaining the evolution of the cosmos and its structure.

Voorwerp imageThere’s a kind of interesting side-note to galaxy studies, and it involves anybody who wants to get online and help astronomers classify galaxies. Surf on over to the Galaxy Zoo and you can learn how to sort galaxies by shape (and other parameters). I’ve done it, and it’s actually a very interesting way to learn a little something about galaxies at the same time you’re contributing to science.  If you’re lucky, you might find something interesting that astronomers have never seen before — just as Dutch school teacher Hanny van Arkel did. While searching through the galaxies at the zoo, Hanny found a “ghost” object that astronomers have now named “Hanny’s Voorwerp” (the green thing in the image at right).  I suspect now that Hanny’s paved the way, a lot of other astronomy-minded folks will want to follow in her path.  Check it out and become part of the “classification team.”