NGC 4921 against a backdrop of more distant galaxies. Taken with the NASA/ESA Hubble Space Telescope. (Click to reframulate.)
Wow, this is one of those pretty pictures in astronomy that just draw you in. The more you look, the more you see. Obviously there’s a huge galaxy staring us in the face, looking as delicate as a jellyfish. But, the longer you look at the picture, the more galaxies you see. It’s astronomy’s version of the gift that keeps on giving.
The galaxy that takes up most of the field of view in this NASA/ESA Hubble Space Telescope image is NGC 4921. It lies about 320 million light-years away from Earth and is part of a giant conglomeration called the Coma Galaxy Cluster. As it turns out, NGC 4921 is a bit of a stranger in its own land. First, it’s spiral-shaped in a cluster where spirals are pretty rare, and second, it looks ghostly and pale and not quite like a “normal” spiral .
The Coma Cluster’s galaxies have been interacting for a long time, and as a result, many of the members have had their shapes sort of “smoothed out” into quiet ellipticals (meaning they are sort of oval-shaped, without spiral arms and without a lot of star formation that you see in active spirals). That makes spiral galaxies something of a rarity in the cluster.
If you look at NGC 4921 for a while you can see that there are kinda, sorta spiral arms in this galaxy, but they don’t seem as well-defined as those that make up our own Milky Way. Those spiral arms are where new stars are being formed in the galaxy, but the star birth activity seems to have been dampened out, perhaps by past interactions. You can see a few bright blue young massive stars in the image, but otherwise there’s just a lot of dust swirling around — and, here’s the coolest part — peeking through that dust are galaxies of all shapes and sizes even farther away than this galaxy! That’s something you don’t always get to see.
Spend some time with this picture — explore the large version when you click on the image. It will amaze and surprise you!
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.
As astronomers get new instruments that are ever more sensitive to different wavelengths of the electromagnetic spectrum, they turn that equipment toward Centaurus A.
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.
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!
