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.

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.

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!

Musings on Miranda

Cat Moon

Miranda Katt

We have a cat named Miranda. As you can see, she’s got a mottled coat, with no two areas exactly the same shape and with an underlying “white” coat overlain by other regions of different-colored hair.

When we got her, we were casting about for a name and I happened to be looking at pictures of the moon Miranda. It occurred to me that, like that moon, our cat had white “surface units” overlaid by other mottled, oddly shaped surface units.

Miranda (the cat) came by her mottled “surface” through a combination of genetic factors inherited from both her parents.

She’s a calico cat, in much the same way that you can think of Miranda the moon as a calico moon, marked with oddly shaped regions and different colors for each of the regions.

The Uranian moon Miranda

The “genetic” components that shaped the surface of Miranda (the moon) have more to do with physical processes like gravitation and the melting point of water ice.

The scientific consensus is that this moon, with its icy grooves and cliffs, has been deformed by a process called tidal heating.  What this means is that Miranda, which is largely water ice, was affected by tidal forces from Uranus early in this moon’s history. The gravitational tugs from Uranus deformed the moon slightly, causing some interior heating.  That heat melted the ice inside of Miranda, and chunks of it likely “floated” to the surface and then froze. That process (which is complex) caused the wrinkles and ridges, troughs and gullies we see today.  No cat fur, but still a mottled, calico surface on a distant moon!

Tales from the Stellar Crypt

Dead Stars Tell No Tales… Or Could They?

The galaxy is full of stars that are dead or dying. You’d think that their stories would be over and we could move on to the next star, right?

Silicates in the dust from alien asteroids circling the white dwarf GD 40; data from Spitzer Space Telescope shows the silicates as seen in recent studies (yellow dots) and older data (orange triangles).

Au contraire… as it turns out, some dead stars are surrounded by the dust of asteroids they chewed up and spat out. And, astronomers are getting a chance to “bite” that dust again by studying the infrared emissions from that material with the spectroscopic “eye” of Spitzer Space Telescope.

The data are intriguing, even if you’re not used to looking at it in X-Y plots like this one.  It shows that the asteroid dust surrounding a dead “white dwarf” star contains silicates. That’s a very common mineral here on Earth — many of our rocks are made of silicates. So are rocks on the Moon and the other rocky planets such as Mars and Venus. And, as it turns out, silicates are found in clouds of debris around other kinds of stars and in the the ghostly shrouds of planetary nebulae.  So, how do asteroids get chewed up and and ground to dust around a white dwarf star?

We all know the story of planetary formation around young stars. Dusty material left over from star birth swirls around the star in a kind of shroud. The dust and particles stick to each other over time, forming larger and large protoplanetary lumps. Eventually, if you stick enough of this stuff together and wait a while (say a few million or so years), you get planets. What’s left over circulates around as chunks of rocks. We call those chunks “asteroids.”

What would shredded asteroids around a white dwarf look like? If you could be close by in a space ship, this artist's conception might be what you'd see. Courtesy CalTech.

Now, fast-forward through the life of the star and its associated planetary system — say, one similar to our Sun. When such a star gets old and cranky, it puffs itself up into a red giant. That action eats up the inner planets and jostles whatever asteroids and outer planets that survive the angry red giant phase. As the star continues to die, it does a most amazing thing:  it blows off its outer layers and then shrinks down into a skeleton of its former self. The end result is a white dwarf, an  object with an intense gravitational field. If anything wanders too close — say an asteroid — it gets shredded to pieces by the gravitational pull of the white dwarf. That scatters around a lot of dust.  The chemical fingerprints of the elements in that dust can be picked out by the infrared spectrograph on Spitzer Space Telescope (for example).

Spitzer looked at eight white dwarf systems and found the dust to be very rich in a glassy silicate material like olivine (which is commonly found here on Earth). The scientists who did the looking, led by Mike Jura at University of California at Los Angeles, plan to search out more of these “dust-bitten” regions around white dwarfs. What they find from the tales these dead stars tell will give us some pretty unique insights into how other star systems treat their planets and asteroids as they form, grow, evolve, and then die.

Note: this story was first released during the AAS meeting a couple of weeks ago. I headlined it at the time, but have been wanting to give it more detailed treatment. For more information, check out the Spitzer press release.