Hubble pokes at a galactic bulge

The entire universe in blog form
Nov. 18 2009 4:01 AM

Hubble pokes at a galactic bulge

When you poke the Pillsbury dough boy in his bulging tummy, he giggles. When you poke the bulge in NGC 4710, however, you get the history of how galaxies form. Voila!

hst_ngc4710

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!  

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Awesome. And you really need to embiggen this one to get a sense of the incredible beauty and resolution of the picture. Try the 4000 x 2000 pixel one on for size!

NGC 4710 is an edge-on spiral galaxy located about 60 million light years away in the Virgo Cluster. That puts it in the next town over, cosmically speaking, so it's a rich target for something like Hubble Space Telescope. This image, newly released (but taken in 2006 before the last servicing mission), reveals spectacular details in the sideways galaxy. Views like this really accentuate the huge sprawling dust complexes littering spiral galaxies.

But it isn't the dust astronomers are interested in here. Spirals have three main parts: a more-or-less spherical bulge in the center, the disk (which has the spiral arms), and a giant halo of stars surrounding them both. We understand a lot about spirals, but lots of big questions remain, including how and when the bulge forms. A galaxy is born out of a vast, collapsing cloud of gas. It's possible that the bulge forms straight away, with the infalling gas of the protogalaxy making stars which build up in the galactic center. It's also possible that the bulge forms later, well after the galaxy itself takes shape, as stars in the inner part of the galactic disk interact gravitationally and fall to the center, building up the bulge.

It turns out there might be a way to distinguish these formation mechanisms, even billions of years after the fact. Globular clusters are small (well, a couple of dozen light years across or so) balls of hundreds of thousands of stars. They orbit bigger galaxies; the Milky Way has well over 100 orbiting it. We know that many globulars formed at the same time as their parent galaxies; the stars in the clusters can be incredibly old. This means that perhaps the formation of the galaxy and its attendant clusters are connected.

In fact, it's thought that the same process that creates the bulge in the "forms at the same time as the galaxy itself" scenario also creates globular clusters, but the other process (stars from the disk falling inward) does not create globulars.

That's where NGC 4710 comes in. Being edge-on, we can see the bulge clearly, so it can be studied. But it also presents a good view of its globulars, so scientists can look at pictures like this one and simply count up the number of globular clusters near the galaxy and then figure out if the number is consistent with one of the two formation mechanisms.

In this case, NGC 4710 sports very few globulars, indicating the bulge formed after the galaxy itself. But NGC 4710 is only one of many galaxies being studied this way. Will they all show the same sluggish beginnings to their central bulges?

Time will tell. But I hope that as more of these galaxies are studied more images as lovely as this one become available.

Image credit: NASA & ESA

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