The cold arms and hot, hot heart of the fuzzy maiden

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Jan. 5 2011 12:15 PM

The cold arms and hot, hot heart of the fuzzy maiden

Hot (and cold) on the heels of my posting the infrared view of the nearby spiral M33, the European Space Agency just published this incredible picture of our other spiral neighbor, M31, the Andromeda Galaxy!

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! Follow him on Twitter.

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[Click to galactinate.]

Oh my. This is a composite of two orbiting observatory images: the far infrared using Herschel (colored orange), and the X-ray emission using XMM-Newton (blue). There's so much to see! That's not surprising, since at 2.5 million light years away, Andromeda is the closest big galaxy to us, and presents itself with loads of detail.

First, shown here is Robert Gendler's magnificent visible-light image of the galaxy. You can see it's tilted almost edge-on to us, but you can see the central bulge of old stars, the spiral arms winding out, the dark lanes of dust. This image has roughly the same orientation and border as the big one above, so you can compare them.

The infrared observations trace the presence of cold dust, created when stars are born and when they die. And by cold, I mean cold: much of it is just a few degrees above absolute zero. That dust is opaque in visible light, as you can see in Gendler's shot. But it glows in infrared! The X-rays, on the other hand, are from incredibly hot gas heated to millions of degrees by neutron stars, black holes, and newly-born massive stars; you can see several individual objects in the galaxy's core. In general, spiral galaxies have cores that long ago shut down when it comes to making new stars -- the lack of dust there is another indication of this -- but the compact remains of massive stars live on, and when they eat material that falls on them they heat up and generate X-rays.

[UPDATE: For some reason, the original picture from Herchel and XMM I put in this post got flipped (something I must have done when I Photohsopped it to change the image size). The orientation looked the same as in the original image, so I didn't notice! The next paragraph, where I describe the companion galaxies as invisible in the IR image, is clearly wrong; I didn't see them because I was looking in the wrong place. Embarrassing, but there you go. I made a mistake, so I'm correcting it. The incorrect paragraph is just below and is struck through; I added a new paragraph below it. Sorry about any confusion!]

Interestingly, the two dwarf galactic companions of Andromeda, named M32 and NGC 205 and which you can see in Gendler's visible image above and just below M31's core, are invisible in the IR/X-ray shot. Both are small elliptical galaxies, and the lack of dust and X-rays means they too are no longer making stars. It's amazing what you can deduce just from glancing at these images!

Andromeda has two dwarf galactic companions named M32 and NGC 205 which you can see in Gendler's visible image (one is above above and to the right, and the other just below M31's core). They're difficult to spot, but definitely in the IR/X-ray image as well. They must be so faint due to a lack of ongoing star formation, which is not surprising given that they are old elliptical galaxies which are generally pretty quiet in that regard. It's amazing what you can deduce just from glancing at these images!

I can't tell you how many times I've seen this galaxy with my own eyes. It's up right now, in fact, high in the northern sky after sunset. The Andromeda Galaxy is about the same size as the Milky Way, 100,000 light years across, and can be seen from dark skies as a small bit of fuzz in the constellation of Andromeda (in mythology (or "Clash of the Titans") the maiden daughter of Cepheus and Cassiopeia, who was saved from the sea serpent by Perseus -- which hopefully explains the title of this article). With binoculars you start to get an idea of the shape, and with even a small telescope it reveals itself as an elongated patch (the companions are easy to spot as well). With bigger telescopes the spiral arms become clearer, and of course deep exposures reveal the magnificence of a grand design spiral galaxy, one of the most awesome and gorgeous objects in the entire Universe.

And yet here is a new view of this old friend. And what a view! Scientists will be poring over these data for a long time to come, finding new ways to interpret this old gal. There's nothing like a fresh perspective to liven up a relationship.



Image credits: IR: ESA/Herschel/PACS/SPIRE/J.Fritz; X-ray: U.Gent/XMM-Newton/EPIC/W. Pietsch, MPE; optical: Robert Gendler

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