(Really) Cool View of Andromeda Galaxy

Nearing the end of its mission, ESA's Herschel Space Observatory has delivered a highly detailed map of extremely cold gas and dust in the iconic Andromeda Galaxy.

"Cool" Andromeda Galaxy
ESA's Herschel Space Observatory obtained this view of cold gas and dust in the Andromeda Galaxy at four far-infrared wavelengths. The blue channel shows emission at 70 microns, green at 100 microns, and red at 160 and 250 microns combined. The image spans roughly 3°. Click here for a larger view.
ESA / PACS & SPIRE Consotrium / O. Krause / H. Linz

We've seen so many images of the grand, stately Andromeda Galaxy (Messier 31) that we sometimes forget that this giant pinwheel is alive with activity and, especially, star formation. But this just-released view, taken by the European Space Agency's Herschel spacecraft, reveals M31 in a new light.

Specifically, the space observatory recorded the galaxy's appearance at far-infrared wavelengths. The longer the wavelength, the colder the matter, and the dark-red lanes in the disk correspond to some of the very coldest dust in the galaxy (only a few tens of degrees above absolute zero). That's where future generations of stars will be born. By contrast, the bluish appearance of M31's central bulge shows that it is somewhat warmer and already dense with older stars.

Located 2½ million light-years away, the Andromeda Galaxy is about 200,000 light-years across (twice the size of our Milky Way Galaxy). Herschel's view reveals at least five concentric rings in its spiral arms. In between these are dark gaps where star-forming regions are absent.

This isn't the first time Herschel's 3.5-meter optics have been directed toward M31. Compare this view with another infrared portrait taken in late December 2010.

An image showing the Herschel Space Observatory set in front of a picture taken by the Hubble Space Telescope
An image showing the Herschel Space Observatory set in front of a picture taken by the Hubble Space Telescope
Launched in May 2009, the spacecraft was originally known as the Far InfraRed and Submillimetre Telescope (FIRST) but later renamed for Sir William Herschel, who in 1800 discovered infrared radiation. The spacecraft has given astronomers their most detailed views to date of cosmic targets in far-infrared and submillimeter wavelengths (from 60 to 670 microns).

While this wasn't Herschel's first look at the Andromeda Galaxy, it will likely be its last. The spacecraft is about to run out of the liquid helium that chills its optics and detectors. Once the helium is gone, now expected to occur within the next two months, the mission will end.

Here's an ESA website where you can get more information about Herschel and its mission.

7 thoughts on “(Really) Cool View of Andromeda Galaxy

  1. Bruce

    The features in these infrared images of Andromida appear ringlike and don’t appear to match up completely with the galaxy’s spiral arms. I wonder what this might be telling us.

  2. Peter WilsonPeter

    Andromeda’s spiral structure is hard to see from our angle, but in general, bright areas in infrared are dark in visible light, and vice versa. When stars ignite, the intense radiation pushes away left over dust. To where? To areas of lower radiation density, with the constraint that the dust cannot escape the gravity of the disk. So dust collects “in the voids,” that is to say, away from brightest stars, which delineate the arms. Hence, the mismatch between arms that show in visible light, and regions of cold dust, as pictured above.

  3. Bruce

    Thanks Peter. That explanation makes great sense. And over time, these cold dusty band regions will light up with newly formed stars and the left-over dust as well as new dust from aging stars will be pushed along in a spiral wavefront of star formation. Very cool.

  4. Option61c

    I suppose it’s too easy of an explanation but….

    Is there a chance that the semicircular black “object” in the center of the image is the partially obscured event horizon of the black hole in the center of M31? Further, is the swirl that overlays the black object a view of the accretion disk?

    I’m just curios because it sure looks like it to me. I look forward to your comments.


  5. Bruce

    In a word, no Option61c, but your question prompted me to try calculating the size of M31’s SMBH’s event horizon. The radius of a black hole’s event horizon is found by the equation R=2GM/c^2. Wikipedia gives a range of 1.1-2.3×10^8 solar masses for M31’s central BH, so even using the upper end of the mass range the diameter of this SMBH works out to be only 1.38×10^9 km. Since 1 light year = 9.46×10^12 km the diameter would be only 0.146 ly. Compare that with M31’s overall diameter of over 200,000 ly and you can see that the central areas you are asking about are way to small to be seen at this scale. Also, this image is of very cold dust, and dust falling into a BH is normally heated enormously. I’m glad you asked though, because since you asked we both have learned more about these objects.

  6. Bruce

    Peter et al, more info has come to my attention about these rings. This Herschel infrared Andromeda (M31) image was also the Astronomy Picture of the Day on 2/4/13, and in the discussion that followed I learned more that has been theorized about these infrared rings. In Kelly Beatty’s article he calls these “concentric rings”, but look closely at the image. Do the rings all really share the same centers? Also, are any of these rings concentric with M31’s actual core? Close examination will show that the answer to both questions is no. The Spitzer infrared Telescope has also imaged M31, and the findings from Spitzer together with Herschel have shown that there is another likely cause at work here. It’s thought that collisions with Andromeda’s smaller satellite galaxies has caused shock waves to ripple through the larger galaxy. So these rings are like ripples in a pond after a rock as been thrown in. There are two sets of rings, the centers of which mark the spots where M31 is thought to have been last penetrated by it’s two satellites.

  7. Bruce

    Oops! Decimal point error correction: The max size of the M31 SMBH event horizon should be 0.000146 ly, which works out to be about 1.28 light hours, making the point even more forcefully when compared to 200,000 light years.

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