Monday, March 19, 2007

The three NASA Great Observatory's view of the starburst galaxy M82

One day I'll try to take the time to write a wikipedia-level description of what a starburst-driven galactic superwind is and why they are important if we are to fully understand the nature of the Universe we evolved in.

But in the mean time it doesn't hurt to show you the archetype: M82, as presented by the Hubble Heritage team. There is even a video (quicktime or mpeg).

These images represent to combination of separate observations with three of the NASA Great Observatories: the Hubble Space Telescope (HST), the Chandra X-ray Observatory, and the Spitzer Space Telescope. These image were put together as part of a press release for Hubble's Sixteenth Birthday, which also marked the release of the deep and wide-field HST imaging of M82 with the (now defunct) ACS instrument.

What you see in these images is disk/spiral galaxy named Messier 82, viewed almost edge-on.

Starting with the HST image (top left), the disk of stars is colored blue. This galaxy is smaller and less massive than our own Milky Way galaxy (also a spiral), by factors of about 3 and 10 respectively, but M82 is not particularly small as far as galaxies go. From side to side the stellar disk is about 30 000 light years in diameter (9 kiloparsecs). The red in this image is H-alpha emission, light emitted from ionized interstellar Hydrogen gas which has a temperature of about 10 000 degrees Kelvin (this is hotter than the surface of the sun). What is immediately apparent is that much of this H-alpha emission in concentrated near the center of the galaxy, with prominent filaments and clumps extended out from the nucleus in to the halo of the galaxy. This is far away from the disk of the galaxy where most interstellar gas normally resides. In fact we know from optical spectroscopic observations that much of this H-alpha emitting gas is flowing out of the galaxy as speeds of about 600 kilometers per second (that is just over 1 million miles per hour).

(Top right). The X-ray view of M82 looks, at first glance, to be totally unrelated to the optical images. The galactic disk of M82, consisting of about 20 billion stars or so, comprising the majority of the baryonic mass of the galaxy, simply isn't visible in X-ray emission. There is vastly more to the Universe than that which can be seen with optical (HST) or near-infra red telescopes (e.g. JWST).

The energy of X-rays we detect tell us about how the X-rays are generated, and the composition and nature of what is emitting them. In these X-ray images from Chandra the red represents X-ray emission from photons having energy in the range 0.3-1.6 keV (kilo electron Volts), the green represents emission in the 1.6-2.8 keV energy range, and blue is 3.0-7.0 keV. By way of comparison, your dental X-rays have a typical energy of about 70 keV, much more energetic than the X-ray emission Chandra is sensitive to. The little dots of brightest X-ray emission are termed "point sources," and most of the ones visible in the center of the image are individual binary stellar systems in M82, comprising high mass stars with neutron star or stellar mass black holes as companions. Normal stars like the Sun do emit X-rays, but as only a tiny fraction of there energy comes out at X-ray wavelengths they're essentially invisible in Chandra images of Galaxies outside the Local Group. The X-ray emission we do detect can tell us useful things about the total amount of massive star formation in the star burst, but that is the topic for another post entirely. Other point sources in the image are most likely much more distant AGN, far in the background.

But the most prominent feature of these X-ray images of M82 is the diffuse X-ray emission, brightest in the central starburst region of M82, fading as it extends outward away from the disk of the galaxy. The red and green in the image is thermal X-ray emission from hot gas (technically a plasma) in the superwind, with a temperature of about 4 million degrees Kelvin in the outer (red) regions. We currently can not measure the speed with which the hot X-ray emitting gas is moving, but based on the theory of winds we would expect this emission to be moving at least as fast as, and maybe twice as fast, as the gas seen in the optical Hubble images. X-ray emission is a much more direct probe of the very violent processes creating the superwind than the optical, UV, or IR emission also seen in the wind.

(Bottom left) The image on the bottom left is the near infra-red (IR) as seen with the IRAC instrument on the Spitzer Space Telescope. In this case each IR photon detected by Spitzer has only a tenth (1/10) of the energy of the photons detected in the optical HST images, and one ten thousandth (1/10000) the energy of the X-ray photons. The blue again shows the stellar disk of M82, as normal stars still emit some decent fraction of their total power in the near IR. The red spread all over the halo of M82 is emission from compounds called Polycyclic aromatic hydrocarbons (PAHs), which are basically organic compounds, and you encounter them in every day situations as one of the components of cigarette smoke, car exhaust, and so on. Consider them to be tiny grains of soot, similar in origin but smaller than your average interstellar dust grain.

Dust and PAHs are a natural component of the interstellar medium (ISM), which normally resides in the disk of spiral or irregular galaxies. So why is it in M82's halo? We know the hot gas in superwinds sweeps up the ambient normal ISM, carrying it out into the halo with it. This is the origin of the H-alpha emission seen in the HST images, for example. Some fraction of the dust and PAHs in M82's halo are probably there because of the currently active wind. But its not as simple as all that.

Firstly, the PAH emission appears to be more widely spread in halo of M82 than the known wind-emission (the H-alpha and soft X-ray emission). Secondly, PAH's are pretty fragile. They're organic molecules, not particularly huge, and they can easily be destroyed by UV or X-ray radiation. So the wide-spread presence of the PAHs in M82's halo is not something we necessarily expected to see before Spitzer was launched. It may be that the numbers will turn out right for the wind to have put them there, and not have destroyed enough of them yet, but it also may well turn out that some other mechanism is needed to create a dusty halo BEFORE the current superwind started (the age of the current wind is perhaps 10 to 30 million years), so there may have been previous superwinds in M82. The PAHs may be a minor detail, as regards the starburst and superwind, but they may tell us something about the history of the wind that otherwise we would never learn.

(Bottom right) The final image is multi-wavelength, multi-observatory, montage of M82 using all three Great Observatories. From the HST ACS observations with have the light blue stellar disk, with the T=10000 K H-alpha emission in yellow (it is barely visible in this image because it strongly overlaps with the red PAH emission), from Chandra we have the soft thermal X-ray emission from the million degree plasma in the superwind, now shown in a darker blue, and from Spitzer the near IR PAH emission in red.

Images like this composite are both scientifically helpful and occasionally misleading. Knowing how the X-ray gas in the wind relates to the H-alpha emission and PAH emission can tell you (if you know the physics) what is actually going on.

I want to take a brief detour from the subject at hand to point out an important caveat for non-professionals to bear in mind. The intensity stretches used to make pretty color images can distort or exaggerate the natural gradations in intensity, and hence fool you into thinking there isn't emission in some place where in a monochromatic image you'd find it is there. Real data analysis is not done with photoshop or other common image formats, after all. For example, in this final image it looks like the X-ray emission comes from within the IR PAH emission, and that there isn't much PAH emission where the X-rays are bright. Yet the bottom left image shows that the PAH emission is pretty bright everywhere.

In practice, in professional astronomy the pretty press release images you end up seeing are never what was actually analyzed scientifically, so the case described above really isn't a problem for real science. On the web its another matter, as there are all sorts of people who take NASA press release images and play with them in photoshop and still somehow think they're getting valid information on the face on mars out of some lossy-compressed jpeg.

In conclusion we're seen the classic starburst galaxy with a superwind, M82, as seen with each of the three remaining NASA Great Observatories. Each covers as different part of the electromagnetic spectrum, and thus each probes very different physical conditions, from cold dusty gas all the way to plasmas at millions of degrees. In some ways the images are very different, yet share some common aspects (such as the superwind flowing out of the disk and into the galaxies halo). Together they provide us with parts of the puzzle for unlocking the nature and implications of starbursts and superwinds.

Image credits:
HST ACS (top left): . Gallagher (University of Wisconsin), M. Mountain (STScI), and P. Puxley (National Science Foundation).
Chandra ACIS (top right): NASA/CXC/JHU/D.Strickland.
Spitzer NIR (bottom left): NASA/JPL-Caltech/C. Engelbracht (University of Arizona).
HST/Spitzer/Chandra composite (bottom right): NASA, ESA, CXC, and JPL-Caltech.

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