Astro: Mid-IR Imaging of M82 from Subaru

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March 31, 2011 -Our ability to understand the Universe has been enhanced immensely by technology that allows us to detect electromagnetic radiation at wavelengths far beyond what our eyes can sense.  For instance when we look out into the night sky and behold the Milky Way, we are seeing the aggregate star light from billions of stars, but also the dark lanes of dust and gas clouds that block background starlight from reaching us.  But if we switch to the longer wavelengths of in the near-infrared (for astronomers, this means wavelengths in the range 1-3 microns), these dark clouds become more transparent, and we can peer through or into them.

Switch to the mid- and far-infared regimes (3-40, and 40-350 microns respectively), and the sky becomes devoid of stars, which emit relatively little radiation at these wavelengths.  However the gas and dust clouds, which have temperatures in the 10s to 100s of Kelvin, visibly glow at this part of the electromagnetic spectrum.  Observing and studying different wavelengths therefore reveal different information about astronomical objects: not only are we looking out at different distances and through or into objects like gas clouds, but we are also probing different physical processes in different environments.  This web of information is important in helping us understand objects that could be halfway across the Galaxy or Universe.

In results discussed in a recent paper by Gandhi et al., the galaxy M82 was imaged using a mid-infrared instrument at the Subaru Telescope on on the peak of Mauna Kea in Hawaii.  M82 is a peculiar looking edge-on spiral galaxy; if you image it in a filter that narrowly includes radiation from the H-alpha line of hydrogen gas (at 656 nanometers), vivid streamers glowing in hydrogen emanate from the core of the galaxy (color coded as red in the next two images below). M82 (right below) and the nearby M81 galaxy (left below) appear to be orbiting and gravitationally interacting with each other.  Tidal forces from near passes between the two galaxies have sent interstellar gas flowing into the center of M82, where it gets jammed up, collapses, and forms stars.

M82 and M81

Credit: Rainer Zmaritsch & Alexander Gross

Lots and lots of stars: hundreds of young stellar clusters have been detected in the core regions of M82.  The star formation rate -- about 10 solar masses of new stars each year -- is several times greater than that estimated in our own Milky Way Galaxy.  In addition to forming from the molecular gas clouds, the massive young stars in these young clusters pump out UV radiation and prodigious winds, before dying in supernovae explosions.  The winds and supernovae expell material back into the interstellar medium, where it can accrete back back into molecular clouds, and the whole process starts all over again.  These are process that have been studied up close in our own Milky Way Galaxy as well as from afar when we look at distant galaxies, so we believe we understand the rough big picture of this galactic ecosystem, although many of the specific details are still being worked out.


Credit: NASA, ESA, The Hubble Heritage Team, (STScI / AURA)

Galaxies with prodigious amounts of starbirth are called "starburst galaxies."  M82 is just one example, and there are countless others where tidal close enounters with near neighbors result in a mass collapse of molecular clouds that lead to a burst of star formation.  When enough young massive stars have formed, their collective winds and supernovae can build up into a "superwind", which tends to flow out of (and hence perpendicular to) the plane of spiral galaxies.  Although the winds and explosions are isotropic for the most part -- they go equally in every direction.  However the disk of a spiral galaxy is filled with gas which is tenuous enough to be considered a vacuum on Earth, but at interstellar distances, there's enough of it to slow down gas flowing from stellar winds and supernova.  It's easier for the expelled material to move perpendicular and out of the disk in a "galactic fountain," which also makes it easier for us to detect, like in M82.


Which brings us back to the recent results discussed in a paper submitted to the Publications of the Astronomical Society of Japan.  The imaging at two different mid-IR wavelengths (12.81 and 11.7 microns) revealed almost two dozen bright cores.  In the mid-IR we are not seeing the stars themselves, but dust clouds that have been heated up by stars.  Presumably the young stellar clusters lay within or very close to these bright cores.

Here's where comparing imaging taken at other wavelengths can be revealing.  Below is a composite of not only the Subaru image (in red), but also near-infrared radiation from young clusters taken from the Hubble Space Telescope's NICMOS instrument (in green), X-ray data from the Chandra X-ray Observatory (blue), and even radio observations that highlight old supernova remnants (magenta).


The mid-IR image shows structures that mimic the streamers that flow perpendicular out of the plane of the M82.  Though the resolution is not enough between any of the data to tie the gas with any one star cluster, the fact that they end near regions with signs of past and present star formation reaffirms our idea of the origins of these structures.  We also see in green the locations of the stellar clusters, the densest of which tend not to coincide with the thickest mid-IR gas in red.  There could be additional clusters hidden beneath the thick veil of dust and gas, so that they are invisible in the near-IR.

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