Antoine de Saint-Exupery wrote, "What is essential is invisible to the eye", and this has certainly been true for examining the details of early galaxies undergoing bursts of star formation. The study of starburst galaxies enables astronomers to effectively peer back into our own galaxy's past. Like human beings, galaxies and the stars within them have a life cycle: They are born, grow and mature, and eventually die. Starburst galaxies are real "baby boomers", creating new, young stars at a rate faster than the combined speed of many Milky Way-like galaxies that are in a later phase of development. The dusty "ash" left over from successive generations of star formation in more mature galaxies blocks out much of the starlight, rendering it invisible to the human eye. The bulk of the energy from starbursts emerges at longer wavelengths. Therefore astronomers must turn to infrared rather than optical observations in order to understand the nature of these cosmic stellar factories.
An international team of scientists, led by Dr. Poshak Gandhi of JAXA (Japan Aerospace Exporation Agency), has used the Subaru Telescope to produce a new view of M 82 (Figure 1 top) at infrared wavelengths that are twenty times longer than those visible to the human eye. M 82 is located close to the ladle of the Big Dipper in the constellation Ursa Major and is the nearest starburst galaxy, at a distance of about 11 million light years from Earth.
The combination of Subaru Telescope’s large 8.2 m primary mirror and its mid-infrared instrument COMICS allowed the team to obtain a sharp, magnified view of the inner area of the galaxy. Team member Dr. Aya Bamba (Dublin Institute for Advanced Studies and JAXA) commented, "Together they give us the capability to capture fine details, equivalent to the ability to see a small coin from a distance of 10 km, or being able to recognize a truck 10,000km away—as far as London is from Tokyo." The final result of the observations is a spectacular image (Figure 1 top) showing the base of a dusty superwind and young star clusters with extraordinary detail, better than could be expected from space telescopes, which have smaller apertures that limit the extent to which they can resolve fine details.
Previous observations of M 82 with infrared telescopes have found a very strong wind emanating from it — a 'superwind' that is composed of dusty gas and extends over many hundreds of thousands of light years (Figure 1 middle and bottom). This high-powered windstorm ejects material from the galaxy at a speed of about a half a million miles per hour, sweeping it up from the central regions and depositing it far and wide over the galaxy and beyond. The contents of this material are seeds for solar systems like our own, and perhaps for life itself. The dusty superwind glows brightly in the infrared, because billions of bright, newly-formed stars heat it up.
The Subaru image gives scientists insight about the sources of the superwind. Dr. Gandhi sums up the new findings: "The wind is found to originate from multiple ejection sites spread over hundreds of light years rather than emanating from any single cluster of new stars. We can now distinguish 'pillars' of fast gas , and even a structure resembling the surface of a 'bubble' about 450 light years wide." COMICS has detectors particularly adept at indicating the presence of warm dust, which it found was more than 100 degrees hotter than the bulk of material filling the rest of the galaxy. The widespread, continuous flow of energy from young stars into the galactic expanse keeps the dust hot.
Further insights from the Subaru mid-infrared image emerge by combining it with the Hubble Space Telescope's near-infrared image and the Chandra X-ray Observatory's X-ray data of M 82. Their integration produces a beautiful mosaic (Figure 2) that provides the first opportunity to isolate M 82's infrared properties. Supported by these data, scientists can study the broad spectrum of radiation of different kinds of objects spread over the galaxy's plane, including supernovae, star clusters, and black holes.
Many questions remain. How many more stars does the galaxy contain? Interestingly, the infrared radiation, which maps out warm gas and dust, appears in regions without stars, meaning that many more stars are likely to be hidden behind the dust.
Another exciting question that remains to be answered is whether or not M 82 hosts an actively growing supermassive black hole. Team members Drs. Mark Birkinshaw and Diana Worrall of the University of Bristol note, "Detailed analysis of the new infrared data combined with X-rays does not find any such object." However, all large galaxies could well contain these monsters, which are known to grow and evolve in conjunction with stars. The team concludes that M 82 may be no exception, and the search for its big black hole must continue.
This discovery is reported in the article "Diffraction-limited Subaru imaging of M82: sharp mid-infrared view of the starburst core" by P. Gandhi, N. Isobe, M. Birkinshaw, D.M. Worrall, I. Sakon, K. Iwasawa & A. Bamba, in the Publications of the Astronomical Society of Japan, volume 63 (2011), in press.
The leader of this research team, P. Gandhi, is a JAXA ITYF Fellow. The JAXA International Top Young Fellowship (ITYF) was established as a prestigious new fellowship program in 2009. The ITYF is designed to attract outstanding, highly motivated, young researchers in any of the space science fields covered by ISAS/JAXA to work in Japan for 3 (and up to 5) years. The excellent remuneration package includes a research budget and travel support so that the Fellow can extend his/her international profile as well as develop collaborations within Japan.
This study was partially supported by the MEXT grant-in-aid "Kakenhi", number 21740152.