Monday, June 15, 2009

Its about time! A new radio supernova in Messier 82
Brunthaler et al (2009, A&A, 499, L17) have just reported the discovery of a new radio transient in the nucleus of the nearby starburst galaxy Messier 82 (M82).

In short, radio telescopes see something now that wasn't there two years ago, and this is most probably a radio supernova, indeed, one of the closest supernova to Earth in the last five years.

But why is this scientifically interesting?

M82 has been called the "exploding galaxy" with good reason: huge filaments and arcs of very hot gas extend out ~12 kiloparsecs (~40,000 light years) perpendicular the plane of this roughly edge-on disk galaxy, connected to the dusty and optically obscured nuclear regions (The image to the right shows the galaxy in cyan, hot X-ray-emitting gas in blue, hydrogen emission in yellow, and dust emission in red).

Spectroscopic observations reveal that the hot gas visible in optical light (e.g. as observed with the Hubble Space Telescope) is flowing outward from the nucleus at a speed of ~600 kilometres per second (~1,350,000 miles per hour), and this is only one tracer of the multi-phase galaxy-sized wind (aka superwind) outflowing from this galaxy. Such superwinds were a particularly common occurrence earlier in the Universe when most galaxies and stars were forming, and indeed winds may have had significant effects on how galaxies formed and evolved.

M82 was first recognized as a peculiar and interesting galaxy in the early 1960's (see e.g. Lynds & Sandage 1963) but it only in the late 1970's and early 1980's that astronomers realized both that it represented a wind rather than a single explosive event, and that the wind was powered by a high rate of core collapse supernovae hidden from optical view in a dusty obscured "starburst" nucleus (M82 is not host an AGN, and does not appear to have a central supermassive black hole, despite many attempts by astronomers to find one).

Indeed, although M82 is perhaps only a tenth the mass of our own Milky Way galaxy it is thought to have a star formation (and hence supernova) rate 2-3 times higher than the entire Milky Way. Furthermore all of this activity is concentrated down within the central 500 parsecs (~1,500 light years) of M82 (by way on contrast, the Milky Way is ~100,000 light years across). Because M82 is one of the closest powerful starbursts it has become an important astrophysical laboratory in which to study the physics of star formation under extreme conditions.

Estimates based on the infra-red and/or radio luminosity of M82 suggest that one supernova should occur in M82's center every twenty years or so, but no-one has ever seen one go off, despite ~40 years of observation. Traditionally most supernova are detected at optical wavelengths, but optical wavelength observations of M82's starburst region are difficult, given the high obscuration. At radio wavelengths the dust that obscures the optical light is effectively transparent, so radio observations have been the primary means of studying many starburst regions, including that of M82. These radio observations reveal approximately 50 compact sources (e.g. see the MERLIN/VLA image shown above, taken from the Jodrell Bank website) that are thought to be the remnants of supernova that occurred in the recent past (although there is some debate as to how young or old they are). However they are not direct detections of stars exploding now.

Brunthaler et al were using the VLA radio telescope (the Very Large Array) to observe M82 a number of times in 2007 and 2008, and noticed the sudden appearance of bright radio transient in the center of M82, which must have occurred some time between two of their observations 2007 October 29 and 2008 March 24 (see the image above, taken from their press release). Initially very bright, the source has faded rapidly by a factor of 10 within a year, roughly exponentially. Brunthaler discuss possible interpretations for this object, and conclude that it is most probably a radio supernova (i.e. its a supernova that is a bright radio source), resulting from the core collapse and subsequent explosion of a massive star. On that basis it is has been given the identifier SN 2008iz.

Indeed, following up from the work described in the paper, observations with Very Long Baseline Array (VLBA) have actually resolved the ring-like expanding remnant.

If so this is the first supernova directly detected in M82 seen observations began (there have been claims that a peculiar source called 41.5+597, that has since disappeared, may have been a radio supernova, but the evidence in not sufficient to make a confident identification of its nature).

However there is more to Brunthaler's detection than just a confirmation that supernova are exploding in M82. The nature of the 50 or so well known compact radio sources in M82, often presumed to be young supernova remnants (SNRs) in free expansion, has been under challenge.

The compact radio sources in M82 have long been assumed to be young SNRs because they appear to fall on the trend of radio surface brightness vs diameter established for older, larger, SNRs within our own galaxy: the so-called Σ-D relation. These sources have diameters in the range ~0.5 to 8 pc, which would imply ages of a few hundred years for the largest sources if they were expanding at ~10,000 km/s. However, if they really are rapidly expanding young SNRs one would expect to see them fade as they expanded, even within the few decades of observations available to astromers on Earth. Yet the M82 sources don't appear to be fading!

As summarized by Brunthaler et al:

Kronberg & Sramek (1985) and Kronberg et al. (2000) monitored the flux densities of 24 radio sources in M 82 from 1980 until 1992. Most sources (75%) remained surprisingly constant. There is some controversy about how the fluxes of these compact radio sources can be stable. Models of supernova remnants expanding into a dense medium may explain this (Chevalier & Fransson 2001). Seaquist & Stankovic (2007) argue that the radio emission could arise from wind-driven bubbles. Studying the evolution of a young source could be very important for understanding these models.
The Chevalier & Franson model posits that the compact sources are actually much older SNRs that are compact because they have been physically confined by high density gas in the starburst region - thus they can't expand and fade quickly. But this causes other problems. SNRs in dense surrounding eventually lose all their expansion energy via radiation, so they cannot contribute to powering the large scale superwind from M82. One is there forced to posit two populations of SNRs in M82, one that creates radio sources but doesn't contribute to the observed superwind, and another population that freely expands to mingle with and power the wind. Furthermore, the relative fraction of these two hypothesized classes aren't known. which complicates any attempt to assess the energy budget of the superwind.

Just to complicate matters further, two of the brightest and most compact radio sources have been monitored with the VLBI for about a decade now, and have been see to expand, one at 1500-2000 km/s and one at 9000-11000 km/s (Beswick et al, 2006). These objects may indeed be young remnants, but then why do all the other compact sources not fade as expected?

SN2008iz can help us resolve some of these issues in the decades to come because we know it is now a very young SN, with a moderately well determined explosion date. Observations of how this young SN remnant expands may help us to understand both the nature, age and environment of the other compact radio sources in M82, and hence the properties of the interstellar medium into which these supernova are exploding. That, in turn, will improve our understanding of how superwinds are created and powered.

Other links to this story:

Other References:
Beswick, R.J., et al, 2006, MNRAS, 369, 1221.
Chevalier, R. A., & Fransson, C. 2001, ApJ, 558, L27 [NASA ADS]
Lynds, C., and Sandage, A., 1963, ApJ, 137, L1005.
Kronberg, P. P., & Sramek, R. A. 1985, Science, 227, 28 [NASA ADS]
Kronberg, P. P., Sramek, R. A., Birk, G. T., et al. 2000, ApJ, 535, 706 [NASA ADS]
Seaquist, E. R., & Stankovic, M. 2007, ApJ, 659, 347 [NASA ADS]

Brunthaler, A., Menten, K., Reid, M., Henkel, C., Bower, G., & Falcke, H. (2009). Discovery of a bright radio transient in M 82: a new radio supernova? Astronomy and Astrophysics, 499 (2) DOI: 10.1051/0004-6361/200912327

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