Taking The Pulse Of A Supernova – NGC 4490

NGC 4490/4485 – T. Grossman, D. Hager and R. Johnson

Way out yonder some 40 to 50 million light years away in the constellation of Canes Venetici is a pair of interacting galaxies sometimes referred to as “The Cocoon”. These two mis-shaped blobs of star stuff have already made their closest approach to each other and are now parting ways. Between them stretches a trail of stars that spans some 24,000 light years as they face each other showing off their numerous star-forming regions. But where there is life… There is death. Let’s put our finger right on the pulse of a supernova.

Interacting galaxies NGC 4485 and NGC 4490 have long been of interest to astronomers as a study field for analysis of the hot interstellar medium. Like a good doctor researching every angle is Chandra – which reveals properties of the diffuse X-ray emission from these galaxies. “The high angular resolution of Chandra enables us to remove discrete sources and perform spatially resolved spectroscopy for the starburst regions and associated outflows.” says Alexander Richings (et al). “This allows us to look at how the physical properties of the hot interstellar medium such as temperature, hydrogen column density and metal abundances vary throughout these galaxies.”

But a good doctor doesn’t stop at just one answer – they look for more – such as imaging in hydrogen alpha and with tools like SCUBA and MERLIN. And did they find more? You know it. “We detect an Ha filament emerging from the disc of NGC 4490 to a projected distance of 3 kpc which has counterparts in both the radio continuum and Hi. The HI counterpart extends to a projected distance of ? 30 kpc from NGC 4490 and we argue that this is evidence that the giant HI envelope in this system has its origins in star formation.” says M.S. Clemens and P. Alexander. “We use SCUBA and radio continuum data to attempt to place constraints on the distribution of dust with respect to the star forming regions. This analysis is limited by the lack of an independent estimate of the dust temperature, something that both ‘SIRTF’ and ‘SOFIA’ will be able to provide, however we find some evidence that most obscuring dust is not located within HII regions themselves.”

Is this news? Not really. As far back as 1997 astronomers were combining images made in different wavelengths and drawing conclusions. According to the early work of Debra Elmegreen (et al); “We also present B- and I-band observations of the interacting pair in order to determine the ages of tidal star-forming regions, including a newly discovered faint tail to the east of NGC 4490. In our discussion, we distinguish this “tail” from the “bridge” connecting the two galaxies, and from the “tidal arm” that spirals out from the bright regions of NGC 4485 toward the bridge.” And astronomers are about to combine images once again…

On March 4, 2008 Swift Ultraviolet/Optical Telescope (UVOT) and X-Ray Telescope (XRT) observed an event in NGC 4490, but they weren’t alone. Amateur astronomer Rick Johnson captured the event as well. But one view isn’t enough and the data was added to a astrophotograph taken by Dietmar Hager prior to the SN. But a good doctor doesn’t stop there and yet another “data merger” occurred when combined with only weeks old RGB data taken by Torsten Grossmann. What happens next is nothing short of magic. Watch this animated gif and put your finger right on the pulse of a supernova….

Click for full size view of…

Supernova 2008ax in NGC 4490 was quite an event. The neophyte supernova was discovered independently by both the Lick Observatory Supernova Search and by astronomer Koichi Itagaki of Japan. At its beginning pulse it was believed to have been a blue variable – but the spectrum doesn’t lie. Soon enough the phenomena went to a young type II supernova and escalated into a type Ib. Although the pulse might have been faint – dangling between magnitude 13 and 16 – it was there and undeniable.

Does a galaxy like NGC 4490 hold more future surprises for us? You bet. And it’s not just a supernova event that sets it apart. “The nearby Sd galaxy NGC 4490 is remarkable in that its hosts one of the most numerous ULX populations within 10 Mpc, only bettered by M51 and M82. Here, we examine the X-ray spectral and temporal variability of these sources over the course of four Chandra and XMM Newton observations spanning the years 2000-2004. We detect all 5 previously identified ULXs in NGC 4490 and that in the tidal tail of NGC 4485. We also find one new transient ULX in the system. The spectral variability is generally characterized by a hardening of the source spectra as their luminosities increase. The sources show a variety of long-term light curves; however, short-term (intraobservational) temporal variability is conspicuous by its absence.” says Jeanette Gladstone and Tim Roberts. “Ultraluminous X-ray Sources (ULXs) are point like, non-nuclear X-ray sources situated outside the nucleus of their host galaxy, that have X-ray luminosities in excess of 1039 erg s-1. Various studies have been carried out on these sources since their discovery ~25 years ago, but their true nature remains uncertain.”

Go on. Take its pulse. I dare you…

Once again, many thanks to (in alphabetical order) Torsten Grossmann, Dietmar Hager and Rick Johnson for this spectacular twist on astro-imaging!