Nova: Galaxy’s Missing Gamma Rays Found

[bystander]

Found: A Galaxy’s Missing Gamma Rays
Nova | American Astronomical Society | 2016 Apr 18

[img3="Recent observations have detected high-energy gamma-ray emission for the first time from the massive, star-forming galaxy Arp 220 (shown here in optical wavelengths,
as imaged by Hubble). [Credit: NASA/ESA/C. Wilson (McMaster University)]"]http://aasnova.org/wp-content/uploads/2016/04/fig16.jpg[/img3][hr][/hr]

Recent reanalysis of data from the Fermi Gamma-ray Space Telescope has resulted in the first detection of high-energy gamma rays emitted from a nearby galaxy. This discovery reveals more about how supernovae interact with their environments.

After a stellar explosion, the supernova’s ejecta expand, eventually encountering the ambient interstellar medium. According to models, this generates a strong shock, and a fraction of the kinetic energy of the ejecta is transferred into cosmic rays — high-energy radiation composed primarily of protons and atomic nuclei. Much is still unknown about this process, however. One open question is: what fraction of the supernova’s explosion power goes into accelerating these cosmic rays?

In theory, one way to answer this is by looking for gamma rays. In a starburst galaxy, the collision of the supernova-accelerated cosmic rays with the dense interstellar medium is predicted to produce high-energy gamma rays. That radiation should then escape the galaxy and be visible to us.

Observational tests of this model, however, have been stumped by Arp 220. This nearby ultraluminous infrared galaxy is the product of a galaxy merger ~700 million years ago that fueled a frenzy of starbirth. Due to its dusty interior and extreme levels of star formation, Arp 220 has long been predicted to emit the gamma rays produced by supernova-accelerated cosmic rays. But though we’ve looked, gamma-ray emission has never been detected from this galaxy … until now. ...

The First Detection of GeV Emission From an Ultraluminous Infrared Galaxy:
Arp 220 As Seen With the Fermi Large Area Telescope - Fang-Kun Peng et al

• Astrophysical Journal Letters 821(2):L20 (20 Apr 2016) DOI: 10.3847/2041-8205/821/2/L20
  arXiv.org > astro-ph > arXiv:1603.06355 > 21 Mar 2016 (v1), 01 Apr 2016 (v2)

[Ann]

Wow, the colors of Arp 220 (IC 1127)!

Click to view full size image

NGC 5394 (top right) and NGC 5395. Photo: Adam Block.

The U-B index of Arp 220 is 0.330, and the B-V index is 0.740. Nothing remarkable there. The colors suggest a galaxy with little star formation, but with a healthy population of A-type stars. Such a stellar makeup suggests a post-starburst galaxy, perhaps slightly similar to small galaxy NGC 5394. The colors of NGC 5394 are 0.120 (U-B) and 0.625 (B-V), bluer than Arp 220, but Arp 220 is much farther away than NGC 5394 and may be more dust-reddened. The far infrared magnitude of NGC 5394 is about two and a half magnitudes brighter than its B magnitude, suggesting it is quite dusty due to its past starburst activity.

Yes, but the far infrared magnitude of Arp 220!!!

The blue light magnitude of Arp 220 is 14.1 or thereabout. But the far infrared magnitude is about 8.3, almost six magnitudes brighter than the B magnitude!!! That's amazing!!!

Arp 220 may well be a post-starburst galaxy, because its U-B and B-V indexes suggest that this might be the case. The galaxy is certainly choking in its own dust, which is clearly a remnant of extremely vigorous star formation in the past.

Arp 220 is perhaps slightly similar to nearby well-known NGC 253, a nearby dusty starburst galaxy whose colors are quite red, 0.380 (U-B) and 0.850 (B-V). Yes, but the far infrared magnitude of NGC 253 is only about two and a half magnitudes brighter than its B magnitude.


Wow!! Imagine how dusty Arp 220 is, and imagine what kind of starburst it took to produce so much dust! In all likelihood, supernovas are popping like popcorn in this galaxy. No wonder Arp 220 is emitting gamma rays!!

Ann