Yet another spectacular image of the Webb telescope teaches us about the wonders of the Universe. On this occasion, we learn about the ultraluminous infrared (IR) galaxy (ULIRGs) Arp 220[1]. This object is 250 million light-years away in the constellation of Serpens, just above what is known as its head.
The center is quite luminous in the infrared, almost saturating this image and creating the famous diffraction spikes of Webb that make this image look almost like a star. This luminosity produced by two spiral galaxies in the process of merging, each with more than a trillion suns, is 100 times more luminous than our Milky Way galaxy.
Arp 220 has been studied for a long time, looking to understand its intricate shape and what is fueling its star formation and active galactic nuclei. In 1997, observations taken with the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) uncovered that the strong IR luminosity was rather produced by the collision between two spiral galaxies, igniting a burst of star formation. In the NICMOS image, the core of one of the objects appears as a bright crescent moon-shaped object[2], with the lower half probably obscured by a disk of dust 300 light-years across.
At the left is the core of the other colliding galaxy, which appears as a bright round object about 1,200 light-years (400 pc) apart. Later observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) revealed that, in spite of the merger, the structure of the nucleus has an impressive symmetry and regularity, implying that the disk of each galaxy had time to relax during the merging process. This means that each disk has rotated several revolutions over which they significantly changed their separation but have now stabilized. With a rotation period calculated to be 1 million years, the observed symmetry implies that the merging of these two galaxies took just about 3 to 5 million years[3].
Reference arXiv:1605.09381v2 [astro-ph.GA] 7 Feb 2017
Optical observation of the Hubble Space Telescope also led to the discovery of more than 200 objects, which appear to be huge young and intermediate population clusters, many with masses 106 to 107 the mass of the Sun. Eight of these clusters were identified in the NICMOS images[3]. The star clusters are the bluish-white bright knots visible in the Hubble image, packed in a dusty region about 5,000 light-years across. Although this region is about 5 percent of the Milky Way Galaxy’s diameter, the amount of gas equals the amount of gas in our entire galaxy.
Reference Lockhart, K.E. et al 2015, The Astrophysical Journal, 810:149
Arp 220 glows brightest in infrared light, with an intensity that earns it the classification of ultra-luminous infrared galaxy. However, the outskirts observed with Webb and represented in blue also reveal material drawn off the galaxies by gravity; these are known as tidal tails and are evidence that a merger occurred here. To make this object more interesting, observations made with the millimetric interferometers reveal that the double nuclei are counter-rotating with respect to each other, each with a very high gas concentration in their centers[5], while at the same time rotate around a massive warped molecular disk [6]. Observations made with NASA s Chandra X-ray Observatory provide an explanation for these observations. They reveal that the X-rays arise from two cores, indicating the presence of two supermassive black holes in the center[7] of these quite interesting merging galaxies.
Arp 220 is also known as IC 1127, VV 540, KPG 470, UGC 09913
References:
[1] NASA, ESA, CSA, STScI [2] Rodger Thompson, Marcia Rieke, Glenn Schneider (University of Arizona) and Nick Scoville (California Institute of Technology), and NASA
[3] Scoville, N. Z., et al. 1998, ApJ, 492, L107
[4] The Astrophysical Journal 2017, Vol 836, p 66
[5] Scoville N. Z. 2009, ASP Conference Series, Vol 408, p95
[6]A. Eckart & D. Downes 2001, The Astrophysical Journal, Vol 551,p730
[7] A. Paggi et al 2017 The Astrophysical Journal, 841 44
[8] Image chandra https://chandra.harvard.edu/photo/2002/1181/