Khagol No.133

Recently, a collisional ring galaxy system, “Kathryn'sWheel”, was discovered in the γ - ray band. This enigmatic system is located at a mere 30 million light years from the Milky Way. These extremely rare systems are formed when a smaller 'bullet' galaxy pierces through another larger galaxy close to its centre. The produced shock wave sweeps up and kicks the interstellar gas out of the system, leaving behind a ring of star-forming regions and a gas-poor galaxy. A computer simulation of the f o rma t i on o f such an en i gma t i c s t r u c t u r e c a n b e s e e n a t www.youtube.com/watch?v=GoD1MSveH gA&t=88s. The best example of such a system is the Cartwheel galaxy. The optical image of “Kathryn's Wheel” is shown in Figure 1 (left panel). “Kathryn's Wheel” is the nearest collisional ring galaxy system to the Milky Way. Located in the low-latitude (|b| = –8°) Galactic neighbourhood, the system includes a dwarf irregular galaxy, LEDA 3080069, acting as the 'bullet' that has collided with an edge-on, late-type, spiral galaxy ESO 179-13. The system also contains another dwarf irregular galaxy LEDA 3080065. The system has an overall physical size of ~15 kpc and a total mass of 6.7x109 Msun (HI + stars). It was observed as a part of the SuperCOSMOS H α Survey of the Southern Galactic plane, which revealed the central galaxy, ESO 179-13, to be surrounded by a ring of star-forming knots and having a 6.1 kpc diameter. Based on the H α and mid-infrared flux measurements, the integrated star formation rate (SFR) of the whole system is reported to be 0.2-0.5 Msun per year, higher than that of the Large Magellanic Cloud (LMC). The most luminous HII region in the system has an H α luminosity of 5 x 1039 erg/s, comparable to supergiant HII region 30 Doradus in LMC, which is also a bright γ -ray emitter. The right panel of Figure 1 shows “Kathryn's Wheel” as observed by the SuperCOSMOS H α survey. The image is a continuum subtracted to highlight the zones of star formation. The spectacular star-forming ring, triggered by galaxy collision, surrounding the central galaxy, ESO 179-13, is evident. The central galaxy shows little star formation, possibly stripped of gas due to interaction with the “bullet” galaxy, which itself is undergoing rapid star formation. The star-forming regions are the sites where the most energetic particles, also known as cosmic rays, are produced. The interaction of cosmic rays with the surrounding interstellar gas and radiation fields could produce the γ -ray emission observed from such objects. Therefore, the γ -ray radiation detected from “Kathryn's Wheel” has been suggested to be produced by star-forming activities. The data taken by NASA's Large Area Telescope instrument onboard the Fermi Gamma-ray Space Telescope was used to confirm the γ -ray detection of this enigmatic galaxy collision. Although located in our Galactic backyard, this object has been little studied because of the presence of a bright nearby star, HD 150915, and its location in a crowded, low- latitude area close to the Galactic plane, where identifying astrophysical sources is challenging due to extreme dust obscuration. “Kathryn's Wheel can be considered a test bed to explore the origin and transport of cosmic rays and their connection with star-forming activities, given its proximity to theMilkyWay. Reference: “A Gamma-ray Emitting Collisional Ring G a l a x y S y s t em i n o u r G a l a c t i c Neighborhood” Vaidehi S. Paliya and D. J. Saikia. The Astrophysical Journal Letters, 9 6 7 , L 2 6 , 2 0 2 4 [ D O I : h t t p s : / / d o i . o r g / 1 0 . 3 8 4 7 / 2 0 4 1 - 8213/ad4999] | 03 | KHAG L | No. 133 - JULY 2024 Dr. Vaidehi S. Paliya is an assistant professor at IUCAA. He completed his doctoral research on high-energy emission from active galactic nuclei (AGN) from the Indian Institute of Astrophysics (IIA) under the joint supervision of Dr C. S. Stalin (IIA) and Dr C. D. Ravikumar (University of Calicut) in 2016. He then moved to Clemson University, USA, for his first postdoctoral position (2016-2018), where he also worked with the Fermi- Large Area Telescope collaboration. In November 2018, as a postdoctoral fellow, he joined the multi-messenger astronomy group at Deutsches Elektronen Synchrotron, DESY, Germany, and worked on the AGN jet-neutrino connection. Before joining IUCAA in April 2022, he worked as Scientist-C at the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, from December 2020 to March 2022. Dr. Paliya's primary research focuses on understanding the relativistic jet phenomenon (launching, propagation, and termination) associated with AGN following a multi-wavelength approach and using observations fromground- and space-based telescopes.