Khagol Bulletin # 132 (Apr 2024) - ENG

| 07 | KHAG L | No. 132 - APRIL 2024 What does HI 21 cm absorption studies tell usaboutgalaxies? Unlike HI 21 cm emission, the detectability of HI 21 cm absorption is not limited by distance of the absorbing gas, rather it depends only on the strength of the background radio source and the HI 21 cm absorption cross-section projected on the sky by the galaxies. Therefore, HI 21 cm absorption line studies can complement the emission line studies to trace the evolution of the atomic gas component in galaxies. HI 21 cm absorption has been used to constrain the thermal state of the atomic gas using the thermal width of the lines as well as the optical depth, which is inversely proportional to the excitation or a powe r f u l t o o l f o r s t ud y i ng t h e distribution, structure, kinematics, and physical conditions of the atomic gas in the Milky Way as well as other galaxies. In the nearby Universe, blind HI 21 cm emission line surveys using single dish radio telescopes, such as Arecibo and Parkes, have provided reliable measurements of the cosmological HI mass density. On the other hand, spatially resolved HI imaging using radio interferometric arrays, such as the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT), have characterised the structure and dynamics of galaxies. These observations have facilitated understanding of the relation of atomic gas with the star formation activity and with the environment of galaxies. Additionally, HI 21 cm emission maps of galaxies have been used to derive rotation curves that give the velocity of matter as a function of distance from the centre of galaxies. The rotation curves thus derived turned out to be remaining flat at large distances from galaxy centres, indicating that there is matter in galaxies that is not visible, and thus providing a very important evidence for the existence of dark matter. However, the flux of the HI 21 cm emission signal is inversely proportional to the square of the distance to the unresolved emitting gas. Hence, sensitivities of most present day radio telescopes make it difficult to directly map HI emission from galaxies far away fromus. How do we study the atomic hydrogen gas ingalaxies? What does HI 21 cm emission studies tell usaboutgalaxies? The Dutch astronomer, Hendrik van de Hulst, first predicted in 1944 that the hydrogen atom could produce a spectral line at a frequency of 1420 MHz or equivalently at a wavelength of 21 cm. This line occurs due to the electron proton spin- flip transition between the two hyperfine levels in the ground state of the hydrogen atom. In other words, a hydrogen atomwith spin of the electron aligned parallel with that of the proton in the hyperfine excited state, undergoes a flip of the electron spin such that it is aligned antiparallel with that of the proton in the ground state, resulting i n t he emi ss i on o f a pho t on wi t h wavelength of 21 cm. Despite being a highly forbidden transition with a very low probability of occurrence, the HI 21 cm line is observable from galaxies thanks to the large amount of hydrogen atoms present in t h e Un i v e r s e . Oc c u r r i ng i n r a d i o frequencies, this radiation from the hydrogen atom penetrates through dust clouds, and provides us a more complete viewof the ISM than that by visible light. The HI 21 cm line from the Milky Way was first detected in 1951 by three independent research groups. Since then, it has become the most important and well studied spectral line in radio astronomy. The HI 21 cmemission line has proved to be mechanical feedback associated with star f o rma t i on . The r e f o r e , i n o r de r t o understand the physical processes that drive the cosmic evolution of star formation activity in galaxies, it is vital to trace the evolution of the HI gas associated with galaxies. Moreover, the HI gas is typically more extended than the stellar component of galaxies, and is therefore highly vulnerable to distortions caused by interactions with other galaxies and with the environment. Hence, the hierarchical structure formation process of galaxies including mergers and accretion leave discernible imprints on theHI gas. Studying Galaxies using the Hydrogen 21 cm spectral line Whataregalaxiesmadeupof? Galaxies are the building blocks of the Universe. Among the primary open questions of current astronomical research are: 'How are galaxies formed?' and 'How do galaxies evolve with cosmic time?'. To answer these questions, it is imperative to understand the different components of galaxies. It is well known that galaxies are made up of stars – our Milky Way is estimated to contain more than 100 billion stars. However, what may not be well appreciated is that stars make up only a tiny fraction of the total volume in a galaxy. The matter and energy present between stars in a galaxy is called the 'Interstellar Medium' or ISM. For example, the ISM constitutes well over 99.99% of the Milky Way's volume. The ISM mainly consists of low density gas, with the average number density being around one hydrogen atom per cubic centimetre. Most of this gas, about 70% by mass and about 90%by number of atoms, is hydrogen – the most abundant element in theUniverse. Why is hydrogen gas important for galaxies? The hydrogen in the ISM is present either in its atomic, ionised ormolecular form. Out of these, atomic hydrogen, denoted as HI, constitutes about 60% of the total hydrogen gas by mass. Galaxies use gas as a fuel for their growth and sustenance. They acquire ionised hydrogen gas from their surroundings or via interactions with other galaxies. Once accreted onto galaxies, the ionised hydrogen gas converts to HI gas, which eventually forms molecular hydrogen gas with the help of dust grains in the ISM. The molecular gas is the reservoir from which stars are formed in galaxies. The HI gas thus acts as a crucial intermediary phase in the lifecycle of a galaxy. It has a direct impact on the observable properties of galaxies such as the total mass of stars, the rate at which s t a r s a r e f o rmi ng , a nd c h emi c a l abundance of the galaxies. While at one hand the HI gas modulates the star formation activity in galaxies, it also gets affected by the radiative, chemical and