Jasjeet Singh Bagla

Atomic hydrogen (Hi) regulates star formation as cold gas fuels star formation. It represents a key phase in the baryon cycle. Redshifted 21 cm line emission serves as a key tracer for investigating Hi in the interstellar medium (ISM) and circumgalactic medium (CGM) in distant galaxies, and enables the study of galaxy evolution. Direct detections of Hi are currently limited to z ≤ 0.4 due to the inherently weak 21 cm emission line. Ongoing and upcoming large radio surveys aim to detect 21 cm emission from galaxies up to z ≳ 1. We present a novel approach for creating optical-Hi joint mock catalogues for upcoming SKA precursor surveys: MIGHTEE-Hi, LADUMA and WALLABY. Incorporation of optical properties along with Hi makes these mock catalogues a powerful tool for making predictions for surveys and provides a benchmark for exploring the Hi science (e.g., conditional Hi MF and optical-to-Hi scaling relations) expected from these surveys. As a case study, we show the use of the joint catalogues for predicting the expected outcome of stacking detection for average Hi mass in galaxies that are below the threshold for direct detection. We show that combining stacking observations with only the number of direct detections puts a strong constraint on the Hi mass function. We also show that this can be done separately for parts of the color-magnitude diagram for galaxies, enabling a fine grained study of galaxy evolution. This may be used to set priors for the full determination of the Hi mass function.

Comprehensive observations of galaxy clusters suggest that gas deficiency in the galaxies could be due to ram pressure stripping due to the high-pressure intra-cluster medium acting on the galactic discs. The presence of gas in galaxies is essential for star formation. The net force due to ram pressure is dependent on the ambient medium and the orbit followed by the galaxy as it moves past the cluster medium. This work deals with the effect of non-radial orbits of galaxies and the inclination of the disc plane of galaxies with the orbital plane on the mass of gas removed due to ram pressure. This gives a realistic approach to understanding the process of ram pressure stripping. The orbital parameters are extracted from eagle simulation data set along with the mass distribution of the galaxies. The analytical model proposed by Singh et. al. is modified appropriately to include the effect of the inclination angle. The non-radial orbits and infalling galaxies not being face-on decrease the amount of gas removed. Moreover, the inclination angle has a pronounced effect on the stripping of gas in low-mass galaxies as compared to high-mass galaxies with similar inclinations. The results show that the efficiency of the ram pressure stripping can be much lower in some cases, and hence gas in infalling galaxies can survive for much longer than expected from a simple analysis.