Despite a wealth of observations, interpreting the SMC’s structure & kinematics has been a challenge. The SMC’s gas exhibits a substantial velocity gradient, yet the stars show minimal rotation. The SMC has an on-sky stellar extent of a few kpc, yet its line of sight depth is > 10 kpc, and the stellar and gas centers are separated by > 1 kpc. The SMC is clearly in a very high state of disequilibrium. Using hydrodynamic simulations, in Rathore+2025(c), we showed that the SMC’s disequilibrium can be attributed to a recent (< 200 Million years ago) direct collision between the SMC and LMC. Tidal forces and ram pressure exerted by the LMC transforms the SMC from a rotationally supported to a dispersion dominated galaxy.

The LMC has a strange bar. The bar is offset from the disk center, tilted with the disk plane and does not drive gas inflows. In Rathore+2025(a), we used Gaia DR3 data to measure the dynamical strength of the LMC's bar with the stellar density field for the first time. To perform this measurement, we developed a novel solution to tackle crowding induced incompleteness in Gaia datasets. Surprisingly, we found that the LMC hosts a strong bar, similar to other barred galaxies in the local universe.

I gave a talk primarily focussed on this work in July 2024 at the Indian Institute of Astrophysics, slides can be found here.

Why does the LMC's bar show such strange properties despite being consistent with bar - galaxy co-evolution ?

Using hydrodynamic simulations of the LMC - SMC - Milky Way interaction history, in Rathore+2025(b) we showed that the LMC bar's strange properties are explainable with a recent (150 - 200 Million years ago) direct collision between the LMC & SMC. This collision also significantly slows down the LMC's bar, potentially explaining the low observed pattern speed values. Further, the LMC bar's tilt is a direct probe of the SMC's dark matter profile, and requires the SMC to be a dark matter dominated galaxy.

I gave a talk primarily focussed on this work in March 2025 at the Durham University (UK), slides can be found here.

S0 galaxies possess well defined stellar disks, but lack spiral arms and are generally quenched. However, there exists a significant number of S0s that show active star-formation. Existence of star-forming S0s poses a significant challenge to galaxy evolution theories. In Rathore+2022, we performed a detailed study of star-forming S0s with resolved spectroscopic observations from the SDSS-MaNGA survey combined with deep optical imaging. Surprisingly, star-forming S0 galaxies in the local universe possess centrally dominated star-formation, which is different from disk dominated star-formation in typical spiral galaxies. Moreover, the star-forming S0s were most likely quenched S0s in the past whose star-formation has been rejuvenated, possibly through mergers with smaller galaxies.

For a relatively non-technical summary of this research, please refer to our article on CosmicVarta. I also gave a talk on this work at IIT Indore in July 2023, slides can be found here.

The Hubble tension is a raging debate in cosmology about the current expansion rate of the universe, as quantified by the Hubble constant. Early universe probes like the Cosmic Microwave Background and late universe probes like the Cepheid-Supernova distance ladder give different values of the Hubble constant. One possible explaination for the Hubble tension could be physics beyond the framework of general relativity, like a time varying gravitational constant (G). In Ruchika, Rathore et al. 2024, we proposed a transition in the value of G at late times, and evaluated its affect on Cepheid variables and Supernova explosions, and thereby the distance ladder. Through a careful statistical analysis, we found that a 4% larger value of the G at lookback times > 70 Million years is preferred by the Cepheid and Supernova data of the local universe. Such a G transition is within existing observational constraints and can resolve the Hubble tension.