Currently, I am investigating the LMC - SMC - Milky Way system. The LMC and SMC (collectively known as the Clouds) are the two most massive satellite galaxies of the Milky Way. The Clouds have likely been interacting with each other since the past 6 billion years ! Vast photometric, spectroscopic and astrometric observations have enabled the utilization of the Clouds as a new Astrophysical laboratory. I am using this laboratory to advance our understanding of galaxy dynamics and dark matter physics, as well as of the Clouds themselves. To achieve this goal, I am building state-of-the-art numerical simulations of the LMC - SMC - Milky Way interaction history alongwith analyzing precision observational datasets.
For the first time, we measure the dynamical strength of the LMC's bar with the stellar density field, and find that the LMC hosts a strong bar. We discover that the LMC is similar to other barred galaxies in the local universe from the viewpoint of bar-galaxy scaling relations.
The LMC's completeness corrected Gaia DR3 red clump disk.
Source: Rathore+2025(a).
The LMC's bar has several strange properties which are typically not shown by barred galaxies. It is offset from the disk center, tilted with respect to the disk plane, and does not drive gas inflows. Characterizing the LMC's bar is important to understand the origin of its strange nature, place the LMC in context with other barred galaxies, and understand the role of bars in the evolution of low mass galaxies.
In Rathore+2025(a), we precisely measure the 2-D geometry of the LMC's bar with Gaia DR3 data. We propose a novel solution to tackle crowding induced incompleteness in Gaia datasets, particularly for color selected Gaia sub-samples.
Using hydrodynamic simulations of the LMC - SMC - Milky Way interaction history, we show that the LMC bar's strange properties are explainable with a recent direct collision between the LMC and SMC. 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.
Schematic showing the SMC's torques on the LMC's bar.
Source: Rathore+2025(b).
Why does the LMC's bar show such strange properties despite being consistent with bar - galaxy scaling relations ?
In Rathore+2025(b), we show that tidal forces from the SMC is what primarily drives the LMC's bar out of equilibrium. In particular, the LMC bar's strange properties (offset, tilt and lack of gas inflows) can be explained if the LMC and SMC collided around 150 - 200 Million years ago ! Further, this collision significantly slows down the LMC's bar, potentially explaining the low observed pattern speed values.
Moreover, the SMC's torques on the LMC's bar during the collision can explain the LMC bar's tilt, provided the SMC's inner regions contain atleast 70% dark matter ! This is the first dynamical constraint on the SMC's pre-collision dark matter profile.
MEGHA (sanskrit for Clouds) is an observationally fine tuned full N-body model of the LMC-SMC-Milky Way interaction.
A snaphot from the MEGHA simulations.
The LMC and SMC are associated with very interesting observables like the gas stream, the bridge, LMC's off-centered bar, LMC's one-armed spiral to name a few, which are a likely consequence of galaxy interactions. The Clouds can give us a detailed understanding of how interactions affect dynamical processes in galaxies; like stellar orbits, bars, spirals and the distribution of dark matter. Accurate models of the Clouds are needed to use precision observations from Gaia, Hubble Space Telescope and other surveys to solve this complex dynamical problem. MEGHA simulations are a path forward ! This is work in progress.
We find that star-forming S0 galaxies in the local universe have centrally dominated star-formation. Moreover, the star-forming S0s were most likely quenched S0s in the past whose star-formation has been rejuvenated, possibly through minor mergers.
Deep Subaru-HSC images of a sample of star-forming S0s.
Source: Rathore+2022.
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. The prevailing theory for the origin of S0s relies on quenching mechanism. Thus, the existence of star-forming S0s poses a significant challenge towards understanding the nature of this class of galaxies. The origin of star-formation and the star-formation properties of such galaxies have been a subject of debate since the last two decades. We did a detailed study of star-forming S0s with resolved spectroscopic observations from the SDSS-MaNGA survey combined with deep optical imaging. We performed the first spatially resolved analysis of the star-formation properties of these galaxies and also attempted to settle the debate on the origin of star-formation in these galaxies.
For details, please refer to our paper Rathore+2022. For a relatively non-technical summary, please refer to our article on CosmicVarta. I gave a talk on this work at IIT Indore in July 2023, slides can be found here.
We find that a 4% larger value of the Gravitational constant (G) at lookback times > 70 Myr is preferred by Cepheid and SNe Ia data of the local universe. Such a G transition is within existing observational constraints and can resolve the Hubble tension.
Effect of a G transion on the Cepheid Period-Luminosity relation.
Source: Ruchika+2024
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, the difference being highly significant statistically. One possible explaination for the Hubble tension could be physics beyond the framework of general relativity, like a time varying gravitational constant (G). In this work, we proposed a transition in the value of G at late times, and evaluated its affect on the physics of Cepheid variables and Supernova explosions, and thereby the distance ladder. We performed a careful statistical analysis to identify whether such a G transition can the solve the Hubble tension.
For details, please refer to our paper Ruchika, Rathore et al. 2024.