Sapna Mishra, Astronomy

Distance to the Magellanic Stream

The Magellanic Stream is the largest gaseous structure visible in the sky, formed by the hydrodynamical interaction between the Magellanic Clouds (the Large and Small Magellanic Clouds, or LMC and SMC) and the Milky Way. However, its distance to the gas from the Sun remains uncertain.To determine this, we analyzed high-resolution VLT/UVES spectra of five blue horizontal branch stars. The method is simple: if we detect absorption features such as Ca II at the velocity of the Magellanic Stream in a star’s spectrum, it means the stream lies in front of that star. If no such features are present, the stream must be behind it. In all five cases, we did not detect any absorption from the stream, even with highly sensitive spectral data. This indicates that the Magellanic Stream is located at least 55 kpc from the Sun. This result provides the first direct observational constraint on the distance to the Magellanic Stream and will play a key role in improving dynamical models of the Milky Way–LMC system. [see Mishra et al 2025 for detail.]

CGM of the LMC

The existence of the circumgalactic medium (CGM) of the Large Magellanic Cloud (LMC) has long been debated due to its "so called" smaller halo mass and proximity to the massive Milky Way. In this study, using HST/COS observations, I investigate the CGM of the LMC. Interestingly, I found that the LMC does possess its own CGM, which is also multiphase in nature. Furthermore, this study demonstrates that the LMC's CGM is truncated to about one-tenth of its virial radius, likely due to strong interaction with the Milky Way via ram pressure. [see Mishra et al 2024b for detail.]

CGM of the Cluster Galaxies

This study investigates how the galaxy cluster environment influences the CGM by stacking HST/COS spectra from 179 galaxies. Spectral properties, like line width and Lyα equivalent width, strongly correlate with cluster mass and distance, with effects observed up to 5 Mpc. This provides the first evidence of cluster influence at such large scales, likely due to galaxies falling along dense filaments extending beyond the cluster center. [see Mishra et al 2024a].

Multiphase outskirts of the Galaxy Clusters

Galaxy cluster outskirts (>R500) remain poorly studied due to their diffuse nature. As the interface between clusters and the IGM, these regions are a "melting pot" where metal-poor infalling gas mixes with metal-rich gas stripped from galaxies via ram pressure and tidal forces. By stacking 80,000 SDSS quasar-cluster pairs for optical MgII absorption and 2,000 HST/COS spectra for UV absorption, we detected MgII, Lyα, and CIV absorption but not OVI. Our results reveal that cluster outskirts are multiphase, with absorption signals not dominated by the CGM of bright galaxies. Covering fractions for Lyα (21%), CIV (10%), and OVI (10%) are significantly lower than in typical CGM, and the patchy distribution of cool gas is evident from weak Lyα signals when strong absorbers are excluded. We propose this cool gas originates from stripped CGM of cluster galaxies and large-scale filaments feeding clusters with cool gas. [see Mishra et al 2024a, Mishra et al 2022].

Photoionization-driven Winds in BALQSOs

Broad absorption line (BAL) quasars exhibit extreme outflows reaching up to 60,000 km/s, with absorption features varying on human timescales. The cause of this variability—whether changes in the covering fraction of outflowing clouds (Gibson et al. 2009) or the quasar's ionizing flux (Vivek 2019)—remains debated. To investigate, I analyzed 84 appearing BAL quasars showing CIV BAL troughs over 0.3–4.8 rest-frame years using SDSS DR-7, DR-12, and DR-14 catalogs. This study shows appearing BAL quasars are brighter with shallower, wider troughs compared to disappearing BALs. The appearance of BALs is accompanied by continuum dimming, following the 'bluer when brighter' trend, and spectral index variations support this anticorrelation. Our findings suggest that BAL appearance/disappearance is primarily driven by changes in the ionization conditions of the absorbing gas, with the intrinsic dust model being less likely. [see Mishra et al 2021].