The Truncated Circumgalactic Medium of the Large Magellanic Cloud

Abstract: The Large Magellanic Cloud (LMC) is the nearest massive galaxy to the Milky Way. Its circumgalactic medium is complex and multi-phase, containing both stripped HI structures like the Magellanic Stream and Bridge, and a diffuse warm corona seen in high-ion absorption. We analyze 28 AGN sightlines passing within 35 kpc of the LMC with archival HST/COS spectra to characterize the cool (T\approx10^4$ K) gas in the LMC CGM, via new measurements of UV absorption in six low ions (OI, FeII, SiII, AlII, SII, and NiII) and one intermediate ion (SiIII). We show that a declining column-density profile is present in all seven ions, with the low-ion profiles having a steeper slope than the high-ion profiles in CIV and SiIV reported by Krishnarao et al. 2022. Crucially, absorption at the LMC systemic velocity is only detected (in all ions) out to 17 kpc. Beyond this distance, the gas has a lower velocity and is associated with the Magellanic Stream. These results demonstrate that the LMC's CGM is composed of two distinct components: a compact inner halo extending to 17 kpc, and a more extended stripped region associated with the Stream. The compactness and truncation of the LMC's inner CGM agree with recent simulations of ram-pressure stripping of the LMC by the Milky Way's extended corona.

• The paper demonstrates a truncated, bimodal CGM structure in the LMC, with an inner compact halo up to 17 kpc and gas stripped into the Magellanic Stream.
• It reveals a steep decline in low-ion column densities at larger radii, contrasting shallow profiles for high ions like C IV and Si IV.
• The study shows that environmental effects, notably Milky Way ram-pressure stripping, critically shape the LMC's CGM compared to isolated dwarf galaxies.

The Truncated Circumgalactic Medium of the Large Magellanic Cloud

Sapna Mishra and collaborators present a comprehensive analysis of the circumgalactic medium (CGM) surrounding the Large Magellanic Cloud (LMC). Utilizing archival spectra from the Hubble Space Telescope’s Cosmic Origins Spectrograph, this study systematically characterizes the cool ionized gas within the LMC's CGM, emphasizing its distinct, truncated structure.

Key Findings

1. Bimodal CGM Structure:
   • The LMC’s CGM is distinctively divided into two main components: an inner compact halo extending up to 17 kpc and a secondary, more extended region of stripped gas associated with the Magellanic Stream. This separation is substantiated by observing absorption at the LMC's systemic velocity only up to 17 kpc, beyond which the gas instead aligns with velocities typical of the Magellanic Stream.
2. Declining Ion Column-Density:
   • A significant finding is the steep decline in column-density for low ions (O, Fe, Si, Al, S, Ni) and the intermediate ion Si observed at larger impact parameters. This contrasts with shallower profiles for high ions like \ion{C}{4} and \ion{Si}{4}, indicative of stratified gas layers perhaps arising from interface regions within a multiphase medium.
3. Environmental Influence:
   • Comparison of the LMC’s CGM to that of isolated dwarf galaxies highlights the steeper column-density profiles of the LMC. This suggests significant environmental effects, notably ram-pressure stripping by the Milky Way’s corona, are at play, truncating the LMC's CGM more aggressively than what is seen in comparable dwarf galaxies.

Implications and Future Directions

The results challenge conventional models of circumgalactic gas distribution around satellite galaxies by capturing the interaction-induced influences shaping the LMC's CGM. From a theoretical perspective, these findings support simulations predicting significant ram-pressure effects from the Milky Way on its satellites, potentially refining models of satellite-galaxy accretion and CGM sustainability.

Future research could explore the temporal evolution of such truncated CGM structures in different interaction scenarios and their broader influence on galaxy evolution, particularly in explaining discrepancies in baryonic mass balance at halo scales. Additionally, spatially resolved simulations could enhance our understanding of CGM dynamics under varying galactic and intergalactic conditions.

Ultimately, this study enriches our understanding of the complex interplay between galactic environments and circumgalactic media, contributing valuable insights into the lifecycle of galaxies in cluster environments and beyond.