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Register a new userRam Pressure Stripping of the Large Magellanic Clouds Disk as a Probe of the Milky Ways Circumgalactic Medium
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Munier Salem
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Recent observations have constrained the orbit and structure of the Large Magellanic Cloud (LMC), implying a well-constrained pericentric passage about the Milky Way (MW) ~ 50 Myr ago. In this scenario, the LMCs gaseous disk has recently experienced maximal ram pressure stripping, suggesting the current extent of its HI disk directly probes the medium in which it is moving. From the observed stellar and HI distributions of the system we find evidence of a truncated gas profile along the windward ``leading edge of the LMC disk, despite a far more extended stellar component. We explore the implications of this ram pressure stripping signature, using both analytic prescriptions and full 3-dimensional hydrodynamic simulations of the LMC. Our simulations subject the system to a headwind whose velocity components correspond directly to the recent orbital history of the LMC. We vary the density of this headwind, using a variety of sampled parameters for a Beta-profile for a theoretical MW circumgalactic medium (CGM), comparing the resulting HI morphology directly to observations of the LMC HI and stellar components. This model can match the radial extent of the LMCs leading (windward) edge only in scenarios where the MW CGM density at pericentric passage is n(R = 48.2 +/- 5 kpc) = 1.1 (+.44/-.45) x 1e-4 cm^-3. The implied pericentric density proves insensitive to both the broader CGM structure and temperature profile, thus providing a model-independent constraint on the local gas density. This result imposes an important constraint on the density profile of the MWs CGM, and thus the total baryon content of the MW. From our work, assuming a Beta-profile valid to ~ Rvir, we infer a total diffuse CGM mass M(300 kpc) = 2.6 +/- 1.4 x 1e10 Msun or approximately 15% of a 1e12 Msun MWs baryonic mass budget.
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(Abridged) We perform high resolution 2D hydrodynamical simulations of face-on ram pressure stripping (RPS) of disk galaxies to compile a comprehensive parameter study varying galaxy properties (mass, vertical structure of the gas disk) and covering a large range of ICM conditions, reaching from high density environments like in cluster centres to low density environments typical for cluster outskirts or groups. We find that the ICM-ISM interaction proceeds in three phases: firstly the instantaneous stripping phase, secondly the dynamic intermediate phase, thirdly the quasi-stable continuous viscous stripping phase. The stripping efficiency depends slightly on the Mach number of the flow, however, the main parameter is the ram pressure. The stripping efficiency does not depend on the vertical structure and thickness of the gas disk. We discuss uncertainties in the classic estimate of the stripping radius of citet{gunn72}, and adapt the estimate used by cite{mori00} for spherical galaxies, (comparison of central pressure with ram pressure). We find that the latter estimate predicts the radius and mass of the gas disk remaining at the end of the second phase very well, and better than the citet{gunn72} criterion. From our simulations we conclude that gas disks of galaxies in high density environments are heavily truncated or even completely stripped, but also the gas disks of galaxies in low density environments are disturbed by the flow and back-falling material, so that they should also be pre-processed.
In the recent literature there is circumstantial evidence that the viscosity of the intracluster medium may not be too far from the Spitzer value. In this letter, we present two-dimensional hydrodynamical simulations of ram pressure stripping of disc galaxies in a viscous intracluster medium. The values of viscosity explored range between 0.1 and 1.0 times the Spitzer value. We find that viscosity affects the appearance and the dimensions of the galactic wakes but has very little effect on the evolution of the gas mass of the galaxy.
While galaxies move through the intracluster medium of their host cluster, they experience a ram pressure which removes at least a significant part of their interstellar medium. This ram pressure stripping appears to be especially important for spiral galaxies: this scenario is a good candidate to explain the differences observed between cluster spirals in the nearby universe and their field counterparts. Thus, ram pressure stripping of disk galaxies in clusters has been studied intensively during the last decade. I review advances made in this area, concentrating on theoretical work, but continuously comparing to observations.
From our position embedded within the Milky Ways interstellar medium (ISM), we have limited ability to detect gas at low relative velocities in the extended Galactic halo because those spectral lines are blended with much stronger signals from dense foreground gas. As a result, the content of the Milky Ways circumgalactic medium (CGM) is poorly constrained at $|v_{rm LSR}|$ $lesssim$ 150 km s$^{-1}$. To overcome this complication, the QuaStar Survey applies a spectral differencing technique using paired quasar-star sightlines to measure the obscured content of the Milky Ways CGM for the first time. We present measurements of the CIV doublet ($lambdalambda$ 1548 r{A}, 1550 r{A}), a rest-frame UV metal line transition detected in HST/COS spectra of 30 halo-star/quasar pairs evenly distributed across the sky at Galactic latitudes $|b|>30^circ$. The 30 halo stars have well-constrained distances (d$approx$5-14 kpc), and are paired with quasars separated by $<$ 2.8$^circ$. We argue that the difference in absorption between the quasar and stellar sightlines originates primarily in the Milky Ways extended CGM beyond $sim$10 kpc. For the Milky Ways extended, low velocity CGM ($|v|<$150 km/s), we place an upper limit on the mean CIV column density of $rm Delta logN_{LVCGM} < 13.39$ and find a covering fraction of $f_{rm CIV,LVCGM} (rm logN>13.65)=$ 20% [6/30], a value significantly lower than the covering fraction for star-forming galaxies at low redshift. Our results suggest either that the bulk of Milky Ways CIV-traced CGM lies at low Galactic latitudes, or that the Milky Ways CGM is lacking in warm, ionized material compared to low-redshift ($z < 0.1$) star-forming galaxy halos.
Observational evidence shows that low-redshift galaxies are surrounded by extended haloes of multiphase gas, the so-called circumgalactic medium (CGM). To study the survival of relatively cool gas (T < 10^5 K) in the CGM, we performed a set of hydrodynamical simulations of cold (T = 10^4 K) neutral gas clouds travelling through a hot (T = 2x10^6 K) and low-density (n = 10^-4 cm^-3) coronal medium, typical of Milky Way-like galaxies at large galactocentric distances (~ 50-150 kpc). We explored the effects of different initial values of relative velocity and radius of the clouds. Our simulations were performed on a two-dimensional grid with constant mesh size (2 pc) and they include radiative cooling, photoionization heating and thermal conduction. We found that for large clouds (radii larger than 250 pc) the cool gas survives for very long time (larger than 250 Myr): despite that they are partially destroyed and fragmented into smaller cloudlets during their trajectory, the total mass of cool gas decreases at very low rates. We found that thermal conduction plays a significant role: its effect is to hinder formation of hydrodynamical instabilities at the cloud-corona interface, keeping the cloud compact and therefore more difficult to destroy. The distribution of column densities extracted from our simulations are compatible with those observed for low-temperature ions (e.g. SiII and SiIII) and for high-temperature ions (OVI) once we take into account that OVI covers much more extended regions than the cool gas and, therefore, it is more likely to be detected along a generic line of sight.
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