Mitali Damle

Our Local Group (LG), i.e. the Milky Way (MW), Andromeda galaxy (M31) and the corresponding satellites remain the best studied ensemble of galaxies, primarily due to their proximity to us, the observers. Yet, there remain quite a few aspects about the LG which we do not completely understand. For instance, the origins and distribution of the gas permeating our LG, the problem of missing satellite galaxies and so on. Owing to the observational constraints, it is not always enough to rely on pure observations in order to get a wholesome understanding of these still-poorly understood phenomena.

Simulations have been an excellent counterpart to the observations since the past few decades. Modern cosmological as well as idealized simulations not only provide with possible guiding directions to the future observational projects, but also act as robust testing grounds for existing and new theories. Cosmological simulations have been able to well-replicate the large-scale structure of the Universe, the Cosmic Web pattern and the morphology of galaxies. However, in the context of the LG, most of these simulations have so far had modest success in getting the local small-scale structure right. The HESTIA suite of constrained simulations goes a step further in this regard, wherein, they manage to get both the large-scale as well as small-scale cosmography of the LG correct.

Our aim here is to use three of the highest-resolution simulations from HESTIA in order to obtain a better understanding about the tenuous gas (more commonly called as the Circumgalactic medium or CGM) that surrounds each galaxy in our LG. The gas comprising this CGM is observed to be multiphase in nature i.e. the presence of a wide range of temperatures and densities gives rise to an ensemble of ions, all of which co-exist in the same physical space. A rigorous knowledge of the CGM, thus, necessitates a thorough ionization modeling implementation; in our case we use the Cloudy photoionization modeling code and model five ions- two of which trace the cold and cool-ionized CGM (HI and SiIII; Low ions) and three, which trace the hot, diffuse CGM (OVI, OVII and OVIII; High ions). Thereafter, we set the midpoint of MW-M31 as our frame of reference and generate All-sky projection maps (Mollweide maps) for all the five ions and three simulations. We also plot the 2D column densities for each ion as a function of the impact parameter (Rvir; a radius characterizing the physical extent of any galaxy), in order to track the changes in respective ion column densities as we move from the inner, denser regions to the outer, more diffuse regions of the galaxies.

We see that the clumpiness of the gas decreases as we go from the low ions to the high ions (i.e. the gas becomes increasingly uniform). Low ions also show a wider range of column density distributions (seen as thicker column density profiles) as compared to High ions (seen as thinner column density profiles). Many of the satellite galaxies, due to the gravitational influence of the bigger MW-M31 systems, show significant distortion and stripping features (as is evident in the 2D projection maps in Page 1). Finally, our results of the SiIII column densities for M31 are seen to match rather well with those from EAGLE cosmological simulations. However, our results are about a factor of two higher than the corresponding results from the Project AMIGA observational survey of the Andromeda galaxy.