We use one of the highest resolution cosmological SPH simulations to date to demonstrate that cold gaseous clouds form around Milky Way size galaxies. We further explore mechanisms responsible for their formation and show that a large fraction of clouds originate as a consequence of late-time filamentary "cold mode" accretion. Here, filaments that are still colder and denser than the surrounding halo gas, are not able to connect directly to galaxies, as they do at high redshift, but are instead susceptible to the combined action of cooling and Rayleigh-Taylor instabilities at intermediate radii within the halo leading to the production of cold, dense pressure-confined clouds, without an associated dark matter component. This process is aided through the compression of the incoming filaments by the hot halo gas and expanding shocks during the halo buildup. Our mechanism directly seeds clouds from gas with substantial local overdensity, unlike in previous models, and provides a channel for the origin of cloud complexes. These clouds can later "rain" onto galaxies, delivering fuel for star formation. Owing to the relatively large cross section of filaments and the net angular momentum carried by the gas, the clouds will be distributed in a modestly flattened region around a galaxy.
Ongoing star formation in the Milky Way requires continuous gaseous fuel from accretion. Previous work has suggested that the accretion of dwarf galaxies could provide the needed gas for this process. In this work we investigate whether dwarf galaxy accretion is consistent with the radial profile of star formation observed in the Milky Way, which is strongly concentrated toward the center of the gaseous disk of the Galaxy. Using realistic parameters for the Galactic potential, gaseous halo, Galactic disk, velocities of dwarf galaxies, and effects of drag on stripped gas in the halo, we model t…
We present a model for the seeding and evolution of magnetic fields in protogalaxies. Supernova (SN) explosions during the assembly of a protogalaxy provide magnetic seed fields, which are subsequently amplified by compression, shear flows and random motions. We implement the model into the MHD version of the cosmological N-body / SPH simulation code GADGET and we couple the magnetic seeding directly to the underlying multi-phase description of star formation. We perform simulations of Milky Way-like galactic halo formation using a standard LCDM cosmology and analyse the strength and distribut…
The dominant physical processes responsible for the formation and longevity of giant gaseous and stellar discs in galaxies remain controversial. Although they are rare (less than 10 confirmed as of now), giant low-surface brightness (gLSB) discy galaxies provide interesting insights given their extreme nature. We describe observations of UGC 1378 including deep spectroscopy with the Russian 6m telescope and multi-band imaging with Binospec at the MMT. Galaxy UGC 1378 has both high surface brightness and an extended low surface brightness discs. Our stellar velocity dispersion data for the high…
There is increasing evidence that a substantial fraction of Milky Way satellite galaxies align in a rotationally-supported plane of satellites, a rare configuration in cosmological simulations of galaxy formation. It has been suggested that other Milky Way substructures (namely young halo globular clusters and stellar/gaseous streams) similarly tend to align with this plane, accordingly dubbed the Vast Polar Structure (VPOS). Using systemic proper motions inferred from Gaia data, we find that globular cluster orbital poles are not clustered in the VPOS direction, though the population with the…
Using 3D hydrodynamic calculations we simulate formation of molecular clouds in the Galaxy. The simulations take into account molecular hydrogen chemical kinetics, cooling and heating processes. Comprehensive gravitational potential accounts for contributions from the stellar bulge, two and four armed spiral structure, stellar disk, dark halo and takes into account self-gravitation of the gaseous component. Gas clouds in our model form in the spiral arms due to shear and wiggle instabilities and turn into molecular clouds after $t\simgt 100$ Myr. At the times $t\sim 100 - 300$ Myr the clouds f…
Galaxy clusters are known to induce gas loss in infalling galaxies due to the ram pressure exerted by the intracluster medium over their gas content. In this paper, we investigate this process through a set of simulations of Milky Way like galaxies falling inside idealised clusters of 10$^{14}$ M$_\odot$ and 10$^{15}$ M$_\odot$, containing a cool-core or not, using the adaptive mesh refinement code RAMSES. We use these simulations to constrain how much of the initial mass contained in the gaseous disk of the galaxy will be converted into stars and how much of it will be lost, after a single cr…
We present a self-consistent hydrodynamical simulation of a Milky Way-like galaxy, at the resolution of 0.05 pc. The model includes star formation and a new implementation of stellar feedback through photo-ionization, radiative pressure and supernovae. The simulation resolves the structure of the interstellar medium at subparsec resolution for a few cloud lifetimes, and at 0.05 pc for about a cloud crossing time. Turbulence cascade and gravitation from the kpc scales are de facto included in smaller structures like molecular clouds. We show that the formation of a bar influences the dynamics o…
We study the gas distribution in the Milky Way and Andromeda using a constrained cosmological simulation of the Local Group (LG) within the context of the CLUES (Constrained Local UniversE Simulations) project. We analyse the properties of gas in the simulated galaxies at $z=0$ for three different phases: `cold', `hot' and HI, and compare our results with observations. The amount of material in the hot halo ($M_{hot}\approx 4-5\times10^{10}\,$M$_{\odot}$), and the cold ($M_{cold}(r\lesssim10\,$kpc$)\approx10^{8}\,$M$_{\odot}$) and HI ($M_{HI}(r\lesssim50\,$kpc$)\approx 3-4\times10^8\,$M$_{\odo…
The long-standing challenge of creating a Milky Way-like disk galaxy from cosmological simulations has motivated significant developments in both numerical methods and physical models in recent years. We investigate these two fundamental aspects in a new comparison project using a set of cosmological hydrodynamic simulations of the formation and evolution of a Milky Way-size galaxy. In this study, we focus on the comparison of two particle-based hydrodynamics methods: the improved smoothed particle hydrodynamics (SPH) code Gadget, and the Lagrangian Meshless Finite-Mass (MFM) code GIZMO. All t…
The satellite galaxies of the Milky Way (MW) align with and preferentially orbit in a vast polar structure (VPOS), which also contains globular clusters, stellar and gaseous streams. Similar alignments have been discovered around several other host galaxies. We test whether recently discovered objects in the MW halo, the satellite galaxy/globular cluster transition object named PSO J174.0675-10.8774 or Crater and three stellar streams, are part of the VPOS. Crater is situated close to the VPOS. Incorporating the new object in the VPOS-plane fit slightly improves the alignment of the plane with…
We analyze circumgalactic medium (CGM) in a suite of high-resolution cosmological re-simulations of a Milky-Way size galaxy and show that CGM properties are quite sensitive to details of star formation--feedback loop modelling. The simulation that produces a realistic late-type galaxy, fails to reproduce existing observations of the CGM. In contrast, simulation that does not produce a realistic galaxy has the predicted CGM in better agreement with observations. This illustrates that properties of galaxies and properties of their CGM provide strong ${\it complementary}$ constraints on the proce…
Using stellar-dynamical calculations it is shown for the first time that counter-orbiting material emerges naturally in tidal interactions of disc galaxies. Model particles on both pro- and retrograde orbits can be formed as tidal debris in single encounters with disc galaxies of 1-to-1 and 4-to-1 mass ratios. A total of 74 model calculations are performed for a range of different initial parameters. Interactions include fly-by and merger cases. The fraction of counter-orbiting material produced varies over a wide range (from a few up to 50 percent). All fly-by models show a similar two-phase …
The TNG50 cosmological simulation produces X-ray emitting bubbles, shells, and cavities in the circumgalactic gas above and below the stellar disks of Milky Way- and Andromeda-like galaxies with morphological features reminiscent of the eROSITA and Fermi bubbles in the Galaxy. Two-thirds of the 198 MW/M31 analogues inspected in TNG50 at z=0 show one or more large-scale, coherent features of over-pressurized gas that impinge into the gaseous halo. Some of the galaxies include a succession of bubbles or shells of increasing size, ranging from a few to many tens of kpc. These are prominent in gas…
We analyze the physical properties of gas in the circumgalactic medium (CGM) of 132 Milky Way (MW)-like central galaxies at $z=0$ from the cosmological magneto-hydrodynamical simulation TNG50, part of the IllustrisTNG project. The properties and abundance of CGM gas across the sample are diverse, and the fractional budgets of different phases (cold, warm, and hot), as well as neutral HI mass and metal mass, vary considerably. Over our stellar mass range of $10^{10.5} < M_\star / \rm{M}_\odot < 10^{10.9}$, radial profiles of gas physical properties from $0.15 < R\rm{ / R_{\rm 200c}} < 1.0$ reve…
We employ hydrodynamical simulations to study the effects of dissipational gas physics on the vertical heating and thickening of disc galaxies during minor mergers. For the first time we present a suite of simulations that includes a diffuse, rotating, cooling, hot gaseous halo, as predicted by cosmological hydrodynamical simulations as well as models of galaxy formation. We study the effect of this new gaseous component on the vertical structure of a Milky Way-like stellar disc during 1:10 and 1:5 mergers. For 1:10 mergers we find no increased final thin disc scale height compared to the isol…
We investigate the impact of cosmic rays (CR) and different modes of CR transport on the properties of Milky Way-mass galaxies in cosmological magneto-hydrodynamical simulations in the context of the AURIGA project. We systematically study how advection, anisotropic diffusion and additional Alfvén-wave cooling affect the galactic disc and the circum-galactic medium (CGM). Global properties such as stellar mass and star formation rate vary little between simulations with and without various CR transport physics, whereas structural properties such as disc sizes, CGM densities or temperatures can…
We present the analysis of the XMM-Newton data of the Circum-Galactic Medium of MASsive Spirals (CGM-MASS) sample of six extremely massive spiral galaxies in the local Universe. All the CGM-MASS galaxies have diffuse X-ray emission from hot gas detected above the background extending $\sim(30-100)\rm~kpc$ from the galactic center. This doubles the existing detection of such extended hot CGM around massive spiral galaxies. The radial soft X-ray intensity profile of hot gas can be fitted with a $β$-function with the slope typically in the range of $β=0.35-0.55$. This range, as well as those $β$ …
The Magellanic Stream (MS) is the nearest example of a gaseous trail formed by interacting galaxies. While the substantial gas masses in these kinds of circumgalactic structures are postulated to represent important sources of fuel for future star formation, the mechanisms whereby this material might be accreted back into galaxies remain unclear. Recent neutral hydrogen (HI) observations have demonstrated that the northern portion of the MS, which probably has been interacting with the Milky Way's hot gaseous halo for close to 1000~Myr, has a larger spatial extent than previously recognized, w…
We present a new approach to study galaxy evolution in a cosmological context. We combine cosmological merger trees and semi-analytic models of galaxy formation to provide the initial conditions for multi-merger hydrodynamic simulations. In this way we exploit the advantages of merger simulations (high resolution and inclusion of the gas physics) and semi-analytic models (cosmological background and low computational cost), and integrate them to create a novel tool. This approach allows us to study the evolution of various galaxy properties, including the treatment of the hot gaseous halo from…
We estimate the potential contribution of M < 10^9 Msun dwarf galaxies to the reionization and early metal-enrichment of the Milky Way environment, or circum-Galactic Medium. Our approach is to use the observed properties of ancient stars (> 12 Gyr old) measured in nearby dwarf galaxies to characterize the star-formation at high-z. We use a merger-tree model for the build-up of the Milky Way, which self-consistently accounts for feedback processes, and which is calibrated to match the present-day properties of the Galaxy and its dwarf satellites. We show that the high-z analogues of nearby dwa…
The wealth of data in the past decades, and especially in the past 15 years has transformed our picture of the gas around the Milky Way and other spiral galaxies. There is good evidence for extraplanar gas that is a few kpc in height and is seen in all gaseous phases: neutral; warm atomic; and hot, X-ray emitting gas. This medium is seen not only around the Milky Way, but other spiral galaxies and it is related to the star formation rate, so it is likely produced by the activity in the disk through a galactic fountain. More extended examples of halo gas are seen, such as the HVC around the Mil…
We study the effect of dissipational gas physics on the vertical heating and thickening of disc galaxies during minor mergers. We produce a suite of minor merger simulations for Milky Way-like galaxies. This suite consists of collisionless simulations as well as hydrodynamical runs including a gaseous component in the galactic disc. We find that in dissipationless simulations minor mergers cause the scale height of the disc to increase by up to a factor of ~2. When the presence of gas in the disc is taken into account this thickening is reduced by 25% (50%) for an initial disc gas fraction of …
