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Register a new userGas accretion and galactic fountain flows in the Auriga cosmological simulations: angular momentum and metal re-distribution
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Using a set of 15 high-resolution magnetohydrodynamic cosmological simulations of Milky Way formation, we investigate the origin of the baryonic material found in stars at redshift zero. We find that roughly half of this material originates from subhalo/satellite systems and half is smoothly accreted from the Inter-Galactic Medium (IGM). About $90 %$ of all material has been ejected and re-accreted in galactic winds at least once. The vast majority of smoothly accreted gas enters into a galactic fountain that extends to a median galactocentric distance of $sim 20$ kpc with a median recycling timescale of $sim 500$ Myr. We demonstrate that, in most cases, galactic fountains acquire angular momentum via mixing of low-angular momentum, wind-recycled gas with high-angular momentum gas in the Circum-Galactic Medium (CGM). Prograde mergers boost this activity by helping to align the disc and CGM rotation axes, whereas retrograde mergers cause the fountain to lose angular momentum. Fountain flows that promote angular momentum growth are conducive to smooth evolution on tracks quasi-parallel to the disc sequence of the stellar mass-specific angular momentum plane, whereas retrograde minor mergers, major mergers and bar-driven secular evolution move galaxies towards the bulge-sequence. Finally, we demonstrate that fountain flows act to flatten and narrow the radial metallicity gradient and metallicity dispersion of disc stars, respectively. Thus, the evolution of galactic fountains depends strongly on the cosmological merger history and is crucial for the chemo-dynamical evolution of Milky Way-sized disc galaxies.
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The stellar disk of the Milky Way shows complex spatial and abundance structure that is central to understanding the key physical mechanisms responsible for shaping our Galaxy. In this study, we use six very high resolution cosmological zoom simulations of Milky Way-sized haloes to study the prevalence and formation of chemically distinct disc components. We find that our simulations develop a clearly bimodal distribution in the $[rm alpha/Fe]$ -- $[rm Fe/H]$ plane. We find two main pathways to creating this dichotomy which operate in different regions of the galaxies: a) an early ($z>1$) and intense high-$rm[alpha/Fe]$ star formation phase in the inner region ($Rlesssim 5$ kpc) induced by gas-rich mergers, followed by more quiescent low-$rm[alpha/Fe]$ star formation; and b) an early phase of high-$rm[alpha/Fe]$ star formation in the outer disc followed by a shrinking of the gas disc owing to a temporarily lowered gas accretion rate, after which disc growth resumes. In process b), a double-peaked star formation history around the time and radius of disc shrinking accentuates the dichotomy. If the early star formation phase is prolonged (rather than short and intense), chemical evolution proceeds as per process a) in the inner region, but the dichotomy is less clear. In the outer region, the dichotomy is only evident if the first intense phase of star formation covers a large enough radial range before disc shrinking occurs; otherwise, the outer disc consists of only low-$rm[alpha/Fe]$ sequence stars. We discuss the implication that both processes occurred in the Milky Way.
We study the z=0 gas kinematics, morphology, and angular momentum content of isolated galaxies in a suite of cosmological zoom-in simulations from the FIRE project spanning $M_{star}=10^{6-11}M_{odot}$. Gas becomes increasingly rotationally supported with increasing galaxy mass. In the lowest-mass galaxies ($M_{star}<10^{8}M_{odot}$), gas fails to form a morphological disk and is primarily dispersion and pressure supported. At intermediate masses ($M_{star}=10^{8-10}M_{odot}$), galaxies display a wide range of gas kinematics and morphologies, from thin, rotating disks, to irregular spheroids with negligible net rotation. All the high-mass ($M_{star}=10^{10-11}M_{odot}$) galaxies form rotationally supported gas disks. Many of the halos whose galaxies fail to form disks harbor high angular momentum gas in their circumgalactic medium. The ratio of the specific angular momentum of gas in the central galaxy to that of the dark-matter halo increases significantly with galaxy mass, from $j_{rm gas}/j_{rm DM}sim0.1$ at $M_{star}=10^{6-7}M_{odot}$ to $j_{rm gas}/j_{rm DM}sim2$ at $M_{star}=10^{10-11}M_{odot}$. The reduced rotational support in the lowest-mass galaxies owes to (a) stellar feedback and the UV background suppressing the accretion of high-angular momentum gas at late times, and (b) stellar feedback driving large non-circular gas motions. We broadly reproduce the observed scaling relations between galaxy mass, gas rotation velocity, size, and angular momentum, but may somewhat underpredict the incidence of disky, high-angular momentum galaxies at the lowest observed masses ($M_{star}=(10^{6}-2times10^{7})M_{odot}$). In our simulations, stars are uniformly less rotationally supported than gas. The common assumption that stars follow the same rotation curve as gas thus substantially overestimates galaxies stellar angular momentum, particularly at low masses.
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 ($M_{hot}approx 4-5times10^{10},$M$_{odot}$), and the cold ($M_{cold}(rlesssim10,$kpc$)approx10^{8},$M$_{odot}$) and HI ($M_{HI}(rlesssim50,$kpc$)approx 3-4times10^8,$M$_{odot}$) components display a reasonable agreement with observations. We also compute the accretion/ejection rates together with the HI (radial and all-sky) covering fractions. The integrated HI accretion rate within $r=50,$kpc gives $sim$$0.2-0.3,$M$_{odot},$yr$^{-1}$, i.e. close to that obtained from high-velocity clouds in the Milky Way. We find that the global accretion rate is dominated by hot material, although ionized gas with $Tlesssim10^5,$K can contribute significantly too. The $net$ accretion rates of $all$ material at the virial radii are $6-8,$M$_{odot},$yr$^{-1}$. At $z=0$, we find a significant gas excess between the two galaxies, as compared to any other direction, resulting from the overlap of their gaseous haloes. In our simulation, the gas excess first occurs at $zsim1$, as a consequence of the kinematical evolution of the LG.
In this work we analyse the structural and photometric properties of 21 barred simulated galaxies from the Auriga Project. These consist of Milky Way-mass magneto-hydrodynamical simulations in a $Lambda$CDM cosmological context. In order to compare with observations, we generate synthetic SDSS-like broad-band images from the numerical data at z = 0 with different inclinations (from face-on to edge-on). Ellipse fits are used to determine the bar lengths, and 2D bulge/disc/bar decompositions with galfit are also performed, modelling the bar component with the modified Ferrer profile. We find a wide range of bar sizes and luminosities in the sample, and their structural parameters are in good agreement with the observations. All bulges present low Sersic indexes, and are classified as pseudobulges. In regard to the discs, the same breaks in the surface brightness profiles observed in real galaxies are found, and the radii at which these take place are in agreement with the observations. Also, from edge-on unsharp-masked images at z = 0, boxy or peanut-shaped (B/P) structures are clearly identified in the inner part of 4 bars, and also 2 more bars are found in buckling phase. The sizes of the B/P match fairly well with those obtained from observations. We thus conclude that the observed photometric and structural properties of galaxies with bars, which are the main drivers of secular evolution, can be developed in present state-of-the-art $Lambda$CDM cosmological simulations.
We show that the stellar surface-brightness profiles in disc galaxies---observed to be approximately exponential---can be explained if radial migration efficiently scrambles the individual stars angular momenta while conserving the circularity of the orbits and the total mass and angular momentum. In this case the discs distribution of specific angular momenta $j$ should be near a maximum-entropy state and therefore approximately exponential, $dNproptoexp(-j/langle jrangle)dj$. This distribution translates to a surface-density profile that is generally not an exponential function of radius: $Sigma(R)proptoexp[-R/R_e(R)]/(RR_e(R))(1+dlog v_c(R)/dlog R)$, for a rotation curve $v_c(R)$ and $R_e(R)equivlangle jrangle/v_c(R)$. We show that such a profile matches the observed surface-brightness profiles of disc-dominated galaxies as well as the empirical exponential profile. Disc galaxies that exhibit population gradients cannot have fully reached a maximum-entropy state but appear to be close enough that their surface-brightness profiles are well-fit by this idealized model.
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