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Register a new userStructural and photometric properties of barred galaxies from the Auriga cosmological simulations
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Guillermo Blazquez-Calero
Publication date
2019
fields
Physics
and research's language is
English
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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.
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We investigate the formation of ultra-diffuse galaxies (UDGs) using the Auriga high-resolution cosmological magneto-hydrodynamical simulations of Milky Way-sized galaxies. We identify a sample of $92$ UDGs in the simulations that match a wide range of observables such as sizes, central surface brightness, S{e}rsic indices, colors, spatial distribution and abundance. Auriga UDGs have dynamical masses similar to normal dwarfs. In the field, the key to their origin is a strong correlation present in low-mass dark matter haloes between galaxy size and halo spin parameter. Field UDGs form in dark matter haloes with larger spins compared to normal dwarfs in the field, in agreement with previous semi-analytical models. Satellite UDGs, on the other hand, have two different origins: $sim 55%$ of them formed as field UDGs before they were accreted; the remaining $sim 45%$ were normal field dwarfs that subsequently turned into UDGs as a result of tidal interactions.
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.
(Aims) In this work, we predict the far-infrared polarisation signal emitted by non-spherical dust grains in nearby galaxies. We determine the angular resolution and sensitivity required to study the magnetic field configuration in these galaxies. (Methods) We post-process a set of Milky Way like galaxies from the Auriga project, assuming a dust mix consisting of spheroidal dust grains that are partially aligned with the model magnetic field. We constrain our dust model using Planck 353 GHz observations of the Milky Way. This model is then extrapolated to shorter wavelengths that cover the peak of interstellar dust emission and to observations of arbitrarily oriented nearby Milky Way like galaxies. (Results) Assuming an intrinsic linear polarisation fraction that does not vary significantly with wavelength for wavelengths longer than 50 micron, we predict a linear polarisation fraction with a maximum of $10-15%$ and a median value of $approx{}7%$ for face-on galaxies and $approx{}3%$ for edge-on galaxies. The polarisation fraction anti-correlates with the line of sight density and with the angular dispersion function which expresses the large scale order of the magnetic field perpendicular to the line of sight. The maximum linear polarisation fraction agrees well with the intrinsic properties of the dust model. The true magnetic field orientation can be traced along low density lines of sight when it is coherent along the line of sight. These results also hold for nearby galaxies, where a coherent magnetic field structure is recovered over a range of different broad bands. (Conclusions) Polarised emission from non-spherical dust grains accurately traces the large scale structure of the galactic magnetic field in Milky Way like galaxies, with expected maximum linear polarisation fractions of $10-15%$. To resolve this maximum, a spatial resolution of at least 1 kpc is required.
We carry out a comparison study on the bar structure in the Illustris-1 and TNG100 simulations. At $z=0$, 8.9% of 1232 disc galaxies with stellar mass $>10^{10.5}M_{odot}$ in Illustris-1 are barred, while the numbers are 55% of 1269 in TNG100. The bar fraction as a function of stellar mass in TNG100 agrees well with the survey $S^4G$. The median redshift of bar formation are $sim 0.4-0.5$ and $sim 0.25$ in TNG100 and Illustris-1 respectively. Bar fraction generally increases with stellar mass and decreases with gas fraction in both simulations. Barred galaxy had higher gas fraction at high redshift tend to form bar later. When the bars were formed, the disc gas fractions were mostly lower than 0.4. The much higher bar fraction in TNG100 probably have benefit from much lower gas fraction in massive disc galaxies since $zsim3$, which may result from the combination of more effective stellar and AGN feedback. The latter may be the primary factor at $z<2$. Meanwhile, in both simulations, barred galaxies have higher star formation rate before bar formation, and stronger AGN feedback all the time than unbarred galaxies. The properties of dark matter halos hosting massive disc galaxies are similar between two simulations, and should have minor effect on the different bar frequency. For individual galaxies under similar halo environment cross two simulations, different baryonic physics can lead to striking discrepancy on morphology. The morphology of individual galaxies is subject to combined effects of environment and internal baryonic physics, and is often not predictable.
We explore the chemodynamical properties of a sample of barred galaxies in the Auriga magneto-hydrodynamical cosmological zoom-in simulations, which form boxy/peanut (b/p) bulges, and compare these to the Milky Way (MW). We show that the Auriga galaxies which best reproduce the chemodynamical properties of stellar populations in the MW bulge have quiescent merger histories since redshift $zsim3.5$: their last major merger occurs at $t_{rm lookback}>12,rm Gyrs$, while subsequent mergers have a stellar mass ratio of $leq$1:20, suggesting an upper limit of a few percent for the mass ratio of the recently proposed Gaia Sausage/Enceladus merger. These Auriga MW-analogues have a negligible fraction of ex-situ stars in the b/p region ($<1%$), with flattened, thick disc-like metal-poor stellar populations. The average fraction of ex-situ stars in the central regions of all Auriga galaxies with b/ps is 3% -- significantly lower than in those which do not host a b/p or a bar. While the central regions of these barred galaxies contain the oldest populations, they also have stars younger than 5Gyrs (>30%) and exhibit X-shaped age and abundance distributions. Examining the discs in our sample, we find that in some cases a star-forming ring forms around the bar, which alters the metallicity of the inner regions of the galaxy. Further out in the disc, bar-induced resonances lead to metal-rich ridges in the $V_{phi}-r$ plane -- the longest of which is due to the Outer Lindblad Resonance. Our results suggest the Milky Way has an uncommonly quiet merger history, which leads to an essentially in-situ bulge, and highlight the significant effects the bar can have on the surrounding disc.
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