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Register a new userChemodynamics of barred galaxies in cosmological simulations: On the Milky Ways quiescent merger history and in-situ bulge
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Francesca Fragkoudi F
Publication date
2019
fields
Physics
and research's language is
English
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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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Numerous studies of integrated starlight, stellar counts, and kinematics have confirmed that the Milky Way is a barred galaxy. However, far fewer studies have investigated the bars stellar population properties, which carry valuable independent information regarding the bars formation history. Here we conduct a detailed analysis of chemical abundance distributions ([Fe/H] and [Mg/Fe]) in the on-bar and off-bar regions to study the azimuthal variation of star formation history (SFH) in the inner Galaxy. We find that the on-bar and off-bar stars at Galactocentric radii 3 $< r_{rm GC}<$ 5 kpc have remarkably consistent [Fe/H] and [Mg/Fe] distribution functions and [Mg/Fe]--[Fe/H] relation, suggesting a common SFH shared by the long bar and the disc. In contrast, the bar and disc at smaller radii (2 $< r_{rm GC} <$ 3 kpc) show noticeable differences, with relatively more very metal-rich ([Fe/H]~0.4) stars but fewer solar abundance stars in the bar. Given the three-phase star formation history proposed for the inner Galaxy in Lian et al. (2020b), these differences could be explained by the off-bar disc having experienced either a faster early quenching process or recent metal-poor gas accretion. Vertical variations of the abundance distributions at small $r_{rm GC}$ suggest a wider vertical distribution of low-$alpha$ stars in the bar, which may serve as chemical evidence for vertical heating through the bar buckling process. The lack of such vertical variations outside the bulge may then suggest a lack of vertical heating in the long bar.
Selecting centrally quiescent galaxies from the Sloan Digital Sky Survey (SDSS) to create high signal-to-noise (>100) stacked spectra with minimal emission line contamination, we accurately and precisely model the central stellar populations of barred and unbarred quiescent disk galaxies. By splitting our sample by redshift, we can use the fixed size of the SDSS fiber to model the stellar populations at different radii within galaxies. At 0.02<z<0.04, the SDSS fiber radius corresponds to ~1 kpc, which is the typical half-light radii of both classical bulges and disky pseudobulges. Assuming that the SDSS fiber primarily covers the bulges at these redshifts, our analysis shows that there are no significant differences in the stellar populations, i.e., stellar age, [Fe/H], [Mg/Fe], and [N/Fe], of the bulges of barred vs. unbarred quiescent disk galaxies. Modeling the stellar populations at different redshift intervals from z=0.020 to z=0.085 at fixed stellar masses produces an estimate of the stellar population gradients out to about half the typical effective radius of our sample, assuming null evolution over this ~1 Gyr epoch. We find that there are no noticeable differences in the slopes of the azimuthally averaged gradients of barred vs. unbarred quiescent disk galaxies. These results suggest that bars are not a strong influence on the chemical evolution of quiescent disk galaxies.
(Abridged) We study the incidence, as well as the nature, of composite bulges in a sample of 10 face-on barred galaxies to constrain the formation and evolutionary processes of the central regions of disk galaxies. We analyze the morphological, photometric, and kinematic properties of each bulge. Then, by using a case-by-case analysis we identify composite bulges and classify every component into a classical or pseudobulge. In addition, bar-related boxy/peanut (B/P) structures were also identified and characterised. We find only three galaxies hosting a single-component bulge (two pseudobulges and one classical bulge). We find evidence of composite bulges coming in two main types based on their formation: secular-built and merger- and secular-built. We call secular-built to composite bulges made of entirely by structures associated with secular processes such as pseudo bulges, central disks, or B/P bulges. We find four composite bulges of this kind in our sample. On the other hand, merger- and secular-built bulges are those where structures with different formation paths coexist within the same galaxy, i.e., a classical bulge coexisting with a secular-built structure (pseudobulge, central disk, or B/P). Three bulges of this kind were found in the sample. We remark on the importance of detecting kinematic structures such as sigma-drops to identify composite bulges. A large fraction (80%) of galaxies were found to host sigma-drops or sigma-plateaus in our sample revealing their high incidence in barred galaxies. The high frequency of composite bulges in barred galaxies points towards a complex formation and evolutionary scenario. Moreover, the evidence for coexisting merger- and secular-built bulges reinforce this idea. We discuss how the presence of different bulge types, with different formation histories and timescales, can constrain current models of bulge formation.
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 analyse from an observational perspective the formation history and kinematics of a Milky Way-like galaxy from a high-resolution zoom-in cosmological simulation that we compare to those of our Galaxy as seen by Gaia DR2 to better understand the origin and evolution of the Galactic thin and thick discs. The cosmological simulation was carried out with the GADGET-3 TreePM+SPH code using the MUlti Phase Particle Integrator (MUPPI) model. We disentangle the complex overlapping of stellar generations that rises from the top-down and inside-out formation of the galactic disc. We investigate cosmological signatures in the phase-space of mono-age populations and highlight features stemming from past and recent dynamical perturbations. In the simulation, we identify a satellite with a stellar mass of $1.2 times 10^9$ M$_odot$, i.e. stellar mass ratio $Delta sim 5.5$ per cent at the time, accreted at $z sim 1.6$, which resembles the major merger Gaia-Sausage-Enceladus that produced the Galactic thick disc, i.e. $Delta sim 6$ per cent. We found at $z sim 0.5-0.4$ two merging satellites with a stellar mass of $8.8 times 10^8$ M$_odot$ and $5.1 times 10^8$ M$_odot$ that are associated to a strong starburst in the Star Formation History, which appears fairly similar to that recently found in the Solar Neighbourhood. Our findings highlight that detailed studies of coeval stellar populations kinematics, which are made available by current and future Gaia data releases and in synergy with simulations, are fundamental to unravel the formation and evolution of the Milky Way discs.
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