172 research outputs found
A close look at secular evolution: Boxy/peanut bulges reduce gas inflow to the central kiloparsec
In this letter we investigate the effect of boxy/peanut (b/p) bulges on
bar-induced gas inflow to the central kiloparsec, which plays a crucial role on
the evolution of disc galaxies. We carry out hydrodynamic gas response
simulations in realistic barred galaxy potentials, including or not the
geometry of a b/p bulge, to investigate the amount of gas inflow induced in the
different models. We find that b/p bulges can reduce the gas inflow rate to the
central kiloparsec by more than an order of magnitude, which leads to a
reduction in the amount of gas available in the central regions. We also
investigate the effect of the dark matter halo concentration on these results,
and find that for maximal discs, the effect of b/p bulges on gas inflow remains
significant. The reduced amount of gas reaching the central regions due to the
presence of b/p bulges could have significant repercussions on the formation of
discy- (pseudo-) bulges, on the amount of nuclear star formation and feedback,
on the fuel reservoir for AGN activity, and on the overall secular evolution of
galaxies.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Letters. 5 pages, 6 figure
The Effects of Boxy/Peanut Bulges on Galaxy Models
We examine the effects that the modelling of a Boxy/Peanut (B/P) bulge will
have on the estimates of the stellar gravitational potential, forces, orbital
structure and bar strength of barred galaxies. We present a method for
obtaining the potential of disc galaxies from surface density images, assuming
a vertical density distribution (height function), which is let to vary with
position, thus enabling it to represent the geometry of a B/P. We construct a
B/P height function after the results from a high-resolution, N-body+SPH
simulation of an isolated galaxy and compare the resulting dynamical model to
those obtained with the commonly used, position-independent "flat" height
functions. We show that methods that do not allow for a B/P can induce errors
in the forces in the bar region of up to 40% and demonstrate that this has a
significant impact on the orbital structure of the model, which in turn
determines its kinematics and morphology. Furthermore, we show that the bar
strength is reduced in the presence of a B/P. We conclude that neglecting the
vertical extent of a B/P can introduce considerable errors in the dynamical
modelling. We also examine the errors introduced in the model due to
uncertainties in the parameters of the B/P and show that even for generous but
realistic values of the uncertainties, the error will be noticeably less than
that of not modelling a B/P bulge at all.Comment: Accepted for publication in MNRA
Bars & boxy/peanut bulges in thin & thick discs: I. Morphology and line-of-sight velocities of a fiducial model
We explore trends in the morphology and line-of-sight (los) velocity of
stellar populations in the inner regions of disc galaxies, using N-body
simulations with both a thin (kinematically cold) and a thick (kinematically
hot) disc which form a bar and boxy/peanut (b/p) bulge. The bar in the thin
disc component is 50\% stronger than the thick disc bar and is more
elongated, with an axis ratio almost half that of the thick disc bar. The thin
disc b/p bulge has a pronounced X-shape, while the thick disc b/p is weaker
with a rather boxy shape. This leads to the signature of the b/p bulge in the
thick disc to be weaker and further away from the plane than in the thin disc.
Regarding the kinematics, we find that the los velocity of thick disc stars in
the outer parts of the b/p bulge can be \emph{larger} than that of thin disc
stars, by up to 40\% and 20\% for side-on and Milky Way-like orientations of
the bar respectively. This is due to the different orbits followed by thin and
thick disc stars in the bar-b/p region, which are affected by the fact that: i)
thin disc stars are trapped more efficiently in the bar - b/p instability and
thus lose more angular momentum than their thick disc counterparts and ii)
thick disc stars have large radial excursions and therefore stars from large
radii with high angular momenta can be found in the bar region. We also find
that the difference between the los velocities of the thin and thick disc in
the b/p bulge () correlates with the initial difference between
the radial velocity dispersions of the two discs () . We
therefore conclude that stars in the bar - b/p bulge will have considerably
different morphologies and kinematics depending on the kinematic properties of
the disc population they originate from.Comment: Accepted for publication in A&A. 15 pages (2 page appendix). 16
figure
A dynamical mechanism for the origin of nuclear rings
We develop a dynamical theory for the origin of nuclear rings in barred
galaxies. In analogy with the standard theory of accretion discs, our theory is
based on shear viscous forces among nested annuli of gas. However, the fact
that gas follows non circular orbits in an external barred potential has
profound consequences: it creates a region of reverse shear in which it is
energetically favourable to form a stable ring which does not spread despite
dissipation. Our theory allows us to approximately predict the size of the ring
given the underlying gravitational potential. The size of the ring is loosely
related to the location of the Inner Lindblad Resonance in the epicyclic
approximation, but the predicted location is more accurate and is also valid
for strongly barred potentials. By comparing analytical predictions with the
results of hydrodynamical simulations, we find that our theory provides a
viable mechanism for ring formation if the effective sound speed of the gas is
low (\cs\lesssim1\kms), but that nuclear spirals/shocks created by pressure
destroy the ring when the sound speed is high (\cs\simeq10\kms). We conclude
that whether this mechanism for ring formation is relevant for real galaxies
ultimately depends on the effective equation of state of the ISM. Promising
confirmation comes from simulations in which the ISM is modelled using
state-of-the-art cooling functions coupled to live chemical networks, but more
tests are needed regarding the role of turbulence driven by stellar feedback.
If the mechanism is relevant in real galaxies, it could provide a powerful tool
to constrain the gravitational potential, in particular the bar pattern speed.Comment: Accepted for publication in MNRA
The dual origin of the Galactic thick disc and halo from the gas-rich Gaia-Enceladus Sausage merger
We analyse a set of cosmological magnetohydrodynamic simulations of the formation of Milky Way-mass galaxies identified to have a prominent radially anisotropic stellar halo component similar to the so-called 'Gaia Sausage' found in the Gaia data. We examine the effects of the progenitor of the Sausage (the Gaia-Enceladus Sausage, GES) on the formation of major galactic components analogous to the Galactic thick disc and inner stellar halo. We find that the GES merger is likely to have been gas-rich and contribute 10-50 per cent of gas to a merger-induced centrally concentrated starburst that results in the rapid formation of a compact, rotationally supported thick disc that occupies the typical chemical thick disc region of chemical abundance space. We find evidence that gas-rich mergers heated the proto-disc of the Galaxy, scattering stars on to less-circular orbits such that their rotation velocity and metallicity positively correlate, thus contributing an additional component that connects the Galactic thick disc to the inner stellar halo. We demonstrate that the level of kinematic heating of the proto-galaxy correlates with the kinematic state of the population before the merger, the progenitor mass, and orbital eccentricity of the merger. Furthermore, we show that the mass and time of the merger can be accurately inferred from local stars on counter-rotating orbits
Orbital analysis of stars in the nuclear stellar disc of the Milky Way
Context. While orbital analysis studies were so far mainly focused on the Galactic halo, it is possible now to do these studies in the heavily obscured region close to the Galactic Centre.Aims. We aim to do a detailed orbital analysis of stars located in the nuclear stellar disc (NSD) of the Milky Way allowing us to trace the dynamical history of this structure.Methods. We integrated orbits of the observed stars in a non-axisymmetric potential. We used a Fourier transform to estimate the orbital frequencies. We compared two orbital classifications, one made by eye and the other with an algorithm, in order to identify the main orbital families. We also compared the Lyapunov and the frequency drift techniques to estimate the chaoticity of the orbits.Results. We identified several orbital families as chaotic, z-tube, x-tube, banana, fish, saucer, pretzel, 5:4, and 5:6 orbits. As expected for stars located in a NSD, the large majority of orbits are identified as z-tubes (or as a sub-family of z-tubes). Since the latter are parented by x2 orbits, this result supports the contribution of the bar (in which x2 orbits are dominant in the inner region) in the formation of the NSD. Moreover, most of the chaotic orbits are found to be contaminants from the bar or bulge which would confirm the predicted contamination from the most recent NSD models.Conclusions. Based on a detailed orbital analysis, we were able to classify orbits into various families, most of which are parented by x2-type orbits, which are dominant in the inner part of the bar
Chasing the impact of the Gaia-Sausage-Enceladus merger on the formation of the Milky Way thick disc
We employ our Bayesian Machine Learning framework BINGO (Bayesian INference for Galactic archaeOlogy) to obtain high-quality stellar age estimates for 68 360 red giant and red clump stars present in the 17th data release of the Sloan Digital Sky Survey, the APOGEE-2 high-resolution spectroscopic survey. By examining the denoised age-metallicity relationship of the Galactic disc stars, we identify a drop in metallicity with an increase in [Mg/Fe] at an early epoch, followed by a chemical enrichment episode with increasing [Fe/H] and decreasing [Mg/Fe]. This result is congruent with the chemical evolution induced by an early-epoch gas-rich merger identified in the Milky Way-like zoom-in cosmological simulation Auriga. In the initial phase of the merger of Auriga 18 there is a drop in metallicity due to the merger diluting the metal content and an increase in the [Mg/Fe] of the primary galaxy. Our findings suggest that the last massive merger of our Galaxy, the Gaia-Sausage-Enceladus, was likely a significant gas-rich merger and induced a starburst, contributing to the chemical enrichment and building of the metal-rich part of the thick disc at an early epoch
Local variations of the stellar velocity ellipsoid - II. The effect of the bar in the inner regions of Auriga galaxies
Theoretical works have shown that off-plane motions of bars can heat stars in the vertical direction during buckling but is not clear how do they affect the rest of components of the stellar velocity ellipsoid (SVE). We study the 2D spatial distribution of the vertical, σz, azimuthal, σφ, and radial, σr velocity dispersions in the inner regions of Auriga galaxies, a set of high-resolution magneto-hydrodynamical cosmological zoom-in simulations, to unveil the influence of the bar on the stellar kinematics. σz and σφ maps exhibit non-axisymmetric features that closely match the bar light distribution with low-σ regions along the bar major axis and high values in the perpendicular direction. On the other hand, σr velocity dispersion maps present more axisymmetric distributions. We show that isophotal profile differences best capture the impact of the bar on the three SVE components providing strong correlations with bar morphology proxies although there is no relation with individual σ. Time evolution analysis shows that these differences are a consequence of the bar formation and that they tightly coevolve with the strength of the bar. We discuss the presence of different behaviours of σz and its connection with observations. This work helps us understand the intrinsic σ distribution and motivates the use of isophotal profiles as a mean to quantify the effect of bars
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