arXiv subjects

Heikki Salo

Publications and source records attributed to Heikki Salo.

At least 19 recordsLinked to original sources

arXiv2026

Rings around irregular bodies. II. Numerical simulations of the 1/3 spin-orbit resonance confinement and applications to Chariklo

Rings have been found around Chariklo, Haumea and Quaoar, three small objects of the Solar System. All these rings are observed near the second-order spin-orbit resonances (SORs) 1/3 or 5/7 with the central body, suggesting an active confinement mechanism by these resonances. Our goal is to understand how collisional rings can be confined near second-order SORs in spite of the fact that they force self-intersecting streamlines.We use full 3D numerical simulations that treat rings of inelastically colliding particles orbiting non-axisymmetric central bodies, characterized by a dimensionless mass anomaly parameter mu. While most of our simulations ignore self-gravity, a few runs include gravitational interactions between particles, providing preliminary results on the effect of self-gravity on the ring confinement. The 1/3 SOR can confine ring material, by transferring the forced resonant mode into free Lindblad modes. We derive a criterion ensuring that the 1/3 SOR counteracts viscous spreading. Assuming meter-sized ring particles, and tau~1, this requires a threshold value mu > 1e-3 in Chariklo's case. The confinement is not permanent as a slow outward leakage of particles is observed in our simulations. This leakage can be halted by an outside moonlet with a mass of ~1e-7 - 1e-6 relative to Chariklo, corresponding to subkilometer-sized objects. With self-gravity, the ring viscosity nu increases by a factor of few in low-tau rings due to gravitational encounters. For large tau, self-gravity wakes enhance nu by a factor of ~100 compared to a non-gravitating ring, requiring ~10-fold larger mu since the threshold value increases proportional to square-root of nu.

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arXiv2025

Rings around irregular bodies I. Structure of the resonance mesh, applications to Chariklo, Haumea and Quaoar

Three ring systems have been discovered to date around small irregular objects of the solar system (Chariklo, Haumea and Quaoar). For the three bodies, material is observed near the second-order 1/3 Spin-Orbit Resonance (SOR) with the central object, and in the case of Quaoar, a ring is also observed near the second-order resonance 5/7 SOR. This suggests that second-order SORs may play a central role in ring confinement. This paper aims at better understanding this role from a theoretical point of view. It also provides a basis to better interpret the results obtained from N-body simulations and presented in a companion paper. A Hamiltonian approach yields the topological structure of phase portraits for SORs of orders from one to five. Two cases of non-axisymmetric potentials are examined: a triaxial ellipsoid characterized by an elongation parameter C22 and a body with mass anomaly mu, a dimensionless parameter that measures the dipole component of the body's gravitational field. The estimated triaxial shape of Chariklo shows that its corotation points are marginally unstable, those of Haumea are largely unstable, while those of Quaoar are safely stable. The topologies of the phase portraits show that only first- (aka Lindblad) and second-order SORs can significantly perturb a dissipative collisional ring. We calculate the widths, the maximum eccentricities and excitation time scales associated with first- and second-order SORs, as a function of C22 and mu. Applications to Chariklo, Haumea and Quaoar using mu ~ 0.001 show that the first- and second-order SORs caused by their triaxial shapes excite large (>~ 0.1) orbital eccentricities on the particles, making the regions inside the 1/2 SOR inhospitable for rings. Conversely, the 1/3 and 5/7 SORs caused by mass anomalies excite moderate eccentricities (<~ 0.01), and are thus a more favorable place for the presence of a ring.

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arXiv2023

Viscous overstability in dense planetary rings -- Effect of vertical motions and dense packing

We investigate the linear axisymmetric viscous overstability in dense planetary rings with typical values of the dynamical optical depth $τ\gtrsim 0.5$. We develop a granular flow model which accounts for the particulate nature of a planetary ring subjected to dissipative particle collisions. The model captures the dynamical evolution of the disc's vertical thickness, temperature, and effects related to a finite volume filling factor of the ring fluid. We compute equilibrium states of self-gravitating and non-self-gravitating rings, which compare well with existing results from kinetic models and N-Body simulations. Subsequently, we conduct a linear stability analysis of our model. We briefly discuss the different linear eigenmodes of the system and compare with existing literature by applying corresponding limiting approximations. We then focus on the viscous overstability, analysing the effect of temperature variations, radial and vertical self-gravity, and for the first time the effects of vertical motions on the instability. In addition, we perform local N-body simulations incorporating radial and vertical self-gravity. Critical values for the optical depth and the filling factor for the onset of instability resulting from our N-body simulations compare well with our model predictions under the neglect of radial self-gravity. When radial self-gravity is included, agreement with N-body simulations can be achieved by adopting enhanced values of the bulk viscous stress.

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arXiv2023

Bar Properties as a Function of Wavelength: A Local Baseline with S4G for High-Redshift Studies

The redshift evolution of bars is an important signpost of the dynamic maturity of disk galaxies. To characterize the intrinsic evolution safe from band-shifting effects, it is necessary to gauge how bar properties vary locally as a function of wavelength. We investigate bar properties in 16 nearby galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G) at ultraviolet, optical and mid-infrared wavebands. Based on the ellipticity and position angle profiles from fitting elliptical isophotes to the two-dimensional light distribution, we find that both bar length and ellipticity - the latter often used as a proxy for bar strength - increase at bluer wavebands. Bars are 9% longer in the B-band than at 3.6 um. Their ellipticity increases typically by 8% in the B-band, with a significant fraction (>40%) displaying an increase up to 35%. We attribute the increase in bar length to the presence of star forming knots at the end of bars: these regions are brighter in bluer bands, stretching the bar signature further out. The increase in bar ellipticity could be driven by the apparent bulge size: the bulge is less prominent at bluer bands, allowing for thinner ellipses within the bar region. Alternatively, it could be due to younger stellar populations associated to the bar. The resulting effect is that bars appear longer and thinner at bluer wavebands. This indicates that band-shifting effects are significant and need to be corrected for high-redshift studies to reliably gauge any intrinsic evolution of the bar properties with redshift.

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arXiv2022

Photometric properties of nuclear star clusters and their host galaxies in the Fornax cluster

We investigate the relations between nuclear star clusters (NSCs) and their host galaxies, and between the structural properties of nucleated and non-nucleated galaxies. We also address the environmental influences on the nucleation of galaxies in the Fornax main cluster and the Fornax A group. We select 557 Fornax galaxies ($10^{5.5} M_{\odot} < M_{\rm *,galaxy} < 10^{11.5} M_{\odot} $) for which structural decomposition models and non-parametric indices are available. We determine galaxy nucleation based on a combination of visual inspection and a model selection statistic, the Bayesian information criterion (BIC). We also test the BIC as an unsupervised method to determine nucleation labels. We find a dichotomy in the properties of nuclei which reside in galaxies more or less massive than $M_{\rm *,galaxy} \approx 10^{8.5} M_{\odot}$. Specifically, the nuclei tend to be bluer than their host galaxies and follow a scaling relation of $M_{\rm *,nuc} \propto {M_{\rm *,galaxy}}^{0.5}$ for $M_{\rm *,galaxy} < 10^{8.5} M_{\odot}$. In galaxies with $M_{\rm *,galaxy} > 10^{8.5} M_{\odot}$ we find that nuclei are redder compared to the host and follow $M_{\rm *,nuc} \propto M_{\rm *,galaxy}$. Comparing early-type galaxies, we find that nucleated galaxies tend to be redder in global ($g'-r'$) colour, have redder outskirts relative to their own inner regions ($Δ(g'-r')$), be less asymmetric ($A$) and exhibit less scatter in the brightest second order moment of light ($M_{20}$) than their non-nucleated counterparts at a given stellar mass. Additionally, we find the nucleation fractions to be typically higher in the Fornax main cluster than in the Fornax A group, and that the nucleation fraction is highest towards the centre of their respective environments. We also find that the BIC can recover our labels of nucleation up to an accuracy of 97\%. (abridged)

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arXiv2021

The Fornax Deep Survey (FDS) with VST XII: Low surface brightness dwarf galaxies in the Fornax cluster

In this work we use Max-Tree Objects, (MTO) on the FDS data in order to detect previously undetected Low surface brightness (LSB) galaxies. After extending the existing Fornax dwarf galaxy catalogs with this sample, our goal is to understand the evolution of LSB dwarfs in the cluster. We also study the contribution of the newly detected galaxies to the faint end of the luminosity function. We test the detection completeness and parameter extraction accuracy of MTO. We then apply MTO to the FDS images to identify LSB candidates. The identified objects are fitted with 2D Sérsic models using GALFIT and classified based on their morphological appearance, colors, and structure. With MTO, we are able to increase the completeness of our earlier FDS dwarf catalog (FDSDC) 0.5-1 mag deeper in terms of total magnitude and surface brightness. Due to the increased accuracy in measuring sizes of the detected objects, we also add many small galaxies to the catalog that were previously excluded as their outer parts had been missed in detection. We detect 265 new LSB dwarf galaxies in the Fornax cluster, which increases the total number of known dwarfs in Fornax to 821. Using the extended catalog, we show that the luminosity function has a faint-end slope of -1.38+/-0.02. We compare the obtained luminosity function with different environments studied earlier using deep data but do not find any significant differences. On the other hand, the Fornax-like simulated clusters in the IllustrisTNG cosmological simulation have shallower slopes than found in the observational data. We also find several trends in the galaxy colors, structure, and morphology that support the idea that the number of LSB galaxies is higher in the cluster center due to tidal forces and the age dimming of the stellar populations. The same result also holds for the subgroup of large LSB galaxies, so-called ultra-diffuse galaxies.

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arXiv2021

Signatures of quenching in dwarf galaxies in local galaxy clusters

The transformation of late-type galaxies has been suggested as the origin of early-type dwarf galaxies in galaxy clusters. Venhola et al. analysed correlations between colour and surface brightness for galaxies in the Fornax cluster binned by luminosity or stellar mass. In the bins with $M_\star<10^8 {\rm M}_\odot$, the authors identified a correlation of redness with fainter surface brightness and interpreted it as a consequence of the quenching of star formation by ram pressure stripping in the dwarf galaxies. We carry out a corresponding analysis for the Virgo cluster and find great similarities in these correlations between surface brightness and colour for the two clusters, despite expected differences in the strength of the ram pressure. Furthermore, we extend the analysis to a wider range of optical colours for both clusters and contrast the results with expectations for fading and reddening stellar populations. Overall the slopes of the surface brightness-colour relations are consistent with these models. In addition the sizes of the early- and late-type galaxies at these low masses are comparable. These two results are compatible with a transformation scenario. However, when analysing early- and late-type galaxies separately, the consistency of the slope of the surface brightness-colour relations with the model expectations for fading and reddening stellar population applies only to the late types. The lack of this imprint for the early-type dwarfs calls for some additional explanation, for which we discuss several possibilities. Finally, the Virgo cluster is an atypical cluster with a low fraction of quiescent early-type galaxies at all galaxy masses despite its large cluster mass. (abridged)

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arXiv2021

The Fornax Deep Survey (FDS) with the VST XI. The search for signs of preprocessing between the Fornax main cluster and Fornax A group

We investigate the structural properties of cluster and group galaxies by studying the Fornax main cluster and the infalling Fornax A group, exploring the effects of galaxy preprocessing in this showcase example. Additionally, we compare the structural complexity of Fornax galaxies to those in the Virgo cluster and in the field. Our sample consists of 582 galaxies from the Fornax main cluster and Fornax A group. We quantified the light distributions of each galaxy based on a combination of aperture photometry, Sérsic+PSF (point spread function) and multi-component decompositions, and non-parametric measures of morphology (Concentration $C$; Asymmetry $A$, Clumpiness $S$; Gini $G$; second order moment of light $M_{20}$), and structural complexity based on multi-component decompositions. These quantities were then compared between the Fornax main cluster and Fornax A group. The structural complexity of Fornax galaxies were also compared to those in Virgo and in the field. Overall, we find significant differences in the distributions of quantities derived from Sérsic profiles ($g'-r'$, $r'-i'$, $R_e$, and $\barμ_{e,r'}$), and non-parametric indices ($A$ and $S$) between the Fornax main cluster and Fornax A group. Moreover, we find significant cluster-centric trends with $r'-i'$, $R_e$, and $\barμ_{e,r'}$, as well as $A$, $S$, $G$, and $M_{20}$ for galaxies in the Fornax main cluster. We find the structural complexity of galaxies increases as a function of the absolute $r'$-band magnitude (and stellar mass), with the largest change occurring between -14 mag $\lesssim M_{r'}\lesssim$ -19 mag. This same trend was observed for galaxies in the Virgo cluster and in the field, which suggests that the formation or maintenance of morphological structures (e.g. bulges, bar) is largely dependent on the stellar mass of the galaxies, rather than their environment.

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arXiv2020

The distribution of star formation in galactic bars as seen with H$α$ and stacked GALEX UV imaging

We investigate the spatial distribution of star formation (SF) within bars of nearby disk galaxies (inclination $< 65^{\circ}$) from the S$^4$G survey. We use archival GALEX far- and near-UV imaging for 772 barred galaxies. We also assemble a compilation of continuum-subtracted H$α$ images for 433 barred galaxies, of which 70 are produced by ourselves from ancillary photometry and MUSE/CALIFA IFU data cubes. We employ two complementary approaches: i) the analysis of bar/disk stacks built from co-added UV images of hundreds of galaxies; and ii) the classification of the morphology of ionised regions in galaxies into three main SF classes: A) only circumnuclear SF, B) SF at the bar ends, but not along the bar, and C) SF along the bar. Lenticular galaxies typically belong to SF class A: this is probably related to bar-induced SF quenching. The distribution of SF class B peaks for early- and intermediate-type spirals: this most likely results from the interplay of gas flow, shocks, and enhanced shear in centrally concentrated galaxies with large bar amplitudes. Late-type galaxies are mainly assigned to SF class C: we argue that this is a consequence of low shear. In bar stacks of spirals, the UV emission traces the stellar bars and dominates on their leading side, as witnessed in simulations. For early-types, the central UV emission is $\sim$0.5 mag brighter in strongly barred galaxies, relative to their weakly barred counterparts: this is related to the efficiency of strong bars sweeping the disk gas and triggering central starbursts. We also show that the distributions of SF in inner ringed galaxies are broadly the same in barred and non-barred galaxies, including a UV/H$α$ deficit in the middle part of the bar: this hints at the effect of resonance rings trapping gas. Distinct distributions of SF within bars are reported in galaxies of different morphological types (Abridged).

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arXiv2020

The reports of thick discs' deaths are greatly exaggerated: thick discs are NOT artefacts caused by diffuse scattered light

Recent studies have made the community aware of scattered light when examining low-surface-brightness galaxy features such as thick discs. In our past studies of the thick discs of edge-on galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S$^4$G) we modelled the point spread function as a Gaussian. We re-examine our results using a revised point spread function model that accounts for extended wings out to more than 2.5arcmin. We study the $3.6μ{\rm m}$ images of 141 edge-on galaxies from the S$^4$G. We decompose the surface brightness profiles of the galaxies perpendicular to their mid-planes assuming that discs are made of two stellar discs in hydrostatic equilibrium. We decompose the axial surface brightness profiles of galaxies to model the central mass concentration - described by a Sérsic function - and the disc - described by a broken exponential disc. Our improved treatment confirms the ubiquity of thick discs. The main difference between our current fits and those presented before is that now the scattered light from the thin disc dominates the surface brightness at levels below $μ\sim26\,{\rm mag\,arcsec^{-2}}$. This does not affect drastically any of our previously presented results: 1) Thick discs are nearly ubiquitous. They are not an artefact caused by scattered light as has been suggested elsewhere. 2) Thick discs have masses comparable to those of thin discs in low-mass galaxies - circular velocities $v_{\rm c}<120\,{\rm km\,s^{-1}}$ - whereas they are typically less massive than the thin discs in high-mass galaxies. 3) Thick discs and central mass concentrations seem to have formed at the same epoch from a common material reservoir. 4) Roughly 60% of the up-bending breaks in face-on galaxies are caused by the superposition of a thin and a thick disc where the scale-length of the latter is the largest. (Abridged)

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arXiv2020

On the origins of up-bending breaks in disk galaxies

Using SPITZER 3.6$μ$m imaging, we investigate the physical and data-driven origins of up-bending (Type III) disk breaks. We apply a robust new break-finding algorithm to 175 low-inclination disk galaxies previously identified as containing Type III breaks, classify each galaxy by its outermost re-classified (via our new algorithm) break type, and compare the local environments of each resulting subgroup. Using three different measures of the local density of galaxies, we find that galaxies with extended outer spheroids (Type IIIs) occupy the highest density environments in our sample, while those with extended down-bending (Type II) disks and symmetric outskirts occupy the lowest density environments. Among outermost breaks, the most common origin of Type III breaks in our sample is methodological; the use of elliptical apertures to measure the radial profiles of asymmetric galaxies usually results in features akin to Type III breaks.

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arXiv2019

The shapes of spiral arms in the S$^4$G survey and their connection with stellar bars

Spiral galaxies are common in the local Universe, but their formation, evolution, and interplay with bars remain poorly understood. We use a sample of 391 nearby galaxies from the S$^4$G survey to characterise the winding angle and amplitude of spiral arms as a function of disc properties, such as bar strength, in all kinds of spirals (grand-design, multi-armed, and flocculent). We derive global pitch angles in 3.6 $μ$m de-projected images from i) average measurements of individual logarithmic spiral segments, and ii) for a subsample of 32 galaxies, from 2-D Fourier analyses. The strength of spirals is quantified from the tangential-to-radial force ratio and from the normalised $m=2$ Fourier density amplitudes. In galaxies with more than one measured logarithmic segment, the spiral pitch angle varies on average by $\sim 10^{\circ}$ between segments, but by up to $\gtrsim 15-20^{\circ}$. The distribution of the global pitch angle versus Hubble type ($T$) is very similar for barred and non-barred galaxies when $1 \lesssim T \lesssim 5$. Most spiral galaxies ($>90\%$) are barred for $T>5$. The pitch angle is not correlated with bar strength, and only weakly with spiral strength. The amplitude of spirals is correlated with bar strength (and less tightly, with bar length) for all types of spirals. The mean pitch angle is hardly correlated with the mass of the supermassive black hole (estimated from central stellar velocity dispersion), with central stellar mass concentration, or with shear, questioning previous results in the literature using smaller samples. We do not find observational evidence that spiral arms are driven by stellar bars or by invariant manifolds. Most likely, disks that are prone to the development of strong bars are also reactive to the formation of prominent spirals, explaining the observed coupling between bar and spiral amplitudes (Abridged).

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arXiv2019

Rings and spiral arms: are they coupled with bars?

Rings and spiral arms are distinctive features of many galaxies, and their properties are closely related to the disk dynamics. They are often associated to stellar bars, but the details of this connection are far from clear. We study the pitch angles of spiral arms and the frequency and dimensions of inner and outer rings as a function of disk parameters and the amplitude of non-axisymmetries in the S$^4$G survey. The ring fraction increases with bar Fourier density amplitude: this can be interpreted as evidence for the role of bars in ring formation. The sizes of inner rings, normalised by the disk size, are positively correlated with bar strength: this can be linked to the radial displacement of the inner 4:1 ultra-harmonic resonance while the bar grows and the pattern speed decreases. The fraction of rings is larger in barred galaxies than in their non-barred counterparts, but still $\sim 1/3$ ($\sim 1/4$) of the galaxies hosting inner (outer) rings are not barred. The amplitudes of bars and spirals are correlated for all types of spirals. However, on average, the pitch angles of spiral arms are roughly the same for barred and non-barred galaxies: this questions the role of bars exciting spiral structure. We conclude that the present-day coupling of rings, spiral arms, and bars is not as robust as predicted by simulations.

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arXiv2019

Inner and outer rings are not strongly coupled with stellar bars

We study the frequency and dimensions of inner and outer rings in the local Universe as a function of disk parameters and the amplitude of non-axisymmetries. We use the 1320 not-highly inclined disk galaxies ($i<65^{\circ}$) from the S$^4$G survey. The ring fraction increases with bar Fourier density amplitude: this can be interpreted as evidence for the role of bars in ring formation. The sizes of inner rings are positively correlated with bar strength: this can be linked to the radial displacement of the 1/4 ultra-harmonic resonance while the bar grows and the pattern speed decreases. The ring intrinsic ellipticity is weakly controlled by the non-axisymmetric perturbation strength: this relation is not as strong as expected from simulations, especially when we include the dark matter halo in the force calculation. The ratio of outer-to-inner ring semi-major axes is uncorrelated with bar strength: this questions the manifold origin of rings. In addition, we confirm that i) $\sim 1/3$ ($\sim 1/4$) of the galaxies hosting inner (outer) rings are not barred; ii) on average, the sizes and shapes of rings are roughly the same for barred and non-barred galaxies; and iii) the fraction of inner (outer) rings is a factor of $1.2-1.4$ ($1.65-1.9$) larger in barred galaxies than in their non-barred counterparts. Finally, we apply unsupervised machine learning (Self-Organizing Maps, SOMs) to show that, among early-type galaxies, ringed or barred galaxies cannot be univocally distinguished based on 20 internal and external fundamental parameters. We confirm, with the aid of SOMs, that rings are mainly hosted by red, massive, gas-deficient, dark-matter poor, and centrally concentrated galaxies. We conclude that the present-day coupling between rings and bars is not as robust as predicted by numerical models (Abridged).

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arXiv2019

The Fornax Deep Survey (FDS) with VST. VI. Optical properties of the dwarf galaxies in the Fornax cluster

The Fornax Deep Survey Dwarf galaxy Catalog (FDSDC) includes 564 dwarf galaxies in the Fornax cluster and the in-falling Fornax A subgroup. We use the FDSDC galaxies for statistical analysis of the structural and stellar population differences in the range of galactic environments within the Fornax cluster. We present the standard scaling relations for the dwarfs and analyze trends as a function of cluster-centric radius. We find a different behavior for the bright dwarfs (-18.5 mag < M$_r$ < -16 mag) as compared to the fainter ones (M$_r$ > -16 mag): While considering galaxies in the same magnitude-bins, we find that, while for fainter dwarfs the g'-r' color is redder for lower surface brightness objects (as expected from fading stellar populations), for brighter dwarfs the color is redder for the higher surface brightness and higher Sérsic n objects. The trend of the bright dwarfs might be explained by those galaxies being affected by harassment and by slower quenching of star formation in their inner parts. As the fraction of early-type dwarfs with respect to late-types increases toward the central parts of the cluster, the color-surface brightness trends are also manifested in the cluster-centric trends, confirming that it is indeed the environment that changes the galaxies. We also estimate the strengths of the ram-pressure stripping, tidal disruption, and harassment in the Fornax cluster, and find that our observations are consistent with the theoretically expected ranges of galaxy properties where each of those mechanisms dominate. We furthermore find that the luminosity function, color-magnitude relation, and axis-ratio distribution of the dwarfs in the center of the Fornax cluster are similar to those in the center of the Virgo cluster.

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arXiv2019

On the varied origins of up-bending breaks in galaxy disks

Aims: Using a sample of 175 low-inclination galaxies from the S$^{4}$G, we investigate the origins of up-bending (Type III) breaks in the 3.6 $μ$m surface brightness profiles of disk galaxies. Methods: We re-analyze a sample of previously identified Type III disk break-hosting galaxies using a new, unbiased break-finding algorithm, which uncovered many new, sometimes subtle disk breaks across the whole sample. We classify each break by its likely origin through close examination of the galaxy images across wavelengths, and compare samples of galaxies separated by their outermost identified break types in terms of their stellar populations and local environments. Results: We find that more than half of the confirmed Type III breaks in our sample can be attributed to morphological asymmetry in the host galaxies. As these breaks are mostly an artifact of the azimuthal averaging process, their status as physical "breaks" is questionable. Such galaxies occupy some of the highest density environments in our sample, implying that much of this asymmetry is the result of tidal disturbance. Additionally, we find that Type III breaks related to extended spiral arms or star formation often host down-bending (Type II) breaks at larger radius which were previously unidentified. Such galaxies reside in the lowest density environments in our sample, in line with previous studies that found a lack of Type II breaks in clusters. Galaxies occupying the highest density environments most often show Type III breaks associated with outer spheroidal components. Conclusions: We find that Type III breaks in the outer disks of galaxies arise most often through environmental influence: either tidal disturbance (resulting in disk asymmetry) or heating through, e.g., galaxy harrassment (leading to spheroidal components). Galaxies hosting the latter break types also show... (abstract continues)

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arXiv2019

The kinematics of local thick discs do not support an accretion origin

Thick discs are nearly ubiquitous components of the discs of present-day galaxies. It has been proposed that a fraction of their stars has been accreted. Here, we aim to find whether accretion of satellites is the main thick disc formation mechanism. To do so, we observed a sample of eight nearby edge-on galaxies with the MUSE integral field unit at the VLT. Six of the galaxies have a distinct thick disc. We derived thick disc velocities and velocity dispersions for the galaxies in our sample. We devise a formalism to estimate the fractions of retrograde material in the thick discs by using kinematical maps and thin/thick dis decompositions. None of the galaxies in our sample shows strong evidence for retrograde material at large distances from the centre. Including those found in the literature, there are seventeen thick discs with studied kinematics, with only one showing unambiguous signatures of retrograde material. Literature numerical studies of dynamical friction allow us to estimate that at the current cosmic time about one in six mergers for which the stars of the accreted galaxy ended in a thick disc were retrograde. This is in tension with the observed fraction of 1/17 of galaxies with a partly retrograde thick disc. We conclude that satellite accretion is not favoured by observations to be the main thick disk formation mechanism.

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arXiv2018

Hydrodynamic simulations of moonlet induced propellers in Saturn's rings: Application to Bleriot

One of the biggest successes of the Cassini mission is the detection of small moons (moonlets) embedded in Saturn's rings which cause S-shaped density structures in their close vicinity, called propellers (Spahn and Sremcevic 2000; Tiscareno et al. 2006; Sremcevic et al. 2007). Here, we present isothermal hydrodynamic simulations of moonlet-induced propellers in Saturn's A ring which denote a further development of the original model (Spahn and Sremcevic 2000). We find excellent agreement between these new hydrodynamic and corresponding N-body simulations. Furthermore, the hydrodynamic simulations confirm the predicted scaling laws (Spahn and Sremcevic 2000) and the analytical solution for the density in the propeller gaps (Sremcevic et al. 2002). Finally, this mean field approach allows us to simulate the pattern of the giant propeller Bleriot, which is too large to be modeled by direct N-body simulations. Our results are compared to two stellar occultation observations by the Cassini Ultraviolet Imaging Spectrometer (UVIS), that intersect the propeller Bleriot. Best fits to the UVIS optical depth profiles are achieved for a Hill radius of 590 m, which implies a moonlet diameter of about 860 m. Furthermore, the model favours a kinematic shear viscosity of the surrounding ring material of $ν_0 = 340$ cm^2/s, a dispersion velocity in the range of 0.3 cm/s $< c_0 <$ 1.5 cm/s, and a fairly high bulk viscosity $7 < ξ_0/ν_0 < 17$. These large transport values might be overestimated by our isothermal ring model and should be reviewed by an extended model including thermal fluctuations.

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