ALBERT

Description: We study the late-time evolution of the central regions of two Milky Way (MW)-like simulations of galaxies formed in a cosmological context, one hosting a fast bar and the other a slow one. We find that bar length, Rb, measurements fluctuate on a dynamical time-scale by up to 100 per cent, depending on the spiral structure strength and measurement threshold. The bar amplitude oscillates by about 15 per cent, correlating with Rb. The Tremaine–Weinberg method estimates of the bars’ instantaneous pattern speeds show variations around the mean of up to $sim !20{{ m per cent}}$, typically anticorrelating with the bar length and strength. Through power spectrum analyses, we establish that these bar pulsations, with a period in the range ∼60–200 Myr, result from its interaction with multiple spiral modes, which are coupled with the bar. Because of the presence of odd spiral modes, the two bar halves typically do not connect at exactly the same time to a spiral arm, and their individual lengths can be significantly offset. We estimated that in about 50 per cent of bar measurements in MW-mass external galaxies, the bar lengths of SBab-type galaxies are overestimated by $sim !15{{ m per cent}}$ and those of SBbc types by $sim !55{{ m per cent}}$. Consequently, bars longer than their corotation radius reported in the literature, dubbed ‘ultrafast bars’, may simply correspond to the largest biases. Given that the Scutum–Centaurus arm is likely connected to the near half of the MW bar, recent direct measurements may be overestimating its length by 1–1.5 kpc, while its present pattern speed may be 5–10 $ m km s^{-1} kpc^{-1}$ smaller than its time-averaged value.

Description: We use a suite of cooling halo simulations to study a new mechanism for rapid accretion of hot halo gas on to star-forming galaxies. Correlated supernova (SN) events create converging ‘superbubbles’ in the halo gas. Where these collide, the density increases, driving cooling filaments of low-metallicity gas that feed the disc. At our current numerical resolution (~20 pc; m gas = 4 x 10 4 M ) we are only able to resolve the most dramatic events; however, as we increase the numerical resolution, we find that the filaments persist for longer, driving continued late-time star formation. This suggests that SN-driven accretion could act as an efficient mechanism for extracting cold gas from the hot halo, driving late-time star formation in disc galaxies. We show that such filament feeding leads to a peak star formation rate of ~3 M yr –1 , consistent with estimates for the Milky Way (MW). The filaments we resolve extend to ~50 kpc, reaching column densities of N ~ 10 18  cm –2 . We show that such structures can plausibly explain the broad dispersion in Mg  ii absorption seen along sightlines to quasars. Our results suggest a dual role for stellar feedback in galaxy formation, suppressing hot-mode accretion while promoting cold-mode accretion along filaments. Finally, since the filamentary gas has higher angular momentum than that coming from hot-mode accretion, we show that this leads to the formation of substantially larger gas discs.

Description: Dwarf spheroidal (dSph) galaxies are prime targets for present and future -ray telescopes hunting for indirect signals of particle dark matter. The interpretation of the data requires careful assessment of their dark matter content in order to derive robust constraints on candidate relic particles. Here, we use an optimized spherical Jeans analysis to reconstruct the ‘astrophysical factor’ for both annihilating and decaying dark matter in 21 known dSphs. Improvements with respect to previous works are: (i) the use of more flexible luminosity and anisotropy profiles to minimize biases, (ii) the use of weak priors tailored on extensive sets of contamination-free mock data to improve the confidence intervals, (iii) systematic cross-checks of binned and unbinned analyses on mock and real data, and (iv) the use of mock data including stellar contamination to test the impact on reconstructed signals. Our analysis provides updated values for the dark matter content of 8 ‘classical’ and 13 ‘ultrafaint’ dSphs, with the quoted uncertainties directly linked to the sample size; the more flexible parametrization we use results in changes compared to previous calculations. This translates into our ranking of potentially-brightest and most robust targets – namely Ursa Minor, Draco, Sculptor – and of the more promising, but uncertain targets – namely Ursa Major 2, Coma – for annihilating dark matter. Our analysis of Segue 1 is extremely sensitive to whether we include or exclude a few marginal member stars, making this target one of the most uncertain. Our analysis illustrates challenges that will need to be addressed when inferring the dark matter content of new ‘ultrafaint’ satellites that are beginning to be discovered in southern sky surveys.

Description: The origin of the gas in between the Magellanic Clouds (MCs), known as the Magellanic Bridge, has always been the subject of controversy. To shed light into this, we present the results from the MAGellanic Inter-Cloud II (MAGIC II) project aimed at probing the stellar populations in 10 large fields located perpendicular to the main ridge-line of H  i in the Inter-Cloud region. We secured these observations of the stellar populations in between the MCs using the WFI (Wide Field Imager) camera on the 2.2 m telescope in La Silla. Using colour–magnitude diagrams, we trace stellar populations across the Inter-Cloud region. In good agreement with MAGIC I , we find significant intermediate-age stars in the Inter-Cloud region as well as young stars of a similar age to the last pericentre passage in between the MCs (~200 Myr ago). We show here that the young, intermediate-age and old stars have distinct spatial distributions. The young stars correlate well with the H  i gas suggesting that they were either recently stripped from the Small Magellanic Cloud (SMC) or formed in situ . The bulk of intermediate-age stars are located mainly in the Bridge region where the H  i column density is higher, but they are more spread out than the young stars. They have very similar properties to stars located ~2 kpc from the SMC centre, suggesting that they were tidally stripped from this region. Finally, the old stars extend to some 8 kpc from the SMC supporting the idea that all galaxies have a large extended metal-poor stellar halo.

Description: We present a novel positive potential-density pair expansion for modelling galaxies, based on the Miyamoto–Nagai disc. By using three sets of such discs, each one of them aligned along each symmetry axis, we are able to reconstruct a broad range of potentials that correspond to density profiles from exponential discs to 3D power-law models with varying triaxiality (henceforth simply ‘twisted’ models). We increase the efficiency of our expansion by allowing the scalelength parameter of each disc to be negative. We show that, for suitable priors on the scalelength and scaleheight parameters, these ‘MNn discs’ (Miyamoto–Nagai negative) have just one negative density minimum. This allows us to ensure global positivity by demanding that the total density at the global minimum is positive. We find that at better than 10 per cent accuracy in our density reconstruction, we can represent a radial and vertical exponential disc over 0.1–10 scalelengths/scaleheights with four MNn discs; a Navarro, Frenk and White (NFW) profile over 0.1–10 scalelengths with four MNn discs; and a twisted triaxial NFW profile with three MNn discs per symmetry axis. Our expansion is efficient, fully analytic, and well suited to reproducing the density distribution and gravitational potential of galaxies from discs to ellipsoids.

Description: According to our current cosmological model, galaxies like the Milky Way are expected to experience many mergers over their lifetimes. The most massive of the merging galaxies will be dragged towards the disc plane, depositing stars and dark matter into an accreted disc structure. In this work, we utilize the chemodynamical template developed in Ruchti et al. to hunt for accreted stars. We apply the template to a sample of 4675 stars in the third internal data release from the Gaia -ESO Spectroscopic Survey. We find a significant component of accreted halo stars, but find no evidence of an accreted disc component. This suggests that the Milky Way has had a rather quiescent merger history since its disc formed some 8–10 billion years ago and therefore possesses no significant dark matter disc.

Description: We describe a N ov el form of A daptive softening ( nova ) for collisionless N -body simulations, implemented in the ramses adaptive mesh refinement code. In ramses – that we refer to as a ‘standard N -body method’ – cells are only split if they contain more than eight particles (a mass refinement criterion). Here, we introduce an additional criterion that the particle distribution within each cell be sufficiently isotropic, as measured by the ratio of the maximum to minimum eigenvalues of its moment of inertia tensor: = max / min . In this way, collapse is only refined if it occurs along all three axes, ensuring that the softening is always of order twice the largest interparticle spacing in a cell. This more conservative force softening criterion is designed to minimize spurious two-body effects, while maintaining high force resolution in collapsed regions of the flow. We test nova using an antisymmetric perturbed plane wave collapse (‘Valinia’ test) before applying it to warm dark matter (WDM) simulations. For the Valinia test, we show that – unlike the standard N -body method – nova produces no numerical fragmentation while still being able to correctly capture fine caustics and shells around the collapsing regions. For the WDM simulations, we find that nova converges significantly more rapidly than standard N -body, producing little or no spurious haloes on small scales. We will use nova in forthcoming papers to study the issue of halo formation below the free-streaming mass M fs ; filament stability; and to obtain new constraints on the temperature of dark matter.

Description: We describe a N ov el form of A daptive softening ( nova ) for collisionless N -body simulations, implemented in the ramses adaptive mesh refinement code. In ramses – that we refer to as a ‘standard N -body method’ – cells are only split if they contain more than eight particles (a mass refinement criterion). Here, we introduce an additional criterion that the particle distribution within each cell be sufficiently isotropic, as measured by the ratio of the maximum to minimum eigenvalues of its moment of inertia tensor: = max / min . In this way, collapse is only refined if it occurs along all three axes, ensuring that the softening is always of order twice the largest interparticle spacing in a cell. This more conservative force softening criterion is designed to minimize spurious two-body effects, while maintaining high force resolution in collapsed regions of the flow. We test nova using an antisymmetric perturbed plane wave collapse (‘Valinia’ test) before applying it to warm dark matter (WDM) simulations. For the Valinia test, we show that – unlike the standard N -body method – nova produces no numerical fragmentation while still being able to correctly capture fine caustics and shells around the collapsing regions. For the WDM simulations, we find that nova converges significantly more rapidly than standard N -body, producing little or no spurious haloes on small scales. We will use nova in forthcoming papers to study the issue of halo formation below the free-streaming mass M fs ; filament stability; and to obtain new constraints on the temperature of dark matter.

Description: We present unpublished data from a tidal disruption candidate in NGC 3599 which show that the galaxy was already X-ray bright 18 months before the measurement which led to its classification. This removes the possibility that the flare was caused by a classical, fast-rising, short-peaked, tidal disruption event. Recent relativistic simulations indicate that the majority of disruptions will actually take months or years to rise to a peak, which will then be maintained for longer than previously thought. NGC 3599 could be one of the first identified examples of such an event. The optical spectra of NGC 3599 indicate that it is a low-luminosity Seyfert/low-ionization nuclear emission-line region (LINER) with L bol  ~ 10 40 erg s –1 . The flare may alternatively be explained by a thermal instability in the accretion disc, which propagates through the inner region at the sound speed, causing an increase of the disc scaleheight and local accretion rate. This can explain the ≤9 yr rise time of the flare. If this mechanism is correct then the flare may repeat on a time-scale of several decades as the inner disc is emptied and refilled.