We use Legacy Survey photometric data to probe the stellar halo in multiple directions of the sky using a probabilistic methodology to identify Blue Horizontal Branch (BHB) stars. The measured ...average radial density profile follows a double power law in the range \( 5 < r_{gc}/{\rm kpc} < 120\), with a density break at \(r_{gc}\approx20\) kpc. This description, however, falls short, depending on the chosen line-of-sight, with some regions showing no signature of a break in the profile and a wide range of density slopes, e.g. outer slope \(-5.5 \lesssim \alpha_{out} \lesssim -4\), pointing towards a highly anisotropic stellar halo. This explains in part the wide range of density profiles reported in the literature owing to different tracers and sky coverage. Using our detailed 3-D stellar halo density map, we quantify the shape of the Pisces overdensity associated with the transient wake response of the Galaxy's (dark) halo to the Large Magellanic Cloud (LMC). Measured in the LMC's coordinate system, Pisces stands above the background, is 60 degrees long and 25 degrees wide and aligned with the LMC's orbit. This would correspond to a wake width of \(\sim 32\) kpc at \(\sim 70\) kpc. We do not find a statistically significant signature of the collective response in density as previously reported in the literature measured with K giant stars, despite our larger numbers. We release the catalogue constructed in this study with 95,446 possible BHB stars and their BHB probability.
We present preliminary results from a study exploring the origin of Milky Way substructures, and show initial evidence of a common “kicked-out” formation mechanism for two low-latitude substructures. ...In this scenario, stars in these substructures formed in the disk and were subsequently “kicked-out” by an external perturbation, such as the merger of an accreted satellite, which created an oscillation in the Galactic disk. To test this origin scenario, we found the fraction of different stellar populations – M giants and RR Lyrae stars – in the Monoceros Ring (also known as GASS) and A13, supplementing a study of stellar populations in the Triangulum-Andromeda cloud. This work provides: (1) the first analysis of the GASS and A13 features based upon their stellar populations; and (2) preliminary evidence of disk stars in the Milky Way that have been relocated to the disk-halo interface due to vertical oscillations of the Milky Way’s disk.
Shallow dark matter cusps in galaxy clusters Laporte, Chervin F. P.; White, Simon D. M.; Naab, Thorsten ...
Monthly notices of the Royal Astronomical Society,
07/2012, Letnik:
424, Številka:
1
Journal Article
We have applied our method for weighing the Galactic disk using phase-space spirals to the Gaia EDR3 proper motion sample. For stars in distant regions of the Galactic disk, the latitudinal proper ...motion has a close projection with vertical velocity, such that the phase-space spiral in the plane of vertical position and vertical velocity can be observed without requiring that all stars have available radial velocity information. We divided the Galactic plane into 360 separate data samples, each corresponding to an area cell in the Galactic plane in the distance range of 1.4-3.4 kpc, with an approximate cell length of 200-400 pc. Roughly half of our data samples were disqualified altogether due to severe selection effects, especially in the direction of the Galactic centre. In the remainder, we were able to infer the vertical gravitational potential by fitting an analytic model of the phase-space spiral to the data. This work is the first of its kind, in the sense that we are weighing distant regions of the Galactic disk with a high spatial resolution, without relying on the strong assumptions of axisymmetry. Post-inference, we fit a thin disk scale length of \(2.2\pm 0.1\) kpc, although this value is sensitive to the considered spatial region. We see surface density variations as a function of azimuth on the order of 10-20 %, which is roughly the size of our estimated sum of potential systematic biases. With this work, we have demonstrated that our method can be used to weigh distant regions of the Galactic disk despite strong selection effects. We expect to reach even greater distances and improve our accuracy with future Gaia data releases and further improvements to our method.
Tidal streams of disrupted clusters are routinely detected in the halo of the Milky Way. It was recently shown that tidal streams of open clusters can now also be detected within the disc. In this ...work, we highlight the fact that tidal streams provide a powerful new diagnostic of the non-axisymmetric disc potential and may provide a new constraint on the pattern speed of the Galactic bar. In particular, we show how the stream-orbit misalignment for an open cluster on a quasi-circular orbit in the solar vicinity varies as a function of the position w.r.t the bar resonances. The angular shift rises beyond corotation, reaching values as high as \(30^\circ\) close to the outer Lindblad resonance (OLR), then dropping again and reversing its sign beyond the OLR. We applied this mechanism to the recently detected the Hyades stream. We note that the stream would be very similar when taking a potential with no bar or with a fast pattern speed of 55 km.s\(^{-1}\) kpc\(^{-1}\). However, we find that the stream is different than previously detected when adopting a potential with a bar pattern speed of \(39\) km.s\(^{-1}\) kpc\(^{-1}\). Previously detected Hyades candidate members would, on the other hand, favour a barless or a fast bar galaxy. Interestingly, the previously reported asymmetry in star counts within the leading and trailing tails of the Hyades tidal stream persists in all cases. Our study conclusively demonstrates that the effect of disc non-axisymmetries cannot be neglected when searching for tidal streams of open clusters and that current candidate members of the Hyades stream should not be trusted beyond a distance of 200 pc from the cluster. Moreover, our study allows for ideal targets to be provided for high-resolution spectroscopy follow-ups, which will enable conclusive identifications of the Hyades stream track and provide novel independent constraints on the bar pattern speed in the MW.
Using the method that was developed in the first paper of this series, we measure the vertical gravitational potential of the Galactic disk from the time-varying structure of the phase-space spiral, ...using data from Gaia as well as supplementary radial velocity information from legacy spectroscopic surveys. For eleven independent data samples, we inferred gravitational potentials that were in good agreement, despite the data samples' varied and substantial selection effects. Using a model for the baryonic matter densities, we inferred a local halo dark matter density of \(0.0085 \pm 0.0039\) M\(_\odot\)/pc\(^3 = 0.32 \pm 0.15\) GeV/cm\(^3\). We were also able to place the most stringent constraint to the surface density of a thin dark disk with a scale height \(\leq 50\) pc: an upper 95 % confidence limit of roughly 5 M\(_\odot\)/pc\(^2\) (compared to previous limit of roughly 10 M\(_\odot\)/pc\(^2\), given the same scale height). For the inferred halo dark matter density and thin dark disk surface density, the uncertainties are dominated by the baryonic model. With this level of precision, our method is highly competitive with traditional methods that rely on the assumption of a steady state. In a general sense, this illustrates that time-varying dynamical structures are not solely obstacles to dynamical mass measurements, but can also be regarded as assets containing useful information.
We present a new method for inferring the gravitational potential of the Galactic disk, using the time-varying structure of a phase-space spiral in the \((z,w)\)-plane (where \(z\) and \(w\) ...represent vertical position and vertical velocity). Our method of inference extracts information from the shape of the spiral and disregards the bulk density distribution that is usually used to perform dynamical mass measurements. In this manner, it is complementary to traditional methods that are based on the assumption of a steady state. Our method consists of fitting an analytical model for the phase-space spiral to data, where the spiral is seen as a perturbation of the stellar number density in the \((z,w)\)-plane. We tested our method on one-dimensional simulations, which were initiated in a steady state and then perturbed by an external force similar to that of a passing satellite. We were able to retrieve the true gravitational potentials of the simulations with high accuracy. The gravitational potential at 400-500 parsec distances from the disk mid-plane was inferred with an error of only a few percent. This is the first paper of a series in which we plan to test and refine our method on more complex simulations, as well as apply our method to Gaia data.
Stars born on near-circular orbits in spiral galaxies can subsequently migrate to different orbits due to interactions with non-axisymmetric disturbances within the disc such as bars or spiral arms. ...This paper extends the study of migration to examine the role of external influences using the example of the interaction of the Sagittarius dwarf galaxy (Sgr) with the Milky Way (MW). We first make impulse approximation estimates to characterize the influence of Sgr disc passages. The tidal forcing from Sgr can produce changes in both guiding radius (\(\Delta R_g\)) and orbital eccentricity, as quantified by the maximum radial excursion, \(\Delta R_ {\rm max} \). These changes follow a quadrupole-like pattern across the face of the disc, with amplitude increasing with Galactocentric radius. We next examine a collisionless N-body simulation of a Sgr-like satellite interacting with a MW-like galaxy and find that Sgr's influence in the outer disc dominates over the secular evolution of orbits between disc passages. Finally, we use the same simulation to explore possible observable signatures of Sgr-induced migration by painting the simulation with different age stellar populations. We find that following Sgr disc passages, the migration it induces manifests within an annulus as an approximate quadrupole in azimuthal metallicity variations (\(\delta_ {\rm Fe/H} \)), along with systematic variations in orbital eccentricity, \(\Delta R_ {\rm max} \). These systematic variations can persist for several rotational periods. We conclude that this combination of signatures may be used to distinguish between the different migration mechanisms shaping the chemical abundance patterns of the Milky Way's thin disc.
Disc galaxies commonly show asymmetric features in their morphology, such as warps and lopsidedness. These features can provide key information regarding the recent evolution of a given disc galaxy. ...In the nearby Universe, up to \(\sim30\) percent of late-type galaxies display a global non-axisymmetric lopsided mass distribution. However, the origin of this perturbation is not well understood. In this work, we study the origin of lopsided perturbations in simulated disc galaxies extracted from the TNG50 simulation of the IllustrisTNG project. We statistically explore different excitation mechanisms for this perturbation, such as direct satellite tidal interactions and distortions of the underlying dark matter distributions. We also characterize the main physical conditions that lead to lopsided perturbations. 50 percent of our sample galaxy have lopsided modes \(m=1\) greater than \(\sim 0.12\). We find a strong correlation between internal galaxy properties, such as central stellar surface density and disc radial extension with the strength of lopsided modes. The majority of lopsided galaxies have lower central surface densities and more extended discs than symmetric galaxies. As a result, such lopsided galaxies are less self-gravitationally cohesive, and their outer disc region is more susceptible to different types of external perturbations. However, we do not find strong evidence that tidal interactions with satellite galaxies are the main driving agent of lopsided modes. Lopsided galaxies tend to live in asymmetric dark matter halos with high spin, indicating strong galaxy-halo connections in late-type lopsided galaxies.
We study azimuthal variations in the mean stellar metallicity, <Fe/H>, in a self-consistent, isolated simulation in which all stars form out of gas. We find <Fe/H> variations comparable to those ...observed in the Milky Way and which are coincident with the spiral density waves. The azimuthal variations are present in young and old stars and therefore are not a result of recently formed stars. Similar variations are present in the mean age and alpha-abundance. We measure the pattern speeds of the <Fe/H>-variations and find that they match those of the spirals, indicating that they are at the origin of the metallicity patterns. Because younger stellar populations are not just more Fe/H-rich and alpha-poor but also dynamically cooler, we expect them to more strongly support spirals, which is indeed the case in the simulation. However, if we measure the radial action, J_R, using the Stackel axisymmetric approximation, we find that the spiral ridges are traced by regions of high J_R, contrary to expectations. Assuming that the passage of stars through the spirals leads to unphysical variations in the measured J_R, we obtain an improved estimate of J_R by averaging over a 1 Gyr time interval. This time-averaged J_R is a much better tracer of the spiral structure, with minima at the spiral ridges. We conclude that the errors incurred by the axisymmetric approximation introduce correlated deviations large enough to render the instantaneous radial action inadequate for tracing spirals.
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