Kinematic Survey Research Articles

Foreground effect on the J-factor estimation of ultrafaint dwarf spheroidal galaxies

Dwarf spheroidal galaxies (dSphs) are promising targets for the gamma-ray dark matter (DM) search. In particular, DM annihilation signal is expected to be strong in some of the recently discovered nearby ultra-faint dSphs, which potentially give stringent constraints on the $\mathcal{O}(1)$ TeV WIMP DM. However, various non-negligible systematic uncertainties complicate the estimation of the astrophysical factors relevant for the DM search in these objects. Among them, the effects of foreground stars particularly attract attention because the contamination is unavoidable even for the future kinematical survey. In this article, we assess the effects of the foreground contamination on the astrophysical $J$-factor estimation by generating mock samples of stars in the four ultra-faint dSphs and using a model of future spectrographs. We investigate various data cuts to optimize the quality of the data and apply a likelihood analysis which takes member and foreground stellar distributions into account. We show that the foreground star contaminations in the signal region (the region of interest) and their statistical uncertainty can be estimated by interpolating the foreground star distribution in the control region where the foreground stars dominate the member stars. Such regions can be secured at future spectroscopic observations utilizing a multiple object spectrograph with a large field of view; e.g. the Prime Focus Spectrograph mounted on Subaru Telescope. The above estimation has several advantages: The data-driven estimation of the contamination makes the analysis of the astrophysical factor stable against the complicated foreground distribution. Besides, foreground contamination effect is considered in the likelihood analysis.

A Spectroscopic Survey of Field Red Horizontal-branch Stars

Abstract A metallicity, chemical composition, and kinematic survey has been conducted for a sample of 340 candidate field red horizontal-branch (RHB) stars. Spectra with high resolution and high signal-to-noise ratio were gathered with the McDonald Observatory 2.7 m Tull and the Hobby–Eberly Telescope echelle spectrographs, and were used to determine effective temperatures, surface gravities, microturbulent velocities, [Fe/H] metallicities, and abundance ratios [X/Fe] for seven α and Fe-group species. The derived temperatures and gravities confirm that at least half of the candidates are true RHB stars, with (average) parameters T eff ∼ 5000 K and log g ∼ 2.5. From the α abundances alone, the thin and thick Galactic populations are apparent in our sample. Space motions for 90% of the program stars were computed from Hipparcos and Gaia parallaxes and proper motions. Correlations between chemical compositions and Galactic kinematics clearly indicate the existence of both thin-disk and thick-disk RHB stars.

Elliptical galaxies spinning on end

Galaxy Structure Most elliptical galaxies have a flattened shape and rotate around their shortest axis, like a wheel. However, some galaxies, known as prolate rotators, are elongated and spin around their longest axis, like a spindle. Tsatsi et al. searched a large survey of galaxy kinematics to discover eight previously unknown examples. They estimate that around 10% of elliptical galaxies are prolate rotators, but the fraction is higher at higher masses. They use simulations to show that prolate rotators can form if two massive spiral galaxies merge in a polar orientation, potentially explaining the evolutionary history of these systems. Astron. Astrophys. 606 , A62 (2017).

Tectonic forcing of evolution and Holocene erosion rate of ravines in the Marginal Ganga Plain, India

Understanding the processes of ravine formation and estimates of Specific Sediment Yields (SSY) of these regions help in formulating mitigation policies significantly. Upto now, in the Ganga Plain, the work done on ravine formation, erosion and SSY estimates has been speculative. Here, utilizing (i) high resolution Digital Elevation Models (DEMs) prepared from Real Time Kinematic (RTK) survey and CARTOSAT images of ravined zones and (ii) luminescence chronology, we provide the first estimate of SSY from the ravines. The results indicate that SSY in the Marginal Ganga Plain (MGP) ravines may range between 600 ± 100t/km2/yr and 1600 ± 200t/km2/yr.The correspondence between the orientations of fractures and that of first order ravine suggests that the ravine growth is genetically related to extensional stress regime of the peripheral forebulge. Further, the clay sedimentology indicates that besides regional tectonics, action of swelling clays may have accelerated development of the badlands. The geomorphic relationship between the length of ravined catchments and incision in the trunk channels indicates that the river-incision associated base level change is not playing an overarching role in the ravine formation in the MGP.

Comparison of Short-Term Changes Caused by Storms along Natural and Protected Sections of the Dziwnow Spit, Southern Baltic Coast

ABSTRACT Bugajny, N. and Furmanczyk, K., 2017. Comparison of short-term changes caused by storms along natural and protected sections of the Dziwnow Spit, southern Baltic coast. This paper focuses on a comparison of changes caused by weak storms (significant wave height [Hs] <2.5 m) along a dune coast on a nontidal sea. Repeated observations were made along two 2 km long sections of the southern Baltic coast; one section is a natural coast, and the other features protective structures. Several real-time kinematic GPS surveys were carried out in 2012. The surveys included cross-shore profiles measured from the dune foot to a water depth of 1 m. The profiles were spaced at constant 100 m intervals. In addition, offshore wave data and water level data were collected. Using statistical methods, three types of hydrodynamic conditions were identified and the data grouped accordingly; these groups had different effects on the coast. The coastal changes that occurred along the natural and protected sections of th...

Analysis and Comparison of Lines Obtained from GNSS and UAV for Large-Scale Maps

AbstractNowadays, obtaining large-scale topographic maps from unmanned aerial vehicle (UAV) photogrammetric projects has become an alternative to traditional surveys based on global positioning systems (GPSs) or total stations. This assumption is based on the reduction of costs, the application to reduced zones, the efficiency of the procedures, and the positional accuracy achieved, etc. In this context, this study analyzes and compares the lines obtained from two sources: a global navigation satellite system–real-time kinematic (GNSS-RTK) survey and an UAV photogrammetric project. Usually, the GNSS data set of lines has a higher positional accuracy, whereas the UAV data set has a richer geometry. The proposed methodology realizes several positional controls (based on lines) to analyze the positional accuracy of the UAV data set of lines and the geometrical representability of the GNSS data set of lines. In addition, an improvement in the representation of lines with poor geometry is suggested by using a ...

Initialization and Occupation Time for Kinematic Surveys using Online Precise Point Positioning

Initialization and Occupation Time for Kinematic Surveys using Online Precise Point Positioning

Coastal wetland response to sea‐level rise in a fluvial estuarine system

AbstractCoastal wetlands are likely to lose productivity under increasing rates of sea‐level rise (SLR). This study assessed a fluvial estuarine salt marsh system using the Hydro‐MEM model under four SLR scenarios. The Hydro‐MEM model was developed to apply the dynamics of SLR as well as capture the effects associated with the rate of SLR in the simulation. Additionally, the model uses constants derived from a 2‐year bioassay in the Apalachicola marsh system. In order to increase accuracy, the lidar‐based marsh platform topography was adjusted using Real Time Kinematic survey data. A river inflow boundary condition was also imposed to simulate freshwater flows from the watershed. The biomass density results produced by the Hydro‐MEM model were validated with satellite imagery. The results of the Hydro‐MEM simulations showed greater variation of water levels in the low (20 cm) and intermediate‐low (50 cm) SLR scenarios and lower variation with an extended bay under higher SLR scenarios. The low SLR scenario increased biomass density in some regions and created a more uniform marsh platform in others. Under intermediate‐low SLR scenario, more flooded area and lower marsh productivity were projected. Higher SLR scenarios resulted in complete inundation of marsh areas with fringe migration of wetlands to higher land. This study demonstrated the capability of Hydro‐MEM model to simulate coupled physical/biological processes across a large estuarine system with the ability to project marsh migration regions and produce results that can aid in coastal resource management, monitoring, and restoration efforts.

Morpho-kinematics ofz∼ 1 galaxies probe the hierarchical scenario

We have studied a representative sample of intermediate-mass galaxies at z~1, observed by the kinematic survey KMOS3D. We have re-estimated the kinematical parameters from the published kinematic maps and analysed photometric data from HST to measure optical disk inclinations and PAs. We find that only half of the z~1 galaxies show kinematic properties consistent with rotating disks, using the same classification scheme than that adopted by the KMOS3D team. Because merger orbital motions can also brought rotation, we have also analysed galaxy morphologies from the available HST imagery. Combining these results to those from kinematics, it leads to a full morpho-kinematic classification. To test the robustness of the latter for disentangling isolated disks from mergers, we confronted the results with an analysis of pairs from the open-grism redshift survey 3D-HST. All galaxies found in pairs are affected by either kinematic and/or morphological perturbations. Conversely, all galaxies classified as virialized spirals are found to be isolated. A significant fraction (one fourth) of rotating disks classified from kinematics by the KMOS3D team are found in pairs, which further supports the need for a morpho-kinematic classification. It results that only one third of z~1 galaxies are isolated and virialized spirals, while 58% of them are likely involved in a merger sequence, from first approach to disk rebuilding. The later fraction is in good agreement with the results of semi-empirical {\Lambda}CDM models, supporting a merger-dominated hierarchical scenario as being the main driver of galaxy formation at least during the last 8 billion years.

KINEMATIC DOWNSIZING AT z ∼ 2

ABSTRACT We present results from a survey of the internal kinematics of 49 star-forming galaxies at in the CANDELS fields with the Keck/MOSFIRE spectrograph, Survey in the near-Infrared of Galaxies with Multiple position Angles (SIGMA). Kinematics (rotation velocity V rot and gas velocity dispersion ) are measured from nebular emission lines which trace the hot ionized gas surrounding star-forming regions. We find that by , massive star-forming galaxies ( ) have assembled primitive disks: their kinematics are dominated by rotation, they are consistent with a marginally stable disk model, and they form a Tully–Fisher relation. These massive galaxies have values of that are factors of 2–5 lower than local well-ordered galaxies at similar masses. Such results are consistent with findings by other studies. We find that low-mass galaxies ( ) at this epoch are still in the early stages of disk assembly: their kinematics are often dominated by gas velocity dispersion and they fall from the Tully–Fisher relation to significantly low values of V rot. This “kinematic downsizing” implies that the process(es) responsible for disrupting disks at have a stronger effect and/or are more active in low-mass systems. In conclusion, we find that the period of rapid stellar mass growth at is coincident with the nascent assembly of low-mass disks and the assembly and settling of high-mass disks.

Developing an algorithm for enhancement of a digital terrain model for a densely vegetated floodplain wetland

Airborne laser scanning survey data were conducted with a scanning density of 4 points/m2 to accurately map the surface of a unique central European complex of wetlands: the lower Biebrza River valley (Poland). A method to correct a degrading effect of vegetation (so-called “vegetation effect”) on digital terrain models (DTMs) was applied utilizing remotely sensed images, real-time kinematic global positioning system elevation measurements, topographical surveys, and vegetation height measurements. Geographic object-based image analysis (GEOBIA) was performed to map vegetation within the study area that was used as categories from which vegetation height information was derived for the DTM correction. The final DTM was compared with a model obtained, where additional correction of the “vegetation effect” was neglected. A comparison between corrected and uncorrected DTMs demonstrated the importance of accurate topography through a simple presentation of the discrepancies arising in features of the flood using various DTM products. An overall map classification accuracy of 80% was attained with the use of GEOBIA. Correction factors developed for various types of the vegetation reached values from 0.08 up to 0.92 m and were dependent on the vegetation type.

Dynamical ejections of massive stars from young star clusters under diverse initial conditions

We study the effects of initial conditions of star clusters and their massive star population on dynamical ejections of massive stars from star clusters up to an age of 3 Myr. We use a large set of direct N-body calculations for moderately massive star clusters (Mecl=$10^{3.5}$ Msun). We vary the initial conditions of the calculations such as the initial half-mass radius of the clusters, initial binary populations for massive stars and initial mass segregation. We find that the initial density is the most influential parameter for the ejection fraction of the massive systems. The clusters with an initial half-mass radius of 0.1 (0.3) pc can eject up to 50% (30%) of their O-star systems on average. Most of the models show that the average ejection fraction decreases with decreasing stellar mass. For clusters efficient at ejecting O stars, the mass function of the ejected stars is top-heavy compared to the given initial mass function (IMF), while the mass function of stars that remain in the cluster becomes slightly steeper (top-light) than the IMF. The top-light mass functions of stars in 3 Myr old clusters in our N-body models agree well with the mean mass function of young intermediate-mass clusters in M31, as reported previously. We show that the multiplicity fraction of the ejected massive stars can be as high as 60%, that massive high-order multiple systems can be dynamically ejected, and that high-order multiples become common especially in the cluster. We also discuss binary populations of the ejected massive systems. When a large kinematic survey of massive field stars becomes available, for instance through Gaia, our results may be used to constrain the birth configuration of massive stars in star clusters. (Abridged)

KROSS: mapping the Hα emission across the star formation sequence atz≈ 1

We present first results from the KMOS Redshift One Spectroscopic Survey (KROSS), an ongoing large kinematical survey of a thousand, z~1 star forming galaxies, with VLT KMOS. Out of the targeted galaxies (~500 so far), we detect and spatially resolve Ha emission in ~90% and 77% of the sample respectively. Based on the integrated Ha flux measurements and the spatially resolved maps we derive a median star formation rate (SFR) of ~7.0 Msun/yr and a median physical size of <r$_{\rm e}$> = 5.1kpc. We combine the inferred SFRs and effective radii measurements to derive the star formation surface densities ({\Sigma}SFR) and present a "resolved" version of the star formation main sequence (MS) that appears to hold at sub-galactic scales, with similar slope and scatter as the one inferred from galaxy integrated properties. Our data also yield a trend between {\Sigma}SFR and {\Delta}(sSFR) (distance from the MS) suggesting that galaxies with higher sSFR are characterised by denser star formation activity. Similarly, we find evidence for an anti-correlation between the gas phase metallicity (Z) and the {\Delta}(sSFR), suggesting a 0.2dex variation in the metal content of galaxies within the MS and significantly lower metallicities for galaxies above it. The origin of the observed trends between {\Sigma}SFR - {\Sigma}(sSFR) and Z - {\Delta}(sSFR) could be driven by an interplay between variations of the gas fraction or the star formation efficiency of the galaxies along and off the MS. To address this, follow-up observations of the our sample that will allow gas mass estimates are necessary.

Galaxy morphology and star formation in the Illustris Simulation atz = 0

We study how optical galaxy morphology depends on mass and star formation rate (SFR) in the Illustris Simulation. To do so, we measure automated galaxy structures in 10808 simulated galaxies at z=0 with stellar masses 10^9.7 < M_*/M_sun < 10^12.3. We add observational realism to idealized synthetic images and measure non-parametric statistics in rest-frame optical and near-IR images from four directions. We find that Illustris creates a morphologically diverse galaxy population, occupying the observed bulge strength locus and reproducing median morphology trends versus stellar mass, SFR, and compactness. Morphology correlates realistically with rotation, following classification schemes put forth by kinematic surveys. Type fractions as a function of environment agree roughly with data. These results imply that connections among mass, star formation, and galaxy structure arise naturally from models matching global star formation and halo occupation functions when simulated with accurate methods. This raises a question of how to construct experiments on galaxy surveys to better distinguish between models. We predict that at fixed halo mass near 10^12 M_sun, disc-dominated galaxies have higher stellar mass than bulge-dominated ones, a possible consequence of the Illustris feedback model. While Illustris galaxies at M_* ~ 10^11 M_sun have a reasonable size distribution, those at M_* ~ 10^10 M_sun have half-light radii larger than observed by a factor of two. Furthermore, at M_* ~ 10^10.5-10^11 M_sun, a relevant fraction of Illustris galaxies have distinct "ring-like" features, such that the bright pixels have an unusually wide spatial extent.

GHASP: an Hα kinematic survey of spiral galaxies – X. Surface photometry, decompositions and the Tully–Fisher relation in theRcband

We present Rc-band surface photometry for 170 of the 203 galaxies in GHASP, Gassendi H-Alpha survey of SPirals, a sample of late-type galaxies for which high-resolution Fabry-Perot H{\alpha} maps have previously been obtained. Our data set is constructed by new Rc-band observations taken at the Observatoire de Haute-Provence (OHP), supplemented with Sloan Digital Sky Survey (SDSS) archival data, obtained with the purpose of deriving homogeneous photometric profiles and parameters. Our results include Rc-band surface brightness profiles for 170 galaxies and $ugriz$ profiles for 108 of these objects. We catalogue several parameters of general interest for further reference, such as total magnitude, effective radius and isophotal parameters -- magnitude, position angle, ellipticity and inclination. We also perform a structural decomposition of the surface brightness profiles using a multi-component method in order to separate disks from bulges and bars, and to observe the main scaling relations involving luminosities, sizes and maximum velocities. We determine the Rc-band Tully Fisher relation using maximum velocities derived solely from H$\alpha$ rotation curves for a sample of 80 galaxies, resulting in a slope of $-8.1 \pm 0.5$, zero point of $-3.0 \pm 1.0$ and an estimated intrinsic scatter of $0.28 \pm 0.07$. We note that, different from the TF-relation in the near-infrared derived for the same sample, no change in the slope of the relation is seen at the low-mass end (for galaxies with $V_{max} < 125$ km/s). We suggest that this different behaviour of the Tully Fisher relation (with the optical relation being described by a single power-law while the near-infrared by two) may be caused by differences in the stellar mass to light ratio for galaxies with $V_{max} < 125$ km/s.

Geomatics activities for monitoring the large landslide of Maierato, Italy

The paper discusses the contribution given by geomatics to monitoring operations regarding a large landslide. The phenomenon affects an entire hill at the edge of the town of Maierato, southern Italy, which bisected the main access road to the town. In the first year after the main event, several surveying methods have been performed by different institutions, often not coordinated. At the end of 2010, an agreement was stated between the Civil Protection Department and the CAMILab of the University of Calabria, regarding the realization of geological, geotechnical, hydrological, and hydraulic studies and surveys in the Municipality of Maierato. The aim is to set up a model of the evolution of the phenomenon and to obtain the periodic assessment of the residual risk level. For understanding the event deeply, the integration of a number of methods of investigation, both geophysical and geotechnical, with geomatics techniques has been performed. The final goal is to set up an early warning system. The used geomatics techniques are the following: total station (operating continuously), Global Navigation Satellite System (GNSS) surveys, laser scanner, and digital photogrammetry. The total station controls and transmits the position of 20 points continuously. The design of the control network and the data acquired during the first 11 months of monitoring are discussed. GNSS receivers are used in static mode for the reference points. Kinematic and RTK surveys have been made to obtain the cross sections, useful to interpret the geoelectrical tomography correctly. Laser scanner has been used to obtain a 3-D model of the area and to evaluate the volume of the landslide. The first 18 months of continuous monitoring show, on the one hand, an excellent repeatability of the measures, and on the other, they confirm the results obtained by geotechnical investigations (inclinometers, etc.) and surface surveys.

Method to Assess Efficiency and Improve Design of Snow Fences

Blowing and drifting of snow is a major concern for transportation efficiency and road safety in regions where their development is common. One common way to mitigate snow drifts on roadways is to install plastic snow fences. Of critical importance for road safety is protection against snow drifting in regions with narrow rights of way, where standard fences cannot be deployed at the recommended distance from the road. The current study proposes a joint experimental and numerical approach to monitor snow deposits around snow fences, and to quantitatively estimate snow deposits in the field, assess efficiency, and improve the design of snow fences. Snow deposit profiles were mapped using global positioning system–based real-time kinematic surveys conducted at the monitored field site during and after snow storms. The monitored site allowed testing various snow fence designs under close to identical conditions. A main goal of the current study was to assess the performance of lightweight plastic snow fences with a lower porosity than the typical 50% porosity used in standard designs of such fences. The field data, collected during the first winter, were used to identify the best design for snow fences with a porosity of 50%. The flow fields obtained from numerical simulations showed that the fence design that worked the best during the first winter induced the formation of an elongated area of small velocity magnitude close to the ground. This information was used to identify other candidates for optimum design of fences with a lower porosity. Two of the designs with a fence porosity of 30% that were found to perform well based on the results of the numerical simulations were tested in the field during the second winter along with the best performing design for fences with a porosity of 50%. The field data showed that the length of the snow deposit away from the fence was reduced by approximately 30% for the two proposed lower-porosity (30%) fence designs compared with the best design identified for fences with a porosity of 50%. Moreover, one of the lower-porosity designs tested in the field showed no significant snow deposition within the bottom gap region beneath the fence. It is expected that this design will continue to work well for even more severe snow events or for successive snow events occurring during the same winter. The approach advocated in the current study allowed making recommendations for optimizing the design of lower-porosity plastic snow fences. This approach can be extended to improve the design of other types of snow fences.

CENTRAL ROTATIONS OF MILKY WAY GLOBULAR CLUSTERS

Most Milky Way globular clusters (GCs) exhibit measurable flattening, even if on a very low level. Both cluster rotation and tidal fields are thought to cause this flattening. Nevertheless, rotation has only been confirmed in a handful of GCs, based mostly on individual radial velocities at large radii. We are conducting a survey of the central kinematics of Galactic GCs using the new Integral Field Unit instrument VIRUS-W. We detect rotation in all 11 GCs that we have observed so far, rendering it likely that a large majority of the Milky Way GCs rotate. We use published catalogs of the ACS survey of GCs to derive central ellipticities and position angles. We show that in all cases where the central ellipticity permits an accurate measurement of the position angle, those angles are in excellent agreement with the kinematic position angles that we derive from the VIRUS-W velocity fields. We find an unexpected tight correlation between central rotation and outer ellipticity, indicating that rotation drives flattening for the objects in our sample. We also find a tight correlation between central rotation and published values for the central velocity dispersion, most likely due to rotation impacting the old dispersion measurements.

THE SINS/zC-SINF SURVEY OFz∼ 2 GALAXY KINEMATICS: EVIDENCE FOR POWERFUL ACTIVE GALACTIC NUCLEUS-DRIVEN NUCLEAR OUTFLOWS IN MASSIVE STAR-FORMING GALAXIES

We report the detection of ubiquitous powerful nuclear outflows in massive (> 10^11 Msun) z~2 star-forming galaxies (SFGs), which are plausibly driven by an Active Galactic Nucleus (AGN). The sample consists of the eight most massive SFGs from our SINS/zC-SINF survey of galaxy kinematics with the imaging spectrometer SINFONI, six of which have sensitive high-resolution adaptive optics (AO) assisted observations. All of the objects are disks hosting a significant stellar bulge. The spectra in their central regions exhibit a broad component in Halpha and forbidden [NII] and [SII] line emission, with typical velocity FWHM ~ 1500 km/s, [NII]/Halpha ratio ~ 0.6, and intrinsic extent of 2 - 3 kpc. These properties are consistent with warm ionized gas outflows associated with Type 2 AGN, the presence of which is confirmed via independent diagnostics in half the galaxies. The data imply a median ionized gas mass outflow rate of ~ 60 Msun/yr and mass loading of ~ 3. At larger radii, a weaker broad component is detected but with lower FWHM ~ 485 km/s and [NII]/Halpha ~ 0.35, characteristic for star formation-driven outflows as found in the lower-mass SINS/zC-SINF galaxies. The high inferred mass outflow rates and frequent occurrence suggest the nuclear outflows efficiently expel gas out of the centers of the galaxies with high duty cycles, and may thus contribute to the process of star formation quenching in massive galaxies. Larger samples at high masses will be crucial to confirm the importance and energetics of the nuclear outflow phenomenon, and its connection to AGN activity and bulge growth.

Conditions of consistency for multicomponent axisymmetric stellar systems

Solving the Boltzmann collisionless equation under the axisymmetric hypothesis introduces serious limitations on describing the kinematics of a single stellar system according to the local Galactic observables. Instead of relaxing the hypothesis of axisymmetry, one alternative is to assume a mixture model. For a finite mixture of ellipsoidal velocity distributions, the coexistence of several stellar populations sharing a common potential introduces a set of conditions of consistency that may also constrain the population kinematics. For only a few potentials, the populations may have independent mean velocities and unconstrained velocity ellipsoids. In this paper, we determine which axisymmetric potentials are connected with a more flexible superposition of the stellar populations. The conditions of consistency are checked against recent results derived from kinematic surveys of the solar neighbourhood that include RAdial Velocity Experiment (RAVE) data. Several key observables are used to determine whether the axisymmetric mixture model is able to account for the main features of the local velocity distribution, such as the vertex deviation associated with the second central moment μ ϖθ , the population radial mean velocities, the radial gradient of the moment μ ϖz , the tilt of the velocity ellipsoids, and the existence of stars with no net rotation. In addition, the mixture moments for an arbitrary number of populations are derived in terms of the one-to-one mean velocity differences in order to study whether a more populated mixture could add any new features to the velocity distribution that remain unnoticed in a two-component mixture. According to this analysis, the quasi-stationary potential is the only potential allowing arbitrary directions of the population mean velocities. Then, the apparent vertex deviation of the total velocity distribution is due to the difference of the mean velocities of the populations whose velocity ellipsoids have no vertex deviation. For a non-separable potential, the population velocity ellipsoids have the same orientation and point towards the Galactic centre. For a potential separable in addition in cylindrical coordinates, the population velocity ellipsoids may have arbitrary tilt.