Centroid Velocity Research Articles

Mapping of the Structure of the Galactic Magnetic Field with Velocity Gradients: Test Using Star Light Polarization

Abstract We apply Velocity Channel Gradients (VChGs) and Reduced Velocity Centroids Gradients (RVCGs) to H i data (the LAB survey), obtaining the plane-of-sky component (POS) of the magnetic field as a function of the relative velocity. Assuming a galactic rotation curve, we transformed the relative velocities to distances, constructing the first map of the POS magnetic field at every point of the celestial sphere as a function of the distance. To test the accuracy of our 3D distribution, we used a set of stars with known distances from the stellar polarization catalog. We compared the polarization directions that we obtain with the VChGs and RVCGs against the starlight polarization directions. We find a good correspondence between the derived magnetic field and measured polarization directions, which testifies to the accuracy of this new way of probing the 3D galactic POS magnetic field structure.

Magnetic Field Tomography in Two Clouds toward Ursa Major Using H i Fibers

Abstract The atomic interstellar medium (ISM) is observed to be full of linear structures that are referred to as “fibers.” Fibers exhibit similar properties to linear structures found in molecular clouds known as striations. Suggestive of a similar formation mechanism, both striations and fibers appear to be ordered, quasi-periodic, and well aligned with the magnetic field. The prevailing formation mechanism for striations involves the excitation of fast magnetosonic waves. Based on this theoretical model, and through a combination of velocity centroids and column density maps, Tritsis et al. developed a method for estimating the plane-of-sky (POS) magnetic field from molecular cloud striations. We apply this method in two H i clouds with fibers along the same line of sight (LOS) toward the ultra-high-energy cosmic-ray (UHECR) hotspot, at the boundaries of Ursa Major. For the cloud located closer to Earth, where Zeeman observations from the literature were also available, we find general agreement in the distributions of the LOS and POS components of the magnetic field. We find relatively large values for the total magnetic field (ranging from ∼10 to ∼20 μG) and an average projection angle with respect to the LOS of ∼50°. For the cloud located further away, we find a large value for the POS component of the magnetic field of . We discuss the potential of our new magnetic field tomography method for large-scale application. We consider the implications of our findings for the accuracy of current reconstructions of the Galactic magnetic field and the propagation of UHECR through the ISM.

‘The Brick’ is not abrick: a comprehensive study of the structure and dynamics of the central molecular zone cloud G0.253+0.016

In this paper we provide a comprehensive description of the internal dynamics of G0.253+0.016 (a.k.a. 'the Brick'); one of the most massive and dense molecular clouds in the Galaxy to lack signatures of widespread star formation. As a potential host to a future generation of high-mass stars, understanding largely quiescent molecular clouds like G0.253+0.016 is of critical importance. In this paper, we reanalyse Atacama Large Millimeter Array cycle 0 HNCO $J=4(0,4)-3(0,3)$ data at 3 mm, using two new pieces of software which we make available to the community. First, scousepy, a Python implementation of the spectral line fitting algorithm scouse. Secondly, acorns (Agglomerative Clustering for ORganising Nested Structures), a hierarchical n-dimensional clustering algorithm designed for use with discrete spectroscopic data. Together, these tools provide an unbiased measurement of the line of sight velocity dispersion in this cloud, $\sigma_{v_{los}, {\rm 1D}}=4.4\pm2.1$ kms$^{-1}$, which is somewhat larger than predicted by velocity dispersion-size relations for the Central Molecular Zone (CMZ). The dispersion of centroid velocities in the plane of the sky are comparable, yielding $\sigma_{v_{los}, {\rm 1D}}/\sigma_{v_{pos}, {\rm 1D}}\sim1.2\pm0.3$. This isotropy may indicate that the line-of-sight extent of the cloud is approximately equivalent to that in the plane of the sky. Combining our kinematic decomposition with radiative transfer modelling we conclude that G0.253+0.016 is not a single, coherent, and centrally-condensed molecular cloud; 'the Brick' is not a \emph{brick}. Instead, G0.253+0.016 is a dynamically complex and hierarchically-structured molecular cloud whose morphology is consistent with the influence of the orbital dynamics and shear in the CMZ.

Kinematic Links and the Coevolution of MHD Winds, Jets, and Inner Disks from a High-resolution Optical [ ] Survey

Abstract We present a survey of optical [O i] emission at 6300 Å toward 65 T Tauri stars at the spectral resolution of ∼7 km s−1. Past work identified a highly blueshifted velocity component (HVC) tracing microjets and a less blueshifted low-velocity component (LVC) attributed to winds. We focus here on the LVC kinematics to investigate links between winds, jets, accretion, and disk dispersal. We track the behavior of four types of LVC components: a broad and a narrow component (“BC” and “NC,” respectively) in LVCs that are decomposed into two Gaussians which typically have an HVC, and single-Gaussian LVC profiles separated into those that have an HVC (“SCJ”) and those that do not (“SC”). The LVC centroid velocities and line widths correlate with the HVC EW and accretion luminosity, suggesting that LVC/winds and HVC/jets are kinematically linked and connected to accretion. The deprojected HVC velocity correlates with accretion luminosity, showing that faster jets come with higher accretion. BC and NC kinematics correlate, and their blueshifts are maximum at ∼35°, suggesting a conical wind geometry with this semi-opening angle. Only SCs include n 13–31 up to ∼3, and their properties correlate with this infrared index, showing that [O i] emission recedes to larger radii as the inner dust is depleted, tracing less dense/hot gas and a decrease in wind velocity. Altogether, these findings support a scenario where optically thick, accreting inner disks launch radially extended MHD disk winds that feed jets, and where inner disk winds recede to larger radii and jets disappear in concert with dust depletion.

A study of multibody modeling and calibration method for FMVSS 208 rollover test

Finite element simulation technology has been widely used in vehicle safety research. However, compared with other vehicle collision accidents, finite element model (FEM) used for passive safety research of rollover cannot be accepted generally due to its relatively long computation time. Aimed at the issue, this article proposes a kind of multibody modeling method based on FEM and test results. First, taking an example of the minivan, vehicle body is divided into suspension system, deformable upper body, and rigid lower body, and each part is constructed using MADYMO (a multibody dynamic software). Detailed modeling steps of each part are provided. Second, the upper body of model is corrected by static pressure test on vehicle top conducted in accordance with FMVSS 216, and the correspondence of output response of acceleration of B-pillar and centroid angular velocity between simulation model and FMVSS 208 rollover test is evaluated by ISO/TS 18571. The quantitative correspondence score is taken as optimization objective to calibrate uncertain parameter of MADYMO model. Finally, the correctness of MADYMO model and effectiveness of model calibration method are verified after comparison with test results of FMVSS 208. The result verifies that multibody vehicle rollover model created with this method can provide simulation with relatively high accuracy and present key deformation of vehicle body, meanwhile saving massive calculation time, which provides reliable simulation model for passenger damage research and vehicle safety performance optimization in subsequent process. The proposed method is general and universal modeling and calibration method, suitable for other vehicle.

Rebounding motion of a highly elastic ball between two parallel matte plates

If a highly elastic ball is thrown into the space between two parallel matte plates, under some circumstances, the ball might be projected back in the opposite direction after bounding a few times. In this paper, a theoretical model, which describes the transformation of the ball's centroid velocity and angular velocity before and after a collision, as well as its motion in the air between two collisions, is established and then verified by experiments. In addition, the coefficient of restitution between the ball and the plates are fitted. Numerical simulations were performed to discuss the conditions under which the ball would rebound back and its various rebounding modes that might exist.

Centroid tracking and velocity measurement of white blood cell in video.

Automated blood cells tracking system has a vital role as the tracking process reflects the blood cell characteristics and indicates several diseases. Blood cells tracking is challenging due to the non-rigid shapes of the blood cells, and the variability in their videos along with the existence of different moving objects in the blood. To tackle such challenges, we proposed a green star based centroid (GSBC) moving white blood cell (WBC) tracking algorithm to measure its velocity and draw its trajectory. The proposed cell tracking system consists of two stages, namely WBC detection and blob analysis, and fine tuning the tracking process by determine the centroid of the WBC, and mark the centroid for further fine tracking and to exclude the bacteria from the bounding box. Furthermore, the speed and the trajectory of the WBC motion are recorded and plotted. In the experiments, an optical flow technique is compared with the proposed tracking system showing the superiority of the proposed system as the optical flow method failed to track the WBC. The proposed system identified the WBC accurately, while the optical flow identified all other objects lead to its disability to track the WBC.

Extended ammonia observations towards the integral-shaped filament

Context. Recent observations suggest a scenario in which filamentary structures in the interstellar medium represent the first step towards clumps/cores and eventually star formation. The densest filaments would then fragment into prestellar cores owing to gravitational instability. Aims. We seek to understand the roles filamentary structures play in high-mass star formation. Methods. We mapped the integral-shaped filament (ISF) located at the northern end of the Orion A molecular cloud in NH3 (1, 1) and (2, 2). The observations were made using the 25 m radio telescope operated by the Xinjiang Astronomical Observatory, Chinese Academy of Sciences. The whole filamentary structure, about 1.2° × 0.6°, is uniformly and fully sampled. We investigate the morphology, fragmentation, kinematics, and temperature properties in this region. Results. We find that the morphology revealed by the map of velocity-integrated intensity of the NH3 (1, 1) line is closely associated with the dust ridge revealed by the Herschel Space Observatory. We identify 6 “lumps” related to the well known OMC-1 to 5 and 11 “sub-clumps” within the map. The clumps and sub-clumps are separated not randomly but in roughly equal intervals along the ISF. The average spacing of clumps is 11.30′ ± 1.31′ (1.36 ± 0.16 pc) and the average spacing of sub-clumps is 7.18′ ± 1.19′ (0.86 ± 0.14 pc). These spacings agree well with the predicted values of the thermal (0.86 pc) and turbulent sausage instability (1.43 pc) by adopting a cylindric geometry of the ISF with an inclination of 60° with respect to the line of sight. We also find a velocity gradient of about 0.6 km s−1 pc−1 that runs along the ISF which likely arises from an overall rotation of the Orion A molecular cloud. The inferred ratio between rotational and gravitational energy is well below unity. Furthermore, fluctuations are seen in the centroid velocity diagram along the ISF. The OMC-1 to 5 clouds are located close to the local extrema of the fluctuations, which suggests that there exist gas flows associated with these clumps in the ISF. The derived NH3 (1, 1) and (2, 2) rotation temperatures in the OMC-1 are about 30–40 K while lower temperatures (below 20 K) are obtained in the northern and southern parts of the ISF. In OMC-2, OMC-3, and the northern part of OMC-4, we find higher and lower temperatures at the boundaries and in the interior, respectively.

B-fields and gas motion in the L1689 region: an interpretation of Planck polarization data

AbstractWith using the Planck polarization data (PR2, 2016A A…596A.109P), we investigate the magnetic fields in L1689 and associated clouds, and compare them with centroid velocities VLSR of 12CO and 13CO from the COMPLETE survey (2006AJ….131.2921R). We observe two components in this elongated region: in one component, the position angle of the magnetic field varies from –10 to 110 degrees in the galactic coordinate, while VLSR is rather constant (=4 ± 0.5 km/s). In the other component with the position angle being constant (=110 ± 15 degrees), the velocity VLSR shows a spatial gradient from 3 to 5 km/s, as one goes from west to east along the direction of elongation. If the east side of the component is more distant from us than the west, this gradient suggests that this component is stretching. This work is supported by JSPS KAKENHI Grant Number JP18H03720 (PI: Koji S. Kawabata).

ALMA observations of a metal-rich damped Ly α absorber at z = 2.5832: evidence for strong galactic winds in a galaxy group

We report on the results of a search for CO(3-2) emission from the galaxy counterpart of a high-metallicity Damped Ly-alpha Absorber (DLA) at z=2.5832 towards the quasar Q0918+1636. We do not detect CO emission from the previously identified DLA galaxy counterpart. The limit we infer on M_gas / M_star is in the low end of the range found for DLA galaxies, but is still consistent with what is found for other star-forming galaxies at similar redshifts. Instead we detect CO(3-2) emission from another intensely star-forming galaxy at an impact parameter of 117 kpc from the line-of-sight to the quasar and 131 km s^-1 redshifted relative to the velocity centroid of the DLA in the quasar spectrum. In the velocity profile of the low- and high-ionisation absorption lines of the DLA there is an absorption component consistent with the redshift of this CO-emitting galaxy. It is plausible that this component is physically associated with a strong outflow in the plane of the sky from the CO-emitting galaxy. If true, this would be further evidence, in addition to what is already known from studies of Lyman-break galaxies, that galactic outflows can be traced beyond 100 kpc from star-forming galaxies. The case of this z=2.583 structure is an illustration of this in a group environment.

ALMA Images of the Host Cloud of the Intermediate-mass Black Hole Candidate CO−0.40–0.22*: No Evidence for Cloud–Black Hole Interaction, but Evidence for a Cloud–Cloud Collision

Abstract This paper reports a reanalysis of archival ALMA data of the high velocity(-width) compact cloud CO−0.40–0.22, which has recently been hypothesized to host an intermediate-mass black hole (IMBH). If beam-smearing effects, difference in beam sizes among frequency bands, and Doppler shift due to the motion of the Earth are considered accurately, none of the features reported as evidence for an IMBH in previous studies are confirmed in the reanalyzed ALMA images. Instead, through analysis of the position–velocity structure of the HCN J = 3–2 data cube, we have found kinematics typical of a cloud–cloud collision (CCC), namely, two distinct velocity components bridged by broad emission features with elevated temperatures and/or densities. One velocity component has a straight filamentary shape with approximately constant centroid velocities along its length but with a steep, V-shaped velocity gradient across its width. This contradicts the IMBH scenario but is consistent with a collision between two dissimilar-sized clouds. From a non-LTE analysis of the multitransition methanol lines, the volume density of the post-shock gas has been measured to be ≳106 cm−3, indicating that the CCC shock can compress gas in a short timescale to densities typical of star-forming regions. Evidence for star formation has not been found, possibly because the cloud is in an early phase of CCC-triggered star formation or because the collision is nonproductive.

A First Look at BISTRO Observations of the ρ Oph-A core

Abstract We present 850 μm imaging polarimetry data of the ρ Oph-A core taken with the Submillimeter Common-User Bolometer Array-2 (SCUBA-2) and its polarimeter (POL-2) as part of our ongoing survey project, -fields In STar forming RegiOns (BISTRO). The polarization vectors are used to identify the orientation of the magnetic field projected on the plane of the sky at a resolution of 0.01 pc. We identify 10 subregions with distinct polarization fractions and angles in the 0.2 pc ρ Oph-A core; some of them can be part of a coherent magnetic field structure in the ρ Oph region. The results are consistent with previous observations of the brightest regions of ρ Oph-A, where the degrees of polarization are at a level of a few percent, but our data reveal for the first time the magnetic field structures in the fainter regions surrounding the core where the degree of polarization is much higher (>5%). A comparison with previous near-infrared polarimetric data shows that there are several magnetic field components that are consistent at near-infrared and submillimeter wavelengths. Using the Davis–Chandrasekhar–Fermi method, we also derive magnetic field strengths in several subcore regions, which range from approximately 0.2 to 5 mG. We also find a correlation between the magnetic field orientations projected on the sky and the core centroid velocity components.

Gravity drives the evolution of infrared dark hubs: JVLA observations of SDC13

Context. Converging networks of interstellar filaments, that is hubs, have been recently linked to the formation of stellar clusters and massive stars. Understanding the relationship between the evolution of these systems and the formation of cores and stars inside them is at the heart of current star formation research. Aims. The goal is to study the kinematic and density structure of the SDC13 prototypical hub at high angular resolution to determine what drives its evolution and fragmentation. Methods. We have mapped SDC13, a ~1000 M⊙ infrared dark hub, in NH3(1,1) and NH3(2,2) emission lines, with both the Jansky Very Large Array and Green Bank Telescope. The high angular resolution achieved in the combined dataset allowed us to probe scales down to 0.07 pc. After fitting the ammonia lines, we computed the integrated intensities, centroid velocities and line widths, along with gas temperatures and H2 column densities. Results. The mass-per-unit-lengths of all four hub filaments are thermally super-critical, consistent with the presence of tens of gravitationally bound cores identified along them. These cores exhibit a regular separation of ~0.37 ± 0.16 pc suggesting gravitational instabilities running along these super-critical filaments are responsible for their fragmentation. The observed local increase of the dense gas velocity dispersion towards starless cores is believed to be a consequence of such fragmentation process. Using energy conservation arguments, we estimate that the gravitational to kinetic energy conversion efficiency in the SDC13 cores is ~35%. We see velocity gradient peaks towards ~63% of cores as expected during the early stages of filament fragmentation. Another clear observational signature is the presence of the most massive cores at the filaments’ junction, where the velocity dispersion is largest. We interpret this as the result of the hub morphology generating the largest acceleration gradients near the hub centre. Conclusions. We propose a scenario for the evolution of the SDC13 hub in which filaments first form as post-shock structures in a supersonic turbulent flow. As a result of the turbulent energy dissipation in the shock, the dense gas within the filaments is initially mostly sub-sonic. Then gravity takes over and starts shaping the evolution of the hub, both fragmenting filaments and pulling the gas towards the centre of the gravitational well. By doing so, gravitational energy is converted into kinetic energy in both local (cores) and global (hub centre) potential well minima. Furthermore, the generation of larger gravitational acceleration gradients at the filament junctions promotes the formation of more massive cores.

Quasars Probing Quasars. IX. The Kinematics of the Circumgalactic Medium Surrounding z ∼ 2 Quasars

Abstract We examine the kinematics of the gas in the environments of galaxies hosting quasars at z ∼ 2. We employ 148 projected quasar pairs to study the circumgalactic gas of the foreground quasars in absorption. The sample selects foreground quasars with precise redshift measurements, using emission lines with precision ≲300 km s−1 and average offsets from the systemic redshift ≲ ∣ 100 km s − 1 ∣ . We stack the background quasar spectra at the foreground quasar’s systemic redshift to study the mean absorption in C ii, C iv, and Mg ii. We find that the mean absorptions exhibit large velocity widths σ v ≈ 300 km s−1. Further, the mean absorptions appear to be asymmetric about the systemic redshifts. The mean absorption centroids exhibit small redshift relative to the systemic δv ≈ +200 km s−1, with large intrinsic scatter in the centroid velocities of the individual absorption systems. We find the observed widths are consistent with gas in gravitational motion and Hubble flow. However, while the observation of large widths alone does not require galactic-scale outflows, the observed offsets suggest that the gas is on average outflowing from the galaxy. The observed offsets also suggest that the ionizing radiation from the foreground quasars is anisotropic and/or intermittent.

Seeds of Life in Space (SOLIS). III. Zooming Into the Methanol Peak of the Prestellar Core L1544*

Abstract Toward the prestellar core L1544, the methanol (CH3OH) emission forms an asymmetric ring around the core center, where CH3OH is mostly in solid form, with a clear peak at 4000 au to the northeast of the dust continuum peak. As part of the NOEMA Large Project SOLIS (Seeds of Life in Space), the CH3OH peak has been spatially resolved to study its kinematics and physical structure and to investigate the cause behind the local enhancement. We find that methanol emission is distributed in a ridge parallel to the main axis of the dense core. The centroid velocity increases by about 0.2 km s−1 and the velocity dispersion increases from subsonic to transonic toward the central zone of the core, where the velocity field also shows complex structure. This could be an indication of gentle accretion of material onto the core or the interaction of two filaments, producing a slow shock. We measure the rotational temperature and show that methanol is in local thermodynamic equilibrium (LTE) only close to the dust peak, where it is significantly depleted. The CH3OH column density, N tot(CH3OH), profile has been derived with non-LTE radiative transfer modeling and compared with chemical models of a static core. The measured N tot(CH3OH) profile is consistent with model predictions, but the total column densities are one order of magnitude lower than those predicted by models, suggesting that the efficiency of reactive desorption or atomic hydrogen tunneling adopted in the model may be overestimated; or that an evolutionary model is needed to better reproduce methanol abundance.

Evolution of the Outflows in NGC 3516

Abstract We analyze the 2011 HST/COS spectrum of the Seyfert 1 galaxy NGC 3516, which demonstrates clear changes in one of the intrinsic absorption troughs (component 5), slight evidence of change in a second trough (component 6), and the appearance of a new absorption trough (component 9). We interpret both the changes and the appearance of the new trough as bulk motion across the line of sight. The implied lower limit on the transverse velocity of component 5 is 360 km s−1, compared to the earlier 2001 HST/STIS spectrum, while the lower limits for components 6 and 9 are 920 km s−1, based on 2009 FUSE data. Component 5 also exhibits a shift in velocity centroid. This is only the second known case of this behavior in a Seyfert galaxy. Due to the high quality of the HST/COS spectrum, we identify a previously undetected trough due to an excited state of Si ii for component 1. In combination with the resonance trough of Si ii and photoionization modeling, we directly determine the distance of the component 1 outflow to be 67.2 pc.

Tracing Magnetic Fields with Spectroscopic Channel Maps

Abstract We identify velocity channel map intensities as a new way to trace magnetic fields in turbulent media. This work makes use of both the modern theory of magnetohydrodynamic (MHD) turbulence, which predicts that magnetic eddies are aligned with the local direction of the magnetic field, and also the theory of spectral line position–position–velocity (PPV) statistics, which describes how velocity and density fluctuations are mapped onto PPV space. In particular, we use the fact that the fluctuations of the intensity of thin channel maps are mostly affected by the turbulent velocity, while the thick maps are dominated by density variations. We study how contributions of the fundamental MHD modes affect the Velocity Channel Gradients (VChGs), and demonstrate that the VChGs arising from Alfvén and slow modes are aligned perpendicular to the local direction of the magnetic field, while the VChGs produced by the fast mode are aligned parallel to the magnetic field. The dominance of Alfvén and slow modes in interstellar media will therefore allow reliable magnetic field tracing using the VChGs. We explore ways of identifying self-gravitating regions that do not require polarimetric information. In addition, we also introduce a new measure, termed “Reduced Velocity Centroids” (RVCGs), and compare its abilities with those of VChGs. We employed VChGs in analyzing GALFA 21 cm data and successfully compared the magnetic field directions with the Planck polarization observations. The applications of the suggested techniques include both tracing the magnetic field in diffuse interstellar media and star-forming regions, and removing the galactic foreground in the framework of cosmological polarization studies.

Effects of the magnetic field direction on the Tsallis statistic

We extend the use of the Tsallis statistic to measure the differences in gas dynamics relative to the mean magnetic field present from natural eddy-type motions existing in magnetohydrodynamical (MHD) turbulence. The variation in gas dynamics was estimated using the Tsallis parameters on the incremental probability distribution function of the observables (intensity and velocity centroid) obtained from compressible MHD simulations. We find that the Tsallis statistic is susceptible to the anisotropy produced by the magnetic field, even when anisotropy is present the Tsallis statistic can be used to determine MHD parameters such as the Sonic Mach number. We quantize the goodness of the Tsallis parameters using the coefficient of determination to measure the differences in the gas dynamics. These parameters also determine the level of magnetization and compressibility of the medium. To further simulate realistic spectroscopic observational data, we introduced smoothing, noise, and cloud boundaries to the MHD simulations.

OUP accepted manuscript

We explore how the velocity gradient technique (VGT) can be applied to absorbing media in the case of$^{13}$CO 2-1 emission. The VGT is a new way to trace magnetic fields in the plane of the sky using only spectroscopic observations. We apply the VGT to magnetohydrodynamic turbulence simulations that have been post-processed to include $^{13}$CO 2-1 emission and we calculate the velocity centroid gradients. We find that the velocity centroid gradients trace the projected magnetic field in media with different $^{13}$CO abundances, densities and optical depths. Our study opens up the possibility of using velocity centroid gradients to trace magnetic fields in molecular clouds using 13CO emission.

Comparison of thermal, salt and dye tracing to estimate shallow flow velocities: Novel triple-tracer approach

The accurate measurement of shallow flow velocities is crucial to understand and model the dynamics of sediment and pollutant transport by overland flow. In this study, a novel triple-tracer approach was used to re-evaluate and compare the traditional and well established dye and salt tracer techniques with the more recent thermal tracer technique in estimating shallow flow velocities. For this purpose a triple tracer (i.e. dyed-salted-heated water) was used. Optical and infrared video cameras and an electrical conductivity sensor were used to detect the tracers in the flow. Leading edge and centroid velocities of the tracers were measured and the correction factors used to determine the actual mean flow velocities from tracer measured velocities were compared and investigated. Experiments were carried out for different flow discharges (32–1813 ml s−1) on smooth acrylic, sand, stones and synthetic grass bed surfaces with 0.8, 4.4 and 13.2% slopes. The results showed that thermal tracers can be used to estimate shallow flow velocities, since the three techniques yielded very similar results without significant differences between them. The main advantages of the thermal tracer were that the movement of the tracer along the measuring section was more easily visible than it was in the real image videos and that it was possible to measure space-averaged flow velocities instead of only one velocity value, with the salt tracer. The correction factors used to determine the actual mean velocity of overland flow varied directly with Reynolds and Froude numbers, flow velocity and slope and inversely with flow depth and bed roughness. In shallow flows, velocity estimation using tracers entails considerable uncertainty and caution must be taken with these measurements, especially in field studies where these variables vary appreciably in space and time.