RotVis user manual
RotVis is a toy forward-modeling tool for exploring how a rotating disk and a two-component merger may appear in spatially and spectrally convolved emission-line observations. Hope it would be helpful if you are working on galaxy kinematics with low resolution data, and if you are worried about how merger can mimic a rotating disk, leading to wrong interpretations.
Quick start
- Choose a disk inclination and rotation curve shape.
- Change the spatial and velocity resolutions to see beam and spectral smearing.
- Adjust the merger separation in position and velocity.
- Compare moment 0, moment 1, moment 2, the major- and minor-axis PV diagrams, and the global 1D spectrum.
What the panels show
| Panel | What to inspect | |
|---|---|---|
| Moment 0 | Integrated intensity | One peak versus two peaks? Elongation? Bridges? |
| Moment 1 | Intensity-weighted line-of-sight velocity | Smooth gradients? Twists? Discontinuities? The thick systemic-velocity contour (V=0)? |
| Moment 2 | Intensity-weighted line-of-sight line broadening, shown with Blues from 0 to 50 km s−1 | A central line broadening peak? Beam smearing? Broad lines where two merger components overlap? |
| Major-axis PV | Position-velocity diagram extracted along the global x axis | Symmetry? Multiple components? Line broadening? Effects of beam and spectral smearing? The Keplerian decline of the SMBH? Whether the merger produces a disk-like pattern? |
| Minor-axis PV | Position-velocity diagram extracted along the global y axis | Velocity asymmetries away from the major axis? Multiple components? Off-axis emission inconsistent with a simple rotating disk? |
| Global spectrum | Flux density integrated within the whole field of view | Single-, double-peaked, or asymmetric line profiles? Spatial convolution conserves total flux in principle, but beam smearing can spread emission beyond the finite field of view and reduce the measured integrated spectrum. |
The vertical colorbars show normalized intensity for moment 0 and the PV diagrams, velocity in km s−1 for moment 1, and line broadening in km s−1 for moment 2.
Disk controls
On a wide screen, the disk and merger sliders are grouped in the left sidebar while the model panels remain on the right. On a narrow screen, the controls move above the panels.
Spatial resolution
The circular Gaussian beam is specified by its FWHM in kpc. The spatial pixel size is one third of the selected FWHM. The field of view remains fixed at 5 kpc (±2.5 kpc), and the brightest intrinsic disk pixel is registered at the centre.
Inclination
The inclination ranges from 1° (nearly face-on) to 89° (nearly edge-on). Projected rotation scales approximately as Vrot sin(i), while the projected minor axis contracts by cos(i).
Velocity resolution
The selected FWHM defines both the Gaussian spectral response and the velocity-channel spacing. The internal velocity coverage expands when necessary so that high-velocity flux is not discarded.
Rotation-curve families
- Pure disk: an empirical rising-to-flat curve with asymptotic speed 220 km s−1 and turnover radius 0.35 kpc.
- Bulge + disk: adds a compact Hernquist-like bulge with scale radius 0.25 kpc. Its mass is adjustable.
- SMBH + bulge + disk: adds a central point-mass contribution softened inside 0.025 kpc. Both bulge and SMBH masses are adjustable.
The equivalent exponential-disk mass is approximately 1.50 × 1010 M☉, inferred by matching the adopted disk rotation speed near 2.2 scale lengths. The displayed bulge-to-disk and SMBH-to-disk fractions update with their sliders.
Dashed PV curve
The cyan dashed line in the major-axis PV diagram is the intrinsic projected rotation curve before spatial or spectral convolution.
PV extraction width
Both PV diagrams are extracted over a width of one beam FWHM. Because the spatial pixel size is one third of the beam FWHM, the extraction sums three pixels: the central pixel and one pixel on either side.
Merger controls and geometry
The merger contains two elliptical Gaussian components with unequal sizes and amplitudes (peak 1 : 0.82). Their centres lie along an axis offset slightly in y rather than along the global x axis. Each component has a mild internal velocity shear, and a faint broader bridge connects them.
- Spatial separation: projected center-to-center distance, adjustable from 0.20 to 1.60 kpc.
- Velocity separation: difference between the component systemic velocities, adjustable from 120 to 420 km s−1.
- Spatial and velocity resolution: can be synchronized with the disk or controlled independently.
How a merger mimics a disk :(
RotVis demonstrates a degeneracy: spatial and spectral blending can turn two dynamically distinct components into a continuous red-to-blue moment-1 gradient. A high disk-fit V/σ therefore does not by itself prove that the source is an equilibrium rotating disk.
Apparent disk-fit V/σ
The value shown for the merger is a simple disk-like interpretation, not the result of fitting a rotating-disk model. RotVis treats half of the merger velocity separation, Δv/2, as the projected rotation speed. It then applies the inclination selected for the disk:
Vfake = (Δv/2) / sin(i).
The denominator is the intrinsic Gaussian velocity dispersion assigned to each merger component, σ = 35 km s−1. The displayed quantity is therefore
Vfake/σ = Δv / [2 σ sin(i)].
For example, Δv = 300 km s−1 and i = 55° give Vfake/σ ≈ 5.2. Spatial and spectral resolution do not enter this numerical estimate; they determine only whether the two components blend sufficiently to make the disk-like interpretation plausible. The displayed ratio should therefore be regarded as an illustrative apparent value, not a recovered dynamical measurement.