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, and the major-axis PV diagram.
- Switch from PV diagrams to global 1D spectra when you want to inspect unresolved line profiles.
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)? |
| PV diagram | Position-velocity diagram extracted along the kinematic major 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? |
| Global spectrum | Flux density integrated within the whole field of view | Single-, double- or asymmetric line profiles? Spatial convolution conserves the flux in principle, but it can dilute the emission to outside of the field of view, and then change the spectrum. |
Disk controls
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 ±1.5 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 PV diagram is the intrinsic projected rotation curve before spatial or spectral convolution.
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 mimic 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.