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Introduction to the Velocity Explorer

Submitted by Kevin Jardine on 23 May, 2013 - 12:01

The Velocity Explorer is an interactive tool for exploring gas velocity in the galactic plane. As mentioned in the section on Velocity, this data, especially when combined with parallax measurements, is a crucial tool for mapping the Milky Way.

Currently the Velocity Explorer data is derived from two sources: the Leiden/Argentine/Bonn (LAB) HI survey of atomic hydrogen gas and the Harvard CO survey of molecular gas. The images were created in Blender from 3D meshes generated from the velocity data using a marching cubes algorithm to create isosurfaces of constant gas temperature.

I have prepared images of the LAB and Harvard data for several different directions as reported in the Velocity section.

For the Velocity Explorer, the data displayed is for the galactic plane (|b|

Isosurface and Max K images

In the case of the LAB data, there are 27 isosurfaces of constant gas temperature: 1 K, and then one at every 5 K increment between 5 K and 130 K inclusive.

This is the colour scale used for the LAB isosurfaces (the colour used is the one to the left of each label):

In addition to individual LAB isosurfaces, the Velocity Explorer contains Max K images for the LAB and CO surveys that combine the velocity data by showing the maximum intensity/temperature for each (l,v) position.

For the CO data, I am currently providing a single Max K image that combines all the CO isosurface data. (I could add the individual CO isosurfaces to the Velocity Explorer if there is demand for it.) The CO gas is much colder that the HI gas, with the isosurface temperature ranging in 0.5 K increments between 0.5 and 10 K inclusive).

This is the colour scale used for the CO isosurfaces (the colour used is the one to the left of each label):

The following images show a detail of the first galactic quadrant LAB data for four example isosurfaces and the combined (Max K) data for all 27 isosurfaces:

  • The 1 K image shows very cold diffuse gas rising well above the galactic plane (and so most of it is truncated at b = 10°).
  • In the 10 K image, most of the emission is restricted to the galactic plane.
  • In the 30 K image, the Norma and Centaurus arms in the outer galaxy are clearly separating from the hydrogen in the solar circle and inner galaxy.
  • The 60 K image shows individual clouds.
  • Finally, the Max K image shows the highest temperature/intensity data for each (l,v) position and gives a good overall representation of the spiral structures.

Object overlay

For both LAB atomic hydrogen and CO molecular gas, an overlay image is available which shows the velocity of objects from three recent catalogs of spiral tracers. These are:
  • the RMS (Red MSX Source) survey (link) (with the planetary nebulae removed), in cyan (blue-green) dots,
  • The 6-GHz methanol multibeam maser catalog (link), in green, and
  • The BeSSeL VLBI maser parallax survey (link and related papers), in yellow.

Model images

In addition to isosurfaces, the Velocity Explorer has overlay images related to the model of the Milky Way presented on this site. The "Highlight" option highlights parts of the velocity image that I have associated with a particular structure and the "Model/Highlight" option shows the projected model velocity edges overlaid on top of the highlighted velocity data.

The Highlight and Model/Highlight images are associated with my particular model and provide a visual argument for its compatibility with the LAB data. Unlike the isosurfaces, these are an interpretation of the data and of course other interpretations are possible.

Clicking on the tabs below will show you examples of the model images for a region around a tangent of the Centaurus arm.

Interactive features

The Velocity Explorer is based on the OpenSeadragon JavaScript library for viewing large images. The application supports a Zoom mode for zooming and panning around one image and a Fade mode for crossfading between two images for easy comparison.

The application starts in Zoom mode. When in Zoom mode, you can pan around the image by clicking and dragging your mouse. You can zoom in and out of the image by using your mouse wheel or by using the round buttons at the upper left. You can also reset the view to include the full image by clicking on the Home button at the upper left.

Your position on the image is shown on the Navigation overview at the upper right.

You can view the longitude and velocity coordinates for the current mouse position at the lower right. The "Link" area contains a url for your current location, data sets and zoom level. If you see an interesting region you can copy the link or bookmark it in order to return to the same view later.

You can toggle between Zoom mode and Fade mode using the control at the bottom left. When in Fade mode, you can crossfade between two data sets at the same location and zoom level by clicking and dragging your mouse left or right. This is useful for comparing two images. In Fade mode, all other controls are disabled. Click on the Zoom toggle at the bottom left to return to Zoom mode.

The controls will disappear after a time if you do not move your mouse.

Accessing the Velocity Explorer

You can use the form below to select Velocity Explorer images. Clicking on the View button will open the Velocity Explorer in a separate window.

Velocity Explorer Settings

Primary data set

LAB Max K with overlay
CO Max K
CO Max K with overlay
LAB Isosurface: K
LAB Highlight:
LAB Model / Highlight:

Secondary data set (for crossfading)

LAB Max K with overlay
CO Max K
CO Max K with overlay
LAB Isosurface: K
LAB Highlight:
LAB Model / Highlight:
Longitude: °
Velocity: km/s