View of a galaxy undergoing ram pressure stripping
Joachim Köppen Praha/Kiel Nov 2017
Some brief explanations
When a spiral galaxy travels through a cluster of galaxies, the hot gas of the
intracluster medium (ICM) interacts with the interstellar medium (ISM)
of the galaxy. If conditions are right, the ISM gas is pushed out of the
galaxy. As a consequence the galaxy loses part or all of its gas and leaves
behind a gaseous tail on its flight path.
This tool simulates this process of ram pressure stripping by a simple model.
The galaxy's gas disk is divided in a number of concentric rings around the
galactic centre. Each ring is represented by a single test particle which
moves in the gravitational potential of the galaxy. Initially, all particles
move on circular orbits around the galactic centre. The test particle is
given a mass proportional to the gas surface density at the initial
galactocentric radius; it is given a random position on the circumference
of its ring. The action of the ram
pressure by the ICM is modeled as a pulse of an additional external force
which acts in a direction perpendicular to the galactic plane. Thus, we
consider the situation of face-on stripping.
After a model has been computed, the positions and velocities of the particles
can be inspected at a number of snapshots at different times. The user has
the choice between various plots, such as plotting any two parameters against
each other. These can be properties in the 3-D system of galaxy, but also
2-D views projected to the celestial sphere available to an outside observer.
The user may apply rotations to the model, as to get the view of the inclined
object. False colour maps are provided to give a 2-D view of the total
intensity, position-velocity maps, channel maps, and the spectra of the
entire object and as observed with a finite beam at some pointing position.
These maps are generated by representing each ring by a number of sub-particles
evenly and randomly distributed within the ring's volume.
The simulator is controlled from the page Operate.
The complete data of a snapshot may be written to the page Output in the
form of an ascii text. From here it can be copied and pasted into a text editor
for further use or analysis.
The controls on the Operate
- The (spiral) galaxy model consists of a central bulge of stars, the
disk of stars and gas, and a halo of dark matter. Each of these components
is described by a number of parameters, such as its mass inside a truncation
radius, and a radial scale length. Bulge and halo are modelled as Plummer
spheres, the disk as a Miyamoto-Nagai disk. For comparison purposes, the
user may place a point mass in the galactic centre. Because of the presence
of molecular gas, the restoring force for the neutral gas is supposed to
be enhanced by a factor (1 + a exp(-r/R)) as by Vollmer et al. (2001).
- Due to the ram pressure force, the test particles will be pushed away from
their initial positions in the galactic plane. Depending on the strength
of the force and its mass a test particle will be accelerated up to the
local escape speed. Particles that escape from the galaxy are shown in the
plot as red dots. Among the particles that still
remain gravitationally bound we distinguish between those that remain
close to the galactic plane (shown as blue dots)
and those which have been pushed away (green dots),
i.e. outside a cylindrical volume assigned for the galactic plane
by galactocentric radius and thickness.
- The Modelling needs parameters.
The Pulse properties are:
Some Modelling details are needed:
- flight speed: is the speed of the galaxy with respect to the ambient ICM.
This will be assumed to be constant throughout the simulation.
- maximum pressure:
- choose between Gauss and Lorentz pulse shape, and a constant pressure ('wind tunnel').
- FWHM duration: of the pulse.
- choose between the stripped gas to follow the galaxy under acceleration by the
ram pressure and to be instantly deposited in the ICM.
The modeling controls are:
- number of particles: 1000 give a quick impression, 10000 make smoother curves,
but 100000 are still possible ...
- snapshot interval: is the time between two snapshots.
- end time: the simulation always starts at two FWHM pulse widths before
the time of peak pressure (at time=0). It will terminate at the end time, or
before if the stop button is clicked.
- time step: is the stepwidth for the numerical solution of the equation of
motion of each particle. For a galaxy like the Milky Way, 1 Myr is a good
choice. If results look weird or if in doubt, use a smaller time step and be
- time now: during modeling it displays the time of the current snapshot.
- (vΣ)ICM: shows the current value of the time-integrated
- start: starts a new model. During the computation the stop button is lit red.
- stop: stops the computations.
- resume: continues with the current model. Note that a model can be extended
by specifying a larger value for the end time.
- Display controls:
- inclination YP, ZP: are the angles of rotation around the YP and ZP axis of the observer's
2-D view. The observer's image has XP and YP as the horizontal and vertical axes,
ZP is the direction along the line of sight.
- for the spectrum at a position these controls are visible:
- point at XP, YP: these are the coordinates of this position.
The point is marked as a plus-sign in the total intensity map, channel map, and
the XP-YP plot.
- antenna HPBW: also, one can specify the HPBW within which all particles are
included for the spectrum.
- number of map/histogram bins: maximum is 100.
- X, Y: if one has chosen all other plots one can select
the parameters to be shown as abscissa and ordinate.
- Set range: In order to use values for the X and Y ranges more convenient
than the standard value, enter the wanted minimum and maximum values in the two fields
and click the circular radio button to show the same plot with the new range. This
applies to all types of plots.
- colour bar (for the false colour maps): one may choose between:
- linear rainbow: between black/violet = 0 and red/white = 1
- rainbow-2: with larger emphasis of high intensities. The colour
bar at the right rim indicates the colour distribution between
0 and 1.
- rainbow2: with stronger emphasis of low intensities
- rainbow4: with even stronger emphasis of low intensities
- rainbow8: with very strong emphasis of low intensities
- log rainbow 0.001-1: logarthmic between black/violet = 0.001
and red/white = 1
- output this snapshot: the current snapshot's data will be written
as an ascii table on the Output page
- time <--- --->: are the controls to navigate among the snapshots:
either by the buttons with arrows or entering the snapshot number
in the text field between them. The proper time of that shapshot
is displayed at right.
- had one chosen the channel map these controls
radial velocity, channel <--- --->: are the controls to navigate
among the radial velocity channels. Δv is the width of the
radial velocity window. Use the buttons with arrows to step forward or
backward by one channel, or enter the desired central value for the
radial velocity in the text field between them.
- Mouse position: displays the coordinates of the present
position of the mouse. In the false colour maps it also shows the
actual surface density as well as the value relative to the image's
The user may choose among these plots:
- create a model: galactic X-Y plot: This is the default plot when the simulator
is loaded, and also the recommended plot to watch the progress of a simulation.
It shows a face-on view of the galaxy. First, all particles are blue, because
all are well inside the volume assigned for the gas disk. In the course of time
particles turn green, as they are pushed out of the disk, and evenually
some appear in red when they have reached escape speed.
- all other plots: here, one may plot any two parameters
against each other, whether they are sensible choices or not :-). One may
follow the evolution of the model calculation for instance by plotting the
distance from the rotation axis as a function of time ...
- total intensity map: is a false colour representation of the density of particle
masses in the observer's XP-YP view. Since the mass of a particle is proportional
to the surface density at its initial position, this maps shows an image of the
surface density of HI (in units of MSUN/pc²), and thus of
the brightness distribution in an image taken in the HI 21 cm line.
- position-velocity map: is a false colour representation of the surface density
in the XP-VRAD plane (in units of MSUN/pc²).
- channel map: false colour map of the surface density in the XP-YP plane of particles
whose radial velocity is near the specified value.
- total spectrum: is a histogram of the radial velocities of all particles.
- spectrum at point (XP,YP) with HPBW: is the histogram of the radial velocities
of all particles whose projected position (XP,YP) is within the HPBW of the
The variables of the model are:
Note that the variables are a mixture of various
types, and therefore it may not be too meaningful to plot certain parameters against
certain others ...
- time: is relative to the instant of maximum pressure.
- ram pressure: permits to plot the dependence of e.g. gas mass fractions
as a function of the instanteous ram pressure.
- time integrated ram pressure (vΣ)ICM: permits to plot the
track in the (vΣ)ICM-ram pressure plane.
- - - - - - - - -
- galactic X,Y,Z: the position of the particles in the galaxy's
- distance from rotation axis:
- initial distance from rotation axis:
- galactic VX,VY,VZ: the velocity components in the galaxy's system.
- radial speed: the component radially away from the axis of rotation.
- tangential speed: the component parallel to a circular orbit.
- gas surface density: in the plane of the galaxy, when viewed face-on
- tail surface density: this is an estimate which applies onlly along the axis
of a tail viewed from the side (at incl.YP = 90°)
- total energy:
- angular momentum density: carried by each ring
- - - - - - - - -
- observer's projected XP,YP: the positions of the particles as seen
in an image by the observer.
- radial velocity: the speed along the line of sight from the observer.
- - - - - - the ordinate has these additional variables - - -
- ram pressure: to be plotted as a function of time
- remaining gas mass fraction: can be plotted against time or ram pressure.
This is the fraction that keeps close to the galactic plane.
These particles are shown as blue dots.
- kicked gas mass fraction: is from particles that have been pushed out of
the galactic plane, but remain bound to the galaxy. Shown as green dots.
- kicking radius: the innermost distance of green particles, i.e. the outer
radius of the particles that remain in the disk.
- stripping radius: the innermost distance of red particles, i.e. the outer
radius of the particles that do not escape from the galaxy.
- mass loss rate: of the gas that is gravitationally bound to the galaxy.
- stripped gas mass fraction: is from particles that escape from the
galaxy (shown as red dots).
- gas deficiency: = log10(initial gas mass/final gas mass)
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Data from a snapshot:
for each particle: surface density [MSUN
/pc²] (at initial position), x, y, z [kpc], vx, vy, vz [km/s], total energy [(km/s)²]