Galaxies falling into a cluster
Joachim Köppen Kiel Sep 2017
Some brief explanations
This Monte Carlo simulation shows the properties of galaxies which are
falling radially but from all directions to the centre of a spherically
symmetric cluster of galaxies. While they follow
this path, the ram pressure by the hot intracluster medium (ICM) removes some
of the interstellar gas of the galaxy. Since this pressure increases with
diminishing distance from the centre, the gas loss also increases. This
can be studied for galaxies of a single type, but also for a population
of galaxies with different properties. The simulation does not follow
the actual flight of a galaxy, the spatial distribution of galaxies does
represent the snapshot computed for a certain instant; instead, one chooses
a radial profile for this distribution. The user may show the various quantities
of the cluster and the galaxies as plots, histograms or false colour maps.
Since the simulation is fairly complex, has many options, and needs various
parameters,
here are some examples of its use.
Cluster properties:
- Dark matter for the radial profile of its density one may use
a β-profile or a NFW (Navarro-Frenk-White) profile, whose parameters
are entered in the fields. Some parameters may be entered in different
measures; the field marked in green is the currently valid input field.
- Intracluster gas (ICM) is modelled in the same way, with the
corresponding parameters.
- within radius gives the masses of dark matter and ICM gas inside
the specified radius. Hitting the Enter key recomputes the values.
Galaxy properties:
- single type all galaxies are of the same type, with
the outer radius of the gas disk, the rotation speed (assuming a flat
rotation curve), and a surface density of the gas. One may also
choose between a 'flat gas disk' with constant surface density
and a 'realistic gas disk' which gives a relation between ram pressure
and deficiency which is closer to that for a gas disk with exponential
profile. (TBD)
- distribution of types the galaxies have randomly picked
outer radii for the gas disk, with a logarithmic distribution between
a minimum and maximum value as given. The gas surface density is assumed
to have the specified value (as above). The model uses an empirical
relation between optical radius and rotational speed, and uses a scatter
factor for the centrifugal acceleration (v²/r). (TBD)
- Galaxy population: Here one chooses the radial profile for
the spatial distribution of galaxies: constant density between an
inner and outer radius, gaussian profile with specified sigma value,
a β profile with parameters, or in 'flight path' with a density
corresponding to the probability of finding there an infalling galaxy.
Simulation display:
- objects per step is the number of galaxies created in each
simulation step between two displays of the data. Choose a small
value (10) to see how a plot is built up; take a high value (1000)
when you're too impatient to wait for the final result in a histogram
or a map.
- objects now displays the number of galaxies created for
the current plot.
- no. of bins is the number of bins in the histogram, and
the number of pixels in X and Y direction in the false colour maps.
The maximum number is 100.
- fly dirt plot: each galaxy is plotted as a small red dot
at the chosen abscissa and ordinate.
- histogram: plots the relative number of occurrences of
the galaxy's abscissa.
- false colour map: plots the relative number of occurrences
of the galaxy's chosen abscissa and ordinate. This number is
coded by colour, with black=0 and red=maximum value, according
to the colour band shown at the right. The scaling is linear
or logarithmic.
- X = and Y = : the abscissa and ordinate of the plot can
be chosen among these parameters:
- X: the coordinate with respect to the cluster centre
- Y: the coordinate with respect to the cluster centre
- Z: the coordinate with respect to the cluster centre
- true distance from centre
- escape speed
- projected distance from centre: as seen from an observer from the end of the X-axis
- radial velocity: as seen by the observer
- estimated true distance from centre: if one assumes that the galaxies travel with the
local escape speed, one may easily deduce the true distance from the projected distance
from the centre and the radial velocity
- ICM density: the density at the position of the galaxy
- ICM surface density: the surface density at the galaxy's projected distance
from the centre, as seen by the observer
- ram pressure: the value that the galaxy experiences
- deficiency: the galaxy's current value
(deficiency = log10(initial gas mass/actual gas mass))
- optical radius: the galaxy's value
- rotation speed: the galaxy's value
- HI surface density: the galaxy's value
- Zoom: when this radio button is clicked, the plotting limits are taken from
the associated fields (min, max)
- start: to begin the simulation
- stop: to halt the simulation
- resume: to continue with the simulation
- clear: to wipe the plot
- Mouse position: displays the coordinates of the present position of the
mouse. In the false colour maps, it also shows the actual number as well as the
relative number of objects in the pixel.