AIPS HELP file for GAL in 31DEC21
As of Sun May 16 11:55:15 2021
GAL: Task to determine parameters from a velocity field
USERID -32000.0 32000.0 User ID - ignored
INNAME Image name
INCLASS Image class
INSEQ 0.0 9999.0 Image seq. #
INDISK 0.0 9.0 Disk drive #
IN2NAME Weight image name
IN2CLASS Weight image class
IN2SEQ 0.0 9999.0 Weight image seq. #
IN2DISK 0.0 9.0 Weight image disk drive #
OUTNAME Output image name (name)
OUTCLASS Output image name (class)
OUTSEQ 0.0 9999.0 Output image name (seq. #)
OUTDISK 0.0 9.0 Output disk drive #
BLC 0.0 4096.0 Bottom left corner of image
TRC 0.0 4096.0 Top right corner of image
SOLCON 0.0 1.0 Convergence criterion.
FUNCTYPE Type of rotation curve: 'BR':
Brandt, 'EX' : exponential,
'CC' : constant, 'SB' : solid
APARM $ First guesses to parameters:
$ (1,2) (x,y) center in pixels
$ (3) position angle major ax
$ (4) inclination (deg)
$ (5) systemic velocity (km/s)
$ (6) maximum velocity (km/s)
$ (7) radius max velocity (")
$ (8) exponent (0=>1).
CPARM (1) >0 residual map written
(see help for choices)
(2) > 0 => make plot
(3) Code for fixed params
(see help for choices)
(4) > 0 use IN2C map for
(5) > 0 copy old history
(6) 0: write one line to
OUTTEXT; -1/1 write
position to OUTTEXT
(7) > 0 just plot using
inputs, else fit too
DPARM (1,2) lower/upper limit to
(3,4) range of cos (position
angle in galaxy);
(5,6) idem for sine;
(7) max. plotted radius;
(8) max. plotted velocity;
(9) min plotted velocity
. (10) width of annuli (pixels)
Text file containing rotation
Text file to which one line
of output results is written.
' ' means don't write one
PIXSTD Estimated rms uncertainty in
the observed radial velocity
(km/s) at one pixel. 0=>10
SYMBOL 0.0 24.0 Plot symbol number: 1 - 24
FACTOR 0.0 Scale plot symbol by FACTOR
DOTV -1.0 1.0 > 0 Do plot on the TV, else
make a plot file
GRCHAN 0.0 8.0 Graphics channel 0 => 1.
Use: GAL calculates eight parameters specifying the orientation (# 1 to
5) and the rotation curve (# 6 to 8) of a galaxy, using its
velocity field as input. If this velocity field consists of M data
points, the parameters are determined by a nonlinear least squares
fitting of eight parameters to M data points. If CPARM(1)> 0, a
residual velocity field (observed minus model) is produced. If
CPARM(2)> 0, the observed rotation curve is plotted together with
the fitted model curve.
USERID......User ID of owner of image. Ignored.
INNAME......Image name(name). blank=>any
INCLASS.....Image name(class). blank=>any
INSEQ.......Image name(seq. #). 0=>any
INDISK......Disk drive # of image. 0=>any
IN2NAME.....Weight image name(name). blank=>any
IN2CLASS....Weight image name(class). blank=>any
IN2SEQ......Weight image name(seq. #). 0=>any
IN2DISK.....Disk drive # of weight image. 0=>any
OUTNAME.....Output image name(name). blank=>INNAME
OUTCLASS....Output image name(class). blank=>'GAL'
OUTSEQ......Output image name(seq). 0=>next unique
OUTDISK.....Output image disk drive #. 0=>any
BLC.........The Bottom Left-hand pixel of the subarray of
the image to be analysed. The value (0,0)
TRC.........The Top Right-hand pixel of the subarray of
the image to be analysed. (0,0) => (1,1).
the top right hand corner of the entire image.
SOLCON......Criterion to stop least squares fitting.
FUNCTYPE....Type of rotation curve to be fitted.
'BR' : Brandt curve, given by :
R / Rmax
V / Vmax = ---------------------- n 3/2n
(1/3 + 2/3 * (R / Rmax) )
'EX' : Exponential flat curve :
- ln(100.0) * (R / Rmax)
V / Vmax = 1 - e
'CC' : Constant curve : V / Vmax = 1
'SB' : Solid Body curve : V / Vmax = R / 60
APARM.......The eight parameters to be fitted:
(1) x pixel central position
(2) y pixel central position
(3) position angle receding major axis
(4) inclination orbital plane
(5) systemic velocity
(6) Vmax, maximum rotation velocity
(7) Rmax, radius(") of maximum rot. velocity
(8) n, measure of extent of 'flat' part of curve, 0=>1.
(8) is only used if FUNCTYPE='BR', and (7) only if
FUNCTYPE='BR' or 'EX'. On input, these parameters are
the initial guesses; on output they are the fit values.
CPARM (1) > 0 => a residual output map is made
if 1, using the whole field; if 2, using a specified
part of the field only; if 3, using INTEXT rather
(2) if >0, the observed and the model rotation curves
(3) parameters to be held fixed in the order given in
APARM. e.g. if parameters i, j, and k are to be held
fixed, CPARM(3) = 2**(i-1) + 2**(j-1) + 2**(k-1).
0 defaults to NO fixed parameters. CPARM(3)=3 holds
the center positions fixed
(4) >0 use IN2C map for weights, else don't use a
weighting image. Zeroth moment often used.
(5) >0 copy old history file; else don't copy it.
(6) 0: write one line with fitted parameters to OUTTEXT;
NOT 0: write position of ends of major major axis
(receding then approaching) to OUTTEXT and write the
full set of plot data - radius, velocity, and rms of
velocity as well. Used only if OUTTEXT not blank.
(7) : no fit, residual field only;
DPARM (1) and (2): lower and upper limit to radius (");
No default except when CPARM(1)=3 where the range of
radii in the input text file is used.
(3) and (4): limits to cos(azimuth angle);
(5) and (6): limits to sin(azimuth angle);
(7) max. radius to be plotted;
(8) and (9) max and min velocity to be plotted
(10) : width annuli in plotting observed rotation curve.
INTEXT......Text file containing rotation curve information. This
is used to determine the residual field when
CPARM(1)=3 and must be specified then.
OUTTEXT.....Not blank => Text file to which one line of output
results is written. When OUTTEXT exists, GAL will
append. Meant to be used in loop when using concentric
rings; OUTTEXT can then become INTEXT of next run.
If CPARM(6) is not zero, the file instead contains the
measured rotation curve and the positions of the two
ends of the major axis.
PIXSTD......Estimated rms uncertainty in observed radial velocity at
one pixel (km/s)
SYMBOL.....1: Plus sign 12: Five pointed star
2: Cross (X) 13: Star of David
3: Circle (default) 14: Seven-pointed star
4: Box 15: Eight-pointed star
5: Triangle 16: Nine-pointed star
6: Diamond 17: Ten-pointed star
7: Pentagon 18: 11-pointed star
8: Hexagon 19: 12-pointed star
9: Septagon 20: 13-pointed star
10: Octagon 21: 14-pointed star
11: Nine-gon 22: Plus with gap
24: Cross with gap
Type 23 (vertical line) is not allowed since a vertical
line is drawn for an error bar. 0 -> 3.
FACTOR......Scale the plot symbol by FACTOR; 0 -> 1.
DOTV........> 0 => plot directly on the TV device, otherwise make a
plot file for later display on one or more devices
(including the TV if desired).
GRCHAN......Graphics channel (1 - 7) to use for line drawing.
0 => multiple.
GAL : Task which analyses a velocity field. In the present
implementation it is assumed that the object is a rotating disk.
The user has to specify initial guesses to the parameters to be
fitted to the velocity field, using the adverb APARM. GAL will
then determine the best least squares' fit to the input velocity
field, and give the final best fit values of the parameters. Of
course a good initial guess will speed up the procedure, whereas
a very bad one will cause divergence and failure.
Some remarks on specific adverbs :
Specifies a map of which the pixel values are weights to be applied to
the velocities in the velocity field map INNAME. It is up to the user
to supply such a map. Several possibilities are open: one can use the
total HI map, or the square of the total HI map (using COMB). The
latter choice is the correct one if the noise in the velocity maps can
be assumed Gaussian with respect to the intensity map which normally
is a reasonable assumption. Other possibilities are the width of the
profile, the skewness of the profile, or - if the velocity field was
determined using XGAUS - the goodness of fit.
BLC , TRC
The velocity field is read in as a whole, which imposes an upper limit
to the area that may be used. A 1024*1024 velocity should be handled
easily. When your image is too big, make sure you are not oversampling
too much; more than two points per beam is unnecessary.
The value of SOLCON determines when to stop the iteration, the default
0.001 should be adequate for most purposes.
Four schematic rotation curves are available now. The Brandt curve
(FUNCTYPE = 'BR') attains a maximum value Vmax at R = Rmax, and
declines Keplerian thereafter. The exponential curve (FUNCTYPE =
'EX') is flat throughout after an initially linear increasing part.
The factor ln(100) in the exponent was chosen to give analogous
meanings to Rmax for the two representations, in fact Rmax is the
radius where Vmax attains 99 percent of its maximum value. Still, the fitted
value of Rmax may well differ depending on the value of FUNCTYPE.
FUNCTYPE='CC' fits a one parameter rotation curve, V = constant. This
is a good approximation in many cases, but a minimum radius DPARM(1)
should be specified since the approximation breaks down at low radii.
It is particularly useful in applying GAL in concentric annuli. GAL
then should be used in a loop with varying DPARM(1) and DPARM(2)
fitting a constant velocity in each ring. FUNCTYPE='SB' fits a solid
body rotation curve. In this case the theoretical velocity field has
an infinite number of axes of symmetry, and the central position
becomes undefined. Furthermore, the steepness and the inclination are
directly related, so one of both (the inclination) is held fixed at
its initial value. Vmax is defined as the circular velocity at a 60"
radius. GAL keeps x,y, and i fixed, although of course you may add
other fixed parameters by means of CPARM(3).
The guess to the central position has to be given in pixels; the
resulting fitted value is also displayed in RA and Dec.
The position angle is defined measured form north eastward, to the
receding part of the major axis (having the largest velocities). An
initial guess which is 180 degrees off is bound to cause divergence.
The inclination angle can have values from 0 (face on) to 90 (edge
on). Don't use values close to 0 or 90 as initial guesses.
A fair initial guess is the mean of the velocities on opposite ends of
the major axis, or values around the minor axis. Specify velocities
in km/s. Note that the units in the map should be in m/s.
The height of the rotation curve can be guessed by halving the
difference of two points on opposite ends of the major axis, and
applying an cosec(i) correction.
The extent of the rising part may be hard to guess, but usually the
program converges if the initial guess is less off than a factor 2 or
This is a measure of the flatness of the curve, if FUNCTYPE = 'BR'.
Very flat curves have APARM(8) well below unity, while curves with a
clear decline in the outer parts have APARM(8) values like 1.5, 2.0,
or even 3.0. If uncertain, start with APARM(8) = 1. Avoid
APARM(8) < 0.1 or > 4.
A successful convergence in the 'BR' case depends critically on
reasonable choices for both APARM(7) and APARM(8).
controls the residual field. If CPARM(1) = 1, 2, or 3, a residual
output map is made. Such a map can be extremely useful to assess
the quality of the fit, and to detect the presence of asymmetries.
In the case CPARM(1)=1, the whole velocity field is used to
generate the residual map, irrespective of the values in DPARM(1)
through DPARM(6). When CPARM(1)=2, only the part of the velocity
field specified in DPARM(1) => DPARM(6) is used. When CPARM(1)=3,
no fitting is done, and the residual field is determined using a
user supplied rotation curve in the textfile INTEXT.
If > 0, the observed rotation curve is plotted (by integrating the
circular velocities in annuli), as well as the fitted model curve.
is a bit map of the parameters to be held fixed. If one wants
to fix the position ( both x and y), and Rmax, specify CPARM(3) =
2**0 + 2**1 + 2**6 = 67. CPARM(3)=0 means NO fixed parameters.
Note that some parameters may be fixed by the program anyway
(x,y, and i if FUNCTYPE='SB').
If <=0, don't use the image in IN2C as weighting image; if >0, use the
IN2C image (if specified).
If <=0, don't copy the old history file; if >0: copy it. The reason
for this is that in the case of very large history files a substantial
fraction of the execution time of GAL is spent in copying the file.
Only important when OUTTEXT is specified. A one liner of information
about the fit obtained is appended to OUTTEXT. If CPARM(6)=0, this
one line contains the values of all fitted parameters. If CPARM(6)
= 1/-1, the one line of output contains the position (RA, Dec) of
the major axis of the ring under consideration. When +1, GAL will
write the position of the receding major axis, and when -1, of the
approaching major axis. When using GAL many times in rings, the
output file contains a range of positions which trace the major axis
of the galaxy. This file can than be turned in a ST extension file,
and the major axis can be plotted on top of other maps. Use only
these 3 values - the exact value of CPARM(6) is used.
If CPARM(7) > 0, no fitting is done, only a plot is generated, based
on the input values in APARM.
The parameters DPARM(1) through DPARM(6) allow the user to spe-
cify a wide variety of subsections of the plane of the galaxy.
DPARM(1) and DPARM(2) specify the minimum and maximum radius to
be used, e.g. to select an annulus, in which case FUNCTYPE='CC'
or 'SB' would be advisable. DPARM(3) and DPARM(4) specify li-
mits on the cosine of the azimuth angle in the galaxy (as pro-
jected on the sky), e.g. 0,1 selects the receding half of the
galaxy. DPARM(5) and DPARM(6) specify the sine of this angle,
e.g. 0,1 selects the half plane traversed when the receding
major axis is rotated in an anticlockwise direction to the ap-
proaching major axis.
DPARM(7) specifies the maximum radius in the plot (",
0=> auto scaling).
DPARM(8) specifies the maximum rotation velocity to be plotted (km/s,
DPARM(9) specifies the minimum rotation velocity to be plotted (km/s,
DPARM(10) specifies the width of the annuli used in plotting the
actual rotation curve, approximately in units of a pixel width (if
DX=DY : exactly one pixel width, DX>>DY : DY * SQRT(2), DX<SQRT(2)). A value of 0 defaults to 3.
This is a text file containing rotation curve information from
concentric rings. This file is used when CPARM(1) = 3. No fitting is
done, but a residual map is produced based on the contents of INTEXT.
Nine columns are used - inner and outer ring radius, X center pixel, Y
center pixel, position angle of the major axis (CCW from N),
inclination, systemic velocity, maximum velocity, radius of maximum
velocity and Brandt index. You may have more than 10 columns with
columns 11 and beyond ignored. A sample INTEXT:
0.0 60.0 308.5 333.4 -60.2 72.0 30.6 123.8 145.3 0.000 0.0 2.3
60.0 120.0 307.2 332.6 -59.6 72.6 24.9 114.4 153.2 0.000 0.0 2.2
120.0 180.0 309.0 333.5 -59.7 72.7 27.5 110.8 160.7 0.000 0.0 3.1
180.0 240.0 309.3 332.9 -60.4 73.2 27.8 112.0 157.3 0.000 0.0 2.6
240.0 300.0 308.7 332.8 -59.6 73.6 27.9 111.8 157.9 0.000 0.0 2.6
300.0 360.0 308.7 332.8 -59.6 73.6 27.9 111.8 158.3 0.000 -0.1 15.4
360.0 420.0 308.7 332.8 -59.6 73.6 27.9 111.8 150.2 0.000 0.2 19.4
420.0 480.0 308.7 332.8 -59.6 73.6 27.9 111.8 148.6 0.000 -0.8 18.8
480.0 540.0 308.7 332.8 -59.6 73.6 27.9 111.8 144.3 0.000 -0.9 53.0
540.0 600.0 308.7 332.8 -59.6 73.6 27.9 111.8 139.8 0.000 -1.4 65.5
If the radius of the maximum velocity is <= 0.0, 1/3 of the sum of the
maximum and minimum radius in INTEXT will be used. Otherwise a zero
This INTEXT defines 10 rings in intervals of 60". The parameter
values are given in the next 6 columns: center pixel x and y, major
axis position angle (CCW from N), inclination, systemic velocity,
maximum velocity, and radius of maximum velocity with average residual
and standard deviation in km/s ignored. Note that the fit is quite
good for the first 5 rings and then deteriorates badly. Such a file
typically is the result of several GAL runs on annuli.
Output results are written to this file, if specified. If CPARM(6)=0,
output parameters are the radii of the ring used, plus the fitted
parameters. To this are added the average difference between the
fitted model and the velocity field (should be close to zero), and the
standard deviation (should be close to the rms in the ring). If
CPARM(6)=+/-1, the location of the major axis is written (see CPARM(6)
PIXSTD is used to calculate the errors associated with the final
results. The default is the velocity dispersion usually observed in
unperturbed spirals, but the value may well be higher as a result of
larger random motions and filter width.
The curve types 'BR', and 'EX' should only be used on the whole
field, and in the initial stage of determining the variables.
Real-world rotation curves deviate from mathematical models. In
the more advanced stages of velocity field analysis, it is
suggested to use 'CC' only, on annuli determined by DPARM(1)
and DPARM(2). The width of these annuli should be a trade-off
between a minimum number of points in the ring to make a good
fit, and a maximum width over which the rotation velocity may
be assumed constant. The whole rotation curve can then be
fitted by a command like:
for i=1:10;dparm(1)=i*30;dparm(2)=(i+1)*30;go gal;wait gal;end
A residual of this fit can be obtained by putting the fitted
values in the textfile INTEXT, and running GAL once more with
CPARM(1)=3. Note that in the above example the very central
portion of the velocity field is not used.