; MODVF ;--------------------------------------------------------------- ;! task to create a warped velocity field ;# Task Imaging Modeling ;----------------------------------------------------------------------- ;; Copyright (C) 1995, 2009 ;; Associated Universities, Inc. Washington DC, USA. ;; ;; This program is free software; you can redistribute it and/or ;; modify it under the terms of the GNU General Public License as ;; published by the Free Software Foundation; either version 2 of ;; the License, or (at your option) any later version. ;; ;; This program is distributed in the hope that it will be useful, ;; but WITHOUT ANY WARRANTY; without even the implied warranty of ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the ;; GNU General Public License for more details. ;; ;; You should have received a copy of the GNU General Public ;; License along with this program; if not, write to the Free ;; Software Foundation, Inc., 675 Massachusetts Ave, Cambridge, ;; MA 02139, USA. ;; ;; Correspondence concerning AIPS should be addressed as follows: ;; Internet email: aipsmail@nrao.edu. ;; Postal address: AIPS Project Office ;; National Radio Astronomy Observatory ;; 520 Edgemont Road ;; Charlottesville, VA 22903-2475 USA ;----------------------------------------------------------------------- MODVF LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC MODVF: Task to create a warped velocity field INFILE Input file name (Non-AIPS) OBJECT Source name IMSIZE Output image size (cells) CELLSIZE Cellsize sec. OUTNAME Output image name (name) OUTCLASS Output image name (class) OUTSEQ -1.0 9999.0 Output image name (seq. #) OUTDISK 0.0 9.0 Output image disk unit #. APARM User supplied array XINC Width of rings in arcsec. ---------------------------------------------------------------- MODVF Adverbs: INFILE.....A file external to the AIPS system from which the rotation curve and warping data is read. OBJECT.....Object name. IMSIZE.....Desired image size in cells. CELLSIZE...Desired cell spacing in arcsec. OUTNAME....Output image name (name). Standard behavior with default = 'MODVF MAP'. OUTCLASS...Output image name (class). Standard defaults. OUTSEQ.....Output image name (seq. #). 0 => highest unique. OUTDISK....Disk drive # of output image. 0 => highest number with sufficient space. APARM......(1),(2): kinematical centre in pixels; (3),(4): position angle major axis and inclination of reference disk. (5) Systemic velocity XINC.......Width of each ring in arcsec ---------------------------------------------------------------- MODVF: Task which creates a warped velocity field. DOCUMENTOR: Gustaaf van Moorsel, NRAO. RELATED PROGRAMS: GAL PURPOSE MODVF creates a modeled galaxy velocity field based on information in APARM and in the external file specified by INFILE. In APARM the user specifies basic parameters of the reference disk, i.e. the disk of radius R within which no warping takes place. INFILE contains the necessary information about the rotation curve and possible warping beyond R. In contrast to earlier modeling tasks, the warp is not specified by variations in position angle and inclination. Though these parameters are easily observable, they are very specific to the position of the observer with respect to the galaxy. Instead, MODVF uses two angles which are more meaningful: 1) the warp angle, which is the angle between the angular momentum vector and the reference disk, and 2) the twisting angle of the warp. For a more extensive discussion and explanation see Christodolou et al. (1993). MODVF divides the actual warped galaxy in cells, the size of which are specified by XINC. Each of these cells are projected on the plane of the sky as seen by the observer. Two things may go wrong using this approach: 1 - certain pixels do not receive a value; there is no cell in the galaxy which projects on such a pixel. The remedy is to choose XINC sufficiently small: one-half the pixel size is recommended, but in strongly warped systems XINC may have to be chosen even smaller, and MODVF takes longer to execute. 2 - certain pixels receive more than one value; several cells in the galaxy end up projected on the same pixel. In this case MODVF will average the contributions of these cells. This is OK if the cells are adjacent in the galaxy as well. Things go wrong if cells from quite different locations in the galaxy end up in the same pixel. This can happen whan the warp is so strong that the galaxy 'folds' and we see several portions of the disk in the line of sight. Averaging then is clearly the wrong thing to do, and the user should always be aware of this danger. A closer look at some of the adverbs: INFILE INFILE contains all the information about the warp and the rotation curve. Each line in INFILE describes one ring in the galaxy. The first item on the line is the radius in arcsec, the second item the velocity in km/s, the third item the warp angle in degrees, and the fourth item the twisting angle in degrees. A simplified INFILE could look like this: 20 132 0 0 40 152 0 0 60 172 0 0 80 192 0 0 100 212 0 0 120 212 5 0 140 212 10 5 180 212 15 10 Blank lines and those beginning with $ or # are treated as comment lines. The rotation curve becomes flat at radius 100". At radius 120", the ring tilts 5" out of the plane of the reference disk (which is the disk up to 100"), and in subsequent rings it tilts further by the same amount. In ring 140", the azimuth of the warp angle as measured in the reference disk is displaced by 5 degrees with respect to the previous ring. At 160" it is twisted by yet another 5 degrees for a total of 10 degrees. Clearly INFILE can be adapted in a way to reflect any warp model, e.g. linear increases in the two angles beyond a certain radius (as in the above example). IMSIZE, CELLSIZE As in many other tasks, IMSIZE and CELLSIZE define the size of the output image in pixels, and the pixel increment in arcsec. APARM APARM is used to 'place' the reference disk in space. APARM(1) and (2) define the pixel coordinates of the kinematical center, (3) and (4) define the position angle of the major axis and the inclination of the galactic plane, and (5) specifies the velocity in km/s. Note that these definitions agree with the ones in GAL. XINC XINC specifies how the galaxy is divided up in cells. The galaxy is divided in many rings of width XINC arcsec (note: these rings are not the same as the ones specified in INFILE), and every ring is divided up in small segments which are as 'square' as possible. A too coarse XINC leads to 'blank' or 'black' spots in the output image. A too fine XINC does no harm apart from increasing the execution time. Reference: Christodolou, D.M., Tohline, J.E., and Steiman-Cameron, T.Y., "Modeling of Warps in HI disks", 1993, Ap.J. 416, 74.