Fitting Redshifted Lines in a Spectrum in EMSAO
For each object spectrum file from the input list
spectra, and/or each
aperture specified in the aperture list
specnum, the fitting
subroutine, EMFIT, is called for the spectrum in the multispec band
specbandspecdir. Full or
relative pathnames may be used in spectra, and specdir may
Sky Spectrum for Noise Calculations (EMFIT)
If skynum is not zero, a sky spectrum
is read from that multispec aperture in the same file as
If skyband is not zero, a sky spectrum
is read from that multispec band for the aperture
The sky spectrum, use to get the noise for error computations, is plotted if
obj_plot is yes.
The spectrum is loaded--and renormalized, if
renormalize is set to
If fixbad is yes,
regions specified in the file named by
badlines are replaced by straight
The spectrum is then smoothed
nsmooth times. If
obj_plot is yes,
the spectrum is plotted and the plot is kept on the screen available for
a q is typed.
A continuum computed by the IRAF interactive curve fitting subroutine driven
by the parameters set in contpars
is subtracted from the spectrum.
If contsub_plot is yes,
the spectrum is plotted with the continuum removed.
An initial redshift source may be specified by
vel_init; if guess,
this is from czguess. If
search, one line in the spectrum is identified using the table
specified by emsearch, which
lists line centers in Angstroms and the wavelength range over which each
one should be the strongest line. This table can be modified by the user
to match a given dataset.
If combination, the initial redshift velocity is read from the
spectrum header parameter VELOCITY; if correlation,it is
read from CZXC; and ifemission) from CZEM.
If (linefit is no,
all of the fitting below is skipped and the results are displayed.
Find Initial Redshift (EMGUESS)
If vel_init is search,
EMFIND is used over each region in the search list
specified by emsearch. The
brightest line in each region is assumed to be the one in the table, and its
observed wavelength is saved. The redshift of the brightest of those lines
is returned as an initial value to be refined by looking at more lines.
Find Bright Emission Lines (EMFIND)
All emission lines within a specified wavelength region are found. A
spectrum pixel is assumed to be a line center if the pixel value is the
max of the npfit neighbors on either
side, and greater than linesig times
the square root of the average counts in those pixels. A second order
fit is then made to the (2*npfit)+1
points centered on the peak to refine the center and peak height.
Search regions in the list specified by
emlines are shifted by the
redshift specified above and expanded in each direction by
Find Emission Lines (EMFIND)
All emission lines within a specified wavelength region are found using the
criteria specified above.
Keep Brightest Emission Lines (EMSRCH)
The brightest line in each region is kept unless it has already been
identified. Order matters--the brightest line in a region should be
listed first, so that if it is the only one present in overlapping regions,
it is correctly named.
A copy of the spectrum is smoothed
should be left at zero unless the data is especially noisy. It is best never
to go above 2.
Remove Continuum for Line Fit (ICSUBCON)
The continuum is removed from the newly smoothed copy of the spectrum
using the same parameters as above.
Check each identified line to see if it is part of one of the groups of close
emission lines listed in the file specified by
If it is, all lines in the combination will be simultaneously fit.
Those members of a line combination which are not found are initialized
at the redshift of the most recently found line of the group with a height
proportional to the most recently found line according to the relative heights
in the emcombine file.
The lines are fit by one to three Gaussians, along with an optional local
continuum level using none to three additional coefficients as set by
Redshift (computed from the wavelength of the center pixel coordinate),
width, height, and errors are returned for each line.
Each emission line is checked to see whether:
The number of the first test a line fails is given after an X
in report_mode=1 output. If a Gaussian has been fitted (i.e., not in 1
and 7), however, all of these rejection criteria can be overridden
interactively using the + and - commands in cursor
mode from the summary display, if
curmode = 1,
If report_mode = 1 and
verbose is yes, or
wavelengths and redshifts are printed for each line, with an X
followed by the rejection code at the end of entries which were omitted
from the fit. If a line has been added or subtracted in cursor mode, a
+ or - at the end of the entry indicates that fact.
If dispmode = 1, the same information
is printed on the summary display.
A mean emission line velocity is computed, weighted by the square of the
error in the line centers and returned.
- 1) it has a fit center greater than zero,
- 2) it is wider than lwmin
times the mean width of all of the lines which are found and
- 3) narrower than lwmax times
that mean width,
- 4) it has an equivalent width more than
lsmin times the error in the
- 5) it is too close to the blue edge of the spectrum,
- 6) it is too close to the red edge of the spectrum, or
- 7) the error in the center of the Gaussian is zero (the fit failed).
Adjust Combined Velocity and Error (EMFIT)
After all of the lines have been fit and a combined velocity has been computed,
A zero-point redshift is computed by adding the solar system barycentric
velocity correction, from a source specified by
If only a single line is found, the error is set to
sigline, which should be set to
the uncertainty in the dispersion function in Angstroms. This is usually
significantly greater that the error in the fit to the center of the Gaussian.
Combine Emission and Cross-Correlation Velocities (VCOMBINE)
If there is only an emission line velocity for this spectrum, it becomes the
final velocity, and the final velocity error is based on the emission error.
If there is only a cross-correlation velocity, that becomes the final
velocity and the final velocity error is based on the correlation error.
If both velocities are present and their difference squared divided by
the sum of the squares of their errors is less than 8, the error is the
two errors added in quadrature and the final velocity is the error-weighed
sum of the velocities. If they differ and the correlation R-value is
greater than 10 or it is greater than 4 and there are only one or two
emission lines found, that velocity is used. Otherwise, the emission line
velocity is used.
In all cases, an assumed dispersion error of 15 km/sec added in quadrature
to the final error value.
The results are displayed as text to the devices specified by
logfiles in the format
report_mode. Options include
1) one line per found emission line under a self-documenting summary,
2) a single line report listing the lines which are found, and
3) a single line report listing a velocity for each searched-for line.
If displot is yes,
the spectrum is plotted to
device in the format specified by
hardcopy is yes,
the same graph is sent to
If nsmooth greater than zero,
the displayed spectrum is smoothed by a 1-2-1 sliding filter
that many times.
Absorption lines listed in the file
are labelled if
Emission lines listed in the file
Both files are found in the directory
If curmode is yes,
the user can interact with the display using the terminal cursor to
zoom in on portions of the spectrum, identify lines and refit the emission
lines, change the display format, edit the spectrum, or several other
If save_vel is yes,
emission line redshift results are written into the spectrum image
header in a form appropriate to the spectrum format: two entries plus
one per line if multispec; otherwise one, value per keyword.
[emsao Flow Diagram]
Last updated 13 December 2007 by