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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
EMSAO: Radial Velocities from Emission Lines in Spectra
D. J. Mink, W. F. Wyatt
Harvard­Smithsonian Center for Astrophysics, 60 Garden St.,
Cambridge, MA 02138
Abstract. Many extragalactic objects for which radial velocities are
desired display strong emission lines. For large surveys, the interactive
determination of line centers and calculation of redshifts is too slow. The
EMSAO task of the RVSAO IRAF package automatically finds emission
lines, fits them with single or multiple Gaussians, and combines the de­
termined redshifts weighting them by the uncertainty in the line centers.
In addition to being used in the CfA and other redshift surveys, EM­
SAO has been used to study sky line shifts in 15 years of observations to
characterize the uncertainty in instrumental measurements.
1. Introduction
When SAO moved its redshift program from minicomputers to workstations, it
was decided that NOAO IRAF, a standard reduction system, be used, rather
than the previous custom Forth system (Tonry & Wyatt 1988). At the time, no
specialized radial velocity software existed inside IRAF, so the RVSAO package
was developed. While the redshifts of many spectra can be computed by cross­
correlating them against templates, that process is more complicated for spectra
exhibiting strong emission lines. For large surveys, the interactive determination
of line centers and calculation of redshifts by a program such as IRAF's SPLOT,
is simply too slow. EMSAO, a companion to the cross­correlation task XCSAO
(Kurtz et al. 1992), was written to find emission lines automatically, compute
redshifts for each identified line, and combine them into a single radial velocity.
The results may be graphically displayed or printed. The graphic cursor may
be used to change fit and display parameters.
2. How EMSAO Works
EMSAO takes a list of 1­dimensional wavelength­calibrated spectra in either
IRAF one­dimensional or MULTISPEC format. Dispersion information is read
from the world coordinate system keywords in the image headers. After reading
the spectrum, an initial velocity guess is used to search for lines from an input
line list. That velocity guess may come from an input parameter, a velocity
in the image header, or an identification of one line in the spectrum. This last
method uses a table of lines 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. The search for the rest of the emission lines is carried out in a similar
1

2
Figure 1. This EMSAO Summary page shows line centers and fit
information for a night sky spectrum.
way, driven by a different table with smaller wavelength tolerances. After the
lines are identified, Gaussian profiles are fit to more exactly determine the line
centers. A third table identifies combined lines, such as the N1--H--N2 triplet at
6548 š A, 6563 š A, and 6584 š A, and multiple Gaussians are fit simultaneously.
After redshift velocities are determined for each of the lines, an error­
weighted mean is computed, omitting the velocities from those lines whose fits
do not meet certain criteria. If good cross­correlation and emission line velocities
exist for a given object, a combined velocity is also computed. The results may
be graphically displayed with emission and/or absorption lines (from a fourth
table) labeled as in Figure 1, logged in any of several formats to a file or text
terminal, and/or written into the spectrum's header. Figure 2 shows the most
verbose tabulation of EMSAO results, with wavelength center and velocity infor­
mation for each line. If the results are displayed graphically, individual identified
lines may be added to or subtracted from the emission line velocity, the spectrum
may be edited, and several other conditions of the fit may be changed.
3. Testing
To test EMSAO adequately, a lot of emission line spectra were needed. The
Smithsonian Astrophysical Observatory has archived 27,000 galaxy and stellar
spectra from its Z­Machine spectrograph. Since a sky spectrum is filed with
each object spectrum, there exist 27,000 easily­available spectra of night sky
emission lines. As these mercury, oxygen, and sodium lines are airglow lines

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rvsao.emsao 1.5 NOAO/IRAF V2.10.3BETA mink@cfa165 Mon 18:09:02 28­Nov­94
12352 Object: N5548 RA: 14:15:43.80 Dec: 25:21:59.0
Observed 13­Feb­1986 13:09:07.00 = JD 2446475.0480 BCV: 0.00
Combined vel = 145.01 +­ 15.66 km/sec, z= 0.0005
Correlation vel = 145.01 +­ 4.50 km/sec, z= 0.0005 R= 40.1
Emission vel = 121.17 +­ 0.96 km/sec, z= 0.0004 for 3/5 lines
Line Rest lam Obs. lam Pixel z vel dvel eqw wt
Hg 5460.74 5460.57 813.09 ­0.000 ­9.06 8.12 2.86 0.000 X
OI 5577.35 5579.64 916.48 0.0004 123.09 0.84 66.60 0.936
Na 5890.40 5893.10 1178.47 0.0005 137.33 5.09 21.10 0.025
OI 6300.23 6301.59 1499.71 0.0002 64.70 4.09 9.93 0.039
OI 6363.88 6365.48 1548.07 0.0003 75.30 10.39 5.01 0.000 X
Figure 2. This is a tabulation, with report mode=1, of the EMSAO
results displayed graphically in Figure 1.
00001 2443575.69726 21 0 2804 44 3.5 2804 44 E 1 1 21.38 0.00 0.00
00002 2445219.24201 13 0 1828 34 4.8 1828 34 E 1 1 13.27 0.00 0.00
00003 2443575.79326 ­37 0 0 0 0.0 0 0 E 1 1 ­37.00 0.00 0.00
. . .
27169 2449342.03237 ­2 1 35477 62 2.1 ­2 62 E 3 3 4.85 ­17.34 ­32.84
27170 2449336.76664 ­28 0 4069 45 2.9 ­28 45 E 3 3 ­27.92 ­28.44 ­38.31
27171 2449300.78227 ­17 0 5465 26 6.2 ­17 26 E 3 3 ­9.00 ­30.71 ­41.82
Figure 3. A portion of the tabulated results for 27,171 night sky
spectra produced by EMSAO with report mode=3.
from the upper atmosphere or emission from artificial sources such as street
lights, they should exhibit a negligible Doppler line shift.
An IRAF program was already being used to convert Z­Machine spectra
from SAO's internal archive format to IRAF .imh and .pix files (Mink & Wyatt
1992). A substitute line list was set up with all of the bright night sky lines. Since
the mercury and sodium line strengths varied significantly, they were dropped in
favor of the more stable oxygen lines. The 6300 š A [O i] line can show significant
variations in intensity within a single night, but that is a story for another paper.
The screen output of EMSAO for an individual night sky spectrum is shown in
Figure 1. An IRAF CL script was written to run through the entire archive, or
portions thereof, with a single command.
The results were tabulated in a file shown in Figure 3, where the columns
are reduced file number, Julian Date of observation, emission line velocity and
error, cross­correlation velocity (from a galaxy template) with error and R­value,
combination velocity and error, quality flag, number of lines found, number of
lines fit, and velocities in km s \Gamma1 for [O i] lines at 5577.35 š A, 6300.23 š A, and
6363.88 š A.
Due to the distribution of lines in the calibration lamp spectrum, the po­
sition of the [O i] line at 6300 š A was most certain. Figure 4 shows the velocity
shift computed from the change in the position of the center of that emission
line over the 15­year lifetime of the Z­Machine spectrograph on the 1.5 m Till­

4
Figure 4. Velocity shift in 6300 š A Oi night sky line over 15 years as
observed from Mt. Hopkins, Arizona.
inghast Reflector. Each vertical grouping is one month's dark­time run, with
larger gaps usually indicating summer telescope shutdown. It is obvious that
some runs had a large scatter in sky ``velocities,'' but for the most part, the
sky ``velocity'' distribution is within the 63 km s \Gamma1 that a one­pixel shift in the
emission line would cause.
4. Access to Software
RVSAO can be obtained via anonymous FTP from cfa­ftp.harvard.edu in the
pub/iraf directory. On­line documentation with examples is available on the
World Wide Web at the RVSAO home page 1 .
References
Kurtz, M. J., Mink, D. J., Wyatt, W. F., Fabricant, D. G., Torres, G., Kriss,
G., & Tonry, J. L. 1992, in Astronomical Data Analysis Software and
Systems I, ASP Conf. Ser., Vol. 25, eds. D. M. Worrall, C. Biemesderfer,
& J. Barnes (San Francisco, ASP), p. 432
Mink, D. J., & Wyatt, W. F. 1992, in Astronomical Data Analysis Software and
Systems I, ASP Conf. Ser., Vol. 25, eds. D. M. Worrall, C. Biemesderfer,
& J. Barnes (San Francisco, ASP), p. 439
1 http://tdc­www.harvard.edu/iraf/rvsao/rvsao.html

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Tonry, J. L., & Wyatt, W. F. 1988, CFA Z­Machine Data Analysis Software
(Cambridge, Smithsonian Astrophysical Observatory)