227 research outputs found
Optimal Techniques in Two-dimensional Spectroscopy: Background Subtraction for the 21st Century
In two-dimensional spectrographs, the optical distortions in the spatial and
dispersion directions produce variations in the sub-pixel sampling of the
background spectrum. Using knowledge of the camera distortions and the
curvature of the spectral features, one can recover information regarding the
background spectrum on wavelength scales much smaller than a pixel. As a
result, one can propagate this better-sampled background spectrum through
inverses of the distortion and rectification transformations, and accurately
model the background spectrum in two-dimensional spectra for which the
distortions have not been removed (i.e. the data have not been
rebinned/rectified). The procedure, as outlined in this paper, is extremely
insensitive to cosmic rays, hot pixels, etc. Because of this insensitivity to
discrepant pixels, sky modeling and subtraction need not be performed as one of
the later steps in a reduction pipeline. Sky-subtraction can now be performed
as one of the earliest tasks, perhaps just after dividing by a flat-field.
Because subtraction of the background can be performed without having to
``clean'' cosmic rays, such bad pixel values can be trivially identified after
removal of the two-dimensional sky background.Comment: 26 pages, 15 figures, accepted for publication in PASP, Figures with
full resolution available at http://www.ociw.edu/~kelso
The Distribution of Active Galactic Nuclei in Clusters of Galaxies
We present a study of the distribution of AGNs in clusters of galaxies with a uniformly selected, spectroscopically complete sample of 35 AGNs in eight clusters of galaxies at z = 0.06 → 0.31. We find that the 12 AGNs with L_X > 10^42 ergs s^-1 in cluster members more luminous than a rest-frame M_+R 10^41 ergs s^-1 do not show evidence for greater central concentration than inactive cluster members, nor evidence for a different kinematic or substructure distribution. As we do see clear differences in the spatial and kinematic distributions of the blue Butcher-Oemler and red cluster galaxy populations, any difference in the AGN and inactive galaxy population must be less distinct than that between these two pairs of populations. Comparison of the AGN fraction selected via X-ray emission in this study to similarly selected AGNs in the field indicates that the AGN fraction is not significantly lower in clusters, contrary to AGNs identified via visible wavelength emission lines, but similar to the approximately constant radio-selected AGN fraction in clusters and the field. We also find significant evidence for variation in the AGN fraction between clusters and explore the dependence of cluster AGN fraction on redshift, velocity dispersion, amount of cluster substructure, and fraction of Butcher-Oemler galaxies. While we see weak evidence for several trends, there are several correlations between these four parameters in our small sample of eight clusters that preclude identification of which one(s) most strongly influence the cluster AGN fraction
- …