81 research outputs found
The environments of Markarian galaxies
The extensively studied Markarian sample of 1500 ultraviolet excess galaxies contains many Seyfert, starburst, and peculiar galaxies. Using the 20 minute V plates obtained for the construction of the Hubble Space Telescope Guide Star Catalog, the authors investigated the morphologies of the Markarian galaxies and the environments in which they are located. The relationship between the types of nuclear activity and the morphologies and environments of the Markarian galaxies is discussed. The authors conclude that the type of nuclear activity present in the galaxies of the Markarian sample is not dependent on either the morphology or the local environment of the galaxy. This is not to imply that nuclear activity per se is not influenced by the environment in which the nucleus is located. Rather the type of nuclear activity (at least in the Markarian population) does not appear to be determined by the environment
Special Section Guest Editorial: Detectors for Astronomy and Cosmology
This guest editorial summarizes the Special Section on Detectors for Astronomy and Cosmology
The young stellar population of NGC 4214 as observed with HST. I. Data and methods
We present the data and methods that we have used to perform a detailed
UV-optical study of the nearby dwarf starburst galaxy NGC 4214 using
multifilter HST/WFPC2+STIS photometry. We explain the process followed to
obtain high-quality photometry and astrometry of the stellar and cluster
populations of this galaxy. We describe the procedure used to transform
magnitudes and colors into physical parameters using spectral energy
distributions. The data show the existence of both young and old stellar
populations that can be resolved at the distance of NGC 4214 (2.94 Mpc) and we
perform a general description of those populations.Comment: 33 pages, 9 figures, and 8 table
Diamond Machining of an Off-Axis Biconic Aspherical Mirror
Two diamond-machining methods have been developed as part of an effort to design and fabricate an off-axis, biconic ellipsoidal, concave aluminum mirror for an infrared spectrometer at the Kitt Peak National Observatory. Beyond this initial application, the methods can be expected to enable satisfaction of requirements for future instrument mirrors having increasingly complex (including asymmetrical), precise shapes that, heretofore, could not readily be fabricated by diamond machining or, in some cases, could not be fabricated at all. In the initial application, the mirror is prescribed, in terms of Cartesian coordinates x and y, by aperture dimensions of 94 by 76 mm, placements of -2 mm off axis in x and 227 mm off axis in y, an x radius of curvature of 377 mm, a y radius of curvature of 407 mm, an x conic constant of 0.078, and a y conic constant of 0.127. The aspect ratio of the mirror blank is about 6. One common, "diamond machining" process uses single-point diamond turning (SPDT). However, it is impossible to generate the required off-axis, biconic ellipsoidal shape by conventional SPDT because (1) rotational symmetry is an essential element of conventional SPDT and (2) the present off-axis biconic mirror shape lacks rotational symmetry. Following conventional practice, it would be necessary to make this mirror from a glass blank by computer-controlled polishing, which costs more than diamond machining and yields a mirror that is more difficult to mount to a metal bench. One of the two present diamond machining methods involves the use of an SPDT machine equipped with a fast tool servo (FTS). The SPDT machine is programmed to follow the rotationally symmetric asphere that best fits the desired off-axis, biconic ellipsoidal surface. The FTS is actuated in synchronism with the rotation of the SPDT machine to generate the difference between the desired surface and the best-fit rotationally symmetric asphere. In order to minimize the required stroke of the FTS, the blanks were positioned at a large off-axis distance and angle, and the axis of the FTS was not parallel to the axis of the spindle of the SPDT machine. The spindle was rotated at a speed of 120 rpm, and the maximum FTS speed was 8.2 mm/s
Special Section Guest Editorial: Detectors for Astronomy and Cosmology
This guest editorial summarizes the Special Section on Detectors for Astronomy and Cosmology
Tip of the Red Giant Branch Distances to NGC 4214, UGC 685, and UGC 5456
We have used WFPC2 VRI observations to calculate the distances to three
nearby galaxies, NGC 4214, UGC 685, and UGC 5456 using the tip of the red giant
branch method. Our values for NGC 4214 (2.94 +/- 0.18 Mpc) and UGC 685 (4.79
+/- 0.30 Mpc) are the most precise measurementes of the distances to these
objects ever made. For UGC 5456 the data do not allow us to reach a decisive
conclusion since there are two possible solutions, one leading towards a short
distance around 3.8 Mpc and another one towards a long distance of 5.6 Mpc or
more.Comment: To appear in the March 2002 issue of the Astronomical Journal. 19
pages, including 4 tables and 8 figures. Due to the size limitations of the
astro-ph service, a reduced resolution version of figures 1-3 is provided
here. For a full resolution version, please go to http://www.stsci.edu/~jmai
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