Engineering topics

Schneider, Donald P.

Publications and source records attributed to Schneider, Donald P..

51 records · Page 3

NASA NTRS1990

A technique for detecting massive collapsed objects in the dark halo of the Galaxy

If the Galaxy possesses a dark halo of massive objects, they will produce small but potentially detectable distortions in the images of distant, extended objects due to gravitational lensing effects. An illustrative analytic and a more realistic numerical model are presented for calculating the number and angular sizes of these distortions, and a specific example of a digital filter is described which could be used for the identification of such distortions. The sensitivity of possible searches for the effect in the image of M31 and the Galactic Center are discussed in some detail. Simulated images of distortions of various angular sizes in these fields as they would be expected to appear in ground-based and Space Telescope CCD frames are presented. A Galactic halo of black holes such as that proposed by Lacey and Ostriker might well be detectable ina practically realizable experiment.

Turner, Edwin L.↗

NASA NTRS1990

What the longest exposures from the Hubble Space Telescope will reveal

NASA's Hubble Space Telescope (HST) is expected to reveal the sizes, shapes, and contents of faint, distant galaxies. Results are presented from detailed simulations of the longest HST exposures of representative galaxy fields, together with predictions of the numbers and types of objects that will emerge on exposures of different duration. Observations made during the first year of HST operations should confirm or disprove the quantitative understanding of telescopic capabilities reflected in the present simulations and predictions; significant departures would indicate that novel populations of astronomical objects may be present.

Bahcall, John N.↗

NASA NTRS1989

PC 1158 + 4635 - An optically selected quasar with a redshift of 4.73

Photometric and spectroscopic observations of PC 1158 + 4635, a quasar with a redshift of 4.73, are presented. This object was discovered by the detection of its Ly-alpha emission line in a low-resolution CCD grism survey. The apparent magnitude of the quasar is r = 20.2; its absolute magnitude is similar to that of 3C 273 (for q0 = 0.5). The spectrum of PC 1158 + 4635 reveals prominent emission lines of Ly-alpha and C IV and a sharp, approximately one magnitude drop in the continuum level across the Ly-alpha emission line (features seen in most greater than 4 quasars). The redshift-continuum depression relation for quasars requires a dramatic change in the 'Lymann alpha clouds' (either in number or intrinsic properties) at a redshift of about 3.5 or else the presence of a new source that is responsible for the majority of the absorption at redshifts obove four.

Schneider, Donald P.↗

NASA NTRS1989

A study of ten quasars with redshifts greater than four

Four quasars with redshifts greater than four were detected in a low-resolution CCD grism survey. CCD photometry and high S/N, moderate resolution spectra are presented for these quasars and the six other known quasars with redshifts above 4. The M sub B values of nine of the objects are between -27.5 and -25, with the tenth quasar having an M sub B value of -29. The emission lines and shapes of the continua of these ten quasars are similar to those of lower-redshift quasars. The results suggest that the C IV emission lines in high-redshift quasars may be weaker than those in lower-redshift quasars. The continua of all of the high-redshift quasars display strong depressions blueward of the Ly-alpha emission line.

Schneider, Donald P.↗

NASA NTRS1989

Evidence for a high-mass white dwarf in nova V1500 Cygni 1975

Time-resolved spectroscopy of V1500 Cyg are presented from 1981, when the nebular lines from the nova ejecta had faded sufficiently to permit reliably radial velocity measurements on emission lines from the binary remnant. The spectra possess a typical AM Herculis signature. The wavelength region 3800-5100 A contains a number of strong lines, in particular the Balmer series and He II 4686 A. All lines show periodic velocity changes with the same period as the photometric variations. On the first night the lines are relatively weak and narrow, are in phase with the photometric cycle, and suffer extreme modulation; two months later they are strong and broad, are roughly 90 deg out of phase with the photometric cycle, and have relatively less variation in their line fluxes. These features support the suggestion that the remnant is an AM Her system. The observed radial velocity of the secondary is 193 + or - 9 km/s. Various constraints suggest that the magnetic white dwarf in this system has a mass over 0.9 solar.

Horne, Keith↗

NASA NTRS1988

Spectroscopic evidence supporting the gravitational lens hypothesis for 1635+267 A,B

The gravitational lens hypothesis is tested for 1613+267 A,B by comparing the detailed line widths and shapes of the 2799-A Mg II and semiforbidden 1909-A C-III lines in each component. Following subtraction of an interpolating polynomial fit to the continua and the determination of a single optimum scaling factor (an amplification ratio of 2.83), reasonable agreement between the profiles of both lines in the two composites is obtained. Comparison of these lines to those in an unrelated quasar with a similar redshift and apparent magnitude does not produce a good match. It is suggested that the observed match in the 1635+267 A,B spectra arises from gravitational lensing.

Turner, Edwin L.↗

NASA NTRS1988

A new technique for measuring extragalactic distances

A relatively direct technique of determining extragalactic distances is described. The method relies on measuring the luminosity fluctuations that arise from the counting statistics of the stars contributing the flux in each pixel of a high signal-to-noise CCD image of a galaxy. The amplitude of these fluctuations is inversely proportional to the distance of the galaxy. This approach bypasses most of the successive stages of calibration required in the traditional extragalactic distance ladder; the only serious drawback to this method is that it requires an accurate knowledge of the bright end of the luminosity function. Potentially, this method can produce accurate distances of elliptical galaxies and spiral bulges at distances out to about 20 Mpc. In this paper, it is explained how to calculate the value of the fluctuations, taking into account various sources of contamination and the effects of finite spatial resolution, and the feasibility and limitations of this technique are demonstrated via simulations and CCD images of M 32 and N 3379.

Tonry, John↗

NASA NTRS1988

Simulations of HST observations of globular clusters

Simulations of Hubble Space Telescope observations to study galactic globular clusters are examined. Studies of the cores of nearby globular clusters to determine the stellar density distributions and luminosity functions in the innermost regions of the clusters are presented. Pictures of M 13 are included to demonstrate the usefulness of these studies. Stellar population studies of a relatively unobscured globular cluster to detect white dwarfs and the faint red end of the cluster luminosity function are discussed and illustrated with pictures of NGC 6397.

Bahcall, John N.↗

NASA NTRS1987

PC 0910 + 5625 - An optically selected quasar with a redshift of 4.04

The discovery of the second quasar with a redshift greater than 4 is reported. This object, which has an r magnitude of 21.0, was detected on a CCD grism survey. The spectrum of the quasar has strong, relatively narrow (but resolved) emission lines; there is a deep absorption trough about 100 A wide on the blue wing of the Lyman-alpha line; and the continuum at wavelengths shorter than the absorption feature is far weaker than the level present redward of Lyman-alpha.

Schmidt, Maarten↗

NASA NTRS1987

Four-shooter - A large format charge-coupled device camera for the Hale telescope

The Hale telescope has employed an astronomical camera using four 800x800 CCDs in an optical arrangement that allows the imaging of a contiguous 1600-pixel-square region of the sky; reimaging optics are then used to yield a scale of 0.33 arcsec/pixel. Optical coatings are incorporated to yield a throughput at peak efficiency of nearly 50 percent, including telescope losses. This camera can be used in a scanning mode, in which the telescope tracking rate is offset and the charge is clocked in the chips in such a way that the charge image remains aligned with the optical image. Attention is given to the results of a survey for high-redshift quasars using this equipment, which has produced images for the most distant galaxy clusters yet discovered.

Gunn, James E.↗

NASA NTRS1987

Nine optically selected quasars with redshifts larger than 3

The detection of nine quasars with redshifts between 3.0 and 3.8 is reported. The quasars were all found in a grism survey from their Lyman-alpha emission line. The r magnitudes of the quasars range from 18.5 to 21.7. The strengths of the Lyman-alpha lines are comparable to the values observed in other quasars (rest equivalent widths from 40 to 160 A). The spectrum of one of the objects possesses complex broad absorption features, similar to those seen in PHL 5200. A preliminary evaluation of these findings suggests that the comoving quasar density declines gradually beyond z = 3.

Schmidt, Maarten↗

NASA NTRS1986

The optical light curve of SN 1985f in NGC 4618

The 4930-A optical light curve obtained for the bizarre 1985f supernova in NGC 4618 covers a six-month interval; care has been taken to eliminate contamination by bright underlying H II regions. The light curve is well approximated by a constant decline of 0.0153 mag/day. The spectroscopic work of Gaskell et al. (1986) suggests that this supernova is a Type Ib object; if so, the present observations represent the first light curve of an object of this type at late times in its evolution, furnishing a standard against which future sightings can be compared.

Filippenko, Alexei V.↗

NASA NTRS1986

Spectroscopic CCD surveys for quasars at large redshift. II - A Pfuei transit survey

A CCD transit survey has been carried out with the 200 in. telescope of a strip of sky 5 arcmin wide and 9 hr long. Direct images and slitless spectra of over 43,000 objects were obtained on two successive nights. An automatic search for emission lines of given minimum equivalent width and signal-to-noise ratio yielded 52 candidate sources. Slit spectra revealed that 24 were emission-line galaxies with z less than 0.4 and eight were quasars with 2 between 1.00 and 2.76. The number of quasars detect agrees with that predicted from luminosity function models for z less than 2.9. The models also indicate that between 30 and 62 quasars with z less than 2.9 should have been found in this survey, but none were detected. This result reconfirms that there is a redshift cutoff near or below redshift three. The apparent conflict of this measurement with the known existence of dozens of quasars with redshifts larger than three is discussed.

Schmidt, Maarten↗

NASA NTRS1986

Spectroscopy of six X-ray-selected BL Lacertae candidates

Results of a continuing program aimed at extending the small list of X-ray-selected BL Lac objects are reported. High-quality spectra have been obtained of six faint blue objects that lie within the positional error boxes of X-ray sources discovered serendipitously by the Einstein Observatory. Three of the objects are found to be previously uncataloged low-red-shift quasi-stellar objects, including one formerly suggested as BL Lac candidate. Two are faint galactic stars, while the final object has a featureless spectrum, and thus remains a candidate. Although X-ray selection may ultimately be an effective means of discovering faint BL Lac objects, such sources are evidently rare at the X-ray flux levels attainable by the Einstein Observatory.

Margon, Bruce↗

NASA NTRS1986

Multiple quasars for multiple images

Consideration is given to the possibility that the two quasar images of 1146 + 111, 1635 + 267, and 2345 + 007 respectively are pairs of physically distinct quasars in galaxy associations. It is shown that, at the same redshifts as the quasars, only galaxy associations of exceptional richness would be identifiable in the existing data. Calculations are performed to assess the probability of pairs of quasars appearing in galaxy associations and it is found that physical pairs could appear with the observed frequency provided that quasars are more common relative to galaxy associations at earlier epochs.

Bahcall, John N.↗