Engineering topics

Bershady, Matthew A.

Publications and source records attributed to Bershady, Matthew A..

9 records

NASA NTRS1995

High-resolution spectra of distant compact narrow emission line galaxies: Progrenitors of spheroidal galaxies

Emission-line velocity widths have been determined for 17 faint (B approximately 20-23) very blue, compact galaxies whose redshifts range from z = 0.095 to 0.66. The spectra have a resolution of 8 Km/s and were taken with the HIRES echelle spectrograph of the Keck 10 m telescope. The galaxies are luminous with all but two within 1 mag of M(sub B) approximately -21. Yet they exhibit narrow velocity widths between sigma = 28-157 km/s, more consistent with typical values of extreme star-forming galaxies than with those of nearby spiral galaxies of similar luminosity. In particular, objects with sigma is less than or equal to 65 km/s follow the same correlations between sigma and both blue and H beta luminosities as those of nearby H II galaxies. These results strengthen the identification of H II glaxies as thier local counterparts. The blue colors and strong emission lines suggest these compact galaxies are undergoing a recent, strong burst of star formation. Like those which characterize some H II galaxies, this burst could be a nuclear star-forming event within a much larger, older stellar population. If the burst is instead a major episode in the total star-forming history, these distant galaxies could fade enough to match the low luminosities and surface brightnesses typical of nearby spheroidals like NGC 185 or NGC 205. Together with evidence for recent star formation, exponential light profiles, and subsolar metallicities, the postfading correlations between luminosity and velocity width and bewtween luminosity and surface brightness suggest that among the low-sigma galaxies, we may be witnessing, in situ, the progenitors of today's spheroidal galaxies.

Koo, David C.↗

NASA NTRS1994

The ensemble variability properties of faint QSOs

A refined sample of 64 variable objects with stellar image structure has been identified in SA 57 to B approximately = 22.5, over a time baseline of 15 yr, sampled at 11 distinct epochs. The photometric data typically have a root-mean-square (rms) error at B = 22 of only 0.05 mag. Thirty-five quasars in this field have already been spectroscopically confirmed, 34 of which are among the sample of variables. Of the other variables, six are known spectroscopically to be stars, 10 additional objects are stars based on reliable detection of proper motion, and one is spectroscopically a narrow emission line galaxy. Of the 13 remaining variables, it is argued that they are a mixture of distant halo subdwarfs and quasars with starlike colors. We compute the ensemble average structure function and autocorrelation function from the light curves in the respective quasar rest frames, which are used to investigate the general dependences on apparent magnitude, absolute magnitude, and redshift.

Trevese, Dario↗

NASA NTRS1994

The optical and near-infrared colors of galaxies, 1: The photometric data

We present optical and near-infrared photometry and spectroscopic redshifts of a well defined sample of 171 field galaxies selected from three high galactic latitude fields. This data set forms the basis for subsequent studies to characterize the trends, dispersion, and evolution of rest-frame colors and image structure. A subset of 143 galaxies constitutes a magnitude-limited sample to B approx. 19.9-20.75 (depending on field), with a median redshift of 0.14, and a maximum redshift of 0.54. This subset is statistically representative in its sampling of the apparent color distribution of galaxies. Thirty six galaxies were selected to have the reddest red-optical colors in two redshift intervals between 0.2 less than z less than 0.3. Photometric passbands are similar to U, B, V, I, and K, and sample galaxy spectral energy distributions between 0.37 and 2.2 micrometers in the observed frame, or down to 0.26 micrometers in the rest frame for the most distant galaxies. B and K images of the entire sample are assembled to form the first optical and near-infrared atlas of a statistically-representative sample of field galaxies. We discuss techniques for faint field-galaxy photometry, including a working definition of a total magnitude, and a method for matching magnitudes in different passbands and different seeing conditions to ensure reliable, integrated colors. Photographic saturation, which substantially affects the brightest 12% of our sample in the optical bands, is corrected with a model employing measured plate-density distributions for each galaxy, calibrated via similar measurements for stars as a function of known saturation level. Both the relative and absolute calibration of our photometry are demonstrated.

Bershady, Matthew A.↗

NASA NTRS1994

HST images of very compact blue galaxies at z approximately 0.2

We present the results of Hubble Space Telescope (HST) Wide-Field Camera (WFC) imaging of seven very compact, very blue galaxies with B less than or equal to 21 and redshifts z approximately 0.1 to 0.35. Based on deconvolved images, we estimate typical half-light diameters of approximately 0.65 sec, corresponding to approximately 1.4 h(exp -1) kpc at redshifts z approximately 0.2. The average rest frame surface brightness within this diameter is mu(sub v) approximately 20.5 mag arcsec(exp -2), approximately 1 mag brighter than that of typical late-type blue galaxies. Ground-based spectra show strong, narrow emission lines indicating high ionization; their very blue colors suggest recent bursts of star-formation; their typical luminosities are approximately 4 times fainter than that of field galaxies. These characteristics suggest H II galaxies as likely local counterparts of our sample, though our most luminous targets appear to be unusually compact for their luminosities.

Koo, David C.↗

NASA NTRS1994

Flatfielding and photometric accuracy of the first Hubble Space Telescope Wide Field Camera

Long exposures with the original Hubble Space Telescope (HST) Wide Field Camera (WFC) through the F555W and F785LP filters show gradients in the background following standard pipeline calibration. We show that these gradients also appear in stellar photometry, and thus must be predominantly the result of inaccurate flatfielding at a level of 10 to 20%. Color errors may be even larger. Applying corrections to the flatfield frames based on the background structure leads to an improved accuracy of approximately 4% for single-measurement photometry within a single CCD chip, compared to the approximately 10% accuracy suggested by previous studies. We have reanalyzed the F555W and F785LP calibration photometry to derive zero points appropriate for corrected data; these new zero points have internal consistency at a level of approximately 1.2%, based on comparison between the chip-to-chip offsets and the sky levels observed in corrected images. This indicates that relative photometry approaching 1 to 2% is achievable with the WFC. The new zero point values for corrected data are 22.90, 23.04, 23.04, and 22.96 (F555W), and 21.56, 21.64, 21.44, and 21.47 (F785LP) for chips WF1-WF4, respectively. Comparison is made with other zero points, and the applicability of 'delta flats' is briefly discussed.

Phillips, Andrew C.↗

NASA NTRS1993

Survey incompleteness and the evolution of the QSO luminosity function

We concentrate on a type of QSO survey which depends on selecting QSO candidates based on combinations of colors. Since QSO's have emission lines and power-law continua, they are expected to yield broadband colors unlike those of stellar photospheres. Previously, the fraction of QSO's expected to be hiding (unselected) within the locus of stellar (U-J, J-F) colors was estimated at about 15 percent. We have now verified that the KK88 survey is at least 11 percent incomplete, but have determined that it may be as much as 34 percent incomplete. The 'missing' QSO's are expected to be predominantly at z less than or = 2.2. We have studied the proper motion and variability properties of all stellar objects with J less than or = 22.5 or F less than or = 21.5 in the SA 57 field which has previously been surveyed with a multicolor QSO search by KK88.

Majewski, Steven R.↗

NASA NTRS1991

The current ability of HST to reveal morphological structure in medium-redshift galaxies

The capabilities of the Faint Object Camera (FOC) and the Wide Field Camera (WFC) are assessed on the basis of a brief program of single-orbit images of medium-faint galaxies. The FOC yielded a good resolved image of a compact galaxy at a blue magnitude J of 20.5 in a single-orbit exposure. WFC images have a survey capability that can include many galaxies per field, with sufficient resolving power to distinguish clearly between galaxies and stars down to the level of 0.2 arcsec, depending on the signal-to-noise ratio, and a reasonable capacity for morphology. Although some morphological detail can be discerned in even the aberrated images, deconvolutions are found to greatly enhance the ability to see structural detail. Even at the low S/N that is provided by single-orbit exposures, the more sophisticated restoration methods offer some advantage over simple Fourier or Lucy techniques.

King, Ivan R.↗

NASA NTRS1991

Telescope search for a 3-eV to 8-eV axion

Axions of mass 3-8 eV should have a cosmological abundance of about 50/cu cm and reside in rich clusters of galaxies. Their decays to two photons will produce a line at a wavelength of about 3100-8300 A. This effort has searched unsuccessfully for such a feature in the tergalactic light of three rich clusters, closing this 'window', and leaving open only the window from 10 to the -6th to 0.001 eV. This implies that if the axion exists, it likely comprises the dark matter. The present flux limits are of relevance to other relics whose decays produce monoenergetic photons.

Bershady, Matthew A.↗

NASA NTRS1991

The faint quasar luminosity function

Preliminary results of an expanded program to determine the faint-quasar luminosity function are described. Quasars have been selected in four fields totaling 1.2 sq deg from four-band photometry. Out of a total of 130 quasars with good spectroscopy, 37 have J greater than 21.5 and 46 have F greater than 21.0. The spectroscopic sample is representative of all of the color-selected candidates. An estimate of the luminosity function as a function of redshift is derived.

Kron, Richard G.↗