Engineering topics

Cohen, Ross D.

Publications and source records attributed to Cohen, Ross D..

24 records · Page 2

NASA NTRS1988

Ultraviolet and optical spectrophotometry of the Seyfert 1.8 galaxy Markarian 609

Ultraviolet and optical observations of the Seyfert 1.8 galaxy Mrk 609 were collected simultaneously. The observations reveal strong line and continuum emission in the UV, an increase in the flux of H-beta and He I 5876, and a decrease in the H-alpha/H-beta value since the measurements by Osterbrock (1978, 1981), as well as an extended population of early-type stars, which is considered to be the source powering the larger part of the far-IR emission. Special attention is given to the origin of steep broad-line Balmer decrement measured by Osterbrock, since the strong UV continuum and the emission lines of Mrk 609 observed rule out reddening as the cause of the Balmer decrement. It is suggested that smaller-than-normal optical depths are likely to be the cause of the decrement.

Rudy, Richard J.↗

NASA NTRS1987

Imaging and spectroscopic studies of the interacting system Markarian 171

V, R, and I broadband CCD images, as well as H-alpha spectroscopic observations of Markarian 171, are presented. Two apparently interacting dust-enshrouded galaxies exhibit relatively normal V-R, V-I galactic stellar-population colors in their outer regions, and spatially extended H-alpha emission. However, the dereddened colors imply a hot stellar population. The dual characteristics of narrow H-alpha lines and spatially extended H-alpha emission clearly distinguish this type of activity from that associated with the compact central regions of active galactic nuclei and QSOs. Apparently, in the case of Mk 171 the interaction has induced star formation throughout a significant fraction of the galaxy (kiloparsecs) - not just in the immediate vicinity of the nucleus (parsecs). A peculiar stellar population of hot stars associated with a knot of weak H-alpha emission appears spatially separate from the other emission-line regions. It is possible that this stellar knot is a severed remnant attributable to the gravitational interaction between the two galaxies.

Friedman, Scott D.↗

NASA NTRS1987

Emission-line variability and the broad-line region of quasi-stellar objects. I - Time scales and photon densities

Spectroscopic data for five low-redshift QSOs taken over several years are presented as evidence for emission-line variability in QSOs. The broad hydrogen emission lines in the QSOs 0026 + 129, 1202 + 281, 1612 + 261, and 2141 + 175 are found to vary in step with their underlying continua. Another QSO, 2135 - 147, shows an increase in the intensities of its hydrogen lines five years after the increase in its continuum level. The inferred upper limit to the size of the broad emission line region is smaller than 3 lt-yr, probably as small as 0.4 lt-yr for 0026 + 129, if the line-emitting region is spherically distributed. Such a small size implies a very high density of ionizing photons, of the order of 10 to the 10th/cu cm. A very high nucleon density must then be assumed in order to obtain a value of the ionization parameter consistent with current photoionization models.

Zheng, Wei↗

NASA NTRS1987

The nature of the BL Lacertae object AO 0235 + 164

Emission features in the optical spectrum of the highly variable BL Lac object AO 0235 + 164 have been detected which are identified as Mg II, forbidden Ne V, and forbidden O II at a redshift of 0.9399, consistent with a cosmological interpretation of the two absorption systems at z = 0.524 and z = 0.851. The spectrum also shows emission features associated with the z = 0.524 system at a projected separation of 20 kpc from the apparent central concentration of the material producing that absorption. This implies a highly extended and very luminous line-emitting region for the material at z = 0.524. A comparison of the new data with previous spectroscopic observations of the BL Lac object when it was faint suggests that the Mg II emission may be variable on a time scale of approximately 2 yr.

Cohen, Ross D.↗

NASA NTRS1987

Detection of the He I 10830-A line in the broadline radio galaxy 3C 120

The first measurement of He I 10830 A flux in a broadline radio galaxy (BLRG), 3C 120, is reported. The flux is 2.7 x 10 to the -13th erg/sq cm/s. The relative strengths of the 10830 A line and other prominent broad emission lines are 10830 A/He I 5876 A/H-beta/H-alpha = 0.64/0.27/1.00/6.3. Reddening is ruled out as the single source of the large 5876 A/H-beta ratio, and the necessity for either an enhanced helium abundance or an extended He II is obviated. Rather, the line ratios indicate that significant collisional enhancement of 5876 A and H-beta occurs, identical to that observed in Seyfert I galaxies and quasars.

Rudy, Richard J.↗

NASA NTRS1986

Variability of Markarian 1018 - Seyfert 1.9 to Seyfert 1

Simultaneous IUE and optical spectrophotometry of the Seyfert 1.9 galaxy Markarian 1018 is presented. The strength of H-beta(broad) has increased by a factor of 6.6 in 4 yr, and the galaxy now shows strong broad UV and optical line emission similar to that seen in normal Seyfert 1 galaxies, but with markedly asymmetric line profiles. As in other variable Seyfert galaxies, H-alpha emission has also increased but by a lesser amount than H-beta. Furthermore, Ly-alpha (not observed during the first epoch) is now quite strong relative to H-beta and H-alpha. Inspection of the line profiles reveals nearly identical blue asymmetric line profiles in all permitted lines; these show evidence of significant profile evolution over a 1 yr time scale. The present line ratios and the nature of their variability suggest that the enhancement in the broad-line emission is not caused by the removal of obscuring dust but by the 'lighting up' of the broad-line region.

Cohen, Ross D.↗