Engineering topics

Neff, S. G.

Publications and source records attributed to Neff, S. G..

33 records · Page 2

NASA NTRS1990

Tidal events and galactic activity

Researchers report some results from recent and ongoing work which relate to the connection between nuclear activity and tidal interactions. They suggest that tidal events are in general a necessary but not sufficient condition for nuclear activation. They also suggest that nuclear activity generally develops at a 'late stage' of a tidal encounter, following star-formation and dust evaporation, and when the most obvious morphological disturbances may have disappeared. The quantitative results from quasi-stellar object imaging, radio source evolution, HI observations, and an Infrared Astronomy Satellite (IRAS) source survey are given in tabular form.

Hutchings, J. B.↗

NASA NTRS1990

Evolution of radio quasars from redshift 0.6-3.7

This paper presents the results of VLA radio imaging of 58 radio-loud quasars with redshift 2.0 or higher, which fill the redshift-luminosity plane as evenly as possible. This work completes a survey of about 250 quasars covering redshifts from 0.6-3.7, which attempts to sample luminosity and look-back time in a uniform way. Within the constraints of possible selection effects it is found that the relative population of extended and unresolved sources changes with redshift in a way that suggests that radio quasars may live longer and spend more time as large triple sources in the present epoch than in the earlier universe. There appear to be few low-luminosity radio quasars at high redshift. Ejection of material appears to occur on one side at a time, with usually at least one reversal of direction in the source lifetime. The velocity of ejection appears to be mildly relativistic at high redshift, but of lower velocity in the present epoch. There is also evidence suggestive of changes in the IGM with cosmic time; however, the data presented do not show the minimum in density at z about 2 that has been suggested for cluster environments.

Neff, S. G.↗

NASA NTRS1990

Imaging and two-dimensional spectra of the IR-bright galaxy NGC 2146 - A recent low-energy merger?

New data are presented on the IR-luminous galaxy NGC 2146 from several sources: direct imaging in B, R, and H-alpha; IR imaging in the J, H, and K bands; long-slit spectroscopy at optical and IR wavelengths; and scanning etalon observations in H-alpha. The results allow measurement of the interstellar extinction in the dust lane, and estimation of the true luminosity of the galaxy and nuclear regions. The spectra indicate that there is no active nucleus, and measure the changing ratio of forbidden to permitted lines across the galaxy. IR images and colors indicate the existence of a significant population of hot young stars in the central regions of the system. The H-alpha velocity maps show the full radial-velocity pattern, and suggest that the system consists of a disturbed disk and a merging or interacting arm which connects to the inner dust and radio structures. Outer H-alpha and H I structures appear to be the earlier remnants of this spiraling merger.

Hutchings, J. B.↗

NASA NTRS1990

Optical environments of radio quasars

CCD images of 36 radio-loud quasars in the redshift range 0.1-0.75 are discussed. The objects were chosen to represent the full range of radio structure found with VLA A-configuration observations. Measured quantities are compared and correlated from both radio and optical observations. At least 70 percent of the quasars show evidence for current or recent interaction. Consideration is given to host galaxy sizes and luminosities, states of tidal interaction and numbers of companions, radio morphology, and luminosity.

Hutchings, J. B.↗

NASA NTRS1990

NGC 1614 - An IR-luminous merger but not (yet?) an active galaxy

New observations of the merging galaxy NGC 1614 are described. The system has a nuclear region of QSO-like luminosity, but shows no direct evidence for an active nucleus. It is heavily and unevenly reddened across its nucleus, while infrared imaging also shows a 'ridge' of dust. The inner spiral structure of the galaxy has normal rotation for an inclined disk, as indicated by the H-alpha emission. A linear 'tail' to the S and extended arms to the E have more positive velocities, and probably are the remains of an interacting companion and the tidal plume(s) caused by the collision. The only H I seen in emission appears to coincide with bright knots of H-alpha and forbidden O III emission of the base of the tail. The lack of direct evidence for an active nucleus indicates that if NGC 1614 is a precursor to a Seyfert-like system the AGN has not yet turned on.

Neff, S. G.↗

NASA NTRS1989

What is the difference between radio-loud and radio-quiet quasi-stellar objects?

Measures of the nuclear and host galaxy luminosities and colors and a morphological discussion of the host galaxies are presented for a sample of low-redshift, high-luminosity, radio-quiet QSOs whose redshift and luminosity distribution matches that of a radio-loud sample previously discussed. Radio-quiet QSOs are found to reside in galaxies which are smaller, fainter, and redder than the host galaxies of radio-loud QSOs. These properties are generally consistent with the suggestion that radio-quiet QSOs are located in spiral-type galaxies and radio-loud QSOs are located in more elliptical-type galaxies. Significantly less evidence is found for tidal interactions among the radio-quiet objects, although they appear to reside in somewhat richer environments in terms of nearby companions.

Hutchings, J. B.↗

NASA NTRS1989

Evolution of radio quasars for z of less than 2

A sample of 72 radio quasars is used to study the cosmic and individual evolution of quasars in the 1-2 redshift range. It is shown that the most luminous sources at all redshifts are unresolved, and that one-sided and two-sided structures correspond to progressively lower core (and total) luminosities. The sizes and fractional population of triple sources are found to fall steadily with increasing redshift to z of about 1.7. A correlation over many orders of magnitude is noted between the optical and maximum radio-core luminosity.

Neff, S. G.↗

NASA NTRS1989

The double-nucleus galactic merger Mkn 463

Deep CCD imaging in broadband and forbidden O III wavelengths of the twin-nucleus active galaxy Markarian 463 is presented. These data show that the system has triple curved tails indicative of a strong tidal interaction and probable merger. Colors, morphology, and their implications are discussed. The forbidden O III line emission is seen to be extended and linear, perhaps filling twin cones with apex at the eastern optical nucleus. It is argued that this indicates nonisotropic radiation from an obscured AGN. Spatially resolved spectroscopy on and off the nuclei reveals that the radial velocities of the gas lie in a small range, unlike some other multiple nucleus systems. The nature of the emission-line regions is discussed, along with the evolutionary state of the system indicated by the present data and IR and radio data.

Hutchings, J. B.↗

NASA NTRS1988

Tidal interactions and infrared-bright QSOs

Deep direct images of five IRAS-selected QSOs with similar IR luminosities and spectral indices have been analyzed. The present objects possess IR luminosities similar to those of the IRAS flux-lined sample of ultraluminous galaxies, but have IR spectral indices similar to those of normal QSOs. Four of the objects are in strong tidal interaction and have blue host galaxies and reddened nuclei. It is suggested that these objects are QSOs and AGN in an intermediate stage of their activity, which lies between that of ultraluminous galaxies and optically selected QSOs.

Hutchings, J. B.↗

NASA NTRS1988

The twin-nucleus merging galaxy Markarian 266 (NGC 5256)

Optical imaging in continuum and line wavelengths, spatially resolved optical spectroscopy, and 21 cm radio imaging of Markarian 266 are presented. The galaxy has faint extended plumes of luminosity, which indicate that it is a recent merger. There are extended regions of blue luminosity suggestive of recent star formation, and remarkable structure in forbidden O III line emission. Spectroscopic data show the velocities and densities in the ionized gas. Radio observations taken with the VLA show that both nuclei are resolved radio sources, and that there is a third radio source between them which has no optical counterpart; 21-cm absorption is seen against the brighter optical nucleus, indicating possible infall. These results are discussed in terms of galaxy mergers, nuclear activation, and ionized gas. Mkn 266 has many similarities to Arp 220 and appears to be a somewhat lower-luminosity version of the IRAS ultraluminous galaxies.

Hutchings, J. B.↗

NASA NTRS1988

Compact radio cores in three Seyfert galaxies - High resolution studies of Mkn 3, Mkn 231, and Mkn 463

The small-scale morphologies of Mkn 231 and Mkn 463 have been investigated using the European VLBI Network (EVN) at 1.6 GHz and the VLA at 15 and 22 GHz. Mkn 3 has been studied using the EVN at 1.4 and 1.6 GHz. All of the galaxies have a substantial fraction of their core radio flux in linear or elongated structures, typically tens of parsecs in extent. Agreement between the parsec- and kiloparsec-scale position angles of continuum radio emission suggests collimated outflow from the galaxy centers that continues over several orders of magnitude in scale. Supernova rates of order one hundred per year on scales of tens of parsecs are required to explain the nuclear radio emission as the result of bursts of star formation. The luminosities, small extents, and linear morphologies of the radio sources all support the idea that the nuclear radio sources are energized by active galactic nuclei and therefore support the more general picture of Seyfert galaxy nuclei being low-luminosity quasars. All of the galaxies appear to have undergone violent interactions in the recent past.

Neff, S. G.↗

NASA NTRS1987

Radio observations of a few selected blazars

The paper presents total-intensity and linear-polarization observations of four selected blazars, 0716+714, 0752+258, 1156+295 and 1400+162, with the VLA A-array, and MERLIN and EVN observations of 1400+162. The sources 0752+258 and 1400+162 which have nearly constant optical polarization, have well-defined double-lobed radio structure, with relatively weak radio cores, and are likely to be at large viewing angles. In addition, 0752+258 appears to be a twin-jet blazar. The position angle (PA) of the VLBI jet in 1400+162 is close to that of the arcsec-scale jet near the nucleus, as well as the optical and 2-cm core polarization PAs. The blazars 0716+714 and 1156+295, which exhibit strongly variable optical polarization, have a core-dominated radio structure and perhaps have their jet axes close to the line-of-sight. From polarization observations at 20, 18, 6, and 2 cm, it is found that the rotation measure of the radio core in 0716+714 is about -20 rad/sq m. It is suggested that low values of core rotation measure in core-dominated sources could be consistent with the relativistic beaming models.

Saikia, D. J.↗

NASA NTRS1987

Morphology and nuclear spectroscopy of Markarian 231

Deep optical imaging of Mkn 231 reveals twin (tidal?) tails, a linear nuclear feature at green wavelengths, and a very blue region 4 arcsec south of the nucleus. Much of the central part of the galaxy is red, but there are complex areas of blue luminosity outside this, and a sharp edge to the luminosity at a distance of about 16 arcsec from the center. Overall, the host galaxy appears to have a normal optical luminosity and blue color (B-R = about 0.7) despite being one of the most luminous galaxies known in the IR. Radio emission in the system is extended on one side on a scale similar to the optical tails, but shows no detailed correspondence with optical structure; in particular, there is no radio counterpart to the optical 'jet'. Examination of IUE archival data indicates that the UV flux is very weak and the UV spectrum is peculiar for a Seyfert galaxy. The UV observations provide evidence for considerable nuclear extinction in the system, in accordance with previously published optical and IR work, but the UV extinction is unlike Galactic absorption and may be more similar to that seen in the LMC. Recent optical spectra of Mkn 231 show changes in both the emission-line spectrum and in the strong broad absorption lines (BAL), compared with previously published observations. It is suggested that Mkn 231 may be a recently merged system which is currently undergoing star formation. The connection with BAL QSOs is also discussed.

Hutchings, J. B.↗

NASA NTRS1987

Radio structure in the inner 1 arcsecond of NGC 1068

High-resolution VLA and VLBI observations of NGC 1068 at several different wavelengths are reported. These observations provide new information about the triple radio source in the central arcsec (75 pc) of the galaxy. The 21-cm European VLBI Network observations at 0.05-arcsec resolution detect a core of flux density 89 + or - 10 mJy. This core is dominated by a component less than 20 marcsec (1.5 pc) in diameter that has a brightness temperature of about 10 to the 8th K. Two more components may have been detected by the VLBI observations, one on either side of the compact core. At 0.07-arcsec resolution, the 1.3-cm VLA map of the central arcsec of NGC 1068 reveals a C-shaped radio morphology on scales from 10 to 50 pc. A comparison of the relative locations of the three components in the VLA and VLBI maps shows that the compact core in the VLBI map probably should be identified with the central component in the 1.3- and 2-cm VLA maps. If this identification is correct, the central component in the VLA maps is the actual nucleus of the galaxy. The core of the inner triple radio source may contain a significant flat-spectrum component. The presence of a compact core and the evidence for bending strengthen the argument that the inner triple radio source is produced by a mechanism similar to that operating in powerful radio galaxies. For a minimum-energy configuration, magnetic-field strengths of about 1 mG and energy densities of about 100 nerg/cu cm are found for the components of this source.

Ulvestad, James S.↗

NASA NTRS1981

VLBI observations of SS 433 at 3.6 and 13 centimeters

SS 433 was detected and partially resolved at 2290 MHz on baselines with fringe spacings of 1.4, 0.1, and 0.003 arcsec. It was also detected at 8420 MHz on a baseline with a fringe spacing of 0.009 arcsec. Simple models of the source, consistent with the limited data, have elongated structures greater than 0.1 arcsec in size with position angles in 1979 May that were within about 10 deg of the position angle of the apparent bulges of the supernova remnant W50. The data also imply that the source contains a core less than 0.002 arcsec in size with a brightness temperature greater than 10 to the 9th K. The bright core and aligned structures that seem to be present in SS 433 and W50 resemble the structures seen in powerful extragalactic radio sources which are many orders of magnitude larger.

Walker, R. C.↗