Features In NGC Research Articles

The masering torus in NGC 4258

The galaxy NGC 4258 displays strong spectral variability in its H_2_O maser emission during a 7 year monitoring program. Variations of line strength and line velocity are particularly prominent. A velocity-epoch diagram of prominent spectral features shows that all features are accelerated systematically with an effective rate of 7.5 km s^-1^ yr^-1^. Combining this result with a recent VLBI result of an angular gradient for the velocity, we find that the masering clouds are located at the inner edge of a molecular torus/disk or a ring at a radial distance of 0.11 pc and have a Keplerian velocity of 900 km s^-1^ around a 2.2 x 10^7^ M_sun_ nuclear mass. The masering clouds amplify the radio continuum during the 15 years it takes to pass in front of the extended (0.014 pc) nucleus. The pumping of the high density masering clouds does not directly result from the nuclear activity itself but rather from short-lived pump sources within the torus. Searches for periodic behavior have not revealed any incidences besides the one reported earlier. The observed high-velocity features in NGC 4258 originate in the tangential sections of the torus.

The 3.28 micron emission feature in NGC 253

view Abstract Citations (25) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The 3.28 Micron Emission Feature in NGC 253 Kalas, Paul ; Wynn-Williams, C. G. Abstract The 3.28 micron polycyclic aromatic hydrocarbon (PAH) emission feature in the galaxy NGC 253 comes from an extended star formation region ~100 pc across. The brightest mid-infrared source in the galaxy, which is displaced ~45 pc from the probable nucleus, does not show the PAH feature; it may be a dust-enshrouded recent supernova. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174756 Bibcode: 1994ApJ...434..546K Keywords: Emission Spectra; Infrared Astronomy; Polycyclic Aromatic Hydrocarbons; Spiral Galaxies; Star Formation; Supernovae; Active Galactic Nuclei; Infrared Astronomy Satellite; Infrared Imagery; Luminosity; Astronomy; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 253; GALAXIES: ISM; GALAXIES: NUCLEI full text sources ADS | data products SIMBAD (7) NED (1)

Simultaneous GINGA and IUE observations of the Seyfert galaxy NGC 3516

A possible connection between the variable X-ray and UV absorption features in NGC 3516 is presently investigated in view of simultaneous X-ray and UV observations. A common origin of the UV and X-ray absorption could not be established, due to the absence of variability in either the X-ray or UV absorption. On the basis of the absorbing properties of a low-luminosity AGN, in the framework of broad absorption-line (BAL) theories, it is suggested that NGC 3516 is consistent with a low-luminosity equivalent of a BAL QSO.

High-resolution spectroscopy of the 3 micron emission features in NGC 7027 and IRAS 21282+5050

High-resolution 3 micron spectra of NGC 7027 and IRAS 21282+5050 are presented. The well-known 3.29 micron emission feature has very similar shapes in the two objects, but a small shift is present between them. The emission features at 3.46 and 3.52 microns recently found in IRAS 21282+5050 are also present in the spectrum of NGC 7027. In addition, there may be two other weak features at 3.55 and 3.58 microns in NGC 7027. These emission features might be related to saturated hydrocarbons. The spectra provide further constraints on the origin of the 'unidentified' emission features.

A detailed study of the C IV 1550-A line profile and adjacent spectral features in NGC 4151 from 1978 to 1983

A detailed study of the C IV 1550-A line profile and adjacent spectral features in NGC 4151 from 1978 to 1983

A detailed study of the CIV 1550 line profile and adjacent spectral features in NGC 4151 from 1978 up to 1983

Fitting technics are applied to study the variations of the 1550 line profile and adjacent features in the IUE spectrum of the Seyfert galaxy NGC 4151, from 1978 up to 1983. New important results are found concerning the kinematics and physics of the emission line gas at a few light-days from the continuum source.

DUST IN THE NUCLEI OF THE SEYFERT GALAXIES MARKARIAN 231 AND NGC 4151.

ABSTRACT Observations carried out with a 8-13 micron grating-spectrometer of Mrk 231 and NGC 4151 are reported. The Mrk 231 data can be fitted to various thermal dust emission models or a single power law, with dust extinction. In all the model fits, except for that of graphite and silicon carbide grain emission, a component of silicate absorption of optical depth of not more than 0.7 is required. Confirming published work, the absorption being at the redshift of the low-redshift absorption-line system is ruled out. The high values of silicate optical depth absorption do not give ratios to the galaxy's visual extinction which are comparable to those of galactic H II regions. Weak evidence for a 10-micron absorption feature in NGC 4151 is also reported. This is somewhat contrary to expectation, since the visual extinction of NGC 4151 is lower than that of Mrk 231, and since there is evidence to support a nonthermal rather than thermal dust origin for the infrared continuum emission.

Spatial studies of the middle infrared spectral features in NGC 7027

We report high spatial resolution spectral observations of the planetary nebula NGC 7027 in the 10 µm region. These show that the unidentified infrared features at 8.65 and 11.25 µm originate from a more extensive region than the shell of ionized gas; the data are consistent with these features arising entirely from the contiguous neutral region. This situation places further constraints upon a fluorescent excitation mechanism (Allamandola, Greenberg & Norman 1979) and it is argued that the features are due to thermal emission from surface bonds on graphite grains in an H I region (Barlow 1982, in preparation; Dwek et al. 1980).

Astrophysical gamma-ray production by inverse Compton interactions of relativistic electrons. III - Cutoff effect for inverse Compton spectra applied to the case of the hard X-ray and gamma-ray emission of NGC 4151

The recently reported spectral features of NGC 4151 in the hard X-ray and ..gamma..-ray energy range are analyzed. A two-component emission model is suggested in which the ultraviolet and soft X-ray photons (E<20 keV) are scattered by relativistic electrons via the inverse Compton process into the hard X-ray and ..gamma..-ray regime. It is shown that the rather flat, hard X-ray spectrum can be explained if a low-energy cutoff in the relativistic electron distribution is postulated. The concept of a low-energy cutoff in the electron distribution is not inconsistent with the observed overall continuum emission of NGC 4151.

High-resolution spectra of the 3.3 micrometer unidentified emission feature in NGC 7027 and HD 44179

High-resolution spectra of the 3.3 micrometer unidentified emission feature in NGC 7027 and HD 44179

A new model for X-ray emission from NGC 4151

We present an analysis of the reported spectral features of NGC 4151 in X-rays. It is shown that the origin of X-rays from the source is inconsistent with a single production mechanism. We suggest a new two-component model in which soft X-rays arise from the black-body emission of a tiny hot nucleus withT∼2×107 K and the hard X-ray photons are generated in an extended region by inverse Compton scattering of electrons with the infrared photons.

Kinematics and spectra of planetary nebulae with O VI-sequence nuclei

Spectral features of NGC 5189 and NGC 6905 are tabulated. Fabry-Perot profiles around H alpha and O III lambda 5007 of NGC 5189, NGC 6905, NGC 246, and NGC 1535, are illustrated. The latter planetary nebula is a non-O VI-sequence, comparison object of high excitation. The kinematics of the four planetary nebulae are simply analyzed. Discussion of these data is motivated by the possibility of collisional excitation by high-speed ejecta from broad-lined O VI-sequence nuclei, and by the opportunity to make a comparison with conditions in the supernova remnant or ring nebula, G2.4 + 1.4, which contains an O VI-sequence nucleus of Population I.

Observations of quasi-thermal and maser phenomena in rotationally excited OH. I

Absorption features arising from $lambda$-doublet transitions in a very short-lived rotationally excited state of OH have been discovered in the direction of the W3 continuum peak. Analyses of these features and of broad emission features in Sgr indicate the presence of massive regions with very high densities. An extensive search for new sources of maser emission from excited- state OH has resulted in the detection of maser features in NGC 7538, OH 69.5-- 1.0, M17, W51, and Orion (OH). Observations with circular polarization suggest Zeeman effects in some spectra, with inferred magnetic field strengths of several milligauss. A rapid time variation has been observed in the maser spectrum of Sgr B2, and evidence is presented for variations in W51 and W3 (continuum).

Observations of Galactic OH Emission

view Abstract Citations (24) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of Galactic OH Emission Ball, J. A. ; Meeks, M. L. Abstract The 120-foot Haystack antenna of Massachussetts Institute of Technology Lincoln Laboratory was used to observe polarized OH emission in the galactic sources W49, NGC 6334, W51, and W75 Both the 1665 and 1667 MHz transitions were observed in W49 and NGC 6334, and oniy the 1665 MHz transition was observed in W51 and W75 The Stokes parameters were determined as functions of radial velocity, and the resulting spectra were decomposed into a set of independent, uniformly polarized features with Gaussian profiles. Half of the fifty-one observed features have a degree of polarization »= 90 per cent. The polarization was circular in 70 per cent of the features Two bright, unpolarized features appear in the spectra of W51, and one feature in NGC 6334 appears to have nearly 100 per cent linear polarization. Observations of NGC 6334 at irregular intervals from March 1966 through June 1967 disclosed intrinsic time variations in the more southern of a pair of OH source points in this region. Not all features showed variations, but two features with radial velocities -6 6 and 7.9 km sec1 (1665 MHz) and one feature at -8 7 km sec' (1667 MHz) varied significantly in intervals of 9 days or longer. The time variations in the 1665 MHz features appear to be correlated. There is little apparent correlation in the radial velocities of 1665 and 1667 Mhz features originating at the same position in W49 and NGC 6334. Of the four cases where such near-coincidence is found, three cases show the same sense of circular polarization in both transitions Publication: The Astrophysical Journal Pub Date: August 1968 DOI: 10.1086/149688 Bibcode: 1968ApJ...153..577B full text sources ADS |

RELATIONSHIP OF THE SMALL-DIAMETER RADIO AND OPTICAL FEATURES IN NGC 1068, NGC 1275, AND NGC 4486

NGC 1068, NGC 1275, and NGC 4486 are all relatively nearby galaxies possessing small-diameter radio and optical features which seem, on the basis of positional coincidence and similarity of size and shape, to be related. The first two are galaxies, and in them the radio feature seems to be associated with the Seyfert nucleus. In the third, the radio feature is associated with the wellknown jet. In addition both NGC 1275 and NGC 4486 have largediameter radio source regions and are thus core-halo radio galaxies. NGC 1068 (radio source 3C 71) -This is a tenth magnitude Sb spiral with a nucleus, which has been discussed recently by Osterbrock and Parker (1965). The optical nucleus is about 2 in angular diameter (Woltjer 1959). As may be seen in Table I, the positional agreement between the radio and optical objects is excellent, leaving little doubt that the two are closely associated. The radio source has two components. Two-thirds of the emission at cm comes from a component about 10 in diameter ( or a few hundred parsecs at the distance of NGC 1068 ) while the remaining third comes from a core < 1'.'5 in diameter (Bash 1967). There is no evidence for any radio component comparable in size to the 5' optical diameter of the galaxy. Osterbrock and Parker and other writers interpret the optical continuum radiation from the nucleus of NGC 1068 as synchrotron radiation. NGC 1275 (radio source 3C 84 or Perseus A) -This is a magnitude 13 elliptical with a nucleus and many other peculiar