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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1997

X-Ray Signatures of Low Luminosity AGN and Non-AGN Activities in Galaxies

This talk will review the present knowledge, based on x-ray observations (Einstein, ROSAT, ASCA) of low-luminosity activity in otherwise normal galaxies. These phenomena cover the x-ray emission of low-luminosity AGN and LINERs found in the nuclei of galaxies. The talk will discuss these phenomena in the context of the more general AGN phenomenon. The other type of activity, which will also be discussed, is connected with enhanced starburst activity. Starburst activity has been found linked in x-rays with hot galactic winds, and gives us direct evidence of the interplay between galaxies and the surrounding medium. The talk will conclude by discussing the opportunities offered by the upcoming launch of AXAF.

Fabbiano, Giuseppina↗
NASA NTRS1993

Double-peaked broad line emission from the LINER nucleus of NGC 1097

We report the recent appearance of a very broad component in the H-alpha and H-beta emission lines of the weakly active nucleus of the Sersic-Pastoriza galaxy NGC 1097. The FWZI of the broad component is about 21,000 km/s, and its profile is double-peaked; the presence of a blue, featureless continuum in the nucleus is also suggested. The broad component was first observed in H-alpha in November 2, 1991, and confirmed 11 months later. The H-alpha profile and flux did not change in this time interval. Comparison with previously published spectral data indicates that the broad lines have only recently appeared. Together with the relatively high X-ray luminosity and the compact nuclear radio source, our results characterize the presence of a Seyfert 1 nucleus in a galaxy which had previously shown only LINER characteristics. Obscuring material along our line of sight to the nucleus appears to have recently cleared, permitting a direct view of the active nucleus. We discuss two possible structures for the broad line region, biconical outflow and an accretion disk, that could give rise to the observed profile.

Storchi-Bergmann, Thaisa↗
NASA NTRS2003

The Sombrero: An Edge-On Spiral: Halo Dynamics, A Massive Dark Object and XRBS

We present an analysis of the XMM-Newton and Chandra ACIS-S observations of the LINER nucleus of the Sombrero galaxy and we discuss possible explanations for its very sub-Eddington luminosity by complementing the X-ray results with high angular resolution observations in other bands. The X-ray investigation shows a hard (Gamma=1.89) and moderately absorbed ( N(sub H) = 1.8 x l0(exp 21)/sq cm) nuclear source of 1.5 x l0(exp 40) erg/s in the 2-10 keV band, surrounded by hot gas at a temperature of approx. 0.6 keV. The bolometric nuclear luminosity is at least approx. 200 times lower than expected if mass accreted on the supermassive black hole, that HST shows to reside at the center of this galaxy, at the rate predicted by the spherical and adiabatic Bondi accretion theory and with the high radiative efficiency of a standard accretion disc. The low luminosity, coupled to the observed absence of Fe-K emission in the nuclear spectrum, indicates that such a disc is not present. This nucleus differs from bright unobscured AGNs also for the lack of high flux variability and of prominent broad Halpha emission. However, it is also too faint for the predictions of simple radiatively inefficient accretion taking place at the Bondi rate; it could be too radio bright, instead, for radiatively inefficient accretion that includes strong mass outflows or convection. This discrepancy could be solved by the possible presence of nuclear radio jets. An alternative explanation of the low luminosity, in place of radiative inefficiency, could be unsteady accretion.

Mushotzky, Richard F.↗
NASA NTRS2004

The Sombrero: An Edge-On Spiral: Halo Dynamics, A Massive Dark Object, and XRBS

We present an analysis of the XMM-Newton and Chandra ACIS-S observations of the LINER nucleus of the Sombrero galaxy, and we discuss possible explanations for its very Sub-Eddington luminosity by complementing the X-ray results with high angular resolution observations in other bands. The X-ray investigation shows a hard (gamma=1.89) and moderately absorbed (NH=1.8x1021 per square centimeter) nuclear source of 1.5x1040 ergs per second in the 2-10 keV band, surrounded by hot gas at a temperature of approximately 0.6 keV. The bolometric nuclear luminosity is at least approximately 200 times lower than expected if mass accreted on the supermassive black hole, which Hubble Space Telescope shows to reside at the center of this galaxy, at the rate predicted by the spherical and adiabatic Bondi accretion theory and with the high radiative efficiency of a standard accretion disk. The low luminosity, coupled to the observed absence of Fe K emission in the nuclear spectrum, indicates that such a disk is not present. This nucleus also differs from bright unobscured active galactic nuclei in the lack of high flux variability and prominent broad H emission. However, it is also too faint for the predictions of simple radiatively inefficient accretion taking place at the Bondi rate; it could instead be too radio bright for radiatively inefficient accretion that includes strong mass outflows or convection. This discrepancy could be solved by the possible presence of nuclear radio jets. An alternative explanation of the low luminosity, in place of radiative inefficiency, could be unsteady accretion.

Fabbiano, G.↗
NASA NTRS1995

A duty cycle hypothesis for the central engines of LINERs

A recent ultraviolet snaphsot imaging survey of the nuclei of nearby galaxies detected a compact nuclear ultraviolet source in only five of the 26 LINERs (low-ionization nuclear emission-line regions) included in the observed sample. Motivated by this observational result, we examine the possibility that all LINERs are powered by photoionization from a nuclear source, which is, however, active only for 20% of the time. We show that decay times of low-ionization species can be of the order of one to a few centuries, and we demonstrate through time-dependent photoionization calculations that if the nuclear ionizing source is active for only a fraction of the time, this would not be readily noticeable in the emission-line spectrum. We suggest that the activity cycle is related to episodic accretion events which are associated with the tidal disruption of stars by a central black hole. The time interval between tidal disruptions is of the same order as the emission-line decay time, with the accretion episode following each disruption lasting a few decades. These estimates appear to support the duty cycle hypothesis. Some observational consequences of the proposed scenario are also discussed.

Eracleous, Michael↗
NASA NTRS1993

The genesis of starbursts and infrared emission in the centers of galaxies

We present 10.8-micron maps of the central regions of 21 IR-luminous galaxies. Each map, obtained using the NASA MSFC bolometer array at the IRTF, has a resolution of about 4 arcsec and spans typically 25 x 20 arcsec. Some of the galaxies have also been mapped at 12.5, 19.2, and 30/microns. The sample consists primarily of starburst galaxies but includes several Seyferts and LINERs. On average, the flux detected in each map is about 58 percent of that detected at 12 microns by IRAS. We see a variety of morphologies, including compact and elongated sources and more complex distributions exhibiting such features as kiloparsec-size rings and double lobes. The mid-IR radiation is emitted by dust particles, and for two of the galaxies we see spatial color variations attributable to the presence of very small grains and consistent with their depletion in the starbursts. The mid-IR emission traces the dust heated by the youngest stars. Comparison of the distributions of intense star formation and the dense neutral interstellar medium for eight of these galaxies demonstrates that the two are intimately linked.

Telesco, C. M.↗
NASA NTRS1994

The discovery of five new H2O megamasers in active galaxies

H2O megamasers with (isotropic) luminosities between 60 and 200 solar luminosity (H(sub 0) = 75 km/s/Mpc) have been detected in the Seyfert 2 galaxies Mrk 1, Mrk 1210, and NGC 5506 and in the LINERs NGC 1052 and NGC 2639. No megamasers have been found in Seyfert 1's. The galaxies have redshifts between 1500 and 4800 km/s and are the most distant H2O sources reported to date. NGC 1052 is also the first elliptical galaxy known to contain an H2O maser. The intensity distribution of an H2O five-point map obtained toward NGC 5506 shows that the H2O emission is pointlike compared to the 40 sec telescope beam. The lack of CO emission in NGC 1052 implies a conservative lower limit to the H2O brightness temperature of 1000 K, thus ruling out a thermal origin for the H2O emission. The success of this survey relative to other recent searches makes it evident that H2O megamasers are preferentially found in galaxies with active nuclei.

Braatz, J. A.↗
NASA NTRS2000

Hard X-Ray Emission and the Ionizing Source in LINERs

We report X-ray fluxes in the 2-10 keV band from LINERs (low-ionization nuclear emission-line regions) and low-luminosity Seyfert galaxies obtained with the ASCA satellite. Observed X-ray luminosities are in the range between 4 x 10(exp 39) and 5 x 10(exp 41) ergs/s, which are significantly smaller than that of the "classical" low-luminosity Seyfert 1 galaxy NGC 4051. We found that X-ray luminosities in 2-10 keV of LINERs with broad H.alpha emission in their optical spectra (LINER 1s) are proportional to their Ha luminosities. This correlation strongly supports the hypothesis that the dominant ionizing source in LINER 1s is photoionization by hard photons from low-luminosity AGNs. On the other hand, the X-ray luminosities of most LINERs without broad H.alpha emission (LINER 2s) in our sample are lower than LINER 1s at a given H.alpha luminosity. The observed X-ray luminosities in these objects are insufficient to power their H.alpha luminosities, suggesting that their primary ionizing source is other than an AGN, or that an AGN, if present, is obscured even at energies above 2 keV.

Terashima, Yuichi↗
NASA NTRS1993

The photoionization mechanism of LINERs - Stellar and nonstellar

We present high quality spectroscopic observations of a sample of 14 LINERs. Starlight removal is achieved by the subtraction of a suitable absorption-line 'template' galaxy, allowing accurate measurements of emission lines. We use these line fluxes to examine the possible excitation mechanisms of LINERs. We suggest that LINERs with weak forbidden O I 6300-A emission may be H II regions photoionized by unusually hot O-type stars. LINERs with forbidden O I/H-alpha approximately greater than 1/6 may be powered by photoionization from a nonstellar continuum. This is supported by the detection of broad H-alpha emission, a correlation between line width and critical density, and pointlike X-ray emission in several of these objects.

Ho, Luis C.↗
NASA NTRS1994

The cosmic X-ray background-IRAS galaxy correlation and the local X-ray volume emissivity

We have cross-correlated the galaxies from the IRAS 2 Jy redshift survey sample and the 0.7 Jy projected sample with the all-sky cosmic X-ray background (CXB) map obtained from the High Energy Astronomy Observatory (HEAO) 1 A-2 experiment. We have detected a significant correlation signal between surface density of IRAS galaxies and the X-ray background intensity, with W(sub xg) = (mean value of ((delta I)(delta N)))/(mean value of I)(mean value of N)) of several times 10(exp -3). While this correlation signal has a significant implication for the contribution of the local universe to the hard (E greater than 2 keV) X-ray background, its interpretation is model-dependent. We have developed a formulation to model the cross-correlation between CXB surface brightness and galaxy counts. This includes the effects of source clustering and the X-ray-far-infrared luminosity correlation. Using an X-ray flux-limited sample of active galactic nuclei (AGNs), which has IRAS 60 micrometer measurements, we have estimated the contribution of the AGN component to the observed CXB-IRAS galaxy count correlations in order to see whether there is an excess component, i.e., contribution from low X-ray luminosity sources. We have applied both the analytical approach and Monte Carlo simulations for the estimations. Our estimate of the local X-ray volume emissivity in the 2-10 keV band is rho(sub x) approximately = (4.3 +/- 1.2) x 10(exp 38) h(sub 50) ergs/s/cu Mpc, consistent with the value expected from the luminosity function of AGNs alone. This sets a limit to the local volume emissivity from lower luminosity sources (e.g., star-forming galaxies, low-ionization nuclear emission-line regions (LINERs)) to rho(sub x) less than or approximately = 2 x 10(exp 38) h(sub 50) ergs/s/cu Mpc.

Miyaji, Takamitsu↗
NASA NTRS1998

The Nuclear Spectrum of the Radio Galaxy NGC 5128 (Centaurus A)

We present near-infrared spectra of the nucleus of the nearby radio galaxy NGC 5128 (Centaurus A). The observed emission line strengths suggest that NGC 5128 should be classified as a LINER, and appear to be powered by shocks.

galaxies active galaxies NGC 5128 Centaurus A nucl↗
NASA NTRS1990

A survey of galaxies in CO

A large survey of galaxies in the CO (J=1-0) line is presented. Among different types of active galaxies, within the sample, the detection rate is found to be: Starbursts (15 percent), Liners (60 percent), Seyferts (30 percent). A look at the subsample that is located within 40 Mpc (Ho = 75 km s(-1) Mpc(-1) regarding local density of galaxies reveals no substantial difference between detections and non-detections.

Elfhag, Torsten↗
NASA NTRS1993

An IUE search for star formation in ellipticals with gaseous disks

The UV spectral energy distribution presently derived from IUE observations for NGC 5077 is presently compared with those of the bulges of two galaxies, M31 and NGC 3115, which exhibit a similar Mg2 absorption-liner strengths and are known to lack gaseous disks. The lack of significant UV spectrum differences imply that there is no star formation activity more recent than 3 million years old in NGC 5077.

Bertola, Francesco↗
NASA NTRS1987

Starburst-driven superwinds from infrared galaxies

New data is presented that indicate that strong far infrared galaxies commonly have largescale emission line nebulae whose properties are suggestive of mass outflows (superwinds), presumably driven by the high supernova rate associated with the central starburst. These data include longslit spectra of M82 which show that the radial variation of the gas pressure in the emission line nebula is in excellent agreement with a previous wind model. The M82 nebula also has a LINER spectrum, consistent with shock heating. Morphologically and spectroscopically similar emission line nebulae were found in NGC253, and Arp 220 and NGC6240. A longslit spectroscopic investigation was conducted of 20 additional very powerful far-infrared galaxies and found that they generally have spatially extended emission line nebulae whose spectra closely resemble that of the M82 nebula. If the superwind interpretation is correct, it could have many important consequences in extragalactic astronomy.

Heckman, Timothy M.↗
NASA NTRS2017

iPTF Discovery of the Rapid "Turn-On" of a Luminous Quasar

We present a radio-quiet quasar at z = 0.237 discovered ''turning on'' by the intermediate Palomar Transient Factory (iPTF). The transient, iPTF 16bco, was detected by iPTF in the nucleus of a galaxy with an archival Sloan Digital Sky Survey spectrum with weak narrow-line emission characteristic of a low-ionization nuclear emission-line region (LINER). Our follow-up spectra show the dramatic appearance of broad Balmer lines and a power-law continuum characteristic of a luminous (L(sub bol) approx. equal 10(exp 45) erg/s) type 1 quasar 12 yr. later. Our photometric monitoring with PTF from 2009-2012 and serendipitous X-ray observations from the XMM-Newton Slew Survey in 2011 and 2015 constrain the change of state to have occurred less than 500 days before the iPTF detection. An enhanced broad H(alpha)/[O III] (lambda)5007 line ratio in the type 1 state relative to other changing-look quasars also is suggestive of the most rapid change of state yet observed in a quasar. We argue that the greater than 10 increase in Eddington ratio inferred from the brightening in UV and X-ray continuum flux is more likely due to an intrinsic change in the accretion rate of a preexisting accretion disk than an external mechanism such as variable obscuration, microlensing, or the tidal disruption of a star. However, further monitoring will be helpful in better constraining the mechanism driving this change of state. The rapid ''turn-on'' of the quasar is much shorter than the viscous infall timescale of an accretion disk and requires a disk instability that can develop around a approx. equal 10(exp 8)solar mass black hole on timescales less than 1 yr.

Gezari, S.↗
NASA NTRS1987

Evidence for large-scale winds from starburst galaxies. II - An optical investigation of powerful far-infrared galaxies

The results of long-split spectroscopy of 20 powerful (about 10 to the 12th solar luminosities) far-infrared galaxies (FIRGs) are discussed, and deep narrowband images of the prototypical tepid FIRGs Arp 220 and NGC 6240 are presented. The results show that, as a class, the emission-line spectra of tepid FIRGs are most similar to LINERs, but about half are intermediate between LINERs and low-excitation H II regions. Huge (10,000-100,000-pc) and morphologically spectacular emission-line nebulae resembling the M82 filament system are associated with both NGC 6240 and Arp 220. It is proposed that the model of Chevalier and Clegg (1985) and McCarthy et al. (1987) for M82 be generalized to encompass powerful FIRGs as a class.

Heckman, Timothy M.↗
NASA NTRS1982

Four years of IUE research on clumpy irregular galaxies

During the first four years of IUE operations seven shifts were used to observe 4 clumpy irregular galaxies: Markarian 7, 8, 297, and 325. All spectra were obtained at low resolution in both short and long wavelength, with exposure time ranging from half to a full shift. The IUE spectra of clumpy irregular galaxies show that the clumps contain a very large number of early O and B type stars, with a large number of supergiants with respect to the main sequence stars. Possibly Wolf-Rayet stars and massive objects of the type of R 136a could also contribute to the clump luminosity. On the average, each clump radiates in the UV 100 times more than 30 Dor. On the other end the liner dimension of a clump is not much larger than the one of 30 Dor.

Benvenuti, P.↗
NASA NTRS1992

Infrared line diagnostics of active galactic nuclei

The CLOUDY photoionization code of Ferland (1991) is employed to predict intensities and ratios of the near-, mid-, and far-IR ionic fine-structure lines from AGN of the LINER and Seyfert types, and starbursts. The depths of varying the important model parameters, including ionization parameter, density, and optical depth, are investigated. The way in which detailed IR spectroscopy can reveal the presence of density inhomogeneities and abundance anomalies is shown, and the slope of the ionizing continuum and the amount of reddening and the reddening law are determined. The predictions of the present model and others are compared to the observed line ratios of the starburst galaxy M82 and of the Galactic center. No single-zone model adequately predicted all of the observed lines, but a simple combination of a starburst plus photodissociation region of higher density have a good fit for M82. The present model predictions are used to estimate the detectability of IR fine-structure lines in various active galaxies.

Spinoglio, Luigi↗