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At least 1,081 records · Page 60
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NASA NTRS1981

Dissipational galaxy formation - Confrontation with observations

An exploration is presented of the hypothesis that a protogalaxy consists of an aggregate of interacting gas clouds which undergo mergers with neighboring systems, as envisaged by both the hierarchical clustering and fragmentation schemes of galaxy formation. Both gaseous dissipation and violet relaxation play fundamental roles in this galaxy formation model, in order to account for such diverse structural and dynamical properties of spheroidal galaxies as velocity anisotropy and metallicity gradients. Protogalaxy mergers during the initial stages of galaxy clustering can account for the observed spatial distribution of spiral, S0, and elliptical galaxies, and galaxy formation can occur slowly and at late epochs, since the time scale for disk formation is less than about 10 to the 10th years.

Silk, J.↗
NASA NTRS1982

X-ray observations of peculiar galaxies with the Einstein Observatory

The results of X-ray observations with the Einstein Observatory of 33 galaxies are presented. Almost all of these galaxies, some of which lie in groups and in interactive pairs, display disturbed morphologies. No uniform, well-defined selection criterion can describe the sample galaxies, but almost all lie in a region of the optical color/color diagram that suggests that they are experiencing bursts of star formation. This region is compatible with the locus of Markarian galaxies. The observed sample and the X-ray data are presented, and the X-ray data are discussed and compared with the optical and radio properties of the galaxies. It is concluded that nuclear activity is not the predominant source of X-rays in late-type peculiar galaxies and that the X-ray emission is likely to originate from the Population I galactic component, in particular binary sources and possible young SNR's.

Fabbiano, G.↗
NASA NTRS1982

Near-infrared photometry of distant radio galaxies - Spectral flux distributions and redshift estimates

Calculations modeling the effects of stellar evolution on the observed colors of elliptical galaxies suggest that the shape of their spectra should remain almost constant in the red and near-infrared for red shifts greater than two. In order to test this idea, JHK photometry of optically faint radio source identifications believed to be faint elliptical galaxies were carried out. Photometric redshift estimates have been made using three different models: a nonevolving elliptical galaxy spectrum, an evolving spectrum based on one of Bruzual's calculations, and an empirical model derived from photometry of distant elliptical galaxies. Redshifts greater than 0.5 are systematically under-estimated by the first two models and for galaxies whose colors are not consistent with the models, deviations occur not only in the optical bands but also in the infrared. The difficulties are not resolved by the addition of a quasar-like nucleus to the galaxies.

Puschell, J. J.↗
NASA NTRS1982

The evolution of galaxies

The recent observational evidence on the evolution of galaxies is reviewed and related to the framework of current ideas for galaxy formation from primordial density fluctuations. Recent strong evidence for the evolution of the stellar population in ellipticals is presented, as well as evidence that not all ellipticals behave as predicted by any simple theory. The status of counts of faint galaxies and the implications for the evolution of spirals is discussed, together with a discussion of recent work on the redshift distribution of galaxies at faint magnitudes and a spectroscopic investigation of the Butcher-Oemler blue cluster galaxies. Finally a new picture for the formation and evolution of disk galaxies which may explain most of the features of the Hubble sequence is outlined.

Gunn, J. E.↗
NASA NTRS1982

Supernovae and the formation of galaxies

Attention is given to the cumulative effect of many SNs in a galaxy, especially a young galaxy, as well as the effect of the blast wave resulting from the combination of all individual SNs on the intergalactic medium (IGM). It is suggested that the IGM can be shock-heated by this process, so that in the late stages of an individual expanding galaxy's shock a shell may form whose mass is many times greater than that of the seed galaxy. If this shell fragments, due to gravitational instability, it will give rise to a new generation of galaxies. SN explosions may thereby provide a hydrodynamic amplifier of some significance in the process of galaxy formation.

Ostriker, J. P.↗
NASA NTRS1983

X-ray and optical studies of emission-line Markarian galaxies

The X-ray and optical properties of a selection of compact nucleus, emission line Markarian galaxies have been studied. Observations with the Einstein Observatory have detected X-rays from all the Seyfert galaxies and none of the narrow emission line galaxies in the sample. This result supports the idea that powerful X-ray emission is correlated with the presence of a broad optical emission line component. The emission line spectra of the Seyfert galaxies show much higher excitation than do the narrow-line galaxies, whose spectra are similar to those of H II regions. Both the optical properties and the radio and X-ray upper limits of the non-Seyfert galaxies are consistent with the suggestion that those objects have undergone bursts of massive star formation.

Ulvestad, J. S.↗
NASA NTRS1983

2-165 keV observations of active galaxies and the diffuse background

HEAO 1 spectral observations of 12 active galaxies in the 12-165 keV and 2-50 keV ranges are reported. The spectra of these galaxies in the 2-165 keV range are well represented by a single power law model; within experimental uncertainties a narrow dispersion in power law index attributable to the individual galaxies is observed, while the 2-165 keV luminosities of these galaxies ranged from 3 x 10 to the 43rd to 3 x 10 to the 45th ergs/s. An apparent universality of the spectral form is found which can be interpreted as due to a common electron distribution with a temperature of tens of keV in the Compton scattering region or as a common nonthermal power-law distribution generating the observed flux through synchrotron-Compton processes. The data indicate that relativistic particles are likely to be responsible for the X-rays from cores of active galaxies through synchroton-Compton processes. In addition, it is noted that only weak number evolution, if any at all, is present in active galaxies.

Rothschild, R. E.↗
NASA NTRS1983

The X-ray emitting gas in poor clusters with central dominant galaxies

The 12 clusters detected in the present study by the Einstein Observatory's X-ray imaging proportional counter show X-ray emission centered on the dominant galaxy in all cases. Comparison of the deduced distribution of binding mass with the light distribution of the central galaxies of four clusters indicates that the mass/luminosity ratio rises to over 200 solar masses/solar luminosity in the galaxy halos. These halos must therefore, like the clusters themselves, posses dark matter. The X-ray data clearly show that the dominant galaxies sit at the bottoms of the poor cluster gravitational potential wells, suggesting a similar origin for dominant galaxies in poor and rich clusters, perhaps through the merger and cannibalism of cluster galaxies. It is the luminosity of the distended cD envelope that reflects the relative wealth of the cluster environment.

Kriss, G. A.↗
NASA NTRS1984

Infrared galaxies in the IRAS minisurvey

A total of 86 galaxies have been detected at 60 microns in the high galactic latitude portion of the IRAS minisurvey. The surface density of detected galaxies with flux densities greater than 0.5 Jy is 0.25 sq deg. Virtually all the galaxies detected are spiral galaxies and have an infrared to blue luminosity ratio ranging from 50 to 0.5. For the infrared-selected sample, no obvious correlation exists between infrared excess and color temperature. The infrared flux from 10 to 100 microns contributes approximately 5 percent of the blue luminosity for galaxies in the magnitude range 14 less than m(pg) less than 18 mag. The fraction of interacting galaxies is between one-eighth and one-fourth of the sample.

Soifer, B. T.↗
NASA NTRS1984

The dusty, luminous broad-line radio galaxy 3C 109

In the present optical polarimetry and IR photometry for the broad line radio galaxy 3C 109, the absence of variability, in conjunction with a polarization in H-alpha that is identical to that of the adjacent continuum, suggests that the strong polarization of this galaxy is due to extinction by aligned dust grains. This is the process which gives rise to interstellar polarization within the Galaxy. The energy output of 3C 109, from the optical to 20 microns, exceeds by a factor of 2-4 that of the Seyfert 1 galaxy Markarian 231. These observations, together with recent polarimetry of other broad line radio galaxies, indicate that their broad line regions are more heavily reddened than those of either the Seyfert 1 galaxies or quasars. Evidence for a gradual decrease in dust content with increasing source luminosity is discussed.

Rudy, R. J.↗
NASA NTRS1983

The formation of galaxies

Current models of galaxy formation are examined in a review of recent observational and theoretical studies. Observational data on elliptical galaxies, disk galaxies, luminosity functions, clustering, and angular fluctuations in the cosmic microwave background are summarized. Theoretical aspects discussed include the origin and early evolution of small fluctuations, matter and radiation fluctuations, the formation of large-scale structure, dissipationless galaxy formation, galaxy mergers, dissipational galaxy formation, and the implications of particle physics (GUTs, massive neutrinos, and gravitinos) for cosmology.

Efstathiou, G.↗
NASA NTRS1985

IRAS observations of Seyfert galaxies

Infrared Astronomy Satellite measurements at 25, 60 and 100 microns were used to analyze the infrared properties of Seyfert galaxies from the Markarian and NGC Catalogs. One hundred and sixteen of 186 Seyfert galaxies were detected. About 50% of all Seyfert galaxies in the sample have 60 micron luminosities in excess of 10 to the 10th power solar luminosity, and the mean 60 micron luminosity increase with the optical B absolute magnitude. The luminosity functions of the Seyfert 1 and Seyfert 2 galaxies appear quite similar. It is possible, however, to statistically separate the two types of galaxies in color-color plots. The 100- to 60- micron energy distributions flatten systematically with increasing 60- micron luminosity. The infrared measurements provide a measure of the bolometric luminosity of the Seyfert galaxies, but do not discriminate between the physical processes involved.

Miley, G. K.↗
NASA NTRS1984

X-ray observations of spiral galaxies. I - Integrated properties

The results of X-ray observations carried out with the Einstein satellite of 14 nearby field spiral and irregular galaxies are reported. The integrated properties of the galaxies are examined in the luminosity range from 10 to the 38th and 10 to the 40th ergs per sec at 0.5-3.0 keV. It is found that the X-ray luminosities of the galaxies are correlated with both the optical (blue) and the radio luminosities (1.4 GHz). The linear relationship found between the X-ray and the optical blue luminosities suggests a relationship to be between the X-ray luminosity and the stellar content of spiral galaxies exhibiting blue luminosities. On the basis of the correlations, it is proposed that most of the spiral galaxies are relatively young systems of not less than about 10 to the 9th years. Data obtained from the Einstein satellite and those of a previous investigation of the blue galaxies are compared in a table.

Fabbiano, G.↗
NASA NTRS1985

Star formation in grand design and flocculent spiral galaxies

An examination is conducted of the colors and neutral hydrogen contents of spiral galaxies which have been classified in the Elmegreen and Elmegreen arm morphology system. Using these data, possible differences between the star formation activity in spiral galaxies with and without classic spiral arms are delineated. At the same revised Hubble type, spiral galaxies with regular global arm patterns (the grand design spirals) are bluer than spiral galaxies lacking such patterns (the flocculent spirals) by a small, but statistically significant amount (approximately 0.05 in B-V and approximately 0.15 in U-V). The neutral hydrogen contents of the two groups are roughly similar, implying the star formation rate averaged over a Hubble time has been approximately the same in grand design and flocculent systems. The color differences can be explained either by using an initial mass function in the flocculent spiral galaxies which is deficient in massive stars by a factor of 2 compared with the grand design systems, or by a decrease of 30 percent in the ratio of recent to past star formation rates in flocculent galaxies. The possibility is discussed that systematic Hubble type misclassifications significantly affect these conclusions.

Romanishin, W.↗
NASA NTRS1985

IUE observations of amorphous hot galaxies

Blue amorphous galaxies are star-forming, irregularlike systems which lack the spatially distinct OB stellar groups that are characteristic of most late-type galaxies. In order to better understand the nature of star-formation processes in these unusual galaxies, short-wavelength IUE spectra of the amorphous galaxies NGC 1705 and NGC 1800 have been obtained. It is found that NGC 1705 contains a normal mix of OB stars, which is consistent with the nearly constant recent star-formation rate inferred from new optical data. NGC 1800 is likely to have similar properties, and blue galaxies with amorphous structures thus do not show evidence for anomalies in stellar populations. The UV spectra of these galaxies and a variety of other hot extragalactic stellar systems in fact have similar characteristics, which suggests OB stellar populations are often homogeneous in their properties.

Lamb, S. A.↗
NASA NTRS1985

IRAS observations of Seyfert galaxies

Infrared Astronomy Satellite measurements at 25, 60 and 100 microns were used to analyze the infrared properties of Seyfert galaxies from the Markarian and NGC Catalogs. One hundred and sixteen of 186 Seyfert galaxies were detected. About 50 percent of all Seyfert galaxies in the sample have 60 micron luminosities in excess of 10 to the 10th power solar luminosity, and the mean 60 micron luminosity increase with the optical B absolute magnitude. The luminosity functions of the Seyfert 1 and Seyfert 2 galaxies appear quite similar. It is possible, however, to statistically separate the two types of galaxies in color-color plots. The 100- to 60-micron energy distributions flatten systematically with increasing 60-micron luminosity. The infrared measurements provide a measure of the bolometric luminosity of the Seyfert galaxies, but do not discriminate between the physical processes involved.

Miley, G. K.↗
NASA NTRS1985

CCD surface photometry of field Galaxies. II - Bulge/disk decompositions

Major- and minor-axis profiles given previously for 105 galaxies of all morphological types are decomposed into bulge and disk components. In addition, three model-independent parameters which measure the mean surface brightness, scale radius, and degree of light concentration are derived. The best correlations are found between Hubble type, concentration, bulge/disk ratio, and mean surface brightness. Correlations between the individual bulge and disk parameters generally show large scatter. The properties of S0 galaxies are inconsistent with their having been formed from spiral galaxies via gas depletion; their properties are intermediate between those of ellipticals and spirals. Most elliptical galaxies probably do not form by the merging of disk galaxies. The difficulty of distinguishing between elliptical and S0 galaxies in some cases is emphasized.

Kent, D. M.↗
NASA NTRS1986

IUE spectra and a resulting model of Seyfert 2 galaxies

Nearly simultaneous optical and ultraviolet observations of several Seyfert 2 galaxies are presented. A mean optical to ultraviolet emission line spectrum for Seyfert 2 galaxies is determined, and these intensities are compared with predictions of photoionization models. The UV to X-ray nonthermal continuum which ionizes this gas is examined. The results suggest that an obscuring layer does not hide an underlying broad-line region, and that narrow-line objects such as Seyfert 2 and narrow-line radio galaxies are physically different from broad-line objects, at least in not having an inner broad-line region. The absence of extinction also distinguishes these classical Seyfert 2 galaxies from the narrow-line X-ray galaxies, which X-ray evidence suggests may be extinguished Seyfert 1 galaxies.

Ferland, G. J.↗