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At least 1,171 records · Page 65
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NASA NTRS1993

What do you gain from deconvolution? - Observing faint galaxies with the Hubble Space Telescope Wide Field Camera

We describe experiments with deconvolutions of simulations of deep HST Wide Field Camera images containing faint, compact galaxies to determine under what circumstances there is a quantitative advantage to image deconvolution, and explore whether it is (1) helpful for distinguishing between stars and compact galaxies, or between spiral and elliptical galaxies, and whether it (2) improves the accuracy with which characteristic radii and integrated magnitudes may be determined. The Maximum Entropy and Richardson-Lucy deconvolution algorithms give the same results. For medium and low S/N images, deconvolution does not significantly improve our ability to distinguish between faint stars and compact galaxies, nor between spiral and elliptical galaxies. Measurements from both raw and deconvolved images are biased and must be corrected; it is easier to quantify and remove the biases for cases that have not been deconvolved. We find no benefit from deconvolution for measuring luminosity profiles, but these results are limited to low S/N images of very compact (often undersampled) galaxies.

Schade, David J.↗
NASA NTRS1993

The morphological catalogue of galaxies equatorial survey

We present 865 redshifts of galaxies located in the equatorial strip delta between -17.5 deg and -2.5 deg in the right ascension range between 20 h and 5 h. Redshifts have been obtained for the complete sample of all 833 galaxies in the Morphological Catalog of Galaxies with magnitudes brighter than m = 14.5 (corresponding approximately to m(Zwicky) = 15.0). This sample also includes three galaxies from other sources with more reliable magnitudes, satisfying this limit, and 29 fainter galaxies, usually companions of the galaxies in the magnitude limited sample. Our maps of a very large volume of nearby space demonstrate a variety of coherent large scale structures which include large voids, 20-50/h Mpc in diameter and large walls at least 70/h Mpc across.

Huchra, John↗
NASA NTRS1993

Molecular gas in powerful radio galaxies detected by IRAS

The present CO(J = 1 to 0) line survey of powerful radio galaxies has led to the detection of five sources in eight IRAS-determined radio galaxies. The range of the computed molecular gas masses is 1-7 times the H2 mass of the Milky Way and strikingly contrasts with the low molecular gas masses found in radio-quiet FIR-selected elliptical galaxies. These new CO observations lend support to the hypothesis that powerful radio galaxies result from disk galaxy collisions that evolve into gas-rich, peculiar E/S0 galaxies in the course of their merging.

Mazzarella, J. M.↗
NASA NTRS1993

Hydrogen and metal absorption lines in PKS 0405-123 from the halos of low redshift galaxies

HST ultraviolet spectra and ground-based optical spectra of the bright quasar PKS 0405-123 are used to identify two absorption-line systems probably produced by the halos of foreground galaxies. The quasar lies in a rich galaxy field. Loose groups of galaxies are present in the field at mean z1 = 0.1669 and mean z2 = 0.3519. Ly-alpha, C IV 1549 A, and probably Mg II 2798 A resonance lines are detected in an absorption system at z = 0.1670. An absorption system consisting of only Ly-alpha is observed at z = 0.3516. We cannot be completely certain which foreground galaxy is responsible for the metal line absorption system at z1. A likely candidate is a large, luminous, early-type spiral 40 arcsec to the east of the quasar. However, the origin of the z1 system could possibly be a small undetected galaxy more precisely aligned along our line of sight to PKS 0405-123, or a very faint galaxy whose redshift has not yet been measured.

Spinrad, Hyron↗
NASA NTRS1993

Deep Hubble Space Telescope imaging of 53W044 - An S0 radio galaxy at z = 0.311

Images of the Wide Field Camera (WFC) and Faint-Object Camera (FOC) of the radio galaxy 53W044 are presented. The WFC images are used to examine the structure of the galaxy, and show evidence for a significant disk, on the basis of which 53W044 is classified as an S0. This radio galaxy is near the maximum radio power associated with sources in S0 host galaxies. The FOC image is combined with ground-based spectroscopy to study 53W044's stellar population, which appears normal for an E/S0 galaxy of modest luminosity. No evidence is found for a significant contribution from a nuclear blue-continuum source, and the stellar population is old with a continuum level at 2100 A, consistent with what is seen in nearby radio galaxies.

Keel, William C.↗
NASA NTRS1993

Star formation in the disks of H I-rich S0 galaxies

We present the results of a H-alpha emission-line imaging survey of a sample of neutral-gas-rich S galaxies. We find evidence of disk H II regions in 14 of our sample of 32 galaxies, detect nuclear or faint diffuse circumnuclear H-alpha + forbidden N II emission in another 11 galaxies without disk H II regions, and obtain upper limits for 8 galaxies. We find a striking dichotomy between Ss with and without H II regions; either a galaxy has a number of H II regions, most often distributed into distinct rings or ringlike structures, or there are none down to detection limits equivalent to a single unreddened H II region ionized by single O stars. We find that the S0s without disk H II regions have a lower median M sub H I/L(B) than those with disk H II regions, but the distributions have a large range of dispersion. Our data suggest that S0s may lie in a regime where local threshold effects, perhaps primarily kinematic in origin, are more important in determining the star formation in these galaxies than the global stability mechanisms that recent empirical models for large-scale star formation have suggested prevail in later-type spirals.

Pogge, Richard W.↗
NASA NTRS1994

ROSAT detection of diffuse hot gas in the edge-on galaxy NGC 4631

ROSAT observation is presented of the edge-on spiral galaxy NGC 4631, a nearby Sc/SBd galaxy best known for its extended radio halo. Because of the low foreground Galactic X-ray-absorbing gas column density, N(sub H) approximately 1.4 x 10(exp 20)cm(exp -2), this observation is sensitive to gas of temperature greater than or equal to a few times 10(exp 5) K. A soft (approximately 0.25 keV) X-ray radiation out to more than 8 kpc above the midplane of the galaxy was detected. The strongest X-ray emission in the halo is above the central disk, a region of about 3 kpc radius which shows high star formation activity. The X-ray emission in the halo is bordered by two extended filaments of radio continuum emission. Diffuse X-ray emission from hot gas in the galaxy's disk was found. The spectrum of the radiation can be characterized by a thermal plasma with a temperature of 3 x 10(exp 6) K and a radiative cooling rate of approximately 8 x 10(exp 39) ergs s(exp -1). This rate is only a few percent of the estimated supernova energy release in the interstellar medium of the galaxy. Analysis of the X-ray spectrum shows evidence for the presence of a cooler (several times 10(exp 5) K) halo gas component that could consume a much larger fraction of the supernova energy. Strong evidence was found for disk/halo interaction. Hot gas apparently blows out from supershells in the galaxy's disk at a rate of approximately 1 solar mass yr(exp -1). This outflow of hot gas drags magnetic field lines up in the halo and forms a magnetized gaseous halo. If the magnetic field lines are still anchored to the disk gas at large disk radii, the outflowing gas may be confined high above the disk by magnetic pressure. A strong X-ray source which coincides spatially with an H I supershell has been identified. However, the source is likely an extremely luminous X-ray binary with L(sub chi)(0.1 - 2 keV) approximately 5 x 10(exp 39) ergs s(exp -1), which makes it a stellar mass black hole candidate.

Wang, Q. David↗
NASA NTRS1994

Extended far-infrared emission and star formation in Seyfert galaxies

An investigation into the extended distribution of far-infrared (FIR) emission associated with nearby Seyfert galaxies is made using a set of MEM reconstructions of IRAS Chopped Photometric Channel (CPC) data (Marston 1993). The data is compared to a set of HII/starburst galaxy images similarly processed in order to compare distributions and FIR color properties. It is shown that the central 1 kpc or so of Seyfert galaxies show extended FIR emission. FIR colors suggest that the bulk of this emission is not directly associated with an active nucleus. They further suggest that the origins of the majority of the emission is from heated dust associated with star formation surrounding the nucleus rather than dust heated by the active nucleus. Nearby Seyfert galaxies are shown to have a higher concentration of far-infrared emission from their centers than the HII/starburst galaxies and a number appear to reside in disk galaxies with relatively low ongoing star formation in their disks. An example of this is NGC 7582 which has a smooth disk but an active nucleus/starbust center.

Marston, A. P.↗
NASA NTRS1994

Dust extinction of the stellar continua in starburst galaxies: The ultraviolet and optical extinction law

We analyze the International Ultraviolet Explorer (IUE) UV and the optical spectra of 39 starburst and blue compact galaxies in order to study the average properties of dust extinction in extended regions of galaxies. The optical spectra have been obtained using an aperture which matches that of IUE, so comparable regions within each galaxy are sampled. The data from the 39 galaxies are compared with five models for the geometrical distribution of dust, adopting as extinction laws both the Milky Way and the Large Magellanic Cloud laws. The commonly used uniform dust screen is included among the models. We find that none of the five models is in satisfactory agreement with the data. In order to understand the discrepancy between the data and the models, we have derived an extinction law directly from the data in the UV and optical wavelength range. The resulting curve is characterized by an overall slope which is more gray than the Milky Way extinction law's slope, and by the absence of the 2175 A dust feature. Remarkably, the difference in optical depth between the Balmer emission lines H(sub alpha) and H(sub beta) is about a factor of 2 larger than the difference in the optical depth between the continuum underlying the two Balmer lines. We interpret this discrepancy as a consequence of the fact that the hot ionizing stars are associated with dustier regions than the cold stellar population is. The absence of the 2175 A dust feature can be due either to the effects of the scattering and clumpiness of the dust or to a chemical composition different from that of the Milky Way dust grains. Disentangling the two interpretations is not easy because of the complexity of the spatial distribution of the emitting regions. The extinction law of the UV and optical spectral continua of extended regions can be applied to the spectra of medium- and high-redshift galaxies, where extended regions of a galaxy are, by necessity, sampled.

Calzetti, Daniela↗
NASA NTRS1994

Ionization cones and radio ejecta in active galaxies

We report radio mapping at three frequencies of the Seyfert 2 galaxy NGC 5252, which is known to exhibit a spectacular pair of 'ionization cones' in optical emission-line images. The radio structure of the galaxy comprises an unresolved (less than 50 pc) source coincident with the optical nucleus, weak, narrow features extending approximately equal to 900 pc to north and south from the nucleus, and an unresolved radio source some 10 kpc from the nucleus. The inner parts of the extended radio structure and the off-nuclear source align well with the axis of the ionization cones. There are currently 11 Seyfert galaxies known to possess an ionization cone or a bi-cone; 8 of these galaxies also contain a linear (double, triple, or jet-like) nuclear radio structure. For this limited, incomplete sample, there is a tight alignment between cone and radi axes: the formal mean difference between the measured projections of these axes on the sky is only 6 deg, and the alignment may well be better than this at the location(s) closer to the nucleus where the collimation occurs. Although the degree of collimation is much worse for the ionizing photons than for the radio plasma, it is clear that they are collimated by the same, or coplanar, nulcear disks or tori. In particular, if the ionization cones result from absorption by dusty tori on the pc scale and the radio ejecta from accretion disks around the central black hole, the absence of differential precession indicates that either the gravitating mass distribution is close to spherical or the dusty torus has settled into a preferred plane. The cones currently known in late-type (but not early-type) spirals show a trend to align with the axis of the galaxy stellar disk. We argue that this alignment is either an observational selection effect or indicates that the gas accreted to power the nuclear activity has an internal origin in late-type spirals, but may have an external origin (e.g., a galaxy merger) in early-types. .

Wilson, A. S.↗
NASA NTRS1994

IRAS F15307+3252: A hyperluminous infrared galaxy at z = 0.93

A new hyperluminous infrared galaxy in the Infrared Astronomy Satellite (IRAS) Faint Source Catalog is identified at a redshift of 0.93. This object has a bolometric luminosity of approximately 10(exp 13) solar luminosity, a very large ratio of infrared-to-optical luminosity, warm dust emission, a ratio of infrared-to-radio flux densities consistent with other infrared galaxies, and an optical spectrum similar to a Seyfert 2 galaxy. IRAS F15307+3552 shares these characteristics and its radio-to-optical spectral energy distribution with two other infrared galaxies, F10214+4724 and P09104+4109. Discovery of a third object with these properties defines an extreme subclass of ultraluminous galaxies powered primarily by star formation. The systematic method used to find this object begins the process of determining the space density of these most luminous examples of the infrared galaxy phenomenon.

Cutri, Roc M.↗
NASA NTRS1994

Hubble Space Telescope photometry of the central regions of Virgo Cluster elliptical galaxies. 1: Observations, discussion, and conclusions

Using the Hubble Space Telescope we have observed at 10 pc resolution the nuclei of a luminosity-limited sample of 14 E and E/SO galaxies in the Virgo Cluster with magnitudes B(sub T) = 9.4 to 13.4. In this paper we present the images, and discuss the results of the detailed analysis of the surface photometry given in two companion papers. We find that the nuclear and near-nuclear morphologies confirm and strengthen the previously recognized dichotomy of 'E' galaxies into 'true' and 'disky' subtypes. The latter, usually classified E4 or later, often show a bright nuclear disk of radius approximately 100 pc. Essentially all early-type galaxies with -18 greater than M(sub B) greater than -20 are disky. Most true E galaxies are classified E4 or earlier. Most galaxies of both types show dust in the nuclear regions, the most remarkable example being a compact dust disk in NGC 4261. Other than dust, no anomalies were detected in the centers of the three galaxies in our sample which show clear kinematic evidence for a decoupled component.

Jaffe, Walter↗
NASA NTRS1994

Radio continuum polarimetric imaging of high redshift radio galaxies

Multifrequency images of total and polarized radio continuum emission from the two high redshift radio galaxies 0902+343 (z = 3.40) and 0647+415 (4C 41.17, z = 3.80) are presented. These images represent the most sensitive polarimetric study of high redshift ratio galaxies to date. The emission from both galaxies is substantially polarized, up to 30% in some regions, and both sources sit behind deep 'Faraday screens,' producing large rotation measures, over 10(exp 3) rad/sq. m in magnitude, and large rotation measure gradients across the sources. Such large rotation measures provide further evidence that high redshift radio galaxies are situated in very dense environments. Drawing the analogy to a class of low redshift powerful radio galaxies with similarly large rotation measures, we suggest that 0902+343 and 0647+415 are situated at the centers of dense, x-ray 'colling flow' clusters, and that the cluster gas is substantially magnetized. The remarkable similarity between the optical and radio morphologies of 0647+415 on scales as small as 0.1 sec is presented. We consider, and reject, both synchrotron and inverse Compton radiation as possible sources of the optical emission. We also consider both scattering of light out of a 'cone' of radiation from an obscured nucleus, and jet-induced star formation, and find that both models encounter difficulties in explaining this remarkably close radio-optical alignment. High resolution spectral index images reveal compact, flat spectrum components in both sources. We suggest that these components are the active nuclei of the galaxies. Lastly, high resolution images of 0902+343 show that the southernmost component forms a 'ring' of 0.2 sec radius. We discuss the possibility that this ring is the result of gravitational lensing, along the lines proposed by Kochanek & Lawrence (1990).

Carilli, C. L.↗
NASA NTRS1994

Possible identification of a cluster of galaxies at redshift z = 3.4

We report the possible detection of a cluster of 16 radio-quiet galaxies at z = 3.4, identified in the field around the optically thick absorption system toward Q0000-263 at z(sub abs) = 3.390. Two of them, a Lyman alpha emitter at z = 3.428 and the galaxy responsible for the absorption system, have redshifts spectroscopically confirmed. The other 14 galaxies identified using a multicolor imaging technique designed to detect sources in the redshift interval 3 approximately less than z approximately less than 3.5 which are characterized by a Lyman discontinuity in an otherwise flat spectrum, have broad-band spectral energy distributions identical to the two galaxies with known redshift. They are spatially distributed in two apparent clumps, around the damped absorber and the Lyman alpha galaxy, respectively. A clustering analysis excludes with 98.8% confidence that this association is a realization of a Poissonian distribution and confirms that the observed clumps are real. The implications are that the 16 galaxies are members of a cluster at z approximately equals 3.4, by far the most distant ever detected. An estimate of the mass bounded in stars of this cluster is 3 x 10(exp 12) solar mass (q(sub 0) = 0 and H(sub 0) = 50 km/s/Mpc throughout this Letter), while the total mass (baryonic + dark) is 6 x 10(exp 14) solar mass. We also estimate that at z = 3.4 the correlation length is 2.2 Mpc, which, compared to the present value of 11 Mpc, suggests that the clustering evolution is still close to the linear regime.

Giavalisco, Mauro↗
NASA NTRS1994

Two-fluid simulations of galaxy formation

We investigate the formation of galaxies and larger structure with a simulation modeling two gravitationally coupled fluids representing dark matter and baryons. The baryon gas dynamics are calculated with a smoothed particle hydrodynamics (SPH) method, and the physics modeled includes thermal pressure, shock heating, and radiative cooling. We simulate a 16 Mpc periodic cube with 64(exp 3) particles in each fluid and 10% baryon mass fraction. We confirm, for the first time experimentally, disk formation as a natural consequence of hierarchical clustering in a large-scale cosmological environment. The majority of isolated galaxies exhibit centrifugally supported disks. A power-law relation between cold baryonic mass and maximum rotation velocity is found, M varies as nu(sub rot)(exp alpha) with alpha = 2.5 after correcting for differential numerical resolution. Both the spatial and velocity distributions of the simulated galaxies are biased with respect to the dark matter. A counts-in-cells analysis indicates that an unphysical degree of merging in the central cluster is likely responsible for the antibias signal in the correlation function. A robust, scale-dependent velocity bias is measured. The ratio of galaxy to dark matter pairwise velocity dispersions on a scale of 1 Mpc is 0.7. The amplitude is only mildly dependent on redshift or mass cutoff and scales with separation as r(exp 0.2). The degree to which these results depend on numerical parameters is discussed. Mass resolution plays a key role in controlling the resulting fraction of cold, dense baryons. The mass fraction associated with galaxies decreases by a factor of approximately greater than 3 when the mass per particle is increased by a factor 8. Photoionization and energy input from supernova will have to be included to determine more carefully the fraction of highly dissipated material and the characteristics of the stellar component of galaxies.

Evrard, August E.↗
NASA NTRS1994

Models of hot galactic coronae around early-type galaxies

We have computed simple non-steady state models of X-ray-emitting, hot galactic coronae which are observed around early-type galaxies. Our models, appropriate for elliptical galaxies, include the effect of the formation of new stars which occurs as the gas radiatively cools and the effect of supernova explosions which serve to heat the cooling gas component from which the stars are forming. We develop scaling relations for the galaxy parameters including the optical galaxy as well as a dark matter halo which we use to generate galaxy models spanning a range of absolute magnitudes from -19.5 to -22.5. We compare our models to a sample of approximately 150 galaxies with measured X-ray luminosities. We show that our simple model does exhibit the observed correlation between X-ray and optical luminosity. However, the model cannot explain the broad range in X-ray luminosity at a given optical luminosity.

Forman, W.↗
NASA NTRS1994

The Reddening law outside the local group galaxies: The case of NGC 7552 and NGC 5236

The dust reddening law from the UV to the near-IR for the extended regions of galaxies is here derived from the spectral distributions of the starburst spiral galaxies NGC 7552 and NGC 5236. The centers of these galaxies have similar absorption and emission line spectra, differing only if the strength of their interstellar lines and in the continuum distribution, with NGC 7552 appearing more reddened than NGC 5236. The disk of NGC 7552 is more inclined, and there is evidence that its center is observed through additional foreground dust and gas clouds, as compared to the center of NGC 5236. While the galaxies can be expected to have similar dust content, they are known to have different dust path lengths to our line of sight. Therefore, differences in the shape of the spectra can be attributed mainly to the effects of dust, allowing us to probe for the first time the properties of the reddening law outside the local group of galaxies. We derive the reddening law based on the optical depth of the emission line of H Alpha and H Beta and also based on the continuum distribtuion. We find that the optical depth from the emission line regions are about twice the optical depth of the continuum regions. Thus, dereddening a starburst galaxy by scaling the Milky Way reddening laws to optical depths obtained from the H Alpha/H Beta line ratio overcompensates for the effect of dust.

Kinney, Anne L.↗
NASA NTRS1995

Morphology of galaxy mergers at intermediate redshift

Using a combination of numerical simulation and synthesized Hubble Space Telescope Wide Field Camera 2 (HST WFC2) images, we follow the detectability of morphological signatures of disk galaxy mergers at intermediate redshifts z = 0.4 and z = 1.0. Rapid evolution in the surface brightness of tidal tails makes their use as an interaction signature limited; long tidal tails will only be detectable relatively early in the encounter, before the galaxies actually merge. Violent relaxation rapidly smooths isophotal irregularities after the galaxies merge, leaving the very low surface brightness tidal debris surrounding the remnant as the only sign of peculiarity after a few hundred Myr. This debris becomes indistinguishable from the main body of the remnant after less than or approximately equal to 0.2 (z = 1.0) to 1 (z = 0.4) Gyr, leaving a morphologically normal elliptical galaxy behind. Using WFC1, all signatures become undetectable almost immediately once the galaxies merge, due to the aberrated Point Speed Function (PSF). We interpret these results in light of recent HST programs investigating the nature of starburst and E + A galaxies in moderate redshift clusters.

Mihos, J. Christopher↗