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At least 343 records · Page 19
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NASA NTRS1984

Fine-scale anisotropy of the cosmic microwave background in a universe dominated by cold dark matter

The fine-scale anisotropy of the cosmic microwave background radiation has been studied in cosmological models with a scale-invariant primordial adiabatic density fluctuation spectrum dominated by cold, weakly interacting particles. Normalization of the present fluctuation spectrum to the observed galaxy distribution results in excessive temperature anisotropy when compared to a recent upper limit on 4.5 arcmin unless the density parameter exceeds 0.4. When this result is combined with the requirement that the universe be at least 13 billion years old, it is found that if the cosmological constant is zero, then the density parameter is between roughly 0.4 and 1 and the Hubble constant is between roughly 60 km/s/Mpc and 50 km/s/Mpc.

Vittorio, N.↗
NASA NTRS1987

Ultra-high-energy cosmic rays in a galactic wind and its termination shock

Results are reported from numerical modeling of the acceleration and transport of ultra-high-energy cosmic rays in a galactic wind and its termination shock. A two-dimensional (azimuthally symmetric) wind and spiral magnetic field, with a spherical termination shock, where the velocity drops suddenly, is assumed. The time-dependent cosmic-ray transport equation, including all major transport effects is solved using an implicit finite-difference scheme. Particles are injected as the shock of low energy, and the subsequent evolution of the distribution function is followed. Iron nuclei are readily accelerated at the shock to energies up to 100 billion GeV, and protons to 10 billion GeV. A major effect aiding the acceleration of these particles is the spiral of the magnetic field carried out by the wind, caused by the rotation of the Galaxy, with the result that the shock is nearly normal over most of its area. Increasing the magnetic field or rotation rate increases the maximum energy attainable. Anisotropies and energy densities of the particles are also discussed. It is concluded that the process is consistent with observations of ultra-high-energy cosmic rays.

Jokipii, J. R.↗
NASA NTRS2002

Seeing Red and Shooting Blanks: A Study of Red Quasars and Blank Field X-Ray Sources

We have identified a population of 'blank field sources' (or 'blanks') among the ROSAT (Roentgen Satellite) bright unidentified X-ray sources with faint optical counterparts. The extreme X-ray over optical flux ratio of blank field sources is not compatible with the main classes of X-ray emitters except for extreme BL Lacertae objects at fx/fv is equal to or less than 35. From the analysis of ROSAT archival data we found evidence for only three sources, out of 16, needing absorption in excess of the Galactic value and no indication of variability. We also found evidence for an extended nature for only one of the five blanks with a serendipitous HRI (High Resolution Imager) detection; this source (1WGA J1226.9+3332) was confirmed as a z=0.89 cluster of galaxies. Palomar images reveal the presence of a red (O - E is equal to or greater than 2) counterpart in the X-ray error circle for six blanks. The identification process brought to the discovery of another high z cluster of galaxies, one (possibly extreme) BL Lac and two apparently normal type 1 AGNs (Active Galactic Nuclei). These AGNs, together with four more AGN-like objects seem to form a well defined group: they present type 1 X-ray spectra but red Palomar counterparts. We discuss the possible explanations for the discrepancy between the X-ray and optical data, among which: a suppressed big blue bump emission, an extreme dust to gas (approximately 40 - 60 the Galactic ratio) ratio value and a high redshift (z is greater than or equal to 3.5) QSO (Quasi-Stellar Object) nature. These AGN-like blanks seem to be the bright (and easier to study) analogs of the sources which are being found in deep Chandra observations. Five more blanks have a still an unknown nature.

Elvis, Martin↗
NASA NTRS1997

A Multiwavelength Study of the Starburst Galaxy NGC 7771

We present a multiwavelength study of the interacting starburst galaxy NGC 7771, including new optical and ultra-violet spectra and a previously unpublished soft X-ray ROSAT image and spectrum. The far-infrared, radio, and X-ray fluxes suggest that a massive burst of star-formation is currently in progress but the small equivalent width of the Balmer emission lines (equivalent width H(alpha approximately equals 100 A), the weak UV flux, the low abundance of ionised oxygen, and the shape of the optical spectrum lead us to conclude that there are few 0 stars. This might normally suggest that star-formation has ceased but the galaxy's barred gravitational potential and large gas reserves imply that this should not be so, and we therefore consider other explanations. We argue that the observations cannot be due to effects of geometry, density bounded nebulae, or dust within the nebulae, and conclude that a truncated IMF is required. The dwarf galaxy NGC 7770 appears to be in the initial stages of a merger with NGC 7771, and the resulting tidal perturbations may have induced the apparent two-armed spiral pattern, and driven a substantial fraction of the disk gas inwards. The presence of a bulge in NGC 7771 may be moderating the starburst so that, while still occuring on a large scale with a supernova rate of 0.8-1/yr, it is less violent and the IMF has a relatively low upper mass limit. We find that there is a cluster of stars obscuring part of the starburst region, and we offer an explanation of its origin.

Davies, Richard I.↗
NASA NTRS2012

The Atacama Cosmology Telescope: Relation Between Galaxy Cluster Optical Richness and Sunyaev-Zel'dovich Effect

We present the measured Sunyaev-Zel'dovich (SZ) flux from 474 optically-selected MaxBCG clusters that fall within the Atacama Cosmology Telescope (ACT) Equatorial survey region. The ACT Equatorial region used in this analysis covers 510 square degrees and overlaps Stripe 82 of the Sloan Digital Sky Survey. We also present the measured SZ flux stacked on 52 X-ray-selected MCXC clusters that fall within the ACT Equatorial region and an ACT Southern survey region covering 455 square degrees. We find that the measured SZ flux from the X-ray-selected clusters is consistent with expectations. However, we find that the measured SZ flux from the optically-selected clusters is both significantly lower than expectations and lower than the recovered SZ flux measured by the Planck satellite. Since we find a lower recovered SZ signal than Planck, we investigate the possibility that there is a significant offset between the optically-selected brightest cluster galaxies (BCGs) and the SZ centers, to which ACT is more sensitive due to its finer resolution. Such offsets can arise due to either an intrinsic physical separation between the BCG and the center of the gas concentration or from misidentification of the cluster BCG. We find that the entire discrepancy for both ACT and Planck can be explained by assuming that the BCGs are offset from the SZ maxima with a uniform random distribution between 0 and 1.5 Mpc. In contrast, the physical separation between BCGs and X-ray peaks for an X-ray-selected subsample of MaxBCG clusters shows a much narrower distribution that peaks within 0.2 Mpc. We conclude that while offsets between BCGs and SZ peaks may be an important component in explaining the discrepancy, it is likely that a combination of factors is responsible for the ACT and Planck measurements. Several effects that can lower the SZ signal equally for both ACT and Planck, but not explain the difference in measured signals, include a larger percentage of false detections in the MaxBCG sample, a lower normalization of the mass-richness relation, radio or infrared galaxy contamination of the SZ flux, and a low intrinsic SZ signal. In the latter two cases, the effects would need to be preferentially more significant in the optically-selected MaxBCG sample than in the MCXC X-ray sample.

Sehgal, Neelima↗
NASA NTRS1988

IRAS observations of an optically selected sample of interacting galaxies

IRAS observations of a large, morphologically selected sample of strongly interacting disk-type galaxies have demonstrated that galaxy-galaxy collisions can lead to enhanced infrared emission, but not in all cases. Infrared luminosities of the interacting galaxies span a large range, but are about a factor of 2 higher, on average, than those of isolated disk galaxies. The data suggest the existence of a cutoff in blue luminosity, below which no galaxies show markedly enhanced infrared emission. Only the most strongly interacting systems in the sample show extreme values of infrared excess, suggesting that deep, interpenetrating collisions are necessary to drive infrared emission to extreme levels. Comparisons with optical indicators of star formation show that infrared excess and color temperatures correlate with the level of star-formation activity in the interacting galaxies. All interacting galaxies in our sample that exhibit an infrared excess and have higher than normal color temperatures also have optical indicators of high levels of star formation. It is not necessary to invoke processes other than star formation to account for the enhanced infrared luminosity in this sample of interacting galaxies.

Bushouse, Howard A.↗
NASA NTRS1981

Galaxy formation in an intergalactic medium dominated by explosions

The evolution of galaxies in an intergalactic medium dominated by explosions of star systems is considered analogously to star formation by nonlinearly interacting processes in the interstellar medium. Conditions for the existence of a hydrodynamic instability by which galaxy formation leads to more galaxy formation due to the propagation of the energy released at the death of massive stars are examined, and it is shown that such an explosive amplification is possible at redshifts less than about 5 and stellar system masses between 10 to the 8th and 10 to the 12th solar masses. Explosions before a redshift of about 5 are found to lead primarily to the formation of massive stars rather than galaxies, while those at a redshift close to 5 will result in objects of normal galactic scale. The model also predicts a dusty interstellar medium preventing the detection of objects of redshift greater than 3, numbers and luminosities of protogalaxies comparable to present observations, unvirialized groups of galaxies lying on two-dimensional surfaces, and a significant number of black holes in the mass range 1000-10,000 solar masses.

Ostriker, J. P.↗
NASA NTRS1995

An HST study of galactic inerstellar zinc and chromium

We present a survey of interstellar Zn II and Cr II absorption extracted from the Hubble Space Telescope Goddard High Resolution Spectrograph (HST GHRS) data archive. We find clear evidence for an enhanced depletion of Zn from the gas phase with increasing fractional abundance of molecular hydrogen f(H2). Our lower limit to the Galactic interstellar metallicity is approximately 65% of the solar value as determined by the measured Zn abundances in the lowest f(H2) sightlines, (N(Zn)/N(H(sup 0)(sub tot)) = -0.19 +/- 0.04. The correspondingly high depletion of Cr with respect to solar (N(Cr/N(H(sup 0)(sub tot)) = -1.44 +/- 0.26 indicates that there are significant amounts of dust present in these lines of sight. The Galactic abundances of Zn and Cr in the ISM provide a fundamental reference point which is used to understand the metal enrichment and dust formation history of damped Lyman alpha QSO absorption-line systems, normally believed to arise from intervening precursors to modern disk galaxies. Although the spread in Zn abundances is large for both the local ISM and in damped Lyman alpha systems, we still find a substantial difference (factor of 4-10) in metallicity between the two sets. This survey and future observations of more distant objects which probe the full extent of the Milky Way halo provide a more complete picture of the enrichment and depletion characteristics of present-day galaxies.

Roth, Katherine C.↗
NASA NTRS1994

The morphology of faint galaxies in Medium Deep Survey images using WFPC2

First results from Hubble Space Telescope (HST) Medium Deep Survey images taken with Wide Field/Planetary Camera-2 (WFPC2) demonstrate that galaxy classifications can be reliably performed to magnitudes I814 approximately less than 22.0 in the F815W band. Published spectroscopic surveys to this depth indicate a mean redshift of bar-z approximately 0.5. We have classified over 200 galaxies in nine WFPC2 fields according to a basic morphological scheme. The majority of these faint galaxies appear to be similar to regular Hubble-sequence examples observed at low redshift. To the precision of our classification scheme, the relative proportion of spheroidal and disk systems of normal appearance is as expected from nearby samples, indicating that the bulk of the local galaxy population was in place at half the Hubble time. However, the most intriguing result is the relatively high proportion (approximately 40%) of objects which are in some way anomalous, and which may be of relevance in understanding the origin of the familiar excess population of faint galaxies established by others. These diverse objects include apparently interacting pairs whose multiple structure is only revealed with HST's angular resolution, galaxies with superluminous star-forming regions, diffuse low surface brightness galaxies of various forms, and compact galaxies. These anomalous galaxies contribute a substantial fraction of the excess counts at our limiting magnitude, and may provide insights into the 'faint blue galaxy' problem.

Griffiths, R. E.↗
NASA NTRS1979

Primordial inhomogeneities in the expanding universe. I - Density and velocity distributions of galaxies in the vicinities of rich clusters

The density profiles and Hubble flow deviations in the vicinities of rich galaxy clusters are derived for a variety of models of initial density and velocity perturbations at the recombination epoch. The galaxy correlation function, measured with respect to the Abell clusters, is used to normalize the theoretical models. The angular scales of the required primordial inhomogeneities are calculated. It is found that the resulting density profiles around rich clusters are surprisingly insensitive to the shape of the initial perturbations and also to the cosmological density parameter, Omega. However, it is shown that the distribution of galaxy radial velocities can provide a possible means of deriving Omega.

Silk, J.↗
NASA NTRS1986

Self-regulated cooling flows in elliptical galaxies and in cluster cores - Is exclusively low mass star formation really necessary?

A self-consistent treatment of the heating by supernovae associated with star formation in a spherically symmetric cooling flow in a cluster core or elliptical galaxy is presented. An initial stellar mass function similar to that in the solar neighborhood is adopted. Inferred star-formation rates, within the cooling region - typically the inner 100 kpc around dominant galaxies at the centers of cooling flows in XD clusters - are reduced by about a factor of 2, relative to rates inferred when the heat input from star formation is ignored. Truncated initial mass functions (IMFs) are also considered, in which massive star formation is suppressed in accordance with previous treatments, and colors are predicted for star formation in cooling flows associated with central dominant elliptical galaxies and with isolated elliptical galaxies surrounded by gaseous coronae. The low inferred cooling-flow rates around isolated elliptical galaxies are found to be insensitive to the upper mass cutoff in the IMF, provided that the upper mass cutoff exceeds 2 M solar mass. Comparison with observed colors favors a cutoff in the IMF above 1 M solar mass in at least two well-studied cluster cooling flows, but a normal IMF cannot be excluded definitively. Models for NGC 1275 support a young (less than about 3 Gyr) cooling flow. As for the isolated elliptical galaxies, the spread in colors is consistent with a normal IMF. A definitive test of the IMF arising via star formation in cooling flows requires either UV spectral data or supernova searches in the cooling-flow-centered galaxies.

Silk, J.↗
NASA NTRS2004

The Soft Excess Emission in the Hercules Supercluster Observed with the ROSAT Position Sensitive Proportional Counter

Recent observations with the XMM-Newton satellite confirmed the existence of the soft excess phenomenon in galaxy clusters, earlier discovered in several EUVE, ROSAT and BeppoSAX observations. Excess of soft X-ray radiation in the spectra of several clusters -- above the contribution from the well-known hot intra-cluster gas -- is normally interpreted as emission from cooler gas (T~10^6 K) in the vicinity of galaxy clusters. In this paper we investigate the emission from three clusters in the Hercules concentration, MKW~S, A2052 and A2063. Available XMM observations for two of them (MKW~3s and A2052) indicate the presence of the excess emission, also associated with O VII emission lines. The clusters lie close to the North Polar Spur region, a portion of the sky which features strong soft X-ray emission. We analyze the ROSAT PSPC data of the three clusters and of eight other neighboring locations, in order to study the level of the local soft X-ray background and its influence on the determination of soft excess fluxes. Our analysis reveals that the North Polar Spur region has strong gradients in the soft X-ray emission, which render the background subtraction process quite cumbersome. Despite these complications, we are able to provide evidence for the presence of soft excess emission in the PSPC data of MKW~S, A2052 and A2063.

Bonamente, Massimiliana↗
NASA NTRS1988

Powerful warm infrared sources in early-type galaxies

IRAS far-infrared sources have been identified with 129 S0, Sa, Sb, and Sc galaxies in a statistically complete sample of 738 galaxies brighter than 14.5 mag and smaller than 4.0 arcmin. In most cases, the far-IR colors and the ratios of far-IR flux to radio flux density are those of normal galactic disks and/or starbursts. The most powerful far-IR sources in S0 and Sa galaxies are just as powerful as the strongest far-IR sources in Sb and Sc galaxies. Bright-IR sources in S0 and Sa galaxies are warm; those in Sc galaxies are cool. Sb galaxies have both warm and cool IR sources. Bright warm IR sources occur much more frequently in barred galaxies than in galaxies without bars for types S0, Sa, and Sb. Bright, cool IR sources are found with increasing frequency along the Hubble sequence, regardless of the presence or absence of a bar. At least some 'S0' galaxies with warm, bright IR sources have peculiar morphologies and ambiguous classifications.

Dressel, L. L.↗
NASA NTRS1993

Long slit spectroscopy of NGC 5506

The galaxy NGC 5506 hosts an active nucleus, that presents characteristics that are intermediate between Sy1 and Sy2. We discuss long slit spectra of NGC 5506 in the ranges 4675-5475 A and 6300-7125 A, that were obtained at three different position angles, in Apr. 1991 at the WHT 4.2 m telescope. The peculiar kinematics of the emitting gas has already been observed by other researchers; following the model proposed by the other researchers, that the emitting gas is located in two cones, we determined the aperture of the cones. The data, moreover, support the hypothesis that the gas is receding from the nucleus. We modelled the intensity and the ratios of the emission lines, and verified that the active nucleus of NGC 5506 can be described as a Sy1 nucleus, with the UV-X source that is partially obscured to our line of sight. On the contrary, a good fraction of the interstellar gas of the galaxy is directly illuminated and photoionized by the central source. Our data show evidence of star formation close to the nucleus; we estimated the star formation rate, that is high with respect to 'normal' spirals, but not high enough to be comparable to star formation rates in a starburst galaxy.

Stanga, R. M.↗
NASA NTRS1987

Molecular gas and star formation in HI-deficient Virgo cluster galaxies

Mapping of the CO emission line in 42 Virgo cluster galaxies reveals that the molecular gas contents and distributions are roughly normal in severaly HI-deficient Virgo spirals. The survival of the molecular component mitigates the impact of the HI-stripping on star formation and subsequent galactic evolution. For spirals which are deficient in HI by a factor of 10, far-infrared, H alpha line, and nonthermal radio continuum luminosities are lower by no more than a factor of 2. The fact that the inner galactic disks are stripped of HI, while CO is normal, suggests that the lifetime of the molecular phase is approximately one billion years in the inner regions of luminous spirals.

Kenney, Jeffrey D.↗
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 NTRS2000

X-Ray Companion Sources to Seyfert 2 Galaxies

The ROSAT observations supported by this grant provided improved positions for the "companion sources" to 3 Seyfert 2 galaxies, as proposed. Follow-up observations at the MMT and CTIO found that the optical counterparts to these sources were a mixture of normal stars (mainly M dwarfs) and background AGN. This surprising result suggests that the reported excess of PSPC sources around Seyfert 2 galaxies was a statistical fluctuation, and does not indicate a new population of bright non-nuclear X-ray sources in these objects. The report of Xia et al., that a companion to MRK273 was a dwarf galaxy at the same redshift containing another Seyfert 2 galaxy was withdrawn in an erratum. (The true redshift of the companion was much larger than that of MRK273.) For the time being then this question is closed. A re-examination of the "companions" using Chandra data will become possible in the next year. If the optical 'counterparts' we found are not the true counterparts, or if the counterparts we found were the background of inevitable interlopers ('serendipitous') sources, there may remain a high X-ray luminosity population associated with Seyfert 2 galaxies.

Elvis, Martin↗
NASA NTRS1987

Induced star formation in interacting galaxies

Measurements of H alpha emission line fluxes and FIR fluxes in approx. 100 interacting spirals were used to investigate the effects of close tidal interactions on the disk and nuclear star formation rates in galaxies. Two samples of interacting spirals were studied, a complete sample of close pairs, and a set of strongly perturbed systems from the Arp atlas. Both the integrated H alpha luminosities and FIR luminosities are enhanced in the interacting galaxies, indicating that the encounters indeed trigger massive star formation in many cases. The response of individual galaxies is highly variable, however. A majority of the interacting spirals exhibit normal star formation rates, while a small fraction are undergoing bursts with luminosities which are rarely, if ever, observed in noninteracting systems. Virtually all of the latter are in the Arp sample, indicating that the Arp atlas is heavily biased to the most active star forming systems.

Kennicutt, R. C.↗