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All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Probing the large-scale velocity field with clusters of galaxies

What is the role of clusters of galaxies in probing the large-scale velocity field of the universe? We investigate the distribution of peculiar velocities of clusters of galaxies in the popular low-density (omega = 0.3) flat cold dark matter (CDM) cosmological model, which best fits many large-scale structure observations. An omega = 1 CDM model is also studied for comparison. We find that clusters of galaxies are efficient tracers of the large-scale velocity field. The clusters exhibit a Maxwellian distribution of peculiar velocities, as expected from Gaussian initial density fluctuations. The cluster three-dimensional velocity distribution for the omega = 0.3 model peaks at nu approximately greater than 400 km/s and extends to high velocities of nu approximately 1200 km/s. The rms peculiar velocity of the clusters is 440 km/s. Approximately 10% of all model clusters move with high peculiar velocities nu greater or equal to 700 km/s. The observed velocity distribution of clusters of galaxies is compared with the predictions from cosmological models. The observed data exhibit a larger velocity tail than seen in the model simulations; however, due to the large observational uncertainties, the data are consistent at approximately equal to 3 sigma level with the odel predictions, and with a Gaussian initial density field. The large peculiar velocities reported for some clusters of galaxies (nu approximately greater than 3000 km/s) are likely to be overestimated, if the current model is viable.

Bahcall, Neta A.↗
NASA NTRS1993

Neutral gas in the central 2 parsecs of the Galaxy

New spectroscopic observations are presented that elucidate the spatial distribution, kinematics, and density structure of atomic and molecular gas within about 2 p from IRS 16/SgrA*. The measurements confirm the presence of a dense clumpy molecular gas ring surrounding a short-lived central cavity of much lower mean gas density. It appears that this ring consists of several distinct and probably short-lived gas streamers that result from accretion from the massive molecular clouds within 10-30 pc of the center toward the central gravitational potential. Within 1 pc of SgrA* a large atomic cloud is found that appears to be falling in from more than 3 pc Galactocentric distance into the central cavity. The northern and eastern 'arms' of ionized gas are very likely dense bright rims at the surface of this atomic cloud that is now exposed to the intense UV radiation from the few central arcsec surrounding IRS 16/SgrA*.

Jackson, J. M.↗
NASA NTRS1999

The Contribution of Ionizing Stars to the Far-Infrared and Radio Emission in the Galaxy

A summary of research activities carried out in this eighth and final progress report. The final report includes: this summary document, copies of three published research papers, plus a draft manuscript of a fourth research paper entitled "The Contribution of Ionizing Stars to the FarInfrared and Radio Emission in the Milky Way; Evidence for a Swept-up Shell and Diffuse Ionized Halo around the W4 Chimney/Supershell." The main activity during the final quarterly reporting period was research on W4, including analysis of the radio and far-infrared images, generation of shell models, a literature search, and preparation of a research manuscript. There will be additional consultation with co-authors prior to submission of the paper to the Astrophysical Journal. The results will be presented at the 4th Tetons Summer Conference on "Galactic Structure, Stars, and the ISM" in May 2000. In this fourth and last paper we show W4 has a swept-up partially ionized shell of gas and dust which is powered by the OCl 352 star cluster. Analysis shows there is dense interstellar material directly below the shell, evidence that that the lower W4 shell "ran into a brick wall" and stalled, whereas the upper W4 shell achieved "breakout" to form a Galactic chimney. An ionized halo is evidence of Lyman continuum leakage which ionizes the WIM (warm ionized medium). It has long been postulated that the strong winds and abundant ionizing photons from massive stars are responsible for much of the large scale structure in the interstellar medium (ISM), including the ISM in other galaxies. However standard HII region theory predicts few photons will escape the local HII region. The significance of W4 and this work is it provides a direct example of how stellar winds power a galactic chimney, which in turn leads to a low density cavity from which ionizing photons can escape to large distances to ionize the WIM.

Terebey, S.↗
NASA NTRS1988

Kinematics and ionization of extended gas in active galaxies. IV - The Seyfert 2 galaxy NGC 4388

The results of direct CCD imaging and long-slit medium-dispersion spectroscopy of the extended narrow-line-emitting gas in the nearby Seyfert 2 galaxy NGC 4388 are presented. The spectra are used to investigate the distribution, kinematics, and ionization structure of the excited gas in the regions surrounding the nucleus out to a distance of about 4.8 kpc. The data support earlier suggestions that the galaxy, which appears to lie near the core of the Virgo cluster, has been tidally distorted, possibly by the nearby cluster core galaxies M84 or IC 3303. Much of the highly ionized gas in the galaxy appears to be distributed in two wide cones originating from the nucleus and extending above and below the disk. Explanations are offered for the distribution of this gas and for the remainder of the gas above and below the disk.

Corbin, Michael R.↗
NASA NTRS1981

Radio emission in the directions of cD and related galaxies in poor clusters. III - VLA observations at 20 cm

VLA radio maps and optical identifications of a sample of sources in the directions of 21 Yerkes poor cluster fields are presented. The majority of the cluster radio sources are associated with the dominant D or cD galaxies (approximately 70 percent). Our analysis of dominant galaxies in rich and poor clusters indicates that these giant galaxies are much more often radio emitters (approximately 25 percent of cD's are radio active in the poor clusters), have steeper radio spectra, and have simpler radio morphologies (i.e., double or other linear structure) than other less bright ellipticals. A strong continuum of radio properties in cD galaxies is seen from rich to poor clusters. It is speculated that the location of these dominant galaxies at the cluster centers (i.e., at the bottom of a deep, isolated gravitational potential well) is the crucial factor in explaining their multifrequency activity. Galaxy cannibalism and gas infall models as fueling mechanisms for the observed radio and X-ray emission are discussed

Burns, J. O.↗
NASA NTRS1983

From dwarfs to giants - Signposts of galaxy formation

A two-dimensional parametrization of galactic evolution from the primordial state involving mean surface density and velocity dispersion is presented. A dissipation diagram for large-scale structure is presented and its evolutionary significance explored while considering both the pancake and the hierarchical clustering models of galaxy formation. Gaseous dissipation is studied by assuming that individual clouds or turbulent elements of gas interact supersonically, producing strong shocks. It is found that many diverse properties of galaxies can be understood in terms of an evolutionary sequence operative during an early gas-rich protogalactic phase in which dissipation played a key role. Dwarf galaxies may be the fossilized link between primordial fluctuations and the giant galaxies observed today, relics of a past era of prolific galaxy formation.

Silk, J.↗
NASA NTRS1978

OSO-8 X-ray spectra of clusters of galaxies. 2: Discussion

X-ray spectral parameters obtained from 2 to 20 keV OSO-8 data on X-ray clusters and optical cluster properties were examined to obtain information for restricting models for hot intracluster gas structures. Topics discussed include the radius of the X-ray core in relation to the galaxy core radius, the viral mass of hotter clusters, and galaxy density and optical central cluster properties. A population of cool, dim X-ray clusters which have not been observed is predicted. The iron abundance determinations recently quoted for intracluster gas are uncertain by 50 to greater than 100 percent from this nonstatistical cause alone.

Smith, B. W.↗
NASA NTRS1974

A study of birefringence in the interstellar medium in the direction of the Crab Nebula

The interstellar medium may be regarded as a weak wave plate, the linear birefringence arising from the aligned grains which produce interstellar linear polarization. Using the Crab Nebula as a background source of linearly polarized light we have investigated this birefringence by measurements of circular polarization. The circular component is found to vary with the intrinsic linear polarization in a sinusoidal fashion characteristic of a wave plate with the orientation expected from independent measurements of the interstellar linear polarization in the same direction. Measurements of the wavelength dependence, together with the sense and magnitude of the circular polarization are interpreted as evidence for the dielectric nature of the interstellar grain materials. These observations provide a firm basis for a similar interpretation of the circular polarization of reddened stars. The observations of the stars can then be used to study the grain composition and the structure of the magnetic field in many directions in the Galaxy.

Martin, P. G.↗
NASA NTRS1983

Discovery of a quasar with a wide angle radio tail in a distant cluster of galaxies

Radio, optical, and X-ray observations of two extended radio sources separated by 99 arc sec (4C 08.66) are reported. Both sources are identified with quasars: z = 0.6227 for the northern source, and z = 0.2282 for the southern. A faint cluster of galaxies is found near the northern source, and the radio structure of this source is interpreted as providing evidence for relative motion between the quasar and the cluster gas, just as Hintzen et al. (1981) interpreted their quasar-cluster association, 3C 275.1. Physical parameters for the core and the extended emission of the northern quasar are estimated. Limits on the magnetic field strength and angular diameter are derived from the radio spectrum of the core and the magnetic field direction in the jet is found from polarization measurements. The morphology of the X-ray source is consistent with inverse-Compton emission from relic radio lobes, but an origin from the quasar itself or cluster gas cannot be ruled out.

Harris, D. E.↗
NASA NTRS1989

The area of isodensity contours in cosmological models and galaxy surveys

The contour crossing statistic, defined as the mean number of times per unit length that a straight line drawn through the field crosses a given contour, is applied to model density fields and to smoothed samples of galaxies. Models in which the matter is in a bubble structure, in a filamentary net, or in clusters can be distinguished from Gaussian density distributions. The shape of the contour crossing curve in the initially Gaussian fields considered remains Gaussian after gravitational evolution and biasing, as long as the smoothing length is longer than the mass correlation length. With a smoothing length of 5/h Mpc, models containing cosmic strings are indistinguishable from Gaussian distributions. Cosmic explosion models are significantly non-Gaussian, having a bubbly structure. Samples from the CfA survey and the Haynes and Giovanelli (1986) survey are more strongly non-Gaussian at a smoothing length of 6/h Mpc than any of the models examined. At a smoothing length of 12/h Mpc, the Haynes and Giovanelli sample appears Gaussian.

Ryden, Barbara S.↗
NASA NTRS1988

The age of the galactic disk

The galactic disk is a dissipative structure and must, therefore be younger than the halo if galaxy formation generally proceeds by collapse. Just how much younger the oldest stars in the galactic disk are than the oldest halo stars remains an open question. A fast collapse (on a time scale no longer than the rotation period of the extended protogalaxy) permits an age gap of the order of approximately 10 to the 9th power years. A slow collapse, governed by the cooling rate of the partially pressure supported falling gas that formed into what is now the thick stellar disk, permits a longer age gap, claimed by some to be as long as 6 Gyr. Early methods of age dating the oldest components of the disk contain implicit assumptions concerning the details of the age-metallicity relation for stars in the solar neighborhood. The discovery that this relation for open clusters outside the solar circle is different that in the solar neighborhood (Geisler 1987), complicates the earlier arguments. The oldest stars in the galactic disk are at least as old as NGC 188. The new data by Janes on NGC 6791, shown first at this conference, suggest a disk age of at least 12.5 Gyr, as do data near the main sequence termination point of metal rich, high proper motion stars of low orbital eccentricity. Hence, a case can still be made that the oldest part of the galactic thick disk is similar in age to the halo globular clusters, if their ages are the same as 47 Tuc.

Sandage, Allan↗
NASA NTRS1992

Optical imaging of QSOs with 0.5 arcsec resolution

Optical imaging data are presented on 28 low redshift QSOs with 0.5 arcsec resolution. The group includes a range of radio and IR luminosities. The high-resolution data reveal significant new morphological information on 16 objects, and yield good luminosity profiles on all of them. Most objects appear to have R exp 1/4 law or peculiar luminosity profiles. The QSOs are almost all found to be in merging or interacting galaxies, most of them with a smaller companion. The observed structural details appear to be similar to lower luminosity AGN at low redshift. There are few unexplained features such as 'jets' near the nucleus. Detailed descriptions are given of all objects, including new high-resolution radio observations of a few of the QSOs.

Hutchings, J. B.↗
NASA NTRS1994

ASCA observations of NGC 1068

With the high sensitivity and spectral resolution of the Advanced Satellite for Cosmology and Astrophysics (ASCA) satellite, we have discovered strong emission lines from the H-like and/or He-like ions of Ne, Mg, Si, and S as well as Fe L and confirmed the complex structure of Fe K line emission in the Seyfert II galaxy NGC 1068. The continuum emission above 3 keV exhibits rather flat shape with no evidence of low energy absorption. The overall X-ray spectrum can be well explained with a model involving starburst activity plus an obscured active galactic nucleus.

Ueno, Shiro↗
NASA NTRS2010

Early Scientific Results and Future Prospects for the Rejuvenated Hubble Space Telescope

Following the extraordinarily successful Servicing Mission 4 (SM4) of Hubble Space Telescope (HST) in May of 2009, the Observatory is now fully equipped with a broad array of powerful science instruments that put it at the pinnacle of its scientific power. Relevant to the subject matter of the Beyond 2010 Conference, HST will be well-placed over the next five-plus years to advance our knowledge of the formation of high-redshift galaxies and their growth with cosmic time; the emergence of structure in the early universe via Dark Matter-driven gravitational instability; and the universe's expansion history and any resulting implications for the temporal character of Dark Energy. These are fitting projects for the iconic facility now celebrating its 20th anniversary in orbit.

Niedner, Malcolm B.↗
NASA NTRS2014

Wide Field X-Ray Telescope Mission Concept Study Results

The Wide Field X-Ray Telescope (WFXT) is an astrophysics mission concept for detecting and studying extra-galactic x-ray sources, including active galactic nuclei and clusters of galaxies, in an effort to further understand cosmic evolution and structure. This Technical Memorandum details the results of a mission concept study completed by the Advanced Concepts Office at NASA Marshall Space Flight Center in 2012. The design team analyzed the mission and instrument requirements, and designed a spacecraft that enables the WFXT mission while using high heritage components. Design work included selecting components and sizing subsystems for power, avionics, guidance, navigation and control, propulsion, structures, command and data handling, communications, and thermal control.

Hopkins, R. C.↗
NASA NTRS2016

Development of the Model of Galactic Interstellar Emission for Standard Point-Source Analysis of Fermi Large Area Telescope Data

Most of the celestial gamma rays detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope originate from the interstellar medium when energetic cosmic rays interact with interstellar nucleons and photons. Conventional point-source and extended-source studies rely on the modeling of this diffuse emission for accurate characterization. Here, we describe the development of the Galactic Interstellar Emission Model (GIEM),which is the standard adopted by the LAT Collaboration and is publicly available. This model is based on a linear combination of maps for interstellar gas column density in Galactocentric annuli and for the inverse-Compton emission produced in the Galaxy. In the GIEM, we also include large-scale structures like Loop I and the Fermi bubbles. The measured gas emissivity spectra confirm that the cosmic-ray proton density decreases with Galactocentric distance beyond 5 kpc from the Galactic Center. The measurements also suggest a softening of the proton spectrum with Galactocentric distance. We observe that the Fermi bubbles have boundaries with a shape similar to a catenary at latitudes below 20deg and we observe an enhanced emission toward their base extending in the north and south Galactic directions and located within approximately 4deg of the Galactic Center.

gamma rays: diffuse background – gamma rays: gen↗
NASA NTRS2021

An Unbiased CO Survey toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. I. Overview: CO Cloud Distributions

We have analyzed the data from a large-scale CO survey toward the northern region of the Small Magellanic Cloud (SMC) obtained with the Atacama Compact Array (ACA) stand-alone mode of ALMA. The primary aim of this study is to comprehensively understand the behavior of CO as an H2 tracer in a low-metallicity environment (Z ∼ 0.2 Zꙩ). The total number of mosaic fields is ∼8000, which results in a field coverage of 0.26 sq.deg (∼2.9 ×10^(5) sq.pc), corresponding to ∼10% of the area of the galaxy. The sensitive ∼2 pc resolution observations reveal the detailed structure of the molecular clouds previously detected in the single-dish NANTEN survey. We have detected a number of compact CO clouds within lower H2 column density (∼10^(20) per sq.cm) regions whose angular scale is similar to the ACA beam size. Most of the clouds in this survey also show peak brightness temperature as low as <1 K, which for optically thick CO emission implies an emission size much smaller than the beam size, leading to beam dilution. The comparison between an available estimation of the total molecular material traced by thermal dust emission and the present CO survey demonstrates that more than ∼90% of H(2) gas cannot be traced by the low-J CO emission. Our processed data cubes and 2D images are publicly available.

Kazuki Tokuda↗