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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS2005

Evolution of Galaxies and the Star Formation Rate in the Infrared

A central goal of extragalactic observational astronomy is to understand how normal galaxies evolve with redshift, and particularly when galaxies formed their stars. While optical and rest-frame UV observations have begun to address these issues, the interpretation of such data is particularly challenging because of the sensitivity to dust obscuration (at optical and UV wavelengths). The absorbed light is re-radiated at IR wavelengths, hence the optimal indicators of the star formation rate (SFR) is at a rest-frame wavelength of approx. 60 microns. The Spitzer Space Telescope mission is revolutionizing the study of the global properties and evolution of galaxies. Spitzer reaches nearly two orders of magnitude more sensitivity than previous IR space missions. This research program is to study the SFR using statistical samples of galaxies in the local universe, at intermediate redshifts, and set the stage for continuing studies up to z=5. The overall research program is divided into three main investigations: A Mid-IR Hubble Atlas and SFR estimators in the local universe, Evolution of the SFR at 0 < z < 1 using pencil beam redshift surveys, and Galaxy formation and evolution at 1 < z < 5. The first papers from Spitzer were published during the last year, including ten refereed journal papers where the PI was first or co-author.

Pahre, Michael A.↗
NASA NTRS1991

Were small galaxies once the dominant cosmological population?

Redshifts and K magnitudes have been obtained for a small but complete sample of 22 galaxies with B magnitudes down to 24. In the luminosity range B = 23-24, the B-band galaxy counts are dominated by a population of small blue galaxies at z roughly 0.25, which may collectively contain as much baryonic matter as the normal galaxies. It is possible either that these earlier galaxies have undergone merging to create the present galaxy population, or that they represent a quite different galactic population which has now faded or disappeared. Either possibility has considerable implications for understanding of galaxy formation.

Cowie, L. L.↗
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 NTRS1992

The X-ray spectra of galaxies. II - Average spectral properties and emission mechanisms

The Imaging Proportional Counter data in the Einstein database is used to study the X-ray spectra of normal galaxies. The X-ray emission temperature of spirals is found to be higher on the average than that of ellipticals. This supports the idea that accreting binaries are a major source of X-rays in spirals, while a hot interstellar medium (ISM) is present in ellipticals. The X-ray spectra of Sa galaxies are intermediate between those of ellipticals and spirals, suggesting that these galaxies contain hot gaseous emission as well as emission from accreting binaries. In E and SO galaxies the emission temperature becomes higher with a decreasing X-ray to optical luminosity ratio, which suggests that the emission of X-ray faint early-type galaxies consists of a large evolved stellar component, while the gaseous emission becomes dominant in X-ray brighter galaxies. The group with the lowest X-ray to optical ratio does not follow this trend; in these galaxies a very soft X-ray component, amounting to about half the total X-ray emission, is found in addition to the hard X-ray component. Possible explanations are integrated emission of M stars and a relatively cool ISM. A very soft component is also found in several spiral galaxies. This may indicate that some spirals contain hot gaseous components similar to those seen in NGC 253 and M82.

Kim, D.-W.↗
NASA NTRS1987

Arecibo observations of IRAS galaxies at 21 and 18 centimeters

Arecibo observations at 21 cm and 18 cm of a sample of galaxies selected for their intense far-infrared (FIR) emission are reported. The objective was to search for H I absorption and OH maser emission. No new cases were found. The significance of this null result is discussed both in terms of spatial distribution of the interstellar gas and dust and in terms of the continuum luminosity of the nucleus. Nevertheless, the H I profiles are extraordinary in that most are convex (single peak). This may suggest disruption of the disk rotation system in some cases. Little correlation is found between the hydrogen mass of these galaxies and their FIR luminosity. Comparison of these quantities suggests that the dust in these galaxies is under conditions very different from the solar neighborhood. Much more of the FIR emission apparently comes from H II regions than in normal galaxies. There is an excellent correlation between IR flux and radio continuum flux. The ratio of these two is the same as for normal spirals, which is surprising considering the many variables which could change this ratio.

Garwood, Robert W.↗
NASA NTRS1986

Ultraviolet spectrophotometry of three LINERs

Three galaxies known to be LINERs were observed spectroscopically in the ultraviolet in an attempt to detect the presumed nonthermal continuum source thought to be the source of photoionization in the nuclei. NGC 4501 was found to be too faint for study with the IUE spectrographs, while NGC 5005 had an extended ultraviolet light profile. Comparison with the optical light profile of NGC 5005 indicates that the ultraviolet source is distributed spatially in the same manner as the optical starlight, probably indicating that the ultraviolet excess is due to a component of hot stars in the nucleus. These stars contribute detectable absorption features longward of 2500 A; together with optical data, the IUE spectra suggest a burst of star formation about 1 billion yr ago, with a lower rate continuing to produce a few OB stars. In NGC 4579, a point source contributing most of the ultraviolet excess is found that is much different than the optical light distribution. Furthermore, the ultraviolet to X-ray spectral index in NGC 4579 is 1.4, compatible with the UV to X-ray indices found for samples of Seyfert galaxies. This provides compelling evidence for the detection of the photoionizing continuum in NGC 4579 and draws the research fields of normal galaxies and active galactic nuclei closer together. The emission-line spectrum of NGC 4579 is compared with calculations from a photoionization code, CLOUDY, and several shock models. The photoionization code is found to give superior results, adding to the increasing weight of evidence that the LINER phenomenon is essentially a scaled-down version of the Seyfert phenomenon.

Goodrich, R. W.↗
NASA NTRS1981

Diffuse radiation

A diffuse celestial radiation which is isotropic at least on a course scale were measured from the soft X-ray region to about 150 MeV, at which energy the intensity falls below that of the galactic emission for most galactic latitudes. The spectral shape, the intensity, and the established degree of isotropy of this diffuse radiation already place severe constraints on the possible explanations for this radiation. Among the extragalactic theories, the more promising explanations of the isotropic diffuse emission appear to be radiation from exceptional galaxies from matter antimatter annihilation at the boundaries of superclusters of galaxies of matter and antimatter in baryon symmetric big bang models. Other possible sources for extragalactic diffuse gamma radiation are discussed and include normal galaxies, clusters of galaxies, primordial cosmic rays interacting with intergalactic matter, primordial black holes, and cosmic ray leakage from galaxies.

Source record↗
NASA NTRS1990

Circumnuclear molecular gas in starburst and Seyfert galaxies

In order to investigate circumnuclear molecular gaseous contents and their relation to the nuclear activity, researchers made a search for circumnuclear (12)CO (J=1-0) emission from 28 starburst-nucleus galaxies (SBNs) and 12 Seyfert galaxies with the recession velocities less than 5000 km/s, using the Nobeyama Radio Observatory 45-m telescope. The full half-power beam width of 17 arcsec covers a region of less than about 5 kpc in diameter for the sample galaxies. The circumnuclear CO emission was detected from twelve SBNs (one is marginal) and four Seyfert galaxies. The main results and conclusions are summarized. Researchers derived the circumnuclear surface density of molecular gas which is corrected for inclination of the galaxies. This analysis shows that the surface density spans a wide range over two orders of magnitude. Further, there is no significant difference in the surface densities between types 1 and 2 Seyfert galaxies. Thus, we may conclude that the circumnuclear molecular content is not a key parameter producing the dichotomy of the Seyfert galaxies. It is also shown that there is no significant difference in the circumnuclear surface densities of molecular gas among the Seyfert, starburst, and normal galaxies. This implies that the circumnuclear gaseous content is not a key parameter determining which activity occurs in nuclei. We may conclude that more centrally condensed (i.e., less than 10 - 100 pc in diameter) gas components play an essential role on the occurrence of nuclear activities. Comparing results with the previous ones, researchers deduced radial distribution of surface density of molecular gases. They cannot obtain evidence for strong central concentration of molecular gas in the sample Seyfert galaxies except for NGC 3227. This is consistent with the previous result by Blitz, Mathieu, and Bally (1986). Comparing the CO emission line profiles with the previous ones taken with the larger beams, researchers discovered circumnuclear components of molecular gases. In particular, they found that molecular gas clouds may be absent in the SE of the nucleus of NGC 7469 where the high-excitation emitting region is discovered by Heckman et al. (1986). It is suggested that the nuclear activity (strong radiation and/or wind) may destruct the molecular clouds in that region.

Taniguchi, Yoshiaki↗
NASA NTRS2016

Connecting the Interstellar Gas and Dust Properties in Distant Galaxies Using Quasar Absorption Systems

Gas and dust grains are fundamental components of the interstellar medium and significantly impact many of the physical processes driving galaxy evolution, such as star-formation, and the heating, cooling, and ionization of the interstellar material. Quasar absorption systems (QASs), which trace intervening galaxies along the sightlines to luminous quasars, provide a valuable tool to directly study the properties of the interstellar gas and dust in distant, normal galaxies. We have established the presence of silicate dust grains in at least some gas-rich QASs, and find that they exist at higher optical depths than expected for diffuse gas in the Milky Way. Differences in the absorption feature shapes additionally suggest variations in the silicate dust grain properties, such as in the level of grain crystallinity, from system-to-system. We present results from a study of the gas and dust properties of QASs with adequate archival IR data to probe the silicate dust grain properties. We discuss our measurements of the strengths of the 10 and 18 micron silicate dust absorption features in the QASs, and constraints on the grain properties (e.g., composition, shape, crystallinity) based on fitted silicate profile templates. We investigate correlations between silicate dust abundance, reddening, and gas metallicity, which will yield valuable insights into the history of star formation and chemical enrichment in galaxies.

Aller, Monique C.↗
NASA NTRS1996

The X-ray emission of galaxies

Results from several satellite observations are used to discuss the discrete emission component of galaxies, including bright binary sources, supernova and supernova remnants. Through X-ray observations, direct evidence of the chemical and dynamical interactions between galaxies and the surrounding medium can be obtained. Although the X-ray emission represents only a small fraction of the bolometric emission of a normal galaxy, it allows the study of the system's component to be performed. The results on low luminosity active galactic nuclei are also discussed.

Fabbiano, G.↗
NASA NTRS1997

Gas and Dust Properties in Dwarf Irregular Galaxies

We present a study of the 158 (micron)meter [C II] fine structure emission line from a sample of 11 low metallicity irregular galaxies using the NASA Kuiper Airborne Observatory (KAO). Our preliminary results demonstrate that the ratio of the 158 (micron)meter [C II] emission to the CO-12(1 yields 0) emission ranges from 6,000 to 46,000. These ratios are significantly enhanced relative to clouds within the Galaxy and to normal metallicity galaxies, which typically have values in the range 2,000 to 6,300. We also find that the [C II] emission in dwarf irregular galaxies can be up to 5% of the far-infrared (FIR) emission, a higher fraction of the FIR than in normal metallicity galaxies. We discuss these results for the dwarf irregular galaxies and compare them to those observed in normal metallicity galaxies. The enhanced 158 (micron)meter [C II] emission relative to CO-12(1 yields 0) emission can be understood in terms of the increased penetration depth of ultraviolet (UV) photons into the clouds in low metallicity environments.

Jones, A. P.↗
DOE OSTI2023

The haloes and environments of nearby galaxies ( HERON ) – III. A 45-kpc spiral structure in the GLSB galaxy UGC 4599

ABSTRACT We use a 0.7-m telescope in the framework of the Halos and Environments of Nearby Galaxies (HERON) survey to probe low surface brightness (LSB) structures in nearby galaxies. One of our targets, UGC 4599, is usually classified as an early-type galaxy surrounded by a blue ring making it a potential Hoag’s Object analogue. Prior photometric studies of UGC 4599 were focused on its bright core and the blue ring. However, the HERON survey allows us to study its faint extended regions. With an 8-h integration, we detect an extremely faint outer disc with an extrapolated central surface brightness of μ0, d(r) = 25.5 mag arcsec−2 down to 31 mag arcsec−2 and a scale length of 15 kpc. We identify two distinct spiral arms of pitch angle ∼6○ surrounding the ring. The spiral arms are detected out to ∼45 kpc in radius and the faint disc continues to ∼70 kpc. These features are also seen in the GALEX far- and near-ultraviolet bands, in a deep u-band image from the 4.3-m Lowell Discovery Telescope (which reveals inner spiral structure emerging from the core), and in H i. We compare this galaxy to ordinary spiral and elliptical galaxies, giant low surface brightness (GLSB) galaxies, and Hoag’s Object itself using several standard galaxy scaling relations. We conclude that the pseudo-bulge and disc properties of UGC 4599 significantly differ from those of Hoag’s Object and of normal galaxies, pointing toward a GLSB galaxy nature and filamentary accretion of gas to generate its outer disc.

Astronomy & Astrophysics↗
NASA NTRS1973

Problem of the redshifts

It was shown by Sandage (1965) that their redshift-apparent magnitude diagram was not incompatible with that obtained for bright galaxies with a displaced magnitude scale. The redshift-angular diameter relation is discussed together with the distribution of redshifts, multiple redshifts, QSOs and nuclei of normal galaxies, questions of variability and relativistic expansion, direct distance measurements, other evidence, and the overall situation.

Burbidge, G. R.↗
NASA NTRS1998

ASCA Observations

This recently expired grant has supported the work of the PI, his students, and his collaborators on a variety of ASCA projects over the past four years. Annual reports have summarized much of the work accomplished; here we provide a brief review of the work resulting from this effort, and a summary of the personnel who have benefited from the grant's support. Starburst Galaxies with Extreme X-ray Luminosities This project began as a careful examination of the claims of Boller et al. (1992) that there were dozens of "normal" galaxies in the ROSAT All-Sky Survey that had X-ray luminosities in excess of 1042 erg sec, higher than that seen in the hundreds of non-AGN galaxies observed with Einstein. If true, this suggested that X-ray emission associated with star formation activity might have a significant contribution to make to the still unexplained cosmic X-ray background (XRB). Since some of our earlier work with the Einstein Observatory Deep Surveys had suggested a similar possibility and several sets of authors over the years had modelled the starburst XRB contribution, these claims were worth pursuing. Our work expanded the examination beyond the RASS to include earlier claims of high-luminosity galaxies powered by starburst emission (selected in this case on the basis of the far-IR luminosities). The result of extensive followup observations under several programs using ROSAT, ASCA, and ground-based facilities was to show that nearly all of these objects in fact have hidden AGN at their cores, and that their luminosities are not in any way extraordinary.

Helfand, David J.↗
NASA NTRS1986

The IRAS view of the extragalactic sky

The global properties of the IR emissions from optically bright galaxies observed with the IRAS are described. A histogram of the morphological types of the galaxies is presented. Analysis of the ratio of far IR to blue luminosity (IR/B) reveals that the ratio covers a large range of about five orders of magnitude; 'normal galaxies' in the IR/B ratio of about 0.1-1.0 and galaxies with an IR/B ratio of greater than 1 are observed. The IR/B ratios for galaxies as a function of the 100 micron-60 micron flux density ratio are examined and it is detected that the galaxies which are more active in the IR display a warmer color temperature.

Soifer, B. T.↗
NASA NTRS1996

Exploration an the Search for Origins: A Vision for Ultraviolet-Optical-Infrared Space Astronomy

Public support and enthusiasm for astronomy have been strong in the final decades of the twentieth century. Nowhere is this better demonstrated than with the Hubble Space Telescope (HCT), a grand endeavor, which is enabling astronomers to make giant strides in understanding our universe, our place in it, and our relation to it. The NASAs first infrared observatory, the Space Infrared Telescope Facility (SIRTF), promises to take the crucial next steps towards understanding the formation of stars and galaxies. Toward their completion, the HST and Beyond Committee identifies major goals, whose accomplishment will justify a commitment well into the next century: (1) the detailed study of the birth and evolution of normal galaxies such as the Milky Way; (2) the detection of Earth-like planets around other stars and the search for evidence of life on them; (3) NASA should develop a space observatory of aperture 4m or larger, optimized for imaging and spectroscopy over the wavelength range 1-5 microns; and (4) NASA should develop the capability for space interferometry.

Dressler, Alan↗
NASA NTRS1996

The X-ray background contributed by QSOs ejected from galaxies

The X-ray background can be explained as coming from the integrated effect of X-ray emitting quasi-stellar objects (QSOs) ejected from spiral galaxies. The model developed to interpret the observations is summarized. The redshift of the QSOs consisted of an intrinsic component and of a cosmological component. The QSOs have a spatial density proportional to that of normal galaxies.

Burbidge, G.↗
NASA NTRS1981

New insight into the physical state of galaxies and quasars

Data from the International Ultraviolet Explorer satellite have revolutionized many concepts in extragalactic astronomy. These include the physical processes at work in the emitting gas characteristic of active objects, the nature of the continuum source itself in those objects, and the constituent hot stellar and gaseous components of normal galaxies. Several problems of extragalactic research investigated with IUE were reviewed.

Green, R. F.↗