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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Precursor active galaxies, active galaxies and the X-ray and gamma-ray background

A description is presented of a model in which the X-ray and gamma-ray background can result from the superposition of radiation from active galaxies in different stages of their evolutionary history. This scheme is based on the generalization of a black-hole model developed for gamma-ray production in active galaxies by Leiter (1980). Both thermal and nonthermal accretion disk processes around massive rotating Kerr black holes are considered. Attention is given to cosmological conditions leading to precursor active galaxies (PAG), PAG as origin of the cosmic X-ray background, and the evolution of PAG into Seyfert galaxies as origin of the gamma-ray background. It is found that after spin-up of the central black hole an active galactic nucleus can generate nonthermal radiation very efficiently up to a few MeV.

Leiter, D.↗
NASA NTRS1985

Studies of the Virgo Cluster. II - A catalog of 2096 galaxies in the Virgo Cluster area. V - Luminosity functions of Virgo Cluster galaxies

The present catalog of 2096 galaxies within an area of about 140 sq deg approximately centered on the Virgo cluster should be an essentially complete listing of all certain and possible cluster members, independent of morphological type. Cluster membership is essentially decided by galaxy morphology; for giants and the rare class of high surface brightness dwarfs, membership rests on velocity data. While 1277 of the catalog entries are considered members of the Virgo cluster, 574 are possible members and 245 appear to be background Zwicky galaxies. Major-to-minor axis ratios are given for all galaxies brighter than B(T) = 18, as well as for many fainter ones.

Binggeli, B.↗
NASA NTRS1985

A statistical analysis of the Einstein normal galaxy sample. I - Spiral and irregular galaxies

The results of a statistical analysis of 48 spiral and irregular galaxies observed with the Einstein Observatory are reported. It is found that the X-ray luminosity is not directly correlated with the mass of the galaxies, but is strongly correlated with the blue luminosity. This suggests that most X-ray sources, including low-mass binaries, are binary systems belonging to both old disk and young arm Population I components. A real bulge population of X-ray sources exists that can contribute significantly to the X-ray luminosity of early-type spirals. The X-ray emission from a sample of 29 relatively isolated, normal elliptical and S0 galaxies is also studied, and the results are compared to those for the above sample to investigate the origin of the X-ray emission in early-type galaxies and the possible emission mechanisms. The influence of a powerful radio source and the onset of nuclear activity are also examined.

Fabbiano, G.↗
NASA NTRS1987

Determination of the masses of elliptical galaxies and clusters of galaxies with AXAF

The procedures to be used in the determination of the mass and mass distribution of elliptical galaxies and clusters of galaxies, using spatially resolved X-ray spectra of galaxies and clusters of galaxies to be obtained by the imaging spectrometers on AXAF, are described. The procedures utilize the unique AXAF features of large band width, large field of view, high angular resolution, long exposure times, and the availability of sophisticated AXAF instrumentation. The techniques described involve the measurement of the surface brightness and temperature profiles of the hot gas trapped in the potential wells of these systems. Then, by using the equation of hydrostatic equilibrium, which relates the total mass inside a given radius to the size, absolute temperature, density gradient, and temperature profile, it will be possible to infer the mass distribution in these objects to large physical scales.

Mushotzky, Richard F.↗
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 NTRS1991

Simulated optical images of galaxies at z about 1 using ultraviolet images of nearby galaxies

Ultraviolet sounding rocket images of several nearby galaxies are used to simulate the appearance in optical bandpasses of similar systems at redshifts z = 0.5-2.67, as observed by the WFPC on HST and by ground-based instruments. Since the morphology of galaxies is a strong function of the wavelength, the appearance of galaxies at large redshifts is subjected to a large k-correction effect. The strong dependence of monochromatic surface brightness on redshift also implies that observed morphology of distant systems will be crucially dependent on the limiting surface brightness set by the sky background. Although the angle subtended by sources depends only weakly on z, the fraction of a galaxy whose surface brightness is above the detection threshold varies strongly with z. Morphological distinctions of spiral from elliptical, spiral from irregular, and barred from unbarred types become more difficult, as does accurate evaluation of interactions or of the local environment.

Bohlin, Ralph C.↗
NASA NTRS1991

The dwarf galaxy UGC 5272 and its small companion galaxy

The present study of optical images and spectroscopy of the dwarf irregular galaxy UGC 5272 notes the presence, at 3.6 kpc, of a small neighboring galaxy which is also of irregular type and has a Holmberg diameter of 0.6 kpc. Attention is given to the possibility that the two galaxies, which are resolved into single stars, may form a physical pair. It is suggested that the blue-to-red supergiant ratio of UGC 5272 is high due to its low metallicity. While its extremely blue colors are suggestive of a recent starburst, the structural parameters of the galaxy are surprisingly normal. The gas contribution to total mass is high.

Hopp, U.↗
NASA NTRS1992

Spectroscopy of galaxies in distant clusters. IV - A catalog of photometry and spectroscopy for galaxies in seven clusters with z in the range of 0.35 to 0.55

Photometry and spectroscopy for seven deep fields containing distant clusters of galaxies with z in the range of 0.35 to 0.55 are presented. Positions and photometric parameters, including r-magnitudes g - r and r - i colors, surface brightnesses, and photometric profile types are given for about 2000 galaxies. Low-resolution spectroscopy is obtained from which redshifts are determined for 289 objects, of which 190 are cluster members. These are classified according to dominant spectral features, and examples are plotted in each cluster. Color-magnitude and color-color diagrams are formed which show trends in the cluster populations, and maps are made of the cluster field using the color-color relations to increase the contrast of cluster over field. Galaxies with spectra typical of old stellar populations cluster most strongly, with active galaxies, those with recent or ongoing star formation, or an active nucleus, distributed more diffusely. The g - r color is well correlated with active star formation as judged from spectral features.

Dressler, Alan↗
NASA NTRS1993

Low redshift star-forming galaxies: What can they teach us about primeval galaxies?

The analysis of the UV plus optical spectra of three star-forming galaxies, Mrk 496, Mrk 357, TOL1924-416, obtained by matching the size of the optical aperture with that of IUE, has given unexpected results. These can be summarized as follows: (1) the dereddened Ly(alpha)/H(beta) ratios are consistent with the prediction of case B recombination for nebular emission, within the uncertainties; (2) the decrease of the Ly(alpha)/H(beta) ratio with increasing metallicities is not confirmed in our three objects, although the sample is too small to consider this result definitive. The first result is surprising, mainly because at least the two Markarian galaxies have a large enough H1 content to markedly increase the optical depth for the Ly(alpha) photons and to trigger their absorption by dust. This finding can probably be explained as an effect of the inhomogeneous distribution of gas and dust within the galaxies. On the basis of these results, we conclude that the detection of the Ly(alpha) emission line in searching for primeval galaxies (PG's) can be still considered a valid technique.

Calzetti, D.↗
NASA NTRS2019

Near-infrared Spectroscopy of Galaxies During Reionization: Measuring C III] in a Galaxy at z = 7.5

We present Keck/MOSFIRE H-band spectroscopy targeting C III] λ1907, 1909 in a z = 7.5056 galaxy previously identified via Lyα emission. We detect strong line emission at 1.621 ± 0.002 mm with a line flux of (2.63 ± 0.52) × 10−18 erg s−1cm−2. We tentatively identify this line as [C III] λ1907, but we are unable to detect C III] λ1909 owing to sky emission at the expected location. This gives a galaxy systemic redshift, zsys = 7.5032 ± 0.0003, with a velocity offset to Lyα of DvLya = 88 ± 27 km s−1. The ratio of combined C III]/Lyα is 0.30–0.45, one of the highest values measured for any z > 2 galaxy. We do not detect Si III] λλ1883, 1892, and place an upper limit on Si III]/C III] < 0.35 (2σ). Comparing our results to photoionization models, the C III] equivalent width (WC III] = 16.23 ± 2.32 Å), low Si III]/C III] ratio, and high implied [O III] equivalent width (from the Spitzer/IRAC [3.6]–[4.5] ~- 0.8 mag color) require subsolar metallicities (Z ~- 0.1–0.2 Zsun) and a high ionization parameter, log U >~ 1.5. These results favor models that produce higher ionization, such as the BPASS models for the photospheres of high-mass stars, and that include both binary stellar populations and/or an IMF that extends to 300 Me. The combined C III] equivalent width and [3.6]–[4.5] color are more consistent with ionization from young stars than active galactic nuclei (AGNs); however, we cannot rule out ionization from a combination of an AGN and young stars. We make predictions for James Webb Space Telescope spectroscopy using these different models, which will ultimately test the nature of the ionizing radiation in this source.

reionization↗
NASA NTRS2013

Stellar Populations in Compact Galaxy Groups: a Multi-wavelength Study of HCGs 16, 22, and 42, Their Star Clusters, and Dwarf Galaxies

We present a multi-wavelength analysis of three compact galaxy groups, Hickson compact groups (HCGs) 16, 22, and 42, which describe a sequence in terms of gas richness, from space- (Swift, Hubble Space Telescope (HST), and Spitzer) and ground-based (Las Campanas Observatory and Cerro Tololo Inter-American Observatory) imaging and spectroscopy.We study various signs of past interactions including a faint, dusty tidal feature about HCG 16A, which we tentatively age-date at <1 Gyr. This represents the possible detection of a tidal feature at the end of its phase of optical observability. Our HST images also resolve what were thought to be double nuclei in HCG 16C and D into multiple, distinct sources, likely to be star clusters. Beyond our phenomenological treatment, we focus primarily on contrasting the stellar populations across these three groups. The star clusters show a remarkable intermediate-age population in HCG 22, and identify the time at which star formation was quenched in HCG 42. We also search for dwarf galaxies at accordant redshifts. The inclusion of 33 members and 27 "associates" (possible members) radically changes group dynamical masses, which in turn may affect previous evolutionary classifications. The extended membership paints a picture of relative isolation in HCGs 16 and 22, but shows HCG 42 to be part of a larger structure, following a dichotomy expected from recent studies. We conclude that (1) star cluster populations provide an excellent metric of evolutionary state, as they can age-date the past epochs of star formation; and (2) the extended dwarf galaxy population must be considered in assessing the dynamical state of a compact group.

Compact Galaxy↗
DOE OSTI2024

Self-interacting Dark Matter, Core Collapse, and the Galaxy–Galaxy Strong-lensing Discrepancy

Abstract Gravitational lensing by galaxy clusters has emerged as a powerful tool to probe the standard cold dark matter (CDM) paradigm of structure formation in the Universe. Despite the remarkable explanatory power of CDM on large scales, tensions with observations on small scales have been reported. Recent studies find that the observational cross section for galaxy–galaxy strong lensing (GGSL) in clusters exceeds the CDM prediction by more than an order of magnitude and persists even after rigorous examination of various possible systematics. We investigate the impact of modifying the internal structure of cluster dark matter subhalos on GGSL and report that altering the inner density profile, given by r γ , to steeper slopes with γ > 2.5 can alleviate the GGSL discrepancy. This is steeper than slopes obtained with the inclusion of the contribution of baryons to the inner regions of these subhalos. Deviating from the γ ~ 1.0 cusps that CDM predicts, these steeper slopes could arise in models of self-interacting dark matter undergoing core collapse. Our results motivate additional study of subhalo core collapse in dense cluster environments.

Dutra, Isaque (ORCID:0000000170404930)↗
DOE OSTI2025

The AGORA High-resolution Galaxy Simulations Comparison Project. VIII. Disk Formation and Evolution of Simulated Milky Way Mass Galaxy Progenitors at 1 < z < 5

We investigate how differences in the stellar feedback produce disks with different morphologies in Milky Way–like progenitors over 1 ≤ z ≤ 5, using eight state-of-the-art cosmological hydrodynamics simulation codes in the AGORA project. In three of the participating codes, a distinct, rotation-dominated inner core emerges with a formation timescale of ≲300 Myr, largely driven by a major merger event, while two other codes exhibit similar signs of wet compaction—gaseous shrinkage into a compact starburst phase—at earlier epochs. The remaining three codes show only weak evidence of wet compaction. Consequently, we divide the simulated galaxies into two groups: those with strong compaction signatures and those with weaker ones. Galaxies in these two groups differ in size, stellar age gradients, and disk-to-total mass ratios. Specifically, codes with strong wet compaction build their outer disks in an inside-out fashion, leading to negative age gradients, whereas codes with weaker compaction feature flat or positive age gradients caused primarily by outward stellar migration. Although the stellar half-mass radii of these two groups diverge at z ∼ 3, the inclusion of dust extinction brings their sizes and shapes in mock observations closer to each other and to observed galaxies. We attribute the observed morphological differences primarily to variations in the stellar feedback implementations—such as delayed cooling timescales, and feedback strengths—that regulate both the onset and duration of compaction. Overall, our results suggest that disk assembly at high redshifts is highly sensitive to the details of the stellar feedback prescriptions in simulations.

Jung, Minyong [Seoul National Univ. (Korea, Republ↗
NASA NTRS1972

The estimation of masses of individual galaxies in clusters of galaxies.

Three different methods of estimating masses are discussed. The 'density method' is based on the analysis of the density distribution of galaxies around the object whose mass is to be found. The 'bound-galaxy method' gives estimates of the mass of a double, triple, or quadruple system from analysis of the orbital motion of the components. The 'virial method' utilizes the formulas derived for the second method to obtain estimates of the virial-theorem masses of whole clusters, and thus to obtain upper limits on the mass of an individual galaxy in a cluster. The analytic formulas are developed and compared with computer experiments, and some applications are given.

Wolf, R. A.↗
NASA NTRS1985

Candidate galaxies for study of the local velocity field and distance scale using Space Telescope. I - The most easily resolved. II - The more difficult cases. III - Galaxies in the Virgo cluster core

A subset of 1246 Shapley-Ames galaxies is presented which holds promise for some degree of useful resolution by the Space Telescope. The 95 candidate galaxies easiest to resolve are described and illustrated first. Many of these can be readily resolved into brightest stars from the ground with presently available telescopes at a few sites, while others might be resolved from the ground into at least the brightest blue supergiants. Then, 152 additional candidates are presented which can only be adequately studied with the Space Telescope. Finally, the most promising galaxies for studies of the resolved stellar content that occur in the central six degree core of the Virgo cluster are given. These are expected to be useful for any direct attack on the distance to the cluster itself.

Sandage, A.↗
NASA NTRS1991

Kinematics and ionization of extended gas in active galaxies. VI - The Seyfert 2 galaxy NGC 1386

Results are presented of direct imaging with a CCD detector and long-slit medium- and low-dispersion spectroscopy of the spatially extended narrow-line-emitting gas in the nearby Seyfert 2 galaxy NGC 1386. The distribution, kinematics, and ionization structure of the excited gas were examined in the regions surrounding the nucleus out to a distance of about 1.0. Results present evidence for a spiral structure in this galaxy and for the presence of H II regions in the disk (supporting the classification of NGC 1386 as a Sa galaxy).

Weaver, K. A.↗
NASA NTRS1991

New Cepheid distances to nearby galaxies based on BVRI CCD photometry. II - The local group galaxy M33

A true distance modulus to the nearby spiral galaxy M33 has been determined based on CCD photometry obtained at BVRI wavelengths. M33 is presently one of five nearby galaxies used in the calibration of the IR Tully-Fisher relation, and thereby in the determination of the Hubble constant. Using period-luminosity relations at several wavelengths offers the advantage that the distance moduli derived can be corrected for the effects of interstellar extinction. These data indicate that there is internal reddening affecting the Cepheid photometry in M33 which must be accounted for if a true distance modulus is to be obtained for this galaxy. Adopting a true distance modulus to the LMC of 18.5 mag, the new CCD data yield a true distance to M33 of 24.64 + or - 0.09 mag, corresponding to a linear distance of 840 kpc. A mean value of the total color excess (foreground and internal) for the Cepheids in M33 is estimated to be E(B - V) = 0.10 + or - 0.09 mag, assuming a value for the total mean LMC Cepheid color excess of 0.10 mag.

Freedman, Wendy L.↗
NASA NTRS1992

Using arcs in clusters of galaxies as probes to cluster potentials and the faint blue galaxies

An extensive study of the statistical properties of highly magnified gravitational images of background galaxies produced by clusters of galaxies (generally known as arcs) is undertaken. After summarizing the results obtained from present observations (described in detail elsewhere), techniques are described for observations to improve upon these results, and to obtain further information on the potentials of the lensing clusters and the faint blue galaxies acting as sources. The most important information that can be obtained from the arcs is, with respect to the clusters, on the total density profile and the substructure at the small scales probed by the arcs, and, with respect to the background sources, on their angular sizes and redshifts.

Miralda-Escude, Jordi↗