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NASA NTRS1991

NGC 1068 - Resolution of nuclear structure in the optical continuum

The HST Planetary Camera has been used to obtain an optical continuum image of the Seyfert galaxy NGC 1068. The image reveals a bright nucleus embedded in an irregular cloudlike structure which is well-differentiated against the background of the galaxy. The nucleus is resolved, with an FWHM of about 0.15 arcsec, or 11 pc. There is no evidence for any unresolved nuclear component. The precise geometry of the scattering region cannot yet be inferred. The cloud surrounding the nucleus is elongated in the NE-SSW direction and has extreme dimensions of 3.5 arcsec x 1.7 arcsec. The brightness centroid is situated 0.4 arcsec SW of the nucleus. It is concluded that the light from the cloud is contributed largely by stars, but that the appearance of the cloud is distinctly dissimilar to what is expected from young or old stellar systems and bears to simple relationship to the numerous features that have heretofore been resolved at other wavelengths.

Lynds, Roger↗

NASA NTRS1984

Far-infrared spectroscopy of the galactic center - Neutral and ionized gas in the central 10 parsecs of the Galaxy

The present mapping of the 3P1-3P2 fine structure line emission from neutral atomic oxygen near the galactic center shows the emission to be extended over more than 12 pc along the galactic plane, centered on the position of Sgr A West. The rotational velocity of the O I gas at R of about 1 corresponds to a mass within the central parsec of about 3 million solar masses. The forbidden O I line probably arises in a predominantly neutral atomic region immediately outside the ionized central parsec of the Galaxy. Gas temperatures are greater than 100 K, and the total integrated luminosity radiated in the line, which is about 100,000 solar luminosities, substantially contributes to the cooling of the gas. The 3P1-3P0 fine structure line of the O III forbidden line has also been detected at 88 microns toward Sgr A West, coming from high density ionized gas.

Genzel, R.↗

NASA NTRS2002

A Chandra X-Ray Study of Cygnus A: The Cluster of Galaxies - 3

We present an analysis of the Chandra Advanced CCD Imaging Spectrometer (ACIS) observation of the intracluster gas associated with the cluster of galaxies surrounding Cygnus A. The dominant gaseous structure is a roughly elliptical (presumably prolate spheroidal in three dimensions) feature with semimajor axis approximately equal to 1.(prime)1 (approximately equal to 100 kpc). This structure apparently represents intracluster gas that has been swept up and compressed by a cavity inflated in this gas by relativistic material that has passed through the ends of the radio jets. The X-ray-emitting gas shows this prolate spheroidal morphology to approximately equal to 1.(prime)2 (110 kpc) from the radio galaxy but is spherical on larger scales. The X-ray emission from the intracluster gas extends to at least 8 prime (approximately equal to 720 kpc) from the radio galaxy, and a second, extended source of X-ray emission (probably associated with a second cluster of galaxies) is seen some 12 prime (approximately equal to 1 Mpc) to the northwest of Cygnus A. The X-ray spectrum of the integrated intracluster gas imaged on the S3 chip (dimensions 8 prime x 8 prime = 720 x 720 kpc), excluding the contribution from the radio galaxy and other compact sources of X-ray emission, has a gas temperature, metallicity, and unabsorbed 2-10 keV rest-frame luminosity of 7.7 keV, 0.34 times solar, and 3.5 x 10(exp 44) ergs per second, respectively. We have projected the X-ray spectra taken from 12 elliptical and circular annuli in order to derive a run of temperature, metallicity, density, and pressure as a function of radius. The temperature of the X-ray-emitting gas drops from approximately equal to 8 keV more than 100 kpc from the center to approximately equal to 5 keV some 80 kpc from the center, with the coolest gas immediately adjacent to the radio galaxy. "Belts" of slightly cooler (approximately equal to 4 keV) X-ray-emitting gas run around the minor dimension of the cavity created by the radio source, while the limb-brightened edges of the cavity are slightly hotter (approximately equal to 6 keV), perhaps as a result of heating by a bow shock driven by the probably expanding cavity into the intracluster gas. There is a metallicity gradient in the X-ray-emitting gas, with the highest metallicities (approximately solar) found close to the center, decreasing to approximately 0.3 times solar in the outer parts. We have used the assumption of hydrostatic equilibrium to derive a total cluster mass within 500 kpc of 2.0 x 10(exp 14) solar mass and 2.8 x 10(exp 14) solar mass for constant and centrally decreasing temperature profiles, respectively. The total mass of X-ray-emitting gas within the same radius is 1.1 x 10(exp 13) solar mass. Thus, the gas fraction of the cluster within 500 kpc is 0.055 and 0.039 for the constant and centrally decreasing temperature profiles, respectively.

Smith, David A.↗

NASA NTRS1990

The interacting pair Mkn 305/306

Direct images and spectra at different slit positions of the interacting system Mkn 305/306 are discussed. Both galaxies show starburst properties due to tidal interaction. The morphology and velocity structure of Mkn 306 reveals the strongest warp of a stellar disk so far known. The galaxies Mkn 305 and Mkn 306 form a double system with 30 arcsec separation and having a common envelope at m sub B greater than or equal to 24.5. Furthermore a small tidal tail west of Mkn 305A, an isophote twist of Mkn 305 and the near identical redshifts of the two galaxies prove that this is a physical pair. Mkn 306 itself was classified as a double nucleus galaxy (Petrosyan et al., 1978). The optical morphology of Mkn 306 has the form of an integral sign which is similar to the radio morphology of strongly warped galaxies (Bottema et al., 1987). But in the optical the warp of the stellar component is normally far weaker than in the radio for the HI-gas. Therefore the authors investigate whether the double nucleus structure of Mkn 306 is real or an artifact due to an extreme warp in the optical. The origin for the warp phenomenon is not clear yet. Direct images of the galaxy system were taken with the Calar Alto 2.2m telescope in the B-band and at La Silla in the r-band using the 2.2m telescope; low dispersion spectra (240 A/mm) as well as high dispersion spectra (56 A/mm) for studying the velocity field were taken with the Calar Alto 3.5m telescope at different position angles. Also a spectrum of Mkn 305 was taken at Calar Alto with the 3.5m telescope covering the whole spectral range (240 A/mm).

Dietrich, Matthias↗

NASA NTRS2014

3D-HST + CANDELS: the Evolution of the Galaxy Size-mass Distribution Since Z=3

Spectroscopic and photometric redshifts, stellar mass estimates, and rest-frame colors from the 3D-HST survey are combined with structural parameter measurements from CANDELS imaging to determine the galaxy size-mass distribution over the redshift (z) range 0 < z < 3. Separating early- and late-type galaxies on the basis of star-formation activity, we confirm that early-type galaxies are on average smaller than late-type galaxies at all redshifts, and find a significantly different rate of average size evolution at fixed galaxy mass, with fast evolution for the early-type population, effective radius is in proportion to (1 + z) (sup −1.48), and moderate evolution for the late-type population, effective radius is in proportion to (1 + z) (sup −0.75). The large sample size and dynamic range in both galaxy mass and redshift, in combination with the high fidelity of our measurements due to the extensive use of spectroscopic data, not only fortify previous results, but also enable us to probe beyond simple average galaxy size measurements. At all redshifts the slope of the size-mass relation is shallow, effective radius in proportion to mass of a black hole (sup 0.22), for late-type galaxies with stellar mass > 3 x 10 (sup 9) solar masses, and steep, effective radius in proportion to mass of a black hole (sup 0.75), for early-type galaxies with stellar mass > 2 x 10 (sup 10) solar masses. The intrinsic scatter is approximately or less than 0.2 decimal exponents for all galaxy types and redshifts. For late-type galaxies, the logarithmic size distribution is not symmetric, but skewed toward small sizes: at all redshifts and masses a tail of small late-type galaxies exists that overlaps in size with the early-type galaxy population. The number density of massive (approximately 10 (sup 11) solar masses), compact (effective radius less than 2 kiloparsecs) early-type galaxies increases from z = 3 to z = 1.5 − 2 and then strongly decreases at later cosmic times.

EVOLUTION↗

NASA NTRS1983

The inner radio structure of Centaurus A - Clues to the origin of the jet X-ray emission

VLA observations at 1.4 and 4.9 GHz of the jet and inner lobes of the nearby radio galaxy Centaurus A have been used to construct maps of total intensity and polarization at resolutions of 31 x 10 and 3.6 x 1.1 arcsec. Surface brightness and pressure distributions in the jet, combined with the apparent X-ray emission from the ISM of NGC 5128, indicate that it is thermally confined. A comparison of the radio structure and the optical galaxy shows that the jet in Cen A emerges nearly along the major axis of the elliptical stellar component that is parallel to the angular momentum vector of the dust lane. The outer radio structure bends toward the galaxy minor axis. Evidence is found for a common synchrotron radiation origin of the full spectrum jet emission.

Burns, J. O.↗

NASA NTRS1990

The polar-ring galaxies NGC 2685 and NGC 3808B (VV 300)

Polar-ring galaxies (PRG) are among the most interesting examples of interaction between galaxies. A PRG is a galaxy with an elongated main body surrounded by a ring (or a disk) of stars, gas, and dust rotating in a near-polar plane (Schweizer, Whitmore, and Rubin, 1983). Accretion of matter by a massive lenticular galaxy from either intergalactic medium or a companion galaxy is usually considered as an explanation of the observed structure of PRG. In the latter case there are two possibilities: capture and merging of a neighbor galaxy, and accretion of mass from a companion galaxy during a close encounter. Two PRG formation scenarios just mentioned are illustrated here by the results of our observations of the peculiar galaxies NGC 2685 and NGC 3808B.

Reshetnikov, V. P.↗

NASA NTRS1993

Biasing and hierarchical statistics in large-scale structure

In this paper we consider the consequences for galaxy formation of an arbitrary, effectively local biasing transformation of a hierarchical underlying matter distribution. We show that a general form of such a transformation preserves the hierarchical properties and the shape of the dispersion in the limit of small fluctuations, i.e., on large scales, although the values of the hierarchical amplitudes may change arbitrarily. We present expressions for the induced hierarchical amplitudes S(g,j) of the galaxy distribution in terms of the matter amplitudes S(j) and biasing parameters for j = 3-7. For higher order correlations, j greater than 2, restricting to a linear bias is not a consistent approximation even at very large scales. To draw any conclusions from the galaxy distribution about matter correlations of order j, properties of biasing must be specified completely to order j - 1.

Fry, J. N.↗

NASA NTRS1979

Gas dynamics in barred spirals - Gaseous density waves and galactic shocks

Steady-state gasdynamical studies, previously limited to tightly wound normal spiral galaxies, are extended to models of barred spirals with a 5% to 10% perturbing potential. The models show that a strong wave manifestation is an important constituent of the bar structure in many barred spirals and that a density-wave shock wave can form a bar structure as pronounced as the narrow bars often evident in optical photographs of barred spirals. The dark narrow dust lanes often observed along the leading edges of bar structures are identified as tracers of shocks, and it is found that strong shocks along a bar structure during even a small part of a galaxy's lifetime might easily deplete a large enough proportion of the gas to cause a lack of gas in the inner annuli encompassing the bar by the time of the present epoch. It is emphasized that even moderate-amplitude barlike perturbations in the disk can drive large noncircular gas motions, typically 50 to 150 km/s.

Roberts, W. W., Jr.↗

NASA NTRS1990

Distribution of galaxies in the Southern Galactic Cap

Observations in the southern galactic hemisphere in the declination range between -17.5 and 2.5 degrees are combined with other available observation. The data set is used to study the spatial distribution of galaxies of a contiguous area of 3.13 sr of the Southern Galactic Cap. An approximately homogeneous magnitude-limited sample of galaxies in the Southern Galactic Cap is constructed from different catalogs. The resulting large scale structure is similar to that of previous surveys in which bright galaxies are distributed on surfaces which intersect at sharp corners and nearly surround voids that are almost empty of galaxies.

Pellegrini, P. S.↗

NASA NTRS1989

Theory of wide-angle photometry from standard stars

Wide angle celestial structures, such as bright comet tails and nearby galaxies and clusters of galaxies, rely on photographic methods for quantified morphology and photometry, primarily because electronic devices with comparable resolution and sky coverage are beyond current technological capability. The problem of the photometry of extended structures and of how this problem may be overcome through calibration by photometric standard stars is examined. The perfect properties of the ideal field of view are stated in the guise of a radiometric paraxial approximation, in the hope that fields of view of actual telescopes will conform. Fundamental radiometric concepts are worked through before the issue of atmospheric attenuation is addressed. The independence of observed atmospheric extinction and surface brightness leads off the quest for formal solutions to the problem of surface photometry. Methods and problems of solution are discussed. The spectre is confronted in the spirit of standard stars and shown to be chimerical in that light, provided certain rituals are adopted. After a brief discussion of Baker-Sampson polynomials and the vexing issue of saturation, a pursuit is made of actual numbers to be expected in real cases. While the numbers crunched are gathered ex nihilo, they demonstrate the feasibility of Newton's method in the solution of this overdetermined, nonlinear, least square, multiparametric, photometric problem.

Usher, Peter D.↗

NASA NTRS1989

Far-infrared spectroscopy of galaxies

Far infrared (FIR) spectral line emission from galaxies is discussed with respect to past, present and near future observations. A review of the importance of the FIR lines as probes of the interstellar medium is presented. The various fine structure emission lines detected from the archetypal starburst galaxy M82, and the (C II) line radiation which is now observed toward a large variety of external galaxies are discussed. The improvements allowed by the advent of the Stratospheric Observatory For Infrared Astronomy (SOFIA), the Infrared Space Observatory (ISO) and the Space Infrared Telescope Facility (SIRTF) are underlined.

Stacey, G. J.↗

NASA NTRS1990

The diffuse ionized interstellar medium perpendicular to the plane of NGC 891

In an attempt to study the structure and the properties of the diffuse ionized interstellar medium perpendicular to the plane of disk galaxies researchers obtained H alpha images and spectra of NGC 891. Perhaps the most remarkable property of the H alpha emission line in NGC 891 is its extension out of the plane of the galaxy: researchers are able to measure the H alpha line out to more than 30 seconds (1.4 kpc) from the midplane. This means that the ionized hydrogen extends at least four times higher than the neutral hydrogen layer. An anomalously large scale-height for the ionized gas of approx. equals 1 kpc is also found in the Milky Way. The echelle spectra show a changing ratio of NII to H alpha. This excludes the possibility that the large scaleheight of the emission is due to scattering of disk emission by dust high above the plane. The z-extent of the H alpha emission is confirmed by the imaging result. The large z-extent of the ionized gas is confined to the inner half of the visible disk. In this inner region the H alpha distribution also shows a filamentary structure of the diffuse ionized medium. These filaments, sticking out of the plane, originate in HII regions in the plane. The H alpha image also shows a large scale asymmetry if the NE and SW parts of the disk are compared. The NE part is more prominent and extended in H alpha. The same asymmetry is also seen in the radio continuum distribution. This correlation between the diffuse ionized medium and the distribution of relativistic electrons is one example of a relation between star formation processes in the disk and the various components of the halo. Thermal filaments or spurs which are related to HII regions are also known in the Galaxy. These filamentary structures perpendicular to the galactic planes may represent the chimneys which result in the supernova dominated model of the Interstellar Medium by Norman and Ikeuchi (1989).

Dettmar, Ralf-Juergen↗

NASA NTRS1993

Spiral density waves resonantly excited by a rapidly rotating bar

Recent observations at millimeter wavelengths have revealed bar-spiral structures in the central regions of several nearby spiral galaxies. Moreover, new infrared data and even COBE data seem to support the idea that there is a bar in the central regions of the Milky Way. All of these new findings have rekindled the interest in bar-driven spirals again. First, the mechanism that is responsible for exciting the spiral density waves near the resonances is reviewed, and then a theory to account for the highly nonlinear behavior of the gas associated with the waves is presented. The new results have again demonstrated that the 3 kpc arm phenomenon can be reproduced by a rapidly rotating bar in the galactic center. Similarly, in terms of the resonance excitation mechanism, the recent high-resolution radio observations of the bar-spiral structure in the central regions of nearby galaxies can be explained.

Yuan, Chi↗

NASA NTRS1985

Far-infrared spectroscopy of galaxies - The 158 micron C(+) line and the energy balance of molecular clouds

Observations of the 158 microns fine-structure line of C(+) toward the nuclei of six gas-rich galaxies are presented. The observations are compared with observations of the CO J = 1-0 and H I 21 cm lines, observations of far-IR continuum emission, and observations of forbidden C II emission with the Galaxy. The forbidden C II line comes from dense, warm gas in UV-illuminated photodissociation regions at the surfaces of molecular clouds. This line is probably optically thin in all but the brightest of galactic sources. The variation of forbidden C II brightness from source to source and its ratio to the integrated infrared continuum intensity agree well with the theoretical prediction that UV absorption by dust controls the C(+) column density. The forbidden C II line is a tracer of molecular clouds, especially those near intense sources of UV radiation.

Crawford, M. K.↗

NASA NTRS1994

Optical observations of NGC 2915: A nearby blue compact dwarf galaxy

This paper presents B and R band Charge Coupled Device (CCD) images and medium resolution spectroscopy of NGC 2915, a relatively isolated BCD (blue compact dwarf) galaxy at a distance of approximately 5 Mpc. NGC 2915 contains two stellar populations: a high surface brightness blue core population and a red diffuse population. The core population contains all of the H II, and numerous embedded objects. It is the locus of current high mass star formation. The brightest embedded objects are likely to be young ionizing clusters, while many of the fainter objects are likely to be individual supergiant stars with masses up to approximately 25 solar mass, or blends of a few such stars. Curious aligned structures on the SE side of the galaxy are seen and their nature discussed. The spectrum of the core is dominated by bright narrow emission lines like that of a high excitation and low metallicity (less than half solar) H II region. The continuum is flat, with Balmer and Ca II features seen in absorption. The velocity of the Ca II features suggest contamination by galactic interstellar absorption. There is a significant velocity gradient in the spectra, probably indicative of rotation. Outside of its core, NGC 2915 resembles a dE (dwarf elliptical) galaxy, in that it has an exponential surface brightness profile, is red ((B-R)(sub 0) = 1.65), and has a low extrapolated central surface brightness (B(0)(sub c) = 22.44). NGC 2915's properties are compared with other BCDs, concentrating on two morphologically similar BCDs that are near enough to resolve into stars: NGC 1705 and NGC 5253. It is noted that the presence of winds in BCDs invalidates closed box chemical evolution models and the remaining constraints on star formation duration are relatively weak. Some BCDs, including NGC 2915, may be able to maintain their present star formation rate for Gyr time scales. This suggests that the overall evolution of these BCDs may be much slower than the approximately 10 Myr burst time scales commonly quoted. However, shortly after the formation of a massive (10(exp 6) solar mass) cluster a BCD will have all the properties of strong starburst galaxy).

Meurer, G. R.↗

NASA NTRS1991

Origin of kinematic subsystems in elliptical galaxies

The formation of a counterrotating central gas disk in a merger of two gas-rich disk galaxies of equal mass is demonstrated. Such a structure may well account for the unusual gas kinematics found in the merger remnant NGC7252. Continued star formation in such gaseous disks may produce central components with decoupled kinematics resembling the cores of some elliptical galaxies.

Hernquist, Lars↗

NASA NTRS2000

Chandra Observations of Hydra A

We present Chandra X-ray Observations of the Hydra A cluster of galaxies, and we report the discovery of structure in the central 80 kpc of the cluster's X-ray-emitting gas. The most remarkable structures are depressions in the X-ray surface brightness, approx. 25 - 35 kpc diameter, that are coincident with Hydra A's radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and there is no evidence for shock-heated gas surrounding the radio lobes. We suggest the gas within the surface brightness depressions was displaced as the radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of the brightest cluster galaxy (BCG), and the cooling time of the gas is approx. 600 Myr in the inner 10 kpc. These properties are consistent with the presence of a approx. 34 solar mass/yr cooling flow within a 70 kpc radius. Bright X-ray emission is present in the BCG surrounding a recently-accreted disk of nebular emission and young stars. The star formation rate is commensurate with the cooling rate of the hot gas within the volume of the disk, although the sink for the material that may be cooling at larger radii remains elusive.

McNamara, Brian↗