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NASA NTRS1987

Radio continuum observations of early- and late-type spiral galaxies

Two complete subsamples of early and late type spiral galaxies were mapped with the VLA in order to explore their radio and optical properties. The choice of the samples is explained, and the observational procedure and data reduction are described. The detection rate was 97 percent, with a detection limit of the order of 1-2 mJy. Total intensity data at 20 cm are presented for the whole sample, and 6 cm data for 11 galaxies. The results are presented in the form of tables and radio contour maps, and are discussed in comparison with previous work.

Gioia, Isabella M.↗
NASA NTRS1989

Far-infrared and submillimeter photometric mapping of spiral galaxies in the Virgo cluster

Three 'normal' Virgo cluster spiral galaxies, NGC 4254, NGC 4501, and NGC 4654, were mapped at 160 and 360 micron wavelengths with arrays of 45 arcsec beams using the NASA Kuiper Airborne Observatory and the NASA Infrared Telescope Facility. Another galaxy, NGC 4321, was mapped at 160 microns. Approximately one-third of the total flux from the galaxies is emitted at wavelengths longer than 12 microns. The maps show centrally peaked, but spatially extended luminosity distributions consistent with disk material.

Stark, A. A.↗
NASA NTRS1994

X-ray-emitting gas surrounding the spiral galaxy NGC 891

We observed the edge-on spiral galaxy NGC 891 with the Position Sensitive Proportional Counter (PSPC) on Roentgen Satellite (ROSAT) to search for how extraplanar gas expected in the galactic fountain model. Diffuse X-ray emission surrounds the disk with a Half Width at Half Maximum (HWHM) for the surface brightness perpendicular to the disk of 50 sec (2.4 kpc) and a radial extent of approximately 6.5 kpc, both of which are similar in extent to the extended H(alpha) and radio halo component; the implied density scale height for the hot gas is 7 kpc. The spectrum is best fitted with a hard stellar component and a soft diffuse gas component of temperature 3.6 x 10(exp 6) K. The density of this gas is 2 x 10(exp -3)/cu cm, the luminosity is 4.4 x 10(exp 39) ergs/s, the mass is 1 x 10(exp 8) solar mass, and the pressure (P/k) is 1.4 10(exp 4) K/cu cm. These data are consistent with this gas participating in a galactic fountain, where the material approaches hydrostatic equilibrium before cooling at a rate of 0.12 solar mass/yr. The cooled material may be responsible for some of the H(alpha) emission.

Bregman, Joel N.↗
NASA NTRS1991

The relationship between the carbon monoxide intensity and the radio continuum emission in spiral galaxies

The relationship between the velocity-integrated CO emission and the nonthermal radio continuum brightness in the disks of normal spiral galaxies is examined on a variety of length scales. On a global scale, the total CO intensity correlates strongly with the total radio continuum flux density for a sample of 31 galaxies. On scales of about 2 kpc or more in the disk of individual galaxies, it is found that the ratio I(CO)/T(20) remains fairly constant over the entire disk as well as from galaxy to galaxy. For the eight spirals in the sample, the disk-averaged values of I(CO)/T(20) range from 0.6-2.4, with the average over all eight galaxies being 1.3 +/- 0.6. It is concluded that what these various length scales actually trace are differences in the primary heating mechanism of the gas in the beam. The observed relationship between CO and nonthermal radio continuum emission can be explained by assuming that molecular gas in galactic disks is heated primarily by cosmic rays. The observed relationship is used to show that the brightness of synchrotron emission is proportional to n(cr) exp 0.4 - 0.9 in galactic disks.

Adler, David S.↗
NASA NTRS1980

Self-gravitating gas flow in barred spiral galaxies

A series of two-dimensional numerical experiments is performed in order to test the response of an isothermal, self-gravitating gas disk to a uniformly rotating, barlike gravitational potential. The barlike potential is an equilibrium stellar model from the n-body calculations of Miller and Smith (1979). In the bar-dominated, central regions of the disk, a gas bar whose phase depends primarily on the location of principal resonances in the disk is formed. This response can be understood in terms of orbit-crowding effects. In the gas-dominated outer regions of the disk, two-armed trailing spiral waves are formed. The local pitch angle of these waves increases with increasing fractional gas mass. These self-gravitating gas waves are not self-sustaining. They are driven from the ends of equilibrium stellar bars, and their phase does not depend on the location of resonances in the disk. The relevance of these self-gravitating waves to observations and models of barred spiral galaxies is discussed. It is concluded that these waves and their associated ringlike structures may be consistent with the morphological distribution of gas features in barred spiral galaxies.

Huntley, J. M.↗
NASA NTRS1990

The 60 micron to 20 centimeter infrared-to-radio ratio within spiral galaxies

A detailed comparison is presented of the distribution of 60 micron IR and 20 cm radio continuum emission within 25 galaxies, mostly disk spirals. Local maxima in the thermal IR and nonthermal radio emission are found to be spatially coincident on scales of less than about 0.4 kpc in the nearest sample galaxies. The IR-red disk in normal spirals appears to be characterized by a shorter scale length than that of the radio continuum disk, suggesting that the IR-to-radio ratio should decrease as a function of radius. A model that successfully accounts for the observations is introduced which is based on the assumptions of steady-state star formation activity within the disk on kpc scales and a tight coupling between the origins of the dust-heating radiation and the radio-emitting cosmic-ray electrons. The underlying source is described as an exponential disk. The results also suggest that a random walk process cannot by itself describe the temporal evolution of cosmic rays.

Bicay, M. D.↗
NASA NTRS2011

Investigating the Nuclear Activity of Barred Spiral Galaxies: The Case of NGC 1672

We have performed an X-ray study of the nearby barred spiral galaxy NGC 1672, primarily to ascertain the effect of the bar on its nuclear activity. We use both Chandra and XMM-Newton observations to investigate its X-ray properties, together with supporting high-resolution optical imaging data from the Hubble Space Telescope (HST) infrared imaging from the Spitzer Space Telescope, and Australia Telescope Compact Array ground-based radio data. We detect 28 X-ray sources within the D25 area of the galaxy; many are spatially correlated with star formation in the bar and spiral arms, and two are identified as background galaxies in the HST images. Nine of the X-ray sources are ultraluminous X-ray sources, with the three brightest (LX 5 * 10(exp 39) erg s(exp -1)) located at the ends of the bar. With the spatial resolution of Chandra, we are able to show for the first time that NGC 1672 possesses a hard (1.5) nuclear X-ray source with a 2-10 keV luminosity of 4 * 10(exp 38) erg s(exp -1). This is surrounded by an X-ray-bright circumnuclear star-forming ring, comprised of point sources and hot gas, which dominates the 2-10 keV emission in the central region of the galaxy. The spatially resolved multiwavelength photometry indicates that the nuclear source is a low-luminosity active galactic nucleus (LLAGN), but with star formation activity close to the central black hole. A high-resolution multiwavelength survey is required to fully assess the impact of both large-scale bars and smaller-scale phenomena such as nuclear bars, rings, and nuclear spirals on the fueling of LLAGN.

COLOR FIGURES↗
NASA NTRS1987

Enhanced star formation: The importance of bars in spiral galaxies

It was found that among an IR luminous subset of nearby spiral galaxies, nearly all of the systems with IRAS colors and luminosities indicative of enhanced star formation are barred. Radio continuum and IR spectroscopic results support the hypothesis that this emission originates within the central 2 kpc; possibly in a circumnuclear ring. It was also found that outer rings are over represented among these barred systems and suggest possible reasons for this phenomena.

Puxley, P. J.↗
NASA NTRS1994

The soft x ray halo of the spiral galaxy NGC4631

ROSAT PSPC observations of the close to edge-on spiral galaxy NGC4631 are presented. This vigorously star forming galaxy shows extented x ray emission perpendicular to the plane, out to about 6 to 8 kpc. The spatial extent is largest at soft x ray energies. The total x ray luminosity of hot gas can be easily supplied by star formation in the disk, and it is likely that the halo is due to outflow of hot gas from the inner disk. Spectral analysis of the x ray data shows that part of the halo emission may be quite cool, well below 10(exp 6)K. Implications of these results are briefly discussed.

Walterbos, Rene A. M.↗
NASA NTRS1994

A ROSAT observation of the nearby spiral galaxy NGC 6946

I present the ROSAT Position-Sensitive Proportional Counter (PSPC) image of the nearby, nearly face-on, spiral galaxy NGC 6946. The galaxy was observed with Einstein in 1980, uncovering a nuclear source, a source on or near the northern spiral arm, SN 1980K, and evidence for non-point-source (i.e., diffuse) emission. The ROSAT image resolves the nuclear region into approximately three sources, yields an accurate position for the north arm source, and reveals diffuse extended emission across the galaxy face, tracing at least the bright northern spiral arm of the galaxy. The diffuse emission is almost certainly the very hot component of the galaxy's interstellar medium and is probably similar to that found in the Milky Way and the Large Magellanic Cloud.

Schlegel, Eric M.↗
NASA NTRS1983

Gravitationally induced spurs in spiral galaxies - An example in M31

Radio and optical morphological data are consistent with a gravitational mechanism for the anomalous structure between the S4 and SS arms of M31, with the large complex of H I and H II in and around NGC 206 being implicated in the creation of the anomalous spur. Computer models of the gravitational effects of the spur show that its gravitational induction explains the observed velocity distortions. It is speculated that spurs in more distant galaxies, for which high resolution data as complete as that presented are not yet available, are also gravitational. This implicitly applies not only to spurs in galaxies with well defined spiral structure, but also for galaxies with more chaotic spiral arm patterns.

Byrd, G. G.↗
NASA NTRS1984

The X-ray emission of normal spiral galaxies

Some results of an Einstein Observatory survey of X-ray emitting spiral galaxies are reported. The sources were normally 10-100 times further away than M31. Luminosities of 10 to the 38th to 10 to the 40th erg/sec were calculated and could have been generated by Pop I and Pop II binary systems, supernova remnants and small nuclear sources. Highly positive correlations were found among properties such as the integrated X-ray, optical and radio emissions. The data will also be compared with similar data from star burst and elliptical galaxies.

Fabbiano, G.↗
NASA NTRS1987

X-ray observations of spiral galaxies. II - Images and spectral parameters of 13 galaxies

The results of a spatial and spectral analysis of a sample of spiral galaxies observed in X-rays with the Einstein satellite indicate that the X-ray emission is generally extended and complex. In some cases, discrete, very bright X-ray sources are detected, some of these coincident with the nuclear regions. The spectral fit of the IPC data to a thermal bremsstrahlung spectrum with a low-energy absorption cutoff shows that the average spectrum could be rather hard with kT greater than 2 keV, and with low-energy cutoff consistent with line-of-sight absorption. Results on NGC 4631, NGC 6946, and the nuclei of IC 342 and NGC 1313 are discussed and compared with previous observations in different energy ranges.

Fabbiano, G.↗
NASA NTRS1987

The formation of the exponential disk in spiral galaxies

A mechanism for the formation of the exponential stellar disk distribution in spiral galaxies is proposed. It is assumed that the gas which falls in to form the disk behaves like an accretion disk which evolves on a viscous time scale t(v) and forms stars on a time scale t(star). It is demonstrated that if t(v) roughly equals t(star) the resulting stellar disk has an exponential distribution independent of the disk rotation law and of the assumed viscosity prescription. Why the viscous and star formation time scales might be related is briefly commented on.

Lin, D. N. C.↗
NASA NTRS1992

The ultraviolet color gradient in the late-type spiral galaxy M33

The ultraviolet surface brightness and color distributions for the late-type spiral galaxy M33 are derived from images at 1520 and 2490 A that were obtained by the Ultraviolet Imaging Telescope (UIT) on the Astro Spacelab mission. Although the surface brightness shows a general decline with radius, the dominant spiral arms cause significant deviations from an exponential fit. Colors in elliptical annuli become 0.2-0.3 mag bluer with increasing radius. Our measures of individual H II regions and sections of spiral arms follow this same trend. Interarm regions are redder than the arms. A metallicity gradient affecting only the colors of stars is inadequate in accounting for the observed color gradient. A plausible explanation invokes a combination of an LMC-type reddening curve and a radial gradient in the internal reddening in M33.

Landsman, Wayne B.↗
NASA NTRS1990

A study of HII regions in spiral galaxies using multiobject spectroscopy

Multiobject spectroscopy results of HII regions in nearby late-type spiral galaxies are given. Results include excitation measurements, log ((O III)/H beta), for 81 regions in M 101, 30 regions in NGC 2403, and 13 regions in M 51. Researchers conclude that late-type spirals can be classified into two distinct populations, examine possible causes of this division, and derive metallicity gradients for these galaxies. M 51 appears to have an anomalously shallow abundance gradient.

Zaritsky, Dennis↗
NASA NTRS1997

The Nature of the Optical "Jets" in the Spiral Galaxy NGC 1097

We present new observations of the jet features in the barred spiral galaxy NGC 1097, including optical spectroscopy of the brightest jet features, two-color optical imagery, new VLA mapping at 327 MHz, and archival 1.4 GHz VLA data reprocessed for improved sensitivity. No optical emission lines appear to an equivalent width limit of 15-30 A (depending on the line wavelength). The jets are uniformly blue, with B - V = 0.45 for the two well-observed jets R1 and R2. No radio emission from the jets is detected at either frequency; the 327-MHz data set particularly stringent limits on "fossil" emission from aging synchrotron electrons. The morphology of the jets is shown to be inconsistent with any conical distribution of emission enhanced by edge-brightening; their combination of transverse profile and relative narrowness cannot be reproduced with cone models. The optical colors, lack of radio emission, and morphology of the features lead us to conclude that they are tidal manifestations, perhaps produced by multiple encounters of the small elliptical companion NGC 1097A with the disk of NGC 1097. We present photometric and morphological comparisons to the tail of NGC 465 1, which is similar in scale and morphology to the northeast "dogleg" feature R1 in NGC 1097.

Wehrle, Ann E.↗