Ionized gas in the nuclei of elliptical, so, spiral, and irregular galaxies.
Interstellar gas in nuclei of elliptical, S0, spiral and irregular galaxies
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Ultraviolet photometry from the Orbiting Astronomical Observatory. XL - The energy distributions of spiral and irregular galaxies
Measurements of the total light of 40 spiral and irregular galaxies are presented. The photometry covers the wavelength range 1550-4250 A and is calibrated on an absolute basis. On the average later-type galaxies are not only bluer at short wavelengths than ellipticals but significantly bluer than visual colors would imply. This reflects a recent history of more vigorous formation of massive stars. The shape of the upper part of the initial mass function apparently varies more among early-type galaxies, producing a wide scatter in their energy distributions. In at least some galaxies interstellar dust appears to have little influence upon the emerging radiation. The local volume luminosity spectrum due to galaxies turns up steeply at short wavelengths, and is shaped largely by contributions from late-type galaxies. The observed ultraviolet background cannot be produced by normal galaxies, although the large corrections implied by theoretical evolutionary models may account for the measurements.
Code, A. D.↗
The H II regions of the galaxy M101
The physical properties and composition of the H II region population of the spiral galaxy M101 (NGC 5457) were examined using spectrophotometrically calibrated CCD imagery of two overlapping 16-arcmin fields in the M101, in which individual H II regions were identified and mapped using an automated algorithm. The characteristics obtained for the H II region population of M101 include the H II region luminosity function and radial variations in extinction; the forbidden O III line/H-beta, O/H, and forbidden S II line/H-alpha ratios; the ionization parameter U; and the numbers of ionizing photons. Global variations of physical properties of the H II regions in M101 are compared to model expectations derived for other galaxies. The evident star formation in the M101 galaxy is discussed.
Scowen, P. A.↗
Blue ellipticals in compact groups
By studying galaxies in compact groups, the authors examine the hypothesis that mergers of spiral galaxies make elliptical galaxies. The authors combine dynamical models of the merger-rich compact group environment with stellar evolution models and predict that roughly 15 percent of compact group ellipticals should be 0.15 mag bluer in B - R color than normal ellipticals. The published colors of these galaxies suggest the existence of this predicted blue population, but a normal distribution with large random errors can not be ruled out based on these data alone. However, the authors have new ultraviolet blue visual data which confirm the blue color of the two ellipticals with blue B - R colors for which they have their own colors. This confirmation of a population of blue ellipticals indicates that interactions are occurring in compact groups, but a blue color in one index alone does not require that these ellipticals are recent products of the merger of two spirals. The authors demonstrate how optical spectroscopy in the blue may distinguish between a true spiral + spiral merger and the swallowing of a gas-rich system by an already formed elliptical. The authors also show that the sum of the luminosity of the galaxies in each group is consistent with the hypothesis that the final stage in the evolution of compact group is an elliptical galaxy.
Zepf, Stephen E.↗
Colors and color gradients in bulges of galaxies
We have obtained surface photometry in U, B, R, and I for a complete optically selected sample of 45 early-type spiral galaxies, to investigate the colors and color gradients of spiral bulges. Color profiles in U-R, B-R, U-B, and R-I have been determined in wedges opening on the semiminor axes. Based on several criteria, like the smoothness of the color profiles, the absence of dust lanes, and the central colors, we have defined a subsample of 18 objects whose colors are largely unaffected by dust. We believe such colors are suitable for inferring properties of the stellar populations of bulges. We find that the colors of bulges are predominantly bluer than those of ellipticals. This result holds even when bulges are compared to ellipticals of the same luminosity, and indicates that bulges are younger and/or more metal poor than old elliptical galaxies. Most bulges do not reach solar metallicities. Bulges show predominantly negative color gradients (bluer outward). For bright bulges (M(sup Bulge sub R) is less than -20.0), the magnitude of the gradient increases with bulge luminosity. For fainter bulges, gradients scatter around large negative values. The behavior of color gradients as a function of bulge luminosity suggests different formation mechanisms for faint and bright spheroids. For bright bulges, the scaling of gradients with luminosity suggests a formation process involving dissipation. The similarity with ellipticals suggests that the formation of the disk did not affect the stellar populations of the bulge in a major way. For small bulges (M(sub R) is greater than -20), the existence of pronounced color gradients suggests a different formation mechanism. For these objects, the presence of the disk may have severely affected the radial population distribution in the bulge.
Balcells, Marc↗
Interpretation of large-scale spiral structure
Large scale spiral galaxy structure interpretation, examining density wave theory, Milky Way System and spiral form origin
Lin, C. C.↗
Star formation in Seyfert galaxies
An analysis of the IRAS data for a sample of classical (optically selected) Seyfert galaxies is presented. The IRAS fluxes at 25 micron, 60 micron, and 100 micron are found to be uncorrelated or only very weakly correlated with the UV/Optical continuum flux and the near and mid IR flux at 3.5 and 10 microns. To investigate the possibility that star formation accounts for the far IR flux, the IRAS measurements for the Seyfert galaxies are compared to IRAS observations of a sample of normal spiral galaxies, and a sample of Starburst galaxies. It is shown that the far IR luminosities and far IR colors of Seyfert galaxies are indistinguishable from those of the Starburst galaxies. Besides, normal galaxies are an order of magnitude less luminous than both the Seyfert and the Starburst galaxies. This indicates that star formation produces the bulk of the far infrared emission in Seyfert galaxies.
Espinosa, J. M. Rodriguez↗
Flyby - Numerical experiments on a galaxy orbiting within a galaxy cluster
Dynamical effects on a self-consistent stellar system in orbit around an attracting force center have been studied by means of numerical experiments. Globular clusters in the Galaxy, galaxies within a cluster, and dwarf spheroidal galaxies in orbit around our Galaxy are examples of this dynamical problem. Stars within the galaxy move in the combined self-consistent and external (cluster) force fields, in an accelerated frame, and the galaxy orbit is integrated at the same time. The principal result found is that, in practice, a spherical galaxy either suffers little mass loss (less than 1 percent) or is totally disrupted. A disruption criterion based on the ratio of tidal stress to internal stress within the galaxy describes the results succinctly. This criterion is used to discuss the distribution of dark mass in galaxy clusters. A galaxy envelope expands, rather than being tidally truncated, following a flyby. Spiral galaxies can show tidal damage after flying past the cluster center, even without a close encounter with another galaxy.
Miller, R. H.↗
The molecular spiral arms of NGC 6946
From CO-12(J=1 to 0) observations at 45 seconds resolution Tacconi and Young (1989) have found evidence for enhancements in both the CO emissivity and the massive star formation efficiency (MSFE) on optical spiral arms of the bright spiral galaxy NGC 6946. In the optically luminous and well-defined spiral arm in the NE quadrant, there are enhancements in both the H2 surface density and MSFE relative to the interarm regions. In contrast, a poorly defined arm in the SW shows no arm-interarm contrast in the MSFE. To further investigate the molecular gas content of these two spiral arms, researchers have made CO-12 J=2 to 1 and 3 to 2 observations with the James Clerk Maxwell Telescope. In the J=2 to 1 line, they made observations of the NE and SW spiral arm and interarm regions in 4 x 9 10 seconds spaced grids (36 points per grid). Because of decreased sensitivity in the J=3 to 2 line, they were limited to mapping the two arm regions in 2 x 3 10 seconds spaced grids (6 points per grid). The centers of each of the grids lie 2.4 minutes to the NE and 2.3 minutes to the SW of the nucleus of NGC 6946. With the CO J=2 to 1 data researchers are able to fully resolve the two observed spiral arms in NGC 6946. In both cases the CO emission is largely confined to the optical spiral arm regions with the peak observed T asterisk sub A being up to 4 times higher on the spiral arms than in the interarm regions. Researchers are currently estimating massive star formation efficiencies on and off the spiral arms through direct comparison of the CO maps with an H alpha image. They are also comparing the CO J=2 to 1 data with an HI map made at similar resolution. Thus, they will be able to determine structure in all components of the IS on scales of less than 20 inches.
Tacconi, L. J.↗
The Reddening law outside the local group galaxies: The case of NGC 7552 and NGC 5236
The dust reddening law from the UV to the near-IR for the extended regions of galaxies is here derived from the spectral distributions of the starburst spiral galaxies NGC 7552 and NGC 5236. The centers of these galaxies have similar absorption and emission line spectra, differing only if the strength of their interstellar lines and in the continuum distribution, with NGC 7552 appearing more reddened than NGC 5236. The disk of NGC 7552 is more inclined, and there is evidence that its center is observed through additional foreground dust and gas clouds, as compared to the center of NGC 5236. While the galaxies can be expected to have similar dust content, they are known to have different dust path lengths to our line of sight. Therefore, differences in the shape of the spectra can be attributed mainly to the effects of dust, allowing us to probe for the first time the properties of the reddening law outside the local group of galaxies. We derive the reddening law based on the optical depth of the emission line of H Alpha and H Beta and also based on the continuum distribtuion. We find that the optical depth from the emission line regions are about twice the optical depth of the continuum regions. Thus, dereddening a starburst galaxy by scaling the Milky Way reddening laws to optical depths obtained from the H Alpha/H Beta line ratio overcompensates for the effect of dust.
Kinney, Anne L.↗
Chandra Observation of Luminous and Ultraluminous X-ray Binaries in M101
X-ray binaries in the Milky Way are among the brightest objects on the X-ray sky. With the increasing sensitivity of recent missions, it is now possible to study X-ray binaries in nearby galaxies. We present data on six ultraluminous binaries in the nearby spiral galaxy, M101, obtained with Chandra ACIS-S. Of these, five appear to be similar to ultraluminous sources in other galaxies, while the brightest source, P098, shows some unique characteristics. We present our interpretation of the data in terms of an optically thick outflow, and discuss implications.
Mukai, K.↗
Spiral structure and star formation. I - Formation mechanisms and mean free paths
The wide variety of optical appearances observed in spiral galaxies has encouraged the growth of two theoretical approaches to explain the spiral patterns exhibited by such young objects as OB associations, giant H II regions, and dark dust lanes. These approaches are related to the density wave theory of spiral structure and 'stochastic, self-propagating star formation'. Levinson and Roberts (1981) tried to reconcile these two theoretical approaches, and considered a disk filled with discrete gas clouds. The present investigation is concerned with refinements and extensions of the model of Levinson and Roberts. Attention is given to gravitational forces and dynamical propagation, cloud-cloud collisions, supernova interactions, computational models, a theory concerning the interactions of the gas cloud and stellar association systems, the time evolution of the gas cloud-stellar association systems, and aspects of collisionally triggered star formation.
Roberts, W. W., Jr.↗
The interstellar medium in galaxies; Proceedings of the 2nd Teton Conference, Grand Teton National Park, WY, July 3-7, 1989
The present conference on the interstellar medium in galaxies discusses the cool phase of the interstellar medium, molecular clouds in spiral galaxies, interstellar dust in galaxies, and the diffuse interstellar medium. Attention is given to cooling flows and X-ray emission in early-type galaxies, the interstellar medium in active galaxies, abundances in extragalactic H II regions, and thermal phases of the interstellar medium in galaxies. Topics considered include large-scale interstellar gasdynamics in disk galaxies, gas during mergers, magnetic fields in galaxies, and large-scale star formation in the interstellar medium. Also discussed are gaseous halos and disks of galaxies at large redshift, the star-gas cycle in galaxies, measuring atomic hydrogen masses using the 21-cm line, and mass determinations from far-infrared and from CO observations.
Thronson, Harley A., Jr.↗
Far-infrared line images of dwarf galaxies
Irregular dwarf galaxies are about ten times more widespread in the universe than regular spiral galaxies. They are characterized by a relatively low metallicity, i.e., lower abundance of the heavier elements (metals) with respect to hydrogen than in the solar neighborhood. These heavier elements in the form of molecules, atoms, or ions, which have radiative transitions in the infrared play a decisive role in the energy balance of the ISM and thereby for the formation of stars. Dwarf galaxies are thus model cases for the physical conditions in the early phase of the universe. Large Magellanic Cloud: 30 Doradus. The two nearest dwarf galaxies are the Magellanic clouds at a distance approximately 50 kpc. The LMC contains 30 Dor, a region with young, extremely massive stars which strongly interact with the surrounding ISM on account of their stellar winds and intense UV radiation. 30 Dor is the brightest object in the LMC at almost all wavelengths.
Poglitsch, A.↗
Far-infrared luminosity functions of normal galaxies
A volume-limited sample is constructed from the Zwicky catalog and IRAS data base to examine the FIR luminosity functions of normal galaxies, and to investigate possible relationships between FIR emission and galaxy morphology. Quantitative and unbiased treatment is provided by 'survival analysis' statistical methods. It is found that the FIR distributions of normal galaxies are better fit by lognormal than Schechter functions. The total FIR emissivity (8 to 115 microns) of normal galaxies is approximately equal to half their emission in the B plus V optical bands. Normal galaxy FIR emission is uncorrelated with the basic S0-Sm Hubble sequence of spiral galaxy morphology, but appears to be affected by de Vaucouleurs' (1959) revised morphological classifications based on inner rings and S-shaped arms. Spirals with bars and inner rings are systematically fainter than unbarred spirals. It is suggested that bars and rings reduce the amount or spatially confine the dust in spiral disks, resulting in lower efficiency conversion of optical and UV photons into the IR.
Isobe, Takashi↗
Disk and ring structure in the universe.
For a gas or dust cloud with a net angular momentum about a central mass, the stable shape is a rotating ring or disk, for a very simple reason. Collisions among the cloud's particles dissipate the energy of the random motions, while the rotational energy is maintained by the conservation of angular momentum. The angular momentum can be very easily imparted to matter. A ring or disk structure is, therefore, found in many celestial objects from asteroids to galaxies. The most impressive and obvious disk structures in the universe are the spiral galaxies. It is thought that the planetary system was formed from a flat disk of dust and gas. Much observational evidence shows that the thin, flat, concentric rings around the planet Saturn are composed of a host of individual particles.
Huang, S.-S.↗
Starbursts triggered by central overpressure in interacting galaxies
A triggering mechanism for the origin of enhanced, massive-star formation in the central regions of interacting spiral galaxy pairs is proposed. Our mechanism is based on the detailed evolution of a realistic interstellar medium in a galaxy following an encounter. As a disk giant molecular cloud (GMC) tumbles into the central region following a galaxy encounter, it undergoes a radiative shock compression via the pre-existing high pressure of the central intercloud medium. The shocked outer shell of a GMC becomes gravitationally unstable and begins to fragment thus resulting in a burst of star formation, when the growth time for the gravitational instabilities in the shell becomes smaller than the crossing time of the shock. The resulting values of typical infrared luminosity agree with observations.
Jog, Chanda J.↗
Discovery of a Nova in the Virgo Galaxy M100
We present the V and I light curves of a nova discovered in the disk of the spiral galaxy M100, located in the Virgo cluster.
Nova Novae cataclysmic variables stars galaxies M1↗