Debra Meloy Elmegreen

Arm classifications for spiral galaxies

On presente la classification de 762 galaxies. Ce systeme de classification est en correlation avec la presence des ondes de densite

Optical tracers of spiral wave resonances in galaxies. II: Hidden three-arm spirals in a sample of 18 galaxies

Computer-enhanced galaxy images reveal underlying spiral structures with three symmetric arms that extend between ∼25% and ∼85% of the outer radii of the two-arm spirals. Many of the galaxies containing multiple arms or other complex spirals are clear superpositions of simple two- and three-arm structures. The three-arm spirals appear to extend exactly from the inner to the outer 3:1 resonances in most cases, and the more prominent two-arm spirals begin outside the inner 2:1 resonance and extend to the outer 2:1 resonance

Do density waves trigger star formation

A comparison of B - V colors, blue and far-IR surface brightnesses, metallicities, and star formation rates calculated from H-alpha and UV fluxes reveals no difference in the average star formation rates for galaxies with and without grand design spiral structure. This implies that strong density waves do not trigger a significant excess of star formation compared to that in similar galaxies without a wave. Density waves organize the gas and young stars into spiral arms because of the flow pattern, and they may influence the formation and destruction of the largest cloud complexes, but they appear to contribute less than 50 percent to the overall star formation rate. 46 references.

Statistical evidence that galaxy companions trigger bars and change the spiral Hubble type

Le nombre de galaxies barrees par rapport au nombre total de galaxies spirales, dans des systemes binaires, des groupes de galaxies et pour les galaxies de champ, est analyse. Les systemes binaires montrent une proportion de galaxies barrees et de galaxies spirales de premier type deux fois superieures a celle des galaxies de champ. Ces resultats sont interpretes en terme de formation de barres galactiques par les galaxies en interaction et de redistribution de masse dans les composantes stellaires et gazeuses.

H I superclouds in the inner Galaxy

Atomic hydrogen clouds in the Weaver-Williams 21 cm survey are found in the vicinity of the largest molecular cloud complexes in the first Galactic quadrant. The atomic masses are estimated in several ways, including a new method where the clouds are assumed to be composed of cool clumps immersed in a warm interclump medium. The clouds are found to contain between a million and 40 million solar masses of atoms at an average density of about 9/cu cm. They appear gravitationally bound, their molecular mass fractions decrease from 70 percent to 5 percent with increasing distance from the Galactic center, and they are located in the spiral arms. The importance of these clouds for the large-scale distributions of interstellar mass, molecular fraction, and star formation is discussed. 103 references.

The Interaction between Spiral Galaxies IC 2163 and NGC 2207. I. Observations

Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society. DOI: 10.1086/176374 [Full text of this article is not available in the UHRA]

Optical tracers of spiral wave resonances in galaxies : applications to NGC 1566

Les resonances orbitales de la galaxie spirale NGC 1566 sont analysees a partir dimages optiques. Les resonnances interieures Lindblad, 6:1 et 4:1, les resonances exterieures Lindblad et 1:1 et la resonance de corotation sont identifiees pour des contraintes rendant plausibles les courbes de rotation.

The Interaction between Spiral Galaxies IC 2163 and NGC 2207. II. Models

Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society. DOI: 10.1086/176375 [Full text of this article is not available in the UHRA]

The Interacting Galaxies NGC 5394/5395: A Post-Ocular Galaxy and Its Ring/Spiral Companion

H I, radio continuum, Fabry-Perot Hα, and 12CO J = 1 → 0 observations and broadband optical and near infrared images are presented of the interacting spiral galaxies NGC 5395 and NGC 5394. Kinematically, there are three important, separate components to the H I gas associated with this galaxy pair: (1) the main disk of NGC 5395, (2) a long, northern tidal arm of NGC 5395 distinct in velocity from its main disk, and (3) the disk of NGC 5394. The H I northern tidal arm of NGC 5395 has a line of–sight velocity as much as 75–100 km s-1 greater than the main disk of NGC 5395 at the same projected location and thus is not in the same plane as the disk. The velocity field of the disk of NGC 5395 is asymmetric and distorted by large-scale and small-scale noncircular motions. In NGC 5395, the encounter appears to be exciting m = 1 and m = 0 modes in what had been a two-armed spiral. The dominant spiral arm of NGC 5395 forms a large ring or pseudo-ring of Hα, radio continuum, and H I emission, somewhat off center with respect to the nucleus. The H I trough in the center of NGC 5395 is not filled in by molecular gas. The Hα velocity contours exhibit an organized pattern of kinks in crossing the ring and also show streaming motions in a large stellar caustic feature. The eastern side of the ring is brighter in radio continuum and Hα; the western side is brighter in H I and contains massive (108 M☉) H I clouds not associated with the most luminous H II regions. The smaller galaxy NGC 5394 is in an immediate post-ocular phase, with a central starburst, an intrinsically oval disk, two long, fairly symmetric, open tidal arms with high arm-interarm contrast, and very bright inner spiral arms, disjoint from the outer tidal arms. Most of the gas in NGC 5394 is in molecular form and concentrated within 3.8 kpc of the center, so is suitable for fueling the starburst. Despite the presence of H I gas, two of the three optically bright inner spiral arms of NGC 5394 show no evidence of ongoing star formation. A galaxy encounter simulation reproduces some of the main features of this system with a collision that is prograde relative to NGC 5394 and retrograde at a high tilt angle relative to NGC 5395. The model finds that the inner spiral structure of NGC 5394 developed from an eye-shaped (ocular) structure at slightly earlier times. NGC 5394 and the two ocular galaxies IC 2163 and NGC 2535, studied earlier, form an evolutionary sequence of structures resulting from prograde encounters and thus confirm the generic models of such collisions. The agreement between the model for NGC 5394/95 and the ring/spiral structures seen in NGC 5395 extends our understanding of collisional ring galaxies.

Observations of the Ocular Galaxy NGC 2535 and its Starburst Companion NGC 2536

H I, radio continuum, and 12CO J=1-->0 observations at resolutions of 12 to 33 (=2.9-8 kpc), and B, I, J, and K-band images, are presented of the galaxy NGC 2535 and its small starburst companion NGC 2536. NGC 2535 has an ocular (eye-shaped) structure with two long tidal arms indicative of a recent, close, nonmerging encounter. Our observations reveal widespread high velocity dispersions (30 km s-1) in the H I gas and five clouds with masses of ˜10^8 Msun in the tidal arms of NGC 2535. These clouds do not correspond to the most luminous blue knots or to major clumps in the old stellar population. Similar phenomena were found previously in other pairs (IC 2163/NGC 2207 and NGC 5774/5), suggesting that enhanced turbulence and massive gas clouds may be a general feature of interacting spiral galaxies during an early phase of post-encounter evolution. A search for 12CO J= 1-->O emission at 11 positions in NGC 2535 and one position in NGC 2536 yielded two clear detections, one at the center of NGC 2535 and the other on its tidal tail but close to the center of NGC 2535. Most of the massive H I clouds in NGC 2535 are not detected in CO emission. The virial masses of these clouds are significantly greater than their measured masses in gas, presumably because of the presence of old disk stars in the clouds. A form of the virial theorem that includes gas and stars is presented. NGC 2535 shows several unusual features, including an intrinsically oval shape to the disk, that are similar to those in the ocular galaxy IC 2163 studied previously. NGC 2535 also contains structures that are not seen in IC 2163. For example, there is an extended (R =48 kpc) H I envelope and a long, outer, elliptically-shaped H I arc in NGC 2535 that may be a gravitational wake produced by the passage of the companion within or close to the envelope. In the radio continuum, NGC 2535 exhibits a bar-like feature that leads the small stellar bar by 50°. The starburst companion, NGC 2536, lies in a 2 X 10^9 Msun clump of H I gas at the outer end of the tidal bridge from NGC 2535. Most of the gas in this clump is associated with the bridge. The H I emission on the anticompanion side of NGC 2536 has the same line-of-sight velocity and projected position as some of the bridge gas there. This observation is consistent with a previous model by Klaric in which NGC 2536 accretes gas from NGC 2535; we even find an H I feature that may represent bridge gas streaming towards NGC 2536. The failure to detect 12CO emission in NGC 2536 places an upper limit of 6 X H2 on its H2 mass if the standard value of the conversion factor between I_CO and N(H2) applies.