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- Elmegreen, B.G., et al.
(författare)
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Hubble Space Telescope Observations of the Interacting Galaxies NGC 2207 and IC 2163
- 2000
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 120:2, s. 630-644
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Tidskriftsartikel (refereegranskat)abstract
- Hubble Space Telescope (HST) images of the galaxies NGC 2207 and IC 2163 show star formation and dust structures in a system that has experienced a recent grazing encounter. Tidal forces from NGC 2207 compressed and elongated the disk of IC 2163, forming an oval ridge of star formation along a caustic where the perturbed gas rebounded after its inward excursion. Gas flowing away from this ridge has a peculiar structure characterized by thin parallel dust filaments transverse to the direction of motion. The filaments become thicker and longer as the gas approaches the tidal arm. Star formation that occurs in the filaments consistently lags behind, as if the exponential disk pressure gradient pushes outward on the gas but not on the young stars. Numerical models suggest that the filaments come from flocculent spiral arms that were present before the interaction. The arms stretch out into parallel filaments as the tidal tail forms. A dust lane at the outer edge of the tidal tail is a shock front where the flow abruptly changes direction. Gas at small-to-intermediate radii along this edge flows back toward the galaxy, while elsewhere in the tidal arm, the gas flows outward.A spiral arm of NGC 2207 that is backlit by IC 2163 is seen with HST to contain several parallel, knotty filaments spanning the full width of the arm. These filaments are probably shock fronts in a density wave. The parallel structure suggests that the shocks occur in several places throughout the arm, or that the interarm gas is composed of spiral-like wisps that merge together in the arms. Blue clusters of star formation inside the clumps of these dust lanes show density-wave triggering in unprecedented detail. The star formation process seems to be one of local gravitational collapse, rather than cloud collisions. Spiral arms inside the oval of IC 2163 have a familiar geometry reminiscent of a bar, although there is no obvious stellar bar. The shape and orientation of these arms suggest they could be the result of inner Lindblad resonance–related orbits in the cos 2θ tidal potential that formed the oval. Their presence suggests that tidal forces alone may initiate a temporary nuclear gas flow and eventual starburst without first forming a stellar bar. Several emission structures resembling jets or conical flows that are 100–1000 pc long appear in these galaxies. In the western arm of NGC 2207, there is a dense dark cloud with a conical shape 400 pc long and a bright compact cluster at the tip, and there is a conical emission nebula of the same length that points away from the cluster in the other direction. This region also coincides with a nonthermal radio continuum source that is ~1000 times the luminosity of Cas A at λ = 20 cm. Surrounding clusters in arclike patterns may have been triggered by enormous explosions.
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| 2. |
- Elmegreen, D.M., et al.
(författare)
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Hubble Space Telescope Observations of Dust and Star-forming Regions in the Ocular Galaxy IC 2163 and Its Spiral Companion NGC 2207
- 2001
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 121:1, s. 182-197
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Tidskriftsartikel (refereegranskat)abstract
- Hubble Space Telescope observations in U, B, V, and I passbands of the interacting spiral galaxies IC 2163 and NGC 2207 are used to measure extinctions in the cloud and intercloud regions and ages and luminosities of the star-forming regions. The extinction in the part of NGC 2207 seen in projection against IC 2163 was determined by using the method of White & Keel. The extinctions there and elsewhere were also determined from radiative transfer models of the magnitude differences between clouds and their surroundings. The intercloud extinction in V band ranges from 0.5 to 1 mag on the line of sight, and the cloud extinction ranges from 1 to 2 mag. The measured star-forming regions in these galaxies have a power-law relation between size and luminosity and a power-law luminosity distribution function. These power laws are consistent with a fractal dimension for the star formation that is the same as that for interstellar gas, D ~ 2.2, extending over scales ranging from 20 to 1000 pc. Fifteen compact massive star clusters that are analogous to super–star clusters found in starburst regions are in the spiral arms of NGC 2207. Nothing is peculiar about these regions except for a high H I velocity dispersion (~50 km s-1). Two more super–star clusters are in the tidally compressed oval of IC 2163. These clusters have masses ranging from ~10^4 to 2 × 10^5 M⊙ and ages of a few times 10^6 yr.
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