Hubble Space Telescope observations of the interacting galaxies NGC 2207 and IC 2163

Citation
Bg. Elmegreen et al., Hubble Space Telescope observations of the interacting galaxies NGC 2207 and IC 2163, ASTRONOM J, 120(2), 2000, pp. 630-644
Citations number
43
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMICAL JOURNAL
ISSN journal
0004-6256 → ACNP
Volume
120
Issue
2
Year of publication
2000
Pages
630 - 644
Database
ISI
SICI code
0004-6256(200008)120:2<630:HSTOOT>2.0.ZU;2-A
Abstract
Hubble Space Telescope (HST) images of the galaxies NGC 2207 and IC 2163 sh ow star formation and dust structures in a system that has experienced a re cent grazing encounter. Tidal forces from NGC 2207 compressed and elongated the disk. of IC 2163, forming an oval ridge of star formation along a caus tic where the perturbed gas rebounded after its inward excursion. Gas flowi ng away from this ridge has a peculiar structure characterized by thin para llel dust filaments transverse to the direction of motion. The filaments be come thicker and longer as the gas approaches the tidal arm. Star formation that occurs in the filaments consistently lags behind, as if the exponenti al disk pressure gradient pushes outward on the gas but not on the young st ars. Numerical models suggest that the filaments come from flocculent spira l 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 o f 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 con tain 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 a rm, or that the interarm gas is composed of spiral-like wisps that merge to gether in the arms. Blue clusters of star formation inside the clumps of th ese dust lanes show density-wave triggering in unprecedented detail. The st ar formation process seems to be one of local gravitational collapse, rathe r than cloud collisions. Spiral arms inside the oval of IC 2163 have a fami liar geometry reminiscent of a bar, although there is no obvious stellar ba r. The shape and orientation of these arms suggest they could be the result of inner Lindblad resonance-related orbits in the cos 2 theta tidal potent ial that formed the oval. Their presence suggests that tidal forces alone m ay initiate a temporary nuclear gas how and eventual starburst without firs t forming a stellar bar. Several emission structures resembling jets or con ical flows that are 100-1000 pc long appear in these galaxies. In the weste rn 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 emiss ion nebula of the same length that points away from the cluster in the othe r direction. This region also coincides with a nonthermal radio continuum s ource that is similar to 1000 times the luminosity of Cas A at lambda = 20 cm. Surrounding clusters in arclike patterns may have been triggered by eno rmous explosions.