[show abstract][hide abstract] 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 ~104 to 2 × 105M⊙ and ages of a few times 106 yr.
The Astronomical Journal 01/2001; · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Hubble Space Telescope 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. Gas flowing away from this ridge has thin parallel dust filaments transverse to the direction of motion. Numerical models suggest that the filaments come from flocculent spiral arms that were present before the interaction. A dust lane at the outer edge of the tidal tail is a shock front where the flow abruptly changes direction. A spiral arm of NGC 2207 that is backlit by IC 2163 is seen to contain several parallel, knotty filaments that are probably shock fronts in a density wave. Blue clusters of star formation inside these dust lanes show density wave triggering by local gravitational collapse. Spiral arms inside the oval of IC 2163 could be the result of ILR-related orbits in the 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 100-1000 pc long appear. There is a dense dark cloud with a conical shape 400 pc long and a bright compact cluster at the tip, and with a conical emission nebula of the same length that points away from the cluster in the other direction. This region coincides with a non-thermal radio continuum source that is 1000 times the luminosity of Cas A at 20 cm. Comment: 20 pgs, 8 figs as jpeg (3 color), to appear in Astronomical Journal, August 2000
[show abstract][hide abstract] ABSTRACT: The Hubble Heritage Project has the aim of providing the public with pictorially striking images of celestial objects obtained with NASA's Hubble Space Telescope. The Heritage team has made a 3-color mosaic of the interacting spiral galaxies IC 2163 and NGC 2207 from three pointings of the WFPC2 camera in UBVI passbands. The scientific investigators for this research (Elmegreen, et al. 1995, ApJ, 453, 100 and 139) previously determined that IC 2163 experienced a close, prograde, in-plane encounter with NGC 2207. Tidal forces compressed and elongated the disk of IC 2163, forming an oval ridge of star formation where the perturbed gas reached its innermost extent. The Hubble Heritage image now shows how accelerated gas flowing away from this ridge developed a peculiar structure characterized by thin parallel dust filaments transverse to the direction of the flow. The filaments thicken as the gas approaches the tidal arm, eventually forming two long thick dust lanes in the arm. A spiral arm in NGC 2207 that is backlit by IC 2163 shows similar filamentary shock structures, which in this case are presumably from the associated density wave. Blue clusters of star formation are forming inside numerous clumps along both sets of filaments. A strong radio continuum source in NGC 2207 is also now seen to be associated with a large region of star formation on a spiral arm. Support for this work was provided by NASA through grant numbers GO-06483-95A and GO-07632.01-96A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
[show abstract][hide abstract] ABSTRACT: IC 2163 has a central eye-shaped ("ocular") structure with two long
tidal arms indicative of a very recent, close, tidal encounter. It is
partially obscured by its spiral companion NGC 2207. Numerical models
(Elmegreen et al. 1995, ApJ, 453,139) suggest that perigalacticon
occurred 40 Myr ago. We use UBVI images taken with the WFPC2 camera of
HST to study the star-forming regions and to make a direct measurement
of extinction through the disk of NGC 2207. We find no evidence for
super star clusters (SSCs) and no difference between the star-forming
regions in the fast-streaming gas on the compressed rim of the ocular
oval and those elsewhere in the system. The star-forming regions have
luminosity functions typical of intermediate and late-type spiral
galaxies, with ages of a few times 106 to a few times
107 years and masses of 103 Msun to 106
Msun. Either the 40 Myr since perigalacticon is too small for SSCs to
have formed yet or the interaction was too weak to produce them. For the
part of NGC 2207 that overlaps IC 2163 along the line of sight,
AV 1.1 mag for the dust lanes and AV 0.4 mag for
the inter-lane disk of NGC 2207. Our values for the extinction of NGC
2207 in B, V, and I are closer to a Whitford reddening curve than
Berlind et al. (1997, AJ, 114, 107) derived from ground-based
measurements because of our higher resolution images. This result
supports their interpretation that extinction becomes grey when patchy
dust is unresolved. Support for this work was provided by NASA through
grant number GO-06483-95A from the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5-26555.
[show abstract][hide abstract] ABSTRACT: We present HI, CO and optical observations of the ocular galaxy NGC 2535 and its starburst companion NGC 2536. NGC 2535 has an ocular (eye--shaped) structure with two long tidal arms. Such morphologies are indicative of a very recent, close, non--merging encounter. They have been reproduced in numerical models when the orbit of the companion is prograde and nearly in--plane. Because the ocular feature is short--lived, it restricts the numerical models and allows the determination of a unique solution to the orbit of the interacting galaxies. We will show that the ocular shape of the disk of NGC 2535 is intrinsically oval and that the velocity anomalies found in HI and in earlier H alpha Fabry--Perot maps can be entirely explained by the interaction. Moreover, the disk gas shows high velocity dispersion throughout. We find five unusually massive HI clouds without any obvious stellar counterpart. We will compare these results with our published work on another ocular galaxy, IC 2163, which is interacting with NGC 2077.
[show abstract][hide abstract] ABSTRACT: Observations with the Hubble Space Telescope reveal an irregular network of dust spiral arms in the nuclear region of the interacting disk galaxy NGC 2207. The spirals extend from ~50 pc to ~300 pc in galactocentric radius, with a projected width of ~20 pc. Radiative transfer calculations determine the gas properties of the spirals and the inner disk, and imply a factor of ~4 local gas compression in the spirals. The gas is not strongly self-gravitating, nor is there a nuclear bar, so the spirals could not have formed by the usual mechanisms applied to main galaxy disks. Instead, they may result from acoustic instabilities that amplify at small galactic radii. Such instabilities may promote gas accretion into the nucleus.
The Astrophysical Journal 08/1998; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Ten H I clouds with masses larger than 10 exp 8 solar masses in the interacting galaxies IC 2163/NGC 2207 are identified. Twenty-eight other interacting pairs of galaxies with large knots or star formation structures in their optical images are also tabulated. It is suggested that interactions can lead to the formation of greater than 10 solar mass clouds and young stellar regions in the outer parts of galactic disks, and that some of these regions may become gravitationally bound dwarf galaxies if they are ejected in tidal arms. It is proposed that the key to the origin of clouds of greater than 10 exp 8 solar mass in interacting systems lies in the high velocity dispersion of the interstellar gas. Numerical N-body simulations of the interacting galaxies suggest that the complete detachment of an unbound dwarf galaxy requires a companion mass comparable to or larger than the galaxy mass.
[show abstract][hide abstract] ABSTRACT: Galaxy interactions that agitate the interstellar medium by increasing
the gas velocity dispersion and removing peripheral gas in tidal arms
should lead to the formation and possible ejection of
self-gravitationally bound cloud complexes with masses in excess of
10(8) Msun. Some of these complexes may eventually appear as
independent dwarf galaxies. The formation of clouds with masses
exceeding 10(8) Msun is the result of gravitational
instabilities in gas disks with high velocity dispersions. Such masses
and high dispersions were observed with the VLA for the interacting pair
IC 2163/NGC 2207, which contains 10 clouds with HI masses >10(8)
Msun and widespread velocity dispersions 4 times larger than
in normal spiral galaxies. A giant cloud that forms by an instability in
a high-dispersion ISM should also have a high internal dispersion, and
it should produce stars with a greater efficiency than in normal
galaxies because of the cloud's greater resistance to self-destruction.
Such clouds should also have a larger fraction of massive stars than
normal clouds because of the larger temperatures that follow from the
high efficiency. Thus agitated galaxies should produce peripheral or
nuclear starbursts partly because of their high gas velocity
dispersions. Numerical N-body simulations of interacting galaxies
illustrate the proposed formation of 10(8) Msun cloud
complexes by gravitational instabilities. The masses and dispersions of
the clouds that form increase with the strength of the perturbation. The
simulations suggest that the complete detachment of an unbound dwarf
galaxy requires a companion mass comparable to or larger than the galaxy
mass. Dwarf galaxies that form this way should contain old stars from
the original disk plus new stars from the cloud complex/starburst phase
of its interaction-induced formation. This work was supported in part by
NSF Grant AST-8914969 to M.K..
[show abstract][hide abstract] ABSTRACT: We made radio, optical, and CO observations of three pairs of spiral galaxies involved in recent, grazing encounters and compared our data with numerical simulations. The tidal structures seen in the galaxy undergoing a prograde, in-plane encounter share a number of key features which are adequately explained by our simulations. It is shown that the objects IC 2163/NGC 2207, NGC 2535/36, and NGC 5394/95 represent distinct stages in the evolution of the type of interactions discussed here.
[show abstract][hide abstract] ABSTRACT: We made VLA HI observations of the young, interacting galaxy pair NGC 2535/36 in C and D Arrays (resolution = 12'' = 2.9 kpc). NGC 2535 has an "ocular" structure similar to that in IC 2163, which we studied previously. In NGC 2535, the intrinsic HI velocity dispersion is high, 30 km s(-1) over much of the disk, as compared to less than 10 km s(-1) in undisturbed spirals. The tidal arms of NGC 2535 contain five unusually massive clouds, with HI masses exceeding 10(8) M_&sun; and radii = 2 -- 3 kpc. These masses are comparable to the masses of HI-rich dwarf galaxies. Most of these clouds do not have prominent counterparts in the older stellar population. The HI column densities and velocity dispersions in these clouds imply a Jeans mass close to the observed HI cloud mass; the high gas turbulence apparently leads to the large cloud mass as a result of gravitational instabilities in the gas. Elmegreen et al. (1993, ApJ, 412, 90; 1995, ApJ, in press) found similar widespread high gas turbulence and very massive HI clouds without strong stellar counterparts in the young interacting pair IC 2163/NGC 2207. This suggests that enhanced turbulence and unusually massive gas clouds produced by gravitational instabilities in the gaseous disk may be a general feature of interacting spiral galaxies during the early phases of post-encounter evolution. The massive HI clouds in NGC 2535, IC 2163, and NGC 2207 give observational support to the gravitational instability model in Elmegreen et al. (1993, ApJ, 412, 90) and are not predicted by the Barnes and Hernquist (1992, Nature, 360, 715) model for the formation of tidal dwarf galaxies.
[show abstract][hide abstract] ABSTRACT: IC 2163 and NGC 2207 are interacting galaxies that have been well studied at optical and radio wavelengths and simulated in numerical models to reproduce the observed kinematics and morphological features. Spitzer IRAC and MIPS observations reported here show over 200 bright clumps from young star complexes. The brightest IR clump is a morphologically peculiar region of star formation in the western arm of NGC 2207. This clump, which dominates the Halpha and radio continuum emission from both galaxies, accounts for 12% of the total 24 micron flux. Nearly half of the clumps are regularly spaced along some filamentary structure, whether in the starburst oval of IC 2163 or in the thin spiral arms of NGC 2207. This regularity appears to influence the clump luminosity function, making it peaked at a value nearly a factor of 10 above the completeness limit, particularly in the starburst oval. This is unlike the optical clusters inside the clumps, which have a luminosity function consistent with the usual power law form. The giant IR clumps presumably formed by gravitational instabilities in the compressed gas of the oval and the spiral arms, whereas the individual clusters formed by more chaotic processes, such as turbulence compression, inside these larger-scale structures. We gratefully acknowledge support from NASA JPL/Spitzer grant RSA 1264471 to M.K. and RSA 1264582 to D.M.E. for Cycle 1 observations.