Michele Kaufman

An interaction model for the formation of dwarf galaxies and 10 exp 8 solar mass clouds in spiral disks

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 M ○. . Some of these complexes may eventually appear as independent dwarf galaxies. The formation of clouds with masses exceeding 10 8 M ○. 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 H I masses >10 8 M ○. and widespread velocity dispersions 4 times larger than in normal spiral galaxies

Dust Spirals and Acoustic Noise in the Nucleus of the Galaxy NGC 2207

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 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.

Spitzer Space Telescope IRAC and MIPS Observations of the Interacting Galaxies IC 2163 and NGC 2207: Clumpy Emission

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 Hα and radio continuum emission from both galaxies, accounts for ~12% of the total 24 μm 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.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 24mu m 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.

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]

Giant H II regions in M81

H-alpha and VLA radio continuum observations at wavelengths of 6 and 20 cm are used to study the distribution of extinction and the distribution of giant radio H II regions along the spiral arms in M81. The radial distribution of visual extinction Av in the plane of M81 shows no trend, with an upper limit 0.1 mag/kpc for the radial extinction gradient and a mean Av of 1.1 + or - 0.4 mag. Nearly all the giant radio H II regions lie along the spiral arms or the inner H I ring. The radial distribution of the set of giant radio H II regions exhibits a strong maximum at a galactocentric distance R roughly 300 arcsec. Unless molecular hydrogen in M81 is also concentrated near R roughly 300 arcsec, the data disagree with Vissers (1980) model for star formation associated with a density wave. Some suggestions are made about how to change the ballistic particle model of Leisawitz and Bash to agree with the observed radial distribution of giant radio H II regions. 47 references.

Hubble Space Telescope Observations of Dust and Star-forming Regions in the Ocular Galaxy IC 2163 and Its Spiral Companion NGC 2207

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.

Hubble Space Telescope Observations of the Interacting Galaxies NGC 2207 and IC 2163

Original article can be found at: http://www.journals.uchicago.edu/AJ/--Copyright American Astronomical Society

The grazing encounter between IC 2163 and NGC 2207 : pushing the limits of observational modelling

We present numerical hydrodynamical models of the collision between the galaxies IC 2163 and NGC 2207. These models extend the results of earlier work where the galaxy discs were modeled one at a time. We confirm the general result that the collision is primarily planar, that is, at moderate inclination relative to the two discs, and prograde for IC 2163, but retrograde for NGC 2207. We list 34 specific morphological or kinematic features on a variety of scales, found with multi-waveband observations, which we use to constrain the models. The models are able to reproduce most of these features, with a relative orbit in which the companion (IC 2163) disc first side-swipes the primary (NGC 2207) disc on the west side, then moves around the edge of the primary disc to the north and to its current position on the east side. The models also provide evidence that the dark matter halo of NGC 2207 has only moderate extent. For IC 2163, the prolonged prograde disturbance in the model produces a tidal tail, and an oval or ocular waveform very much like the observed ones, including some fine structure. The retrograde disturbance in the model produces no strong waveforms within the primary galaxy. This suggests that the prominent spiral waves in NGC 2207 were present before the collision, and models with waves imposed in the initial conditions confirm that they would not be disrupted by the collision. With an initial central hole in the gas disc of the primary, and imposed spirals, the model also reproduces the broad ring seen in HI observations. Model gas disc kinematics compare well to the observed (HI) kinematics, providing further confirmation of its validity. An algorithm for feedback heating from young stars is included, and the feedback models suggest the occurence of a moderate starburst in IC 2163 about 250 Myr ago. We believe that this is now one of the best modeled systems of colliding galaxies, though the model could still be improved by including full disc self-gravity. The confrontation between observations and models of so many individual features provides one of the strongest tests of collision theory. The success of the models affirms this theory, but the effort required to achieve this, and the sensitivity of models to initial conditions, suggests that it will be difficult to model specific structures on scales smaller than about a kiloparsec in any collisional system.

HST observations of the interacting galaxies NGC 2207 and IC 2163

Original article can be found at: http://www.journals.uchicago.edu/AJ/--Copyright American Astronomical Society

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]