Radio studies of southern interacting galaxies

Gordon, Scott Douglas (2003). Radio studies of southern interacting galaxies PhD Thesis, School of Physical Sciences, The University of Queensland.

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Author Gordon, Scott Douglas
Thesis Title Radio studies of southern interacting galaxies
School, Centre or Institute School of Physical Sciences
Institution The University of Queensland
Publication date 2003
Thesis type PhD Thesis
Supervisor Dr. Barbel Koribalski
Dr. Keith Jones
Total pages 460
Language eng
Subjects 020104 Galactic Astronomy
Formatted abstract
The work presented in this thesis is a survey of a sample of six known, or suspected, interacting groups of galaxies in the southern sky, for the purpose of studying their structure, kinematics, and extent and distribution of star formation activity. This sample covers a wide range including the spectacular, well- studied Antennae merging pair (NGC 4038 and 4039), through less-studied groups such as the NGC 6845 quartet, to groups about which very little was previously known, such as IC 2554 and its neighbours. The other galaxies and groups studied are NGC 3882, NGC 6810, and the NGC 6769/6770/6771 group, with their neighbours. Along the way a number of previously unknown galaxies have been discovered and described.

This study uses extensive new H ı and radio continuum observations made by myself (except where noted in the Acknowledgements section and in individual chapters). They total 350 hours duration, and were made from 1996 to 1998 using the Australia Telescope Compact Array, a southern radio interferometer. Superior images are obtained relative to previous observations of these groups. They are integrated here with earlier radio, optical, infrared and other, images and prior studies. Additional work presented describes a project undertaken to develop software for the analysis of optical spectral images made using Fabry-Perot interferometers (which has been published as Gordon et al. 2000).

Results vary substantially between the six groups studied, and cover a number of topics. The results for The Antennae (NGC 4038/9; Chapter 4) have already been published as Gordon et al. (2001), and those for NGC 6845 (Chapter 5) are expected to be in the near future.

Seven previously unknown H ı-rich dwarf galaxies have been discovered, residing in all but one of the groups studied, with no obvious correlation to the properties of the known group members. These reveal a substantial population at the smaller end of the galaxy size distribution, which previously has largely gone undetected, and which has not been as well studied as the larger spirals and ellipticals. This agrees with other recent observations, and with cosmological models in which large galaxies form through mergers of many smaller objects.

Strong tidal interaction is seen conclusively in three of the six groups; one (NGC 6769/70/71) is more mildly interacting; and two (NGC 6810 and NGC 3882) interact only slightly or not at all. Radio continuum measurements reveal greatly enhanced star formation (relative to 'normal' levels in spiral galaxies) in the target spirals in every group studied except for IC 2554, but the degree of enhancement is not closely correlated with the degree of interaction.

In The Antennae (a very well studied group), the well-known tidal tails have been observed in H ı with greater clarity, revealing an extended northern tail and clearer evidence of gas pooling at the tip of the southern tail; the merging starburst disk has been revealed to have a detached H ı component produced in the merger, but half of the H ı is now in the tails; and a small neighbouring spiral has been identified as a companion.

In the NGC 6845 quartet, the two lenticular members (C and D) appear to have no gas or radio continuum sources, and no involvement in ongoing interaction. The two spiral members (A and B) have substantial H ı masses totalling 2.3xlO10M⊙
, and starburst activity. They are strongly interacting with each other, with prominent tidal arms and serious morphological changes, particularly in the smaller B galaxy. The H ı is partly ambiguous, but most likely each galaxy possesses single large H ı rich tidal arms, with additional arms more prominent optically. IC 2554 has previously been interpreted as a chaotic merger, but closer examination of optical, continuum, and H ı images suggest that instead it has a partially dust- shrouded disk and an extensive single tidal arm which is rich in H ı but optically invisible. It is clearly interacting, but its interaction partner is uncertain, and its star formation rate (in spite of the presence of optical knots like H ıı-regions) is normal for a spiral. The nearby smaller spiral ESO 92-G9 shows generally normal features but may be slightly extended in H ı. It has two adjacent H ı objects; one (RKK 1959) is a newly- discovered optical galaxy, while the other appears to have no optical image. The NGC 6769/70/71 triplet has been observed. NGC 6769 and NGC 6770 overlap on the sky and appear to be interacting with one another (but not merging), with distorted or bifurcated spiral arms and a common light envelope. They exhibit starburst rings within their disks, additional radio continuum in their arms, and NGC 6770 (the eastern one) also has bright nuclear radio emission of a different origin. The radio shows no sign of interaction involving lenticular NGC 6771 in the south, although it may have a very faint optical bridge; this galaxy also has no detectable H ı and very little radio continuum emission. Two additional field spirals, IC 4845 and IC 4836, have been observed also to have extensive radio continuum emission, and in IC 4845 an extensive but two- lobed H ı disk; an optically faint irregular galaxy in the south was also discovered.

NGC 6810 is an edge-on spiral galaxy which is shown to have an approximately regular H ı disk, although possibly distorted at its edges. It has centrally concentrated radio continuum which could be from an active nucleus or from star formation; in the latter case a substantial starburst is occurring. The galaxy shows no clear evidence of interaction. Neighbouring irregular galaxy ESO 142-G32 has a rotating, but warped, H ı component, with an extended Hi and optical tail, and could be interacting, although interaction with NGC 6810 is doubtful. An additional optically faint (and probably dwarf) galaxy was discovered from its compact H ı emission; it may have a rotating disk.

NGC 3882 is a spiral galaxy with abundant H ıı regions throughout its disk, due to a starburst event. In the radio continuum some form a ring structure. The galaxy has a rotating H ı disk which is warped beyond the edge of the optical emission. These features suggest an interaction event, however an interaction partner cannot be identified due to confusing foreground stars resulting in minimal knowledge of neighbouring galaxies (especially of their velocities). Any partner is probably distant. An optically invisible companion galaxy has been newly discovered from its compact H ı emission.

Keyword Galaxies

Document type: Thesis
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