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Scientific Proposal
The significance of spiral galaxies needs no explanation to any astronomer, and as barred spirals form around half the total spiral population, it is not necessary to explain how a study aimed at understanding the structure and dynamics of barred spirals can make a real advance in our comprehension of the Universe. In this introduction we will only highlight some of the problems implicit in our International Time Project and how they fit into the big picture.
The dynamical problem of bar formation is intimately linked to that of disc formation, and has implications for the presence and structure of dark galactic halos. It is well known that the thin rotating discs of spirals would be
unstable without the presence of massive galactic halos, or of some equivalent dynamic component. Rotating discs of gas and stars, even so, are subject to gravitational instabilities, of which the warp instability and the bar instability are among the most widespread. Modern N-body simulations readily yield stable bars. However, there is a collection of dynamical phenomena
associated with bars which theory addresses semi-quantitatively, and for which excellent data sets are urgently required. These can be listed as :
Although these problems have not been suddenly discovered, there are two powerful reasons why they should be studied now. One is that theoretical modelling has reached a stage of considerable realism, where all the dynamical
factors: stars, neutral atomic and molecular gas, ionized gas, dark halo, can be well included, and in three-dimensional schemes, so that clear-cut testable predictions are now available (this would not have been the case only some three or four years ago). Secondly the balanced combination of CCD mapping photometry, in broad and narrow bands, two-dimensional emission-line spectroscopy, and
classical slit spectroscopy available on the La Palma telescopes means that a
self-contained project can now be expected to yield significant results. If we can add infrared measurements from Izana, and neutral gas observations from other observatories (as will be possible from the make-up of our proposing team) the attack on the problems posed will be even more powerful. In fact the team
contains some of the leading theorists in the field as well as observers with a variety of skills and experience. In the following sections we present a more detailed accounts of the justification for the proposed observations, followed by the selection of
individual objects to observed and an outline of how the observations, their analysis and their interpretation will be planned and carried out. We also give a list of participants with their institutions, and their contributing skills
and any associated colleagues, as required by the terms of reference of the call for proposals for International Time Projects.
During the past five years evidence has slowly accumulated linking activity in
the nuclear zones of galaxies with the presence of a bar, or an oval distortion, in the distribution of the gravitational potential. An early study by Simkin et al. (1980) suggested the association of the presence of bars in galaxies with activity in the form of Seyfert nuclei. This study was not considered
statistically convincing, i.e. not at all complete, but the underlying idea was taken seriously. Although the detailed mecanisms of nuclear activity remain in a state of partial controversy (above all the "black hole v. starburst" issue is
not at all well sorted out, nor is this proposal the correct place to do so) it is physically evident that to feed any kind of outburst of energy, matter has to flow into the centre, and that for this to happen non-conservative non-central forces must operate. The existence of a non-axisymmetric structure around the nucleus foments exactly this possibilty; a stellar bar can absorb angular
momentum from gas, or transfer it outwards in the more conservative case, thus allowing the gas to fall down the potential gradient into the nuclear potential well (Roberts et al. 1979, Schwarz 1985).
There has been a sequence of studies, both observational, and theoretical, around this most interesting topic. Among the most compelling were observations of molecular gas forming "gas bars" around the nuclei of starburst galaxies, often with a circumnuclear ring of molecular gas at the centre ( Ishizuki et al. 1990, Turner & Hurt 1992). Evidence for inflow, with velocities of tens of km/s, has been cited, but the interpretation of this has been questioned (Beckman
1993) and a more plausible phenomenology is that of gas in "counterflow" with elongated orbits along the bar, leading to a net inflow at rates of order of km/s.
The most completely studied of this type found in the literature is the Sc spiral NGC4321 (M100) which has a circumnuclear starburst. We originally studied this object for the star forming properties of its disc, and for this purpose high quality imaging in the optical (H(alpha), U, B, V, R and I bands) was obtained with excellent resolution (-0.8 arcsec seeing over the whole object), at the WHT on La Palma. To understand the parameters governing global star
formation in the disc, we went on to obtain a 21 cm map with the VLA, and a map in the 1-0 line of CO, which gave information on the atomic and molecular components of the galaxy: their column of density and velocity distributions.
During our analysis of the 21 cm data we were impressed with the evidence of gas in highly non-circular orbits, aligned at a certain position angle (Knapen et al. 1993a). Careful examination of our own, and previous optical images revealed the presence of a bar on a scale of 6 kpc in length, and infrared
imaging, in the H band, strongly confirmed this. Intensive morphological study of the inner 1 kpc including a K-band NIR image then revealed a miniature bar, aligned with the principal bar, but separated from it by an axisymmetric zone of stars.
The complex morphology of the circumnuclear zone, with a clear inner bar in the NIR which is not even hinted at on HST images in the optical bands, has been
dealt with observationally and theoretically in two recent papers (Knapen et al. 1995a,b) by the research group of the author of the present proposal.
We have dealt in detail with this object here because from the point of view of the International Time Project it represents a paradigm of the types of observations we will need to make in order to examine the effects of bars, and non-axisymmetric in general, in producing nuclear activity. In any sample of objects we would need to obtain:
It would be of great interest in the context of this part of the project to make complementary measurements of neutral i.e. atomic and molecular gas in the circumnuclear zones of a sub-set of our galaxies using radioastronomical techniques. In the case of HI membership in the project includes several experts in HI observations (in particular the Groningen group which implies access to
the Westerbork Radio Synthesis Telescope). We would also plan to apply for VLA time for HI observations. In the case of H2, which is mapped via CO, we have experience within the group of the use of the Nobeyama milimeter interferometer and the IRAM interferometer, and would wish to make selected high angular resolution studies of circumnuclear CO, especially with the latter instrument. We can group these effects into two broad themes, both in terms of their astrophysical nature and the observational techniques employed. The first is the distribution of star formation in the galactic discs of barred galaxies. From our previous work in this field (starting with Cepa & Beckman 1990, and Knapen et al. 1992) we have detected a tendency, in grand design spirals, for the star formation regions to the distributed in two-dimensionally symmetric patterns, reflecting disc-wide resonance structures governed by density wave systems. The presence of long-lived resonances indicate that there must exist a long-lived wave-mode in these galaxise. Bars can be considered as steady density waves, or a long-lived mode in a galaxy. Corresponding resonance features are often observable in the form of nuclear, inner and outer rings at the ILR, UHR and OLR respectively. These patterns form one of the more important pieces of evidence for the existence of such systems, and also set interesting constraints on the mechanism of massive star formation. One aspect of the bisymmetric patterns is that they appear relatively strongly in galaxies with apparent absence of bars, and the symmery appears to be destroyed in barred galaxies (note that this does not refer to the arm symmetry itself, but to the pattern of star formation along the arms). It will be important to quantify this effect and set it on a statistical basis. The second theme is the apparent absence of metallicity gradients in barred galaxies as pointed out in a recent paper by Vila Costas & Edmunds (1992). Here again the implications are of considerable importance, because models of galaxy evolutionpredict such gradients, and they have been well observed in non-barred galaxies. In this context, Martin, Roy & Belley (1993) claimed to find some barred galaxies with metallicity gradients. Thus the initial results referred to above must be generalized if we are to deepen our understanding of gas-stellar interactions on a galactic scale. The group proposing has ample and deep experience in the chemodynamical evolution of galaxies, and the ability to make systematic use of the parameter of non-axisymmetry will be eagerly seized upon. The specific questions to be addressed here can be summarized as follows: To tackle this programme we will require the following type of observations: Using direct imaging one can make a very fair approximation to the form of the gravitational potential of a barred galaxy using simple assumptions about mass-light ratios the wider the range of broad-band photometric imaging the better the approximation. From this one can carry out a set of theoretical studies: the existence and location of resonances can be determined and a predicted stationary gas-flow pattern established. These can be compared directly with the positions and shapes of observed dust-lanes which should follow the off set shocks associated with a bar (see Athanassoula 1991) as well as with observed two-dimensional radial velocity patterns in ionized and neutral gas. Developing further, a self-consistent model for the bar can then be constructed using a library of orbits, via a Schwarzschild type method; this model contains predictions of the stellar velocity field which can be checked against observations (long-slit spectra). Among the specific properties of bars which would be investigated are: We have a number of theorists in the proposing group (Athanassoula, Combes, Heller, Kuijken, Shlosman) who are eager to try out their increasingly powerful computer codes on a set of well-founded homogeneously obtained data. The optical observations for this selection: multi-band broad-band photometric imaging, long-slit spectroscopy, and two-dimensional emission spectroscopy, are all implicit in the observations explained in the previous sections, and do not imply extra types of observing, rather the systematic use of the observations obtained. The supplementary HI and CO observations will be also employed for the dynamical studies presented in this section. The direct support received for the project from members of leading European astronomical institutions (and the general interest shown also by colleagues in the United States) leads us to believe that the work proposed is considered of key importance and reinforces the view of the initiating group of investigators that it would mark a well-recognized step forward in extragalactic research. It is our determination, if we are granted the opportunity to make this a dynamic international project which would following the inspiring example of the late Mike Penston, give rise to new collaborations and new and permanent working friendships.
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