LRRP A Proposal to Extend the Canadian Galactic Plane Survey
Tom Landecker National Research Council

SUMMARY

A substantial fraction of the Canadian radio astronomy community is now engaged in the Canadian Galactic Plane Survey (CGPS). The CGPS Consortium (55 researchers including international partners, graduate students and postdocs) is making startling new discoveries related to the states and physical processes within the Interstellar Medium (ISM). HIA is providing the major Canadian observational component of the CGPS through dedication of the DRAO Synthesis Telescope 90% to the project in the period 1995 to 2000.

In Phase 1 of the CGPS, Consortium researchers have (a) captured in action the flow of material and energy from the disk to the halo of the Galaxy, (b) used polarimetry to measure magnetic fields and ionized material at densities and field strengths lower than previously achievable, (c) laid the groundwork for statistical studies of the turbulent morphology of the ISM, (d) revealed detailed coupling of stellar winds with the ISM, and is (e) providing arcminute-resolution panoramic images of the major components of the ISM (atomic, ionized, molecular, relativistic and dust).

Based on the results of Phase 1, the CGPS Consortium, working with HIA, proposes a Phase 2, designed to probe new questions. This project will capitalize on the scientific momentum, and on the investment in the telescope and in skills. Over a period of five years (2000 to 2005) the CGPS Consortium will turn its attention to three major goals. The first is an extensive investigation of the disk-halo interaction. Second, we plan to complete a study of the local spiral arm. This area of the Galaxy has already been a rich vein of discovery because of the many instances of astrophysical processes which can be observed in detail there. Third, we propose the first detailed mapping of atomic gas in a major star-forming region, the nearby Cepheus complex, which contains forming and recently formed stars over a wide range of initial masses, which, as point sources, couple energy and momentum to large-scales in the ISM.

HIA can provide four of the nine positions needed to accomplish this work, but will require new funding to support the remainder. Phase 2 of the CGPS will provide the Canadian community of centimetre-wave astronomers with the observing facility they need to continue the community's scientific productivity until the new cm-wave radio astronomy facilities come on line beyond 2010. The DRAO Synthesis Telescope is training young astronomers, at the graduate student and postdoctoral level, in the aperture synthesis techniques which will be vital to Canada's participation in the Millimetre Array and the Square Kilometre Array.

 

1. THE CANADIAN GALACTIC PLANE SURVEY

The Canadian Galactic Plane Survey (CGPS) is a project of a consortium of scientists from Canada and other countries who set out in 1995 to create high-resolution (about 1 arcminute) images of the major constituents of the Interstellar Medium (ISM) in our Galaxy with the aim of improving our understanding of the physical states of matter in the ISM and the interaction of its many components. The original consortium of 35 has now grown to about 55 as graduate students and postdoctoral researchers have joined the project. The consortium was awarded an NSERC Collaborative Special Project Grant in 1995 amounting to $796,000, spread over the years 1995 to 2000.

The principal Canadian observational component is being delivered by HIA through the DRAO Synthesis Telescope, which is dedicated 90% to the imaging of a region of more than 600 square degrees along the Galactic plane in the second quadrant, acquiring data in the radio continuum (408 and 1420 MHz) and, most notably, in the 21-cm H I line. The telescope is unique in its ability to image the atomic hydrogen with 1 arcminute resolution and full sampling of all structures from this size up to the largest scales (with the inclusion of data from the DRAO 26-m Telescope). Collaborators are contributing images of the same region (Galactic longitude from 75 to 145, latitude from -3.5 to +5.5) in (a) the infrared (reprocessed IRAS 60 and 100 micron data, contributed by IPAC and reprocessed IRAS 12 and 25 micron data, contributed by CITA), (b) the CO (J=1-0) transition (Five Colleges Radio Astronomy Observatory), (c) 151 MHz radio continuum (Cambridge University, England), and (d) 232 and 327 MHz radio continuum (Beijing Astronomical Observatory). These observations in different wavebands portray the atomic, the molecular, the ionized and the relativistic components of the ISM, as well as the dust, including small particles and PAHs. The consortium is now using this powerful combination of data to reveal new phenomena and features of the ISM.

The DRAO observations are on target to complete the imaging of the region specified above by April 2000. We refer to these observations as Phase 1 of the CGPS. We now propose to build on the results of Phase 1 by extending the project into a second phase which will capitalize on our investment in the telescope, in the expertise, and in the team.

2. WHAT HAVE WE LEARNED?

In Phase 1 we have developed highly efficient techniques for observing with the Synthesis Telescope. The telescope now operates virtually 24 hours a day, unattended. We have increased its reliability, and we are able to operate it with fewer people. We have solved many technical problems in imaging. With this instrument we are regularly producing noise-limited continuum images with dynamic ranges of 5000 to 10000. We have observed the Survey region with the DRAO 26-m Telescope to provide information on the largest structures; these data have extremely high fidelity, and are corrected very precisely for instrumental effects. We have developed accurate and efficient techniques for combining single-antenna and synthesis telescope data, and for the rapid processing of multi-channel spectral-line data into mosaics. We have learned how to make mosaics of regions tens of degrees across with consistent uniformity and quality. We are developing visualization and analysis software for scientific interpretation of the three-dimensional data. We are regularly distributing data from DRAO to the Consortium, and will soon begin to distribute data to the wider astronomical community through HIA's CADC.

We have forged an outstanding collaboration between HIA and our university partners, which embraces both data reduction and science: we are truly working together. A few scientific highlights which have emerged from the CGPS are outlined in the following paragraphs. In each case, the collaborating institutions are listed in parentheses at the end of the description. The emphasis in this list is on the truly new phenomena that have been uncovered.

  1. The first discovery and characterization of a Galactic Chimney in the Milky Way (Normandeau et al 1996). Apparently a conduit for energy and material from the disk to the halo, this chimney is driven by O stars in W4, part of a major H II complex in the Perseus Arm. Such conduits have often been postulated, but this is the first detection. (U. Calgary, DRAO).
  2. An extraordinary H I feature, the "mushroom'', another manifestation of disk-halo interaction. This object rises 5 degrees above the plane in the Perseus Arm, a height of a few hundred parsecs, and resembles the mushroom cloud from a nuclear blast. The energy source is not clear, and theory groups in the Consortium at CITA and Laval are working on models. (Queen's, U. Calgary, Laval, CITA).
  3. When we built the polarimetry facility, we expected that the polarization images would be dominated by extended synchrotron emitters such as supernova remnants (SNRs). We were astounded to find that SNRs are barely significant, against a background of widespread structures in polarization, which are apparently produced by Faraday rotation of the galactic synchrotron emission by a magneto-ionic screen. This channel is opening a new window on ISM studies through its ability to measure magnetic fields in the ISM, and through its very high sensitivity to low-density ionized material. The first result from this new view of the Galaxy was the detection of an unusual large elliptical feature, which is very uniform in structure in contrast to its chaotic surroundings (Gray et al 1998). Of unknown origin, this may be a manifestation of ionized material in the interarm region. Other polarization results (Gray et al 1999) have led to the detection of the extended envelope of a large HII region, and the measurement of the magnetic field in that envelope.  (DRAO, U. Calgary).
  4. H I self-absorption features that become visible at the high resolution of this survey. They are highly structured, and resemble molecular clouds in topology. The velocity of the absorbing clouds matches the spiral-shock velocity of the Perseus Arm, and at least some are associated with CO clouds and dust. Our hypothesis is that we have located sites where compression of the atomic hydrogen by the spiral arm shock has led to cooling (and thus self-absorption) and subsequent formation of molecules and, eventually, formation of stars. This possibility addresses a fundamental issue in the evolution of the ISM in galaxies. (U. Calgary, DRAO, UBC, U. Hamburg).
  5. The view of the interstellar atomic and molecular gas presented by these highly sensitive observations, capable of revealing both small and large structures, is revolutionary. Groups at Laval and at FCRAO are developing new statistical tools to characterize the structures present (H I - Ghazzali et al 1999; CO - Heyer and Schloerb 1997, Brunt, 1998). (Laval, FCRAO).
  6. Stellar winds appear to have a major influence on the morphology of the H I. The strength of the survey is its ability to deliver comparative studies of these features in H I, CO, and IR. One example is in the H II region IC1396 (Moriarty-Schieven et al 1996) where globules of molecular material are being destroyed by the wind of an O star. Many structures, apparently wind-blown, are being found (Knee and Wallace 1998). Stellar winds are perhaps the most important mechanism in sculpting the ISM in regions of massive star formation, either by disrupting the placental environment, or perhaps triggering further star formation. (DRAO).

We embarked on Phase 1 with the conviction that we had to take the big view of the ISM and that the interactions of its components would be complex. The survey mode of observing, as distinct from pointing the telescope at selected objects, was chosen as the only reasonable attack on the problem. The results have justified this approach. In many cases we have seen that objects are "bigger'' than we could have imagined they might be, in the sense that their influence extends far beyond what we had hitherto thought to be their boundaries, and they are influenced by a large volume of space around them. It is the large ``spatial dynamic range'' of the survey, the large ratio between the extent of the images and the resolution element, that has yielded the most exciting scientific results. An excellent example is the new understanding of the W3/W4/W5/HB3 complex that has emerged from Phase 1. The influence of the stellar winds of the exciting stars is now seen to extend at least 150 pc up from the plane.

It appears that many objects, H II regions and supernova remnants, occur in complexes. This is not surprising, given the tendency of massive stars to form in groups. From Phase 1 it is becoming clear that it is far better to approach the study of individual objects within their wider context. In other words, we are convinced of the success of the survey approach, particularly of the multi-wavelength survey approach, in pushing forward our understanding of the ISM. We now plan to apply this approach to solving some new problems.

3. THE PROPOSAL

We propose to continue observations with the DRAO Synthesis Telescope for a further five years, from April 2000 to April 2005. In Phase 2, our primary goal is to build on what we have learned in Phase 1 and to tackle new scientific challenges.

The time allocation for the Synthesis Telescope will be changed from the present mix (90% CGPS, 10% other highly-rated proposals) to 70% to 80% CGPS, 20% to 30% other proposals. Proposals will be invited from the world astronomical community, and the fraction to be dedicated to non-CGPS observations will depend on the number of proposals received, and on their quality as assessed by a rigorous reviewing process (already in place). Present demand indicates that good proposals will be received for both Galactic and extragalactic work. The telescope has a following of researchers who value its ability to detect diffuse HI of large extent in nearby external galaxies (e.g. Carignan and Purton 1998).

The telescope is currently capable of observing 45 fields every year with a comfortable safety margin and we base our plans on that rate (in 1998 we will probably achieve 48 fields, projecting from performance up to the end of October). In 5 years, we will observe between 160 and 180 fields, about 600 square degrees of sky. Large though this is, we must choose the directions in which we observe deliberately. As with Phase 1, some sort of compromise between competing science objectives will have to be reached in planning Phase 2.

Detailed planning will be done in consultation with the community. The first phase of the CGPS is now in high gear, with rapidly developing research output, and these ideas will, over the next two years, shape the detailed plan of observations for Phase 2. The ideas that follow indicate some strong contenders.

3.1 DISK-HALO INTERACTIONS

A major focus of the proposal is our plan to extend our studies of disk-halo phenomena by extending the observations to higher Galactic latitudes. The discovery of two striking disk-halo phenomena in Phase 1 has already led us to observe a number of additional fields beyond the original limit in latitude. We will develop exact observing plans based on the results of the remaining work in Phase 1, and we will closely examine all existing wide-area HI surveys for clues to other structures. Looking, for example, at the survey of Burton (1985) we can see HI structures with substantial column density extending to latitudes of 10 degrees and higher. Such structures are visible in the second quadrant, where the Phase 1 CGPS is focussed, at Perseus arm velocities. Unfortunately, the Burton survey has an angular resolution of only 21 arcminutes, and is quite sparsely sampled. From these data alone, or from the other single-antenna surveys (which have even coarser resolution) one can say little about the high-latitude H I features beyond the fact that they exist. Now that we have recognized the "mushroom'' feature, we can find it in Burton's data, but with Burton's data alone it would have been impossible to recognize its significance. Exactly the same statement can be made about the W4 Chimney.

The Burton survey also reveals high-latitude features in the vicinity of Cas A, again at Perseus arm velocities. There is a large grouping of H II regions and SNRs in the Cas A vicinity. From Phase 1 we will establish whether they are related. In Phase 2 we hope to investigate whether the high-latitude gas is related to them.

Fourteen years ago, Heiles (1984) was able to detect abundant high-latitude H I in the existing coarse-resolution single-antenna surveys, and he catalogued many "shells, supershells and worms'' rising from the Galactic plane (ApJS, 55, 585) and he postulated that these features represented a link between the disk and the halo of the Galaxy. This paper is much quoted, but its important hypothesis has stood untested simply because no telescope existed which had the angular resolution and the sensitivity to extended structure to provide the needed observations. We can now do that.

3.2 THE LOCAL SPIRAL ARM

We also plan to complete the observations of an astrophysically important region at lower latitudes. In Phase 1 we will not be able to completely observe the Cygnus-X region, where our line of sight traverses the Local spiral arm. This region contains a fascinating menagerie of objects, many of which are prototypical, ranging from Cygnus X3 to the Cygnus superbubble. The great concentration of WR stars in the Northern sky is found here.

To provide data for a complete study of the structure and phenomena we need to extend the survey at least to longitude 65. Between longitudes 72 and 87 the latitude coverage should also be extended to at least -6 and +8 degrees. The detailed study of Cygnus-X is already underway, led by H. Wendker, University of Hamburg.

3.3 STAR FORMATION

Star formation research continues to capture a large fraction of astronomical effort worldwide. Phase 2 of the CGPS offers the opportunity to make a major contribution by mapping with arcminute resolution the distribution of atomic gas around significant star-forming regions in Cepheus (longitude 100 and 120 degrees, latitude 11 to 22 degrees). In the past Cepheus has been overshadowed by the observational attention paid to the Orion and Taurus regions, but it is now receiving renewed attention from observers. One advantage of observing an area at high galactic latitude is the relative absence of confusion, which can be a problem for this work in the Galactic plane.

The Cepheus region contains a number of star forming regions relatively close to us, and displays a wide variety of star formation-related phenomena. For instance, the Cepheus Flare cloud (distance about 300 pc) is comparable in size and mass to the Orion or Taurus regions, but, surprisingly, shows little evidence of star formation. Being less dense than these other regions, the Flare clouds appear to be an observationally rare and poorly studied example of a giant molecular cloud (GMC) without associated star formation, perhaps intermediate in type between dense star forming clouds and the translucent high latitude clouds. However, recent large studies of the Cepheus Flare cloud reveal pre-protostellar cores and low-mass star formation activity (Kun 1998). Earlier HI studies reveal a very wide range in velocities for clouds in this region (about 15 km/s), a range which is also seen in the molecular gas. Possibly the result of cloud-cloud collisions and/or a swept-up supernova shell, the high energy dynamics of this region makes for low-mass star formation in a very different environment than is the case for Taurus.

In contrast, the molecular clouds known as Cepheus A to F (distance about 800 pc) are associated with high mass star formation (the Cepheus OB2 and OB3 associations, for example). This region although active in forming stars, is different from Orion in the sense that the cloud has a very low filling factor of molecular gas and is thus extremely clumpy. It has been suggested that the discrete groupings of OB associations in this regions are a classic example of sequential star formation. Parts of these regions lie within the CGPS Phase 1 region, so an extension to higher latitudes in Phase 2 would complete the continuum/H I survey relatively quickly.

12CO and 13CO surveys of the various Cepheus clouds are available made with 1.2 and 2.5 metre class telescopes (e.g. Remy et al 1997) and smaller surveys of the densest molecular regions with telescopes in the 15-metre class. There is an opportunity here to complement the DRAO observations with data from the focal-plane arrays now being developed for the JCMT.

We note that almost no detailed H I mapping of the large star-forming regions has been done. The best data for Orion, for example, has angular resolution 36 arcminutes (Green, 1991) - this survey, which covers 28.5 x 28.5 degrees, was made with the DRAO 26-m Telescope.

3.4 POLARIMETRY

The polarization channel detects Faraday rotation of the background of Galactic synchrotron emission, yielding the product of magnetic field strength and electron density. Using data at other wavebands it is often possible to separate this product, obtaining the field strength. At high Galactic latitudes, the new H-alpha surveys will be an extremely powerful aid (as well as the traditional tools of thermal radio continuum and pulsar dispersion measure). We are poised to provide vital data which will help us understand the degree of control of magnetic fields in ISM processes. This could not be done until now because of the extreme difficulty of measuring fields.

It will be very important to trace the polarization structures detected in Phase 1 at 1420 MHz to the limit of their extent in latitude.

Investigation of the magnetic field has strong relevance to the two major themes of Phase 2. It is conceivable that the field mediates disk-halo interactions. In general, the exchange of energy and mass between the disk and the halo of a galaxy may play a role in global mechanisms for inciting star formation complexes. Systematic studies in our Galaxy at high latitudes are needed to explore this possibility.

We plan to extend polarimetry to 408 MHz as well as 1420 MHz. Polarimetry at 1420 MHz is ideally suited to probing regions close to the Galactic Plane, while the 408 MHz channel is well-suited to higher latitude studies (because of the shorter lines of sight).

4. SOME TECHNICAL ISSUES

We will maintain rigidly certain survey parameters set for Phase 1. We will use the same field separation (112 arcminutes) to allow us to use the present mosaicing algorithms. All observations will be made on an extension of the present grid. Wherever possible, the needs of observers outside the CGPS will be met by observing at positions on the survey grid.

At the same time, we plan to remedy some known defects of the telescope which are limiting its dynamic range. The list of improvements depends on the time and staff effort that can be dedicated to the work. The high priority items include antenna drive modifications to improve pointing accuracy and modifications to make the seven antennas more exactly similar.

Since observations are planned at higher latitudes where the HI intensity is falling off, sensitivity is an issue. Receiver and antenna improvements are planned to reduce sytem temperature from 65 to 35 K.

Polarimetry at 408 MHz will be added as a facility, through a graduate student (Ph.D.) starting in 1999. This facility can be expected to come into operation early in Phase 2.

5. OTHER VALUES OF THE DRAO SYNTHESIS TELESCOPE

Quite apart from the scientific reasons, there are other strong justifications for continuing observations with the DRAO Synthesis Telescope. In particular, the Canadian community of centimetre-wave radio astronomers must be supported with an ongoing facility until the new radio astronomy facilities of the 21st century come on line. (Canada's share of the JCMT would be unable to support the present radio astronomy community in Canadian universities).

Further, as Canadian radio astronomy proceeds into the era of the Millimetre Array and the Square-Kilometre Array, HIA will require expertise in aperture synthesis. That expertise is currently being maintained and developed at DRAO, where graduate students and postdoctoral researchers are being trained in the discipline. The skills required include the design and operation of such telescopes, as well as the processing of the data that they produce.

The Synthesis Telescope also offers opportunities for the training of graduate students with skills in instrumentation. In this way the DRAO group has made a contribution over many years to the growth of the technical infrastructure that supports Canadian astronomy.

Finally, the Synthesis Telescope has been, and will continue to be, a test-bed for techniques applicable to the telescopes of the future. Many innovative features of the present telescope have found their way into other instruments.

6. ORGANIZING, STAFFING AND FINANCING THE WORK

The present operation requires approximately nine full-time equivalent staff, drawn from a larger number of people who spend some or all of their time on other DRAO projects. This team schedules and operates the telescope, maintains and repairs it, processes and archives the data, and processes the images and prepares them for distribution to the Consortium. It should be emphasized that staff at DRAO deliver images to the Consortium ready for scientific analysis and interpretation: the data require no further processing by the users. This is a very unusual mode of operation for an observatory, but is absolutely necessary in a survey to ensure a uniform product. Data and image processing is at least half the effort.

This staff complement includes the effort involved in maintaining a large Export Software Package that is running on many types of computer systems in universities across Canada, and in many institutions in other countries.

This staff complement currently includes one position supported by NSERC (0.5 CGPS production, 0.5 research). One further NSERC position at the University of Calgary is also involved in CGPS production work.

Several individuals contributing to the effort described above are also involved in research with the CGPS data; this research effort is not counted in the figure given above. However, it is an essential part of the operation of a front-line facility to have the staff who operate the telescope also carry on research with it.

Equally vital to the concept of the survey is the Consortium. We believe that the CGPS has created a unique partnership between HIA and University partners. The existing management structure, in which DRAO and university researchers share the effort, will be maintained.

We suggest that membership in the CGPS Consortium should become open to new members at the start of Phase 2 (This idea must, however, be subject to the wishes of the present Consortium.).

From ongoing funding, HIA can identify support for four of the nine FTE positions in the available budget after the completion of the present Phase 1. We cannot continue the present level of effort on the Survey as well as all the other ongoing activities at DRAO in the list in Appendix 1. If the Survey is to continue, we must identify funds for 5 more positions, each costing about $100K/yr for salaries, benefits, overheads, computers and travel. At the beginning of Phase 2 we also need about $100K for upgrades to the telescope. We expect the consortium to apply for a further NSERC grant, but those funds would be used principally to support Consortium research in the universities. We cannot count on NSERC money being available to help with the basic observing costs. Thus to extend the Canadian Galactic Plane Survey for 5 years HIA will need an extra $2600K.

7. REFERENCES

C. Brunt, "Multifractal Analysis and Modelling of the Molecular Interstellar Medium'', 1998, Ph.D.Thesis, University of Massachusetts.

W.B. Burton, "Leiden-Greenbank Survey of Atomic Hydrogen in the Galactic Disk. I. l,v and b,v Maps'', 1985, AAS, 62, 365.

C. Carignan and C.R. Purton, "The ėTotal' Mass of DDO 154'', 1998, ApJ, 506, 125.

N. Ghazzali, G. Joncas and Jean, S., "Statistical Characterization of Complex Structures in HI Maps '', 1999, ApJ, in press.

S.J. Gibson, A.R. Taylor, P.E. Dewdney, L.A. Higgs, W.H. McCutcheon, and H.J. Wendker, "Neutral Hydrogen Self-Absorption Features in the Canadian Galactic Plane Survey'', 1999, in "New Perspectives on the Interstellar Medium'', editors A.R. Taylor, T.L. Landecker and G. Joncas, ASP Conference Series, in press.

A.D. Gray, T.L. Landecker, P.E. Dewdney and A.R. Taylor, "A Large-Scale Interstellar Faraday-Rotation Feature of Unknown Origin'', 1998, Nature, 393, 660.

A.D. Gray, T.L. Landecker, P.E. Dewdney and A.R. Taylor, A.G. Willis and M. Normandeau, ``Radio Polarimetric Imaging of the Interstellar Medium: Magnetic Field and diffuse Ionized Gas Structures Near the W3/W4/W5/HB3 Complex'', 1999, ApJ, in press (to be published March 1999).

D.A. Green, "A Survey of H I in Orion: I. the Data'', 1991, MNRAS, 253, 350.

C. Heiles, "H I Shells, Supershells, Shell-Like Objects, and Worms'', 1984, ApJS, 55, 585.

M.H. Heyer and F.P. Schloerb, "Application of Principal Component Analysis to Large Scale Spectral Line Imaging Studies of the Interstellar Medium'', 1997, ApJ, 475, 173.

L.B.G. Knee and B.J. Wallace, "Atomic Gas Streaming from a Molecular Cloud in the Perseus Arm'', 1999, in ``New Perspectives on the Interstellar Medium'', editors A.R. Taylor, T.L. Landecker and G. Joncas, ASP Conference Series, in press.

M. Kun, "Star Formation in the Cepheus Flare Molecular Clouds: I. Distance Determination and the Young Stellar Object Candidates'', 1998, ApJS 115, 59.

G.H. Moriarty-Schieven, T. Xie and N.A. Patel, "H I ėTails' from Cometary Globules in IC1396'', 1996, ApJ, 463, L105.

M. Normandeau, A.R. Taylor, and P.E. Dewdney, "A Galactic Chimney in the Perseus Arm of the Milky Way'', 1996, Nature, 380. 387.

G.P. Remy, I.A. Grenier, G. Duvert, and P. Thaddeus, "The Cepheus Molecular Cloud: I. Multi-transition Observations in CO and 13CO'', 1997, A&AS, 125, 303.