CGPS News: Volume 8

CGPS News Volume 8
Nouvelles du RCPG Volume 8
March / mars 1999

From the editor

Magdalen Normandeau

Fame and Glory! At the January 1999 AAS meeting in Austin, there were three press releases featuring CGPS work. Jayanne and company told the world about the mushroom, Brad & Lewis wowed the press with pretty pictures of HI streamers and cones, and Steve & Russ told tales of HI self-absorption. Excellent publicity which has made external colleagues much more aware of the survey.

As with any project that spans several years, new members appear and some members leave us. Richard Christie has unfortunately had to withdraw from the PhD programme at UBC. Richard teaches full-time at Okanagan University-College and there just wasn't enough time left over for thesis work. He will however continue to try to carry on with some of the research projects which he has started. Fred Vaneldik has fully retired and will no longer be participating in CGPS projects. Fred was one of the first long-term CGPS consortium visitors at DRAO, spending a summer at the observatory to work on pointing related issues in the early days of the CGPS.

Fourth Annual CGPS Science Meeting
May 14/15, 1999
University of British Columbia

Bill McCutcheon

The fourth annual CGPS science meeting will be held on 14/15 May at the beautiful University British Columbia.


There will be a reception on the evening of Thursday, May 13, 7:30-10:00 pm. All day Friday and Saturday will be devoted to meeting sessions. A draft program will be available in March.


30 single rooms with shared washroom facilities at Walter Gage residence have been reserved for nights of May 13,14 and 15. The rate is $30.00 + 8% Provincial Hotel Tax + 7% GST. There is one washroom for every six rooms. Check-in time is 2:00 pm May 13. Check-out time is 11:00 am.

All rooms will be held until April 13, after which time all unreserved rooms will automatically be released to the general public.

Participants will reserve and pay for their own rooms. Please mention that you are part of the CANADIAN GALACTIC PLANE SURVEY GROUP. The Reservation Office can be reached as follows:

Reservation Office, UBC Conference Centre
5961 Student Union Blvd.
Vancouver, B.C.
V6T 2C9

Tel: (604) 822-1010
Fax: (604) 822-1001

Also see the website:

For more information, contact Bill McCutcheon
phone: (604) 822-6234

For local planning is it important to get a fairly accurate sense of how many people will attend. Please respond as soon as possible to Bill to confirm that you will attend this meeting, and if you plan to attend the reception.

Please also indicate whether you plan to give a presentation and, if so, include a tentative title (or subject area) and indicate whether it will be a poster or an oral presentation.

Fred Vaneldik

Tom Landecker

Fred Vaneldik recently retired from his position of Professor of Electrical Engineering at the University of Aberta. Fred has made many contributions to DRAO over his career, and his support for DRAO over the years has been very strong.

It all started in 1979/1980, when Fred spent a sabbatical year at DRAO. At the time, we were just expanding the Synthesis Telescope from two antennas to four, still working only at 1420 MHz. During that year, Fred worked on the local oscillator system that ensures the coherency of the array at 1420 MHz. The same system serves us today on seven antennas.

This period at DRAO launched a long collaboration between Fred and Dave Routledge and the DRAO crew. Graduate students, working with Fred and Dave at the University of Alberta, carried through a succession of development projects that turned the Synthesis Telescope into what it is today. First came the 408 MHz channel, with two students, Wing Lo and Bruce Veidt, working on the digital correlator and the antenna and receiver. Next came David Karpa, working on the continuum correlator for the 1420 MHz channel. This correlator doubled the telescope bandwidth and also allowed axpansion to seven antennas, and made possible polarimetry at 1420 MHz. Development of polarimetry itself was another U of A graduate student project, pushed through by Rick Smegal. Somewhere in there, the U of A lab built low-noise 1420 MHz front-end amplifiers for the Telescope.

Fred's knowledge and experience cover a vast area of engineering, and he has been able to contribute to many areas of Canadian radio astronomy, including our efforts in millimetre-wave technology. He has the same reputation as a teacher, of someone who is at home in many disciplines, from antennas and electromagnetics to digital engineering to electric motors, and can teach effectively in all of them. We'll miss him.

CGPS observing and processing at DRAO: status report

Andrew Gray, Lewis Knee

Observing with the Synthesis Telescope went exceptionally well in 1998, with nearly 47 fields observed during the year, up by 1.5 fields from 1997. This was despite persistent problems with the main observing computer (which are now mostly in hand) and a 3 week session devoted to collecting holography data for Anne Thorsley's (U.Alberta) Masters project. Such an achievement does not happen by accident, and it a testament to the hard work and dedication of the DRAO staff.

The CGPS fields observed in 1998 have filled in the gap at the Perseus end, giving a contiguous block of about l=126°-146°, and extended the Cygnus end to about l=96°. We have also added a block in the middle between l=107°-117°, excluding a small buffer zone (5 fields) immediately adjacent to Cas A. These fields will be observed once we are confident that the dynamic range problem is as far under control as we are likely to get it. At present it seems that 10000:1 is the dynamic range limit we can expect in this region (Cas A is a kilo-jansky source).

The observing status at the end of 1998 stood at 140 of the 190 CGPS fields completed, or 74%, with 76% of the allotted time elapsed. Since the observing period ends in March 2000, only a year away, the time has come to make up this small discrepancy between the amount observed and the time taken. This requires us to observe at a rate of 41 CGPS fields per year for the remainder of the project, which is clearly within a our capabilities, and may not even require us to sacrifice any of the 6 fields usually set aside for external users.

Data processing is also progressing well. HI continuum subtraction is complete for 89 fields. Data for the MX1, MX2, MY1, and MY2 mosaics are fully processed, with the MX1 and MX2 mosaics currently being assembled, and MY1 and MY2 to follow soon after. Data processing for the MA1 mosaic is also nearing completion, and work is also progressing at the Cygnus end, with continuum subtraction complete for MM1, MN1, and MO1 (final mosaicing in Cygnus awaits the resolution of some position and flux registration problems). In short, expect to see plenty of data products appearing over the next few months, joining the already released MV1, MV2, MW1, MW2, and interim A/B mosaics.

In the area of personnel, Roland Kothes (from Bonn) joined the processing team in October 1998, bridging the gap left by Sean Dougherty's departure. He got up to speed quickly and has been working on reducing the continuum processing backlog at the Cygnus end, producing a very nice preliminary mosaic of this area. Chris Brunt has also just arrived from FCRAO, and will soon be reporting for processing duty, so we can expect the processing to pick up further speed in the coming months.

Processing in the Cygnus region

Roland Kothes

During the last months, the data processing effort has been concentrated on the Cygnus region at 1420 MHz (see also CGPS Newsletter of June 1997 for an earlier report of work on this region). The fields containing most of the extended emission of the local spiral arm have been completed. Thus at the low-longitude end of the CGPS only the processing of the O4 field, in which Cygnus A is located about 90' away from the pointing centre, remains a problem. The figure below represents the current status of data processing of the Cygnus region at 1420 MHz. This preliminary mosaic (without short-spacing data) contains all O fields (except O4), all N fields, all M fields, and three L fields (L6, L8, and L9). The strongest source is the HII region DR21 with a peak flux of 6.4 Jy. With a background rms noise of about 0.5 mJy/beam, we get a maximum dynamic range of at least 10000:1 for this mosaic. The background rms moise is slightly higher than the usual 0.3 mJy/beam obtained in fields with less extended emission; there is low-level extended structure right to the edge of the mosaic presented here.

Notes from the Polarisation Group

Marta Peracaula

Status of the Polarisation Data Calibration:

Status of the Instrumental Polarisation Corrections:

In order to measure the Instrumental Polarisation Corrections that have to be applied to the individual CGPS 1420 MHz fields, known polarised and unpolarised sources were observed at different offsets from the field center in different directions.

After several observational tests in radial directions along the field, we decided that the optimum approach was to make the observations along an RA and DEC grid with a spacing of 15 arcmin. In that way, we obtained 88 points across the field up to a distance of 75 arcmin from the field center. In Fig. 1 we show the contours representing percentage of each of the measured Stokes parameters with respect to the total intensity of the unpolarised calibrators 3C295 and 3C147 across the field.

Figure 1 Tony Willis has written a code that interpolates in the tables obtained from the grid of offset observations and subtracts them from the uncorrected Stokes Q and Stokes U maps. If Qobs is the flux of a pixel in the uncorrected Stokes Q map, I is its corresponding value in the Stokes I map, and t is the corresponding value of the table represented in the upper left panel of Fig. 1 the corrected map is formed by applying to all the pixels:

Qcorr = Qobs - t x I/100

The program, called INSPOLCORR, is now part of the DRAO software.

We performed different tests to prove the effectiveness of these corrections. One of them was to get offset observations of a known polarised source (3C286) in 5 different directions and apply to them the corrections indicated by the contours. We recovered satisfactorily, within the error bars, the known values for 3C286. Another test was to study 5 sources that are situated in intersecting fields of the final MV1 mosaic product. Each of these sources lies in at least two intersecting fields and the measured flux in the mosaic is the weighed mean of the values in the two or three fields. We verified the consistency between the fluxes measured at different radius and directions of a same source.

The feasibility of offset observations beyond 75 arcmin from the field center gets complicated due to the low signal to noise ratio and the large integration times every point would need in those positions. Since the observational contours shown in Fig. 1 are consistent with the relevant antenna theory, Tom Landecker, with the help of a summer student, will study the predicted behavior of the DRAO ST antennas at offsets larger than 75 arcmin, for possible corrections at larger radii.

At the moment we are building the Stokes Q and U mosaics setting the weights to zero beyond 75 arcmin (the value used for Stokes I is 100 arcmin). In Fig. 2 we show how the fields overlap with this truncation, using the Stokes Q, MV1 mosaic. We checked the overlapping is still satisfactory to build the mosaic and we do not include in this way the sources that haven't been corrected for instrumental polarisation.

Figure 2

Data production meetings

Lloyd Higgs

As of February 11, a regular series of monthly coordination meetings of the CGPS data-production team at DRAO and the University of Calgary was initiated. The meetings will be held on the second Thursday of each month, starting at 15:00 PST (PDT in the summer). In addition to those workers who are deeply involved in the data processing, representatives of the CGPS Management Committee (Dewdney, Landecker and Taylor) will sit in on these meetings when they are available. Although these coordination meetings are most relevant to the Synthesis Telescope data-production team (Brunt, Gray, Knee, Kothes and Wallace at DRAO and Gibson and Peracaula at U. of Calgary), Consortium members may from time to time like input to the agendas. (Data production is taking place or is planned to take place elsewhere in the Consortium!). I have volunteered to chair these meetings, so anyone wanting input or feedback should contact me. The next two meetings will be on March 11 and April 8.

Turbulence in the Interstellar Medium
(PhD thesis abstract)

Chris Brunt via Mark Heyer

[Editor's note: Mark sent this in for Chris who is apparently en route for his new posting at DRAO]

The multivariate technique of principal component analysis (PCA) is a powerful statistical tool with which to describe spectral line imaging observations of the molecular interstellar medium. In particular, as formulated by Heyer & Schloerb (1997), PCA can retrieve statistical information about the velocity fields within the interstellar gas. However, the nature of the transformation of the intrinsic velocity field onto an observable velocity axis is extremely complex if the velocity field is macroturbulent.

In this work, PCA is used to show that interstellar velocity fields are characterized by stochastic fluctuations on all measurable scales (ie are macroturbulent) and to obtain a quantitative measure of the turbulent velocity dispersion as a function of scale. To relate the measurable statistical information to intrinsic velocity field statistics, an ensemble of artificial density and velocity fields are translated onto the observational domain, utilizing non-LTE radiative transfer calculations. The intrinsic statistical properties of these fields are well-defined and accurately known, which allows the retrieved information to be calibrated to the intrinsic information. Additional results dealing with the instrumental noise and telescope beam-smearing effects on the PCA method are derived and demonstrated.

An application of the reformulated method is carried out on an ensemble of molecular regions observed within the FCRAO CO Survey of the Outer Galaxy. to obtain the first calibrated measurements of interstellar cloud velocity fields. Macroscopic velocity fluctuations within the molecular clouds are detected. The dependence of velocity fluctuations (Delta_V) on spatial scale delta is well characterized by the relationship : Delta_V = c (delta/1 pc)alpha with alpha ~ 0.62 and c ~ 1 kms-1. According to the calibrated biases of the PCA method, this translates into an intrinsic "size-linewidth" relationship of the form : Delta_V \propto R1/2 for the velocity field within molecular clouds, in good agreement with size-linewidth relationships obtained from an ensemble of potentially physically-diverse clouds. We also find a range of exponents around the mean of gamma = 1/2, with minimum observed gamma = 0.33 and maximum observed gamma = 0.81. The upper end of this range involves steeper spectra than can be easily accounted for by idealized compressible modifications of the Kolmogorov cascade phenomenology. Coupled with the eigenimage signatures, which motivate stochastic motions, but not strongly shock-like motions in a supersonic medium, these exponent measurements provide useful constraints to hydrodynamical simulations of molecular clouds.

MIGA Mosaics Now Available

Charles Kerton

The Mid-Infrared Galaxy Atlas (MIGA) mosaics are now available on the CGPS ftp site. A paper describing the atlas is almost complete. This paper, combined with Cao et al. 1997, will give users a complete guide to the HIRES infrared data sets contained in the CGPS. The format of the MIGA mosaics is similer to that of the IGA mosaics. For example, here are the 12 micron MIGA mosaics for the MA1 region:

The _bem mosaics are equivalent to the _bsm mosaics from the IGA (sorry about the nomenclature confusion - there's a good reason for it that has to do with the naming of the output files in HIRES, but you really don't want to read about that!). Just remember they show the HIRES beam shape at different locations in the image. The text listing of 2D Gaussian fits to these beams (the bsm_fwhm.txt files) will be available soon. The _cfv and _phn maps are a measure of the correction factor variance and photometric noise - see the Aumann et al. 1990 paper for details on these quantities. The _cvg map is a measure of the number of data points that were used in estimating the flux at that point, and the _det map shows the tracks of the various IRAS detectors across the image. Finally, the _img files are the first (co-add) and twentieth iteration HIRES images.

As mentioned in the last newsletter, I've made a set of GIF versions of the 20th iteration images that may be useful for a quick look at the image data. Hopefully sometime in the near future the individual MIGA fields will be available via the web on a server set-up similar to the current IGA server. This will give users access to the complete MIGA which goes from -6 to +6 degrees in the CGPS region.

One nice aspect of both the MIGA and the IGA is that, since they are based on an (almost) all sky survey, they can be seamlessly extended to higher or lower galactic latitudes. This should prove useful for the proposed Phase 2 of the CGPS with its focus on disk-halo interaction and the planned extension of the survey around Cygnus X and Cepheus. In the Cygnus X region plans are to extend the coverage of the CGPS so that it will cover -6 to +8 degrees. The MIGA actually already extends from -6 to +6 degrees so an extension to +8 degrees would be trivial. Finally, the mapping of the Cepheus region will cover the latitude range from +11 to +22 degrees; this can be added on to existing MIGA and IGA data between longitudes 100 - 120 degrees or mapped as a completely separate region.

HI absorption and emission towards W3

Magdalen Normandeau

A paper entitled "Probing the Interstellar Medium using HI absorption and emission toward the W3 HII region" has been accepted for publication in the Astronomical Journal (estimated publication date is May 99).

You can find electronic copies of the paper on the CGPS publications page, on my homepage, and through astro-ph.

This seems like a good opportunity to remind you to let Steve Gibson know of CGPS papers which you have submitted and to keep him informed of their status in order that the publications web page be kept current. Note that this page is also accessible from the public CGPS page and therefore has some PR value. As well, if you're not already in the habit of doing so, please consider sending your papers in to astro-ph once they have been accepted. Again, it increases the project's visibility.

HI Self-Absorption near the Perseus Spiral Shock

Steve Gibson

The HI self-absorption (HISA) team has broadened its investigation of cold HI clouds to examine not only the physical properties and geometries of these objects, but also their distribution in the Galactic environment.

Population Census

An accurate census of HISA features requires the use of software pattern-recognition techniques to ensure consistency and repeatability. Lloyd Higgs has recently developed algorithms for this purpose which examine 2-D channel maps with the largest-scale background structures removed. In this article, I discuss the results of an earlier, simpler census attempt using 1-D wavelet searches of spectra in the MVW supermosaic HI cube. These results are summarized below (Fig. 1). Noise and confusion problems limited the sensitivity of this search method to only the darkest features, so the total amount of HISA in a given channel is severely underestimated. However it still shows qualitative agreement with the amount of HISA found in visual inspections of the HI cube. In particular, the strong enhancement of HISA near -40 km/s (LSR) is verified by eye-brain ``detections'' of numerous dark features around that velocity.

Fig. 1: Upper panel: average OFF brightness temperature for each HISA pixel at each velocity (solid) along with the average value of all pixels (blue dashes). The lowest OFF pixel values are around 40 K, which may represent an average lower limit to the spin temperature of the absorbing HI (zero values occur in channels with no detected absorption). Lower panel: HISA optical depth integrated over channel area (solid), assuming a spin temperature of 60 K, and converted to HISA mass assuming a distance of 2000 pc -- a gross oversimplification intended only to give an order-of-magnitude mass estimate. Average FCRAO 12CO brightness is shown on an arbitrary scale for comparison (red).

A basic question about the space distribution of HISA is whether it is uniform or concentrated in certain regions. Since both CO and HI emission peaks occur around -40 km/s in the Perseus arm, a similar enhancement of HISA would be expected if HISA is distributed uniformly in the general HI. However the amplitude of HISA at -40 km/s relative to levels at other velocities is much stronger than that of the general HI emission, arguing for a real concentration of HISA in this vicinity.

Shock Signature?

One interesting possibility is that this concentration is brought about by gas compression in the environment of the Perseus Arm spiral shock, and that the material we see as HISA is condensing to form molecular hydrogen, and eventually stars, downstream (Figs. 2 & 3).

Fig. 2 (left): Schematic view of relevant spiral arm geometry, as seen from the North Galactic Pole. Fig. 3 (right): Gas radial velocity model of Roberts (1972, Ap.J., 173, 259), with possible absorbing and emitting HI components marked. Clicking on either image will select a larger version.

In the MVW field sightline, one would expect the shock to lie on the near side of the Perseus Arm. Material overtakes the arm from behind, since the orbital speed exceeds the pattern speed at this Galactocentric radius. Gas decelerates abruptly in the shock and is compressed, allowing for rapid cooling after the initial shock heating. The gas then reaccelerates as it leaves the shock, appearing to slow down again at larger distances due to differential rotation and perspective. The multi-valued nature of the resulting velocity vs. distance curve permits a consistent model of the radiative transfer geometry, in which more distant HI (reheated by star formation?) is able to illuminate the HISA just downstream of the shock, where molecular cloud formation would be most likely to occur.

This hypothesis obviously needs further testing. The next step is to employ Lloyd's algorithm for a more accurate census, and to examine a larger longitude range as soon as additional HI mosaics become available, to see if the HISA peak velocity shifts with longitude in the same fashion predicted for the spiral shock.

These results were presented at the January AAS meeting in Austin; a complete copy of the poster is available. For general information on the HISA project, please see the CGPS HISA page.

Triggered star-formation associated with a new super-bubble in the Perseus arm?

Brad Wallace, Peter Dewdney, Tom Landecker

A new superbubble has been discovered in archival HI line data from the Effelsberg and Green-Bank 100m radio telescopes. This superbubble, which measures some 6 x 8 degrees in size is centered near l=131d, b=+1d. The CGPS data for this region are in the final stages of processing at DRAO, and these data are being used to construct a higher resolution HI cube. The superbubble can also be seen in the CGPS HiRes and CO data, where emission coincident with the bottom of the HI shell exists.

The HI shell which bounds the super-bubble is not uniform, showing instead significant signs of disruption. The shell also appears to be associated with an OB association, two known SNRs, a number of smaller HI shells, the HI feature known as "The Dragon's Breath", and a number of IRAS sources with colors suggesting they are young stellar objects. We are investigating the possibility that the formation of this super-bubble has resulted in a new generation of star formation along its edge, and that the newly formed stars are acting to destroy the shell.

We are actively soliciting assistance in studying this object. The large number of very diverse objects are best studied by a team of people who are experts in the specific types of objects. If you are interested in supernova remnants, stellar winds and their bubbles, superbubble formation theory, or star formation, and have the time and resources to assist in understanding this feature, we want to hear from you. More information on this object can be found here.

Dissociating stars homepage

Magdalen Normandeau

A web page for the Dissociating Stars project has been linked to the CGPS homepage. Some initial searches are documented and one object, in the so-called Dragon's Breath, is now receiving Chris Purton's attention. He will be telling us more about this object at the spring science meeting.

The W4 loop

Susan Terebey

S. Terebey is working on a paper provisionally entitled "Infrared and Radio Images of the W4 Supershell" with CGPS coauthors M. Fich and R. Taylor. This paper analyzes the infrared and radio continuum emission from the W4 loop region, which forms the lower half of the W4 Supershell. Previous work on the W4 Supershell includes the initial Galactic chimney paper by Normandeau, Taylor, Dewdney (1996) and theoretical modeling by Basu, Johnstone, and Martin (1999, in press).

Stars in the W4 chimney's globule

Magdalen Normandeau

Within the W4 chimney/superbubble there are two CO globules. The lower one was the subject of a paper by Heyer et al (1996) based on the FCRAO survey and the DRAO pilot project data. Follow up observations were carried with the JCMT and the details of this study are given by Taylor et al (1999) who pointed out that the presence of an IRAS point source at the apex of the cloud, where the molecular gas density reaches 104 cm-3, indicates the presence of embedded star formation. An attempt to image the region at higher resolution with the BIMA array failed in June 98 when the observations were cut short by a thunderstorm.

Alan Watson of UNAM has been carrying out UBVRIJHK observations of the W3 region in hopes of detecting all the OB stars except where Av > 30 or so. He contacted me to obtain a 1420 MHz image of W3 for upcoming observations at the Mexican National Observatory. The subsequent exchange lead to observations last autumn of the cometary globule in the chimney. The data have not yet been examined but we should be getting around to it in March.


Francois Cazzolato

This is just an update of what I have been doing for my M. Sc. thesis. I am currently finishing the study of the possible interaction between WR143 and its environment.

Continuum emission at 1420MHz has revealed no trace of WR143 nor diffuse emission in the vicinity. WR143 is also not detected at 60 and 100 microns (IRAS/HIRES).

After a thorough examination of the HI data cube, I have come to the conclusion that there seems to be a deficiency of HI within the velocity range -13 to -25 km s-1 with WR143 near the border closest to the galactic plane. This HI cavity seems to be opened toward the direction OPPOSITE to the galactic plane. The kinematical distance of 6kpc corresponding to those velocities (all velocities are referred to LSR) is NOT in agreement with the value of 0.82kpc quoted by van der Hucht (1988).

The HI data cube also reveals features at velocities "forbidden" by the galactic rotation curve. There is a lot of motion in that direction so kinematical distances have to be treated with caution (WR143 lies in the line of sight of the Cygnus OB1/OB3 superbubble).

Commentaires ou suggestions? Comments or suggestions?
Ecrivez à / Write to