SPARO Updates (1996-2003)

February 2003: SPARO Galactic Center results published in the Astrophysical Journal

The first science results from SPARO, obtained during Austral Winter 2000, are described in a paper published in this month's issue of the Astrophysical Journal Letters. The abstract appears below, followed by a brief summary.

Title:

FIRST RESULTS FROM SPARO: EVIDENCE FOR LARGE-SCALE TOROIDAL MAGNETIC FIELDS IN THE GALACTIC CENTER

Abstract:

We have observed the linear polarization of 450 micron continuum emission from the Galactic center, using a new polarimetric detector system that is operated on a 2 m telescope at the South Pole. The resulting polarization map extends ~ 170 pc along the Galactic plane and ~ 30 pc in Galactic latitude, and thus covers a significant fraction of the central molecular zone. Our map shows that this region is permeated by large-scale toroidal magnetic fields. We consider our results together with radio observations that show evidence for poloidal fields in the Galactic center, and with Faraday rotation observations. We compare all of these observations with the predictions of a magnetodynamic model for the Galactic center that was proposed in order to explain the Galactic Center Radio Lobe as a magnetically driven gas outflow. We conclude that the observations are basically consistent with the model.

A brief summary of these Austral Winter 2000 results follows:

Shown below are magnetic field vectors implied by SPARO's 450 micron polarization measurements superimposed upon a 90 cm VLA map (LaRosa, et al 2000) tracing the hot ionized gas in the region of the Galactic Center. The white contours show the 850 micron continuum emission from SCUBA/JCMT(Pierce-Price, et.al).


Click image to enlarge

Due to the operation of the magnetic alignment mechanism, the direction of polarization is perpendicular to the magnetic field direction. The measured polarization is predominantly perpendicular to the Galactic plane, implying that the magnetic field is aligned with the Galactic plane, as viewed from our vantage point in the Galactic disk. This in turn implies an azimuthal field geometry, i.e. the magnetic field lines run in circles that lie in the plane of the Galaxy.

The field that SPARO samples pertains to the cool neutral regions within our Galactic nucleus, but there is also hot ionized gas in the nucleus, coexisting with the cool neutral gas. Interestingly, the magnetic field in this hot gas is known to run perpendicular to the Galactic plane. One way this can be seen is to note the orientation of the radio filaments shown (in yellow) in the radio continuum image that we have shown superposed on our data. These non-thermal filaments trace magnetic fields. We conclude that there are two orthogonal magnetic field structures coexisting in the center of our Galaxy. In our recent paper (see above) we compare the SPARO results, radio filament results, and Faraday rotation results, and show that these are all basically consistent with a magnetodynamic model for the Galactic center proposed in 1985 by Y. Uchida and co-workers.

We also obtained polarimetry of NGC 6334 during Austral Winter 2000 (see below).

  • click for index to photos

    • Early 2002: Success of SPARO optical upgrade is confirmed!

    During Austral Summer 2001-2002, observations towards the peak of the Galactic source RCW 57 confirmed that SPARO optics upgrade carried out in Austral Summer 2000-2001 did indeed improve the optical efficiency by about a factor of four, as predicted from changes in bolometer IV characteristics and measured transmission of filters. As a result, one day of Austral winter 2002 observations will be equivalent to one week of Austral winter 2000 observations! For details click here.

    Austral Winter 2000: First SPARO polarimetry at South Pole

    Click here here to see preliminary analysis of the first polarimetry data obtained with SPARO at the South Pole. The polarimetry vectors were taken on May 18,(black), July 7(green), and July 26(blue). The map corresponds to SPARO photometry data taken on May 18. The thick vectors represent polarimetric detections of 3 sigma or greater, while the thin vectors are below 3 sigma detection.

    Austral Winter 2000: SPARO's first image of a Galactic source

    RCW 57
    The top image is SPARO's 450 micron image of RCW 57. The lower image is the IRAS 100 micron image of the same region. The SPARO Data are a result of median filtering of 16 1.25 hour images of this region. This is the first true image of a galactic source taken by SPARO. The dark blue spots surrounding the two images in the 450 micron data are artifacts of the small (.2 degree) chopper throw used in this scan.

    Fall 1999: SPARO first light at Pole!

    Following the SPARO winter-over of 1999, during which engineering tests were done but no observations were possible due to telescope problems, first-light observations were made by a summer-season team consisting of Dave Chuss and Giles Novak of NU, Greg Griffin and Matt Newcomb of CMU, and Bob Loewenstein of U. Chicago. We observed the Moon on November 11th, and saw the bright Galactic cloud RCW57 several days later.

    During this SPARO observing run, that lasted through the third week of November, we carried out tests of focus, pointing, optical efficiency, detector noise, and many any other aspects of the experiment. The testing went well (see details below). We are looking forward to our first science data in a few months: SPARO will be operated on the telescope again beginning in late January and continuing into the Austral Winter 2000. We plan to observe with SPARO for the first three months of the winter season (through May 2000).
    February 20, 1999-Despite a long wait for liquid Helium, the testing of the SPARO experiment during Austral Summer 1998-99 was a success. Tests with the instrument in the lab and also with it mounted on the Viper telescope showed that SPARO and its interface to Viper are working well, and the instrument is now in the care of winter-over personnel: Greg Griffin and John Davis. John and Greg will install it on Viper in midwinter, after the Carnegie-Mellon HEMT receiver ("Corona") has had a chance to carry out another round of CMBR anisotropy observations.

  • For details click here.

  • For photos click here.

    • November 18, 1998- Twenty-two crates left Northwestern University's Technological Institute bound for the South Pole Station via New Zealand, after a busy push to complete laboratory systems tests in which such things as instrumental polarization, system noise, and array alignment were tested. The final laboratory shakedown of SPARO was encouraging. The hold times for both 4He and 3He are of the order of 3 days with improvement expected in lower Antarctic ambient temperatures.

    The first SPARO crew consisting of Dr. Giles Novak(P.I.), Tom Renbarger, Jill Hanna, and Prashant Malhotra departed for New Zealand on Sunday, December 6. While there, they will keep busy integrating SPARO to the Viper Telescope, installing the tertiary mirror on Viper, assembling and cooling SPARO, and running total instrumental polarization tests using planetary targets.

    August 11, 1998- Northwestern University. A committee consisting of Robert Pernic, Director of Polar Operations, Stephan Meyer, Director of CARA, and Mark Thoma, Engineer at Yerkes observatory, decided to give the green light to the SPARO experiment for deployment at the South Pole this Austral summer to prepare it for winter 1999. A status report was given for the instrument, data from the successful May CSO run were presented, and a plan was presented for the completion of the instrument.
    Those present at the meeting were: Giles Novak, Tom Renbarger, David Chuss, Jim Jaeger, Robert Loewenstein, Jesse Wirth, Jill Hanna, Prashant Malhotra, and Erik Limpaecher.
    CSO data

    May 12, 1998- With the "T" array electronics having been installed, SPARO performed remarkably well during its first light tests from May 1 through May 6 at the Caltech Submillimeter Observatory(CSO) located at Mauna Kea in Hawaii. Though there was high water vapor content in the atmosphere for the duration of the run, the polarimeter and its accompanying data system responded favorably in a gamut of tests which helped to characterize the infant instrument. This CSO "shakedown" helped to prepare SPARO for its eventual deployment at the South Pole. Februray, 26, 1998 - Jim Jaeger (NU shop) is nearly finished with the SPARO optics on the "optics mock-coldplate". From left to right: snout with field lens, half-wave plate cell, mirror holder, filter and pupil lens and holders, grids and one grid holder. Mounted at the top of the "optics shelves" is the blank-off wheel. February 5, 1998 - We've cycled 3He in both systems! The final temperatures of the inner and outer stages are 0.29K and 0.40K respectively. The inner stage can remain cold for 46 hours, the outer stage for 18.4 hours. We've also successfully cycled the dewar while it was tilted - demonstrating that it can be cycled on the telescope and that convection is not a heat load in the 3He systems. We also measured a critical cooling capacity of 2 mW for the pumped pot.

    December 19, 1997 - The bolometers are installed in the arrays (by J. Wirth). The tensioned wires from the arrays to the work surface are also done (by B. Hirsch). December 11, 1997 - We cooled the dewar down this past week. We were unable to condense much 3He. The pumped pot didn't have enough cooling power. The rest of the 3He system performed very well. The 4He system behaved about as expected - the cryogen use rate implied a hold time of 4.5 days even though we dumped a lot of heat into the system running tests on the 3He system. November 25, 1997 - We packed the dewar up and moved it from Yerkes to Northwestern. Time to start doing serious lab testing! November 19, 1997 - The He3 system is complete and leak checked! He3 gas has been loaded into the system (20psi in each system for initial tests). August 10, 1997 - We successfully mounted the dewar on the viper telescope at Carnegie Mellon! July 21, 1997 - July 16, 1997 - June 10, 1997 - May 28, 1997 -

    April 9, 1997 - Darren has made up our bolometers. Thanks Darren!

    April 8, 1997 - Cryogenic performance data for the last cool down.

    March 26, 1997 -The dewar is still cold! Further monitoring of the LHe usage suggests a He4 hold time of just over 7 days. The inner vapor cooled shield is at 100K, the outer vapor cooled shield is at 200K.

    March 24, 1997 - I cooled the dewar again last week. There were 10 layers of MLI on every surface. The only wiring was for 5 temperature sensors. Preliminary monitoring implies a He4 hold time of 8 days (approximately 2.5 liters per day).

    March 3, 1997 - I finally assembled the dewar and cooled it down last week. The dewar only had 5 temperature sensors in it. I didn't even install MLI. The inner vapor cooled shield ran at 125K, the outer vapor cooled shield at 225K. The He4 hold time was approximately 3 days (which corresponds to around 6 liters per day).

    I will model the cryogenic performance this week (watch this space for results). Preliminary modeling implies that the vapor cooling was very efficient, even with the large neck tube. Also the bare metal emissivities were better than anticipated.

    February 15, 1997 - This past week we have manufactured a new fill tube and a new optics shield. The remaining suspension parts have been designed and fabricated. We have begun wiring thermometers.

    Next week we hope to assemble the cryostat and cool to He4. The helium is due to arrive monday!

    February 8, 1997 - All of the He4 cryostat parts have been delivered. In the past two weeks all parts have been fit checked. Several pieces (notably ends of the vapor cooled shields) have been altered for improved fit. We have already redesigned the fill tube eliminating the rotatable bolt circle on the indium seal to the reservoir.

    Next week we hope to assemble the cryostat and install the suspension system.

    October 9, 1996 - Our dewar reservoir was delivered!

    September 16, 1996 - Our dewar case was delivered!