Key GPIPS Science

Plain language summary: The GPIPS project is aimed at revealing and studying the very weak magnetic field that is present in our Milky Way Galaxy in the diffuse material located between the stars. The magnetic field could be an important agent in regulating how clouds of this diffuse material collect into the dense 'molecular' clouds that can lead to new stars forming. The magnetic field could also be important in the processes within these molecular clouds that lead to formation of 'dense cores' and new stars within these cores. Surprisingly, very little examination of these magnetic fields has taken place to date, mostly because the required observations are difficult, time-consuming, and require specialized instruments and/or telescopes. Viewing this state of affairs as an opportunity, we designed and developed the 'Mimir' instrument to be able to accurately and efficiently obtain magnetic field information by detecting the weak linear polarization signal starlight acquires as it passes through diffuse and dense interstellar clouds containing magnetic fields. GPIPS used Mimir on the 1.83 m (72 inch) Perkins telescope owned by Lowell Observatory in Flagstaff, AZ to map the magnetic field in the cool interstellar medium across a huge swath of the Milky Way. The GPIPS project has taken about eight+ years, using about 50 nights each year on the Perkins telescope, and finished its main observing effort in late 2013. The over one million infrared images obtained by Mimir were calibrated and processed at Boston University and the science-quality data products have returned to the astronomical community as quickly as possible. The number of measures of the magnetic field, embodied in the infrared linear polarization measurements of starlight, increase the available data in this region by a multiplicative factor of 100,000 (i.e., a 10,000,000% increase!). In addition to our group's scientific studies with this marvelous data set, other astronomers will find new and powerful ways to incorporate GPIPS data in their investigations and so multiply the scientific legacy of this effort and its supporting agencies, the NSF (for GPIPS and Mimir), NASA (Mimir), and the W. M. Keck Foundation (Mimir).

Magnetic Fields - Nearby and not-so-nearby

A picture of iron filings being oriented by a bar magnet. The magnet is located beneath the transparent plastic sheet and the iron filings are atop the sheet. The bar magnet has 'North' and 'South' magnetic poles and the lines of magnetic field connecting the poles are traced by the iron filings.

Zoom of a small region in the upper picture, taken from just below middle center of the upper picture. This highlights how the magnetic field lines are both parallel to each other and over larger distances show curvature toward the magnetic poles.

One of the 3,237 sky fields making up GPIPS. This shows the background image of distant stars on an inverted gray-scale, so that dark sky is white and the stars are black. The lines show the several hundred successful linear polarization measurements made by Mimir in this sky field. Each line is a representation of the polarization properties impressed on the starlight by the magnetic field between us and each star. The length of each line represents the degree of polarization of the starlight (compared to the 2% legend in the lower left). The orientation of each line encodes the polarization position angle, as projected onto the plane of the sky. The magnetic field lines run parallel to the polarization lines, so that in this field, the magnetic field seems to be running mostly parallel and mostly along a North-South direction, though with a bit of tilt to the East at the top. The tiny interstellar dust grains that help the magnetic field produce the infrared polarizations that Mimir sees have a role very similar to that of the iron filings in the images above, of helping reveal and delineate the otherwise hidden magnetic field that is present. The dust grains are about a factor of 1,000 times smaller than the iron filings and the magnetic field sensed in this sky field is almost one billion times weaker than that of the magnet in the images above. Nevertheless, operating over huge volumes of space and interacting with gas clouds posessing thousands of times the mass of our Sun, the weak interstellar field may be an important agent in our Milky Way Galaxy.

The Galactic magnetic field has barely been characterized and the importance of the field in most aspects of Galaxy structure, cloud formation, and star formation is largely unknown.

The Galactic Plane Infrared Polarization Survey (GPIPS) is producing a huge increase in the data available for probing the Milky Way’s magnetic field.

The inner Galactic midplane region offers a wealth of correlative data including superb molecular spectroscopic mapping (from the 13CO Galactic Ring Survey, see link below), Spitzer Space Telescope mid-infrared imaging (from the GLIMPSE project, see link below) as well as 2MASS point source catalog and images.

The new magnetic field insight GPIPS will uncover will be used to test models across many size scales to gauge the role of the magnetic field in a wide range of ISM environments in the Galaxy.

Project Summary

The interstellar medium (ISM) of the Galaxy is filled with atomic and molecular clouds arranged into enormous loops, shells, and bubbles and is threaded by magnetic fields. Yet to date, optical and infrared probes of the fields in the Northern inner disk midplane have detected only 6 linear polarizations. GPIPS will expand this by almost five orders of magnitude to about one million stellar polarizations, probing the field to, and beyond, the nearest spiral arm.

GPIPS obtained H-band (1.6 microns wavelength) linear polarimetry for 76 sq degrees in the L=18-56°, B= ± 1° region, to H=12th mag or deeper.

GPIPS data products include cataloged of stellar polarizations, deep photometric catalogd (to H ~ 16th), the deep, coadded images, polarization overlays for the images, and plots (Postscript file form) showing the polarizations on the field images.

(at Right) Example Deep GPIPS image, with polarization stick overlays (red) and Galactic Ring Survey column density contours (blue)

Example deep coadded GPIPS images for two neighboring fields, with GRS column density contour overlays (blue), showing regions of low, moderate, and high extinction -- all traced by GPIPS stars.

GPIPS data products are bing used by our group to answer key questions about magnetic fields as they pertain to large-scale fields, for example in and outside spiral arms, to the medium-scale fields associated with nearby atomic and molecular clouds, and to small-scale fields in regions with active star formation.

All GPIPS data products are available (e.g., are non-proprietary) to the astronomical community and to the public from this dedicated web server. See the Data_Release web page (to download data products in bulk) and/or the Search_Data web page (to perform directed seaches of the data products).

Project Tools and Approach

GPIPS used the 1.8m Perkins telescope located on Anderson Mesa outside Flagstaff, Arizona, operated in a 50:50 partnership between Boston University and Lowell Observatory.

The instrument used was Mimir, a multi-function wide-field imager, spectrometer, and polarimeter. Mimir (see the web link at the bottom of this page) was developed at Boston University and Lowell observatory with support from NSF, NASA, and the W.M. Keck Foundation. It offered a large, 10x10 arcmin field of view onto an ALADDIN III InSb Array of 1024x1024 pixels at a plate scale of 0.58 arcsec per pixel.


(Top) The Mimir instrument, shown prior to departing the Boston University lab. It consisted of the large cryostat and the electronics box at left.

(Right) Mimir on the Perkins telescope

Polarimetry was performed using a cold, rotating compound half-wave plate for the H-band feeding a fixed wire grid polarization analyzer ahead of the detector. Mimir’s polarimetry mode was accurately calibrated to levels below 0.1%.

(Top Left) Wire-grid polarization analyzer, in its filter cell.

(Bottom Left) H-Band Compound Half-Wave Plate, in its geared & rotateable cell.

(Right) Configuration of the four light analysis wheels in Mimir. The left-most wheel holds the rotateable HWPs. The center two wheels hold filters, while the last wheel holds grisms and the wire grid.

Conducting the observing portion of GPIPS required about 25 clear nights (40-50 allocated nights) per year, an amount endorsed by the Boston University Time Allocation Committee for this key project on the Perkins Telescope.

Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE)

Galactic Ring Survey