Harvard University

Astronomy Department

Graduate Student Research Forum (9/17/97)

Alyssa A. Goodman

"Is a Map of the Milky Way's Magnetic Field Worth $66,000,000?"

1. What can magnetic fields do, in the (mostly) neutral ISM?

• "tie" ions (and neutrals by collisions) to B
• 
• 
• k~0.3 to 0.5 and, the magnitude of this ratio is very large: ~10⁷ for n~100 cm^-3 and ~10² for n~10⁹ cm^-3

Large ratio implies gas motions (relatively) simply related to field structure.

2. Energetically:

• Zeeman observations of field strength show M=K(=G) (in unbound regions, M=K)

3. So, what do we expect structure to look like?

• spiral arms--poloidal component--supershells--filamentary clouds--outflows/disks--H II regions

?

4. Progress on mapping the field (structure) so far:

• Our Galaxy: mostly optical polarimetry of background starlight (e.g. Mathewson & Ford--point out trouble at large distances) (+ some Pulsar RM/DM in ionized ISM)
• External galaxies: mostly polarization of (radio) synchrotron emission & some optical polarimetry

5. The "trouble" with the dense ISM: (see forum 3 yrs ago + Héctor)

6. How do we do better??

• Use the thermal emission from aligned grains, rather than the absorption.

Advantages: can identify target regions along l.o.s. more accurately (origin of emission better understood) Disadvantages: peak wavelength radiation not visible from the ground! (see handout)

Past/future themal emission polarimetry:

Balloons: Cudlip et al. 1982

KAO: Hildebrand et al. 1984-1996

• sensitivity only enough for very bright regions (e.g. H II region environs & Galactic Center)
• showed much more ordered fields than background starlight polarimetry in distant dense clouds (e.g. M17)

Ground-based sub-mm/mm: CSO, JCMT, and now OVRO have been used to measure polarization at 350, 450, & 1300 microns--shows ordered fields in small, dense, cold regions OVRO 5" resolution shows field parallel to outflow in NGC 1333, and not to large-scale field

ISO: Goodman et al. -- 22 hrs. allocated

• low probability of any data, given ISOPHOT troubles--would have mapped star-forming dark clouds, with sensitivity ~100x better than KAO

SOFIA: should have polarimetric capability 30x better than KAO, but not nearly as good as cooled (space-based) aperture--which is 10,000 times more sensitive than SOFIA (note: two proposed SOFIA instruments just turned down!--need to wait 3 yrs.)

SIRTF: no polarimetric capability planned!!

7. The M4 Proposal

• Clemens is P.I., AG was chair of "Science Advisory Group," which became the large collaboration: see handout
• Dedicated 100-micron polarimetry satelllite
  • 20-cm aperture telescope, superfluid liquid He cooled
  • no moving parts: polarimetry done w/wire grid & satellite rolls
  • two 32 x 32 Ge:Ga arrays, central l=95 mm
  • polarimetric sensitivity from S/N~150 to 600:1 gives position angle errors <10 deg (usually much less)
  • sky background-limited in most directions
  • 0.8 to 2 arcmin resolution
  • 4 to 6 month lifetime--will map >1000 sq. deg.

8. What will M4 do?

1. Inner Milky Way Survey (distances needed for 3-D picture will come from separate spectral-line obsv'ns.)
2. Magnetic Field in a Star-Forming Complex (survey of Ophiuchus, will show patterns, uniform/non-uniform, outflow orientation w.r.t. B)
3. Map of Cirrus/Supershell Region (North Polar Spur) (shows field behavior in unbound gas--geometry)

9. How did M4 come to be this mission?

Cost restrictions cause:

• restrictions on launch vehicle>
• restrictions on weight/volume>
• small-ish dewar>
• instrument must fit inside dewar>
• small telescope (but at at least you get a large field of view... good for survey!)

Science causes:

• wavelength choice (this is near peak for much dust)

10. Will M4 be funded?

• ask me mid-October