Image Credit: Christine Wilson

Detection of Extragalactic Magnetic Fields

Using radio emission from the 1667 MHz rotational transition of extragalactic hydroxyl, we have made the first direct detection of magnetic fields in a galaxy other than our own Milky Way by means of Zeeman splitting. We've found milligauss-strength fields in 4 starburst galaxies including Arp 220 (left).

The radio emission that we detect comes from clouds of gas deep inside these far-away star-forming galaxies. These clouds emit extremely bright laser emission at microwave wavelengths and therefore called masers. Actually, because they're so wicked bright, they're called megamasers.

I'll be conducting a 400-hour Arecibo survey with Carl, Eliot, & Jeremy Darling (Boulder) that will include both of the satellite lines (1612 & 1720 MHz) as well as the doublet (1665 & 1667 MHz). We also have 40 hours of High-Sensitivity Array VLBA time to try to directly detect Zeeman splitting in the megamaser spots in the nucleus of Arp 220 and III Zw 35 with Geoff Bower (Berkeley), Jeremy Darling (Boulder), & Anuj Sarma (DePaul).

Image Credit: Bill Saxton

Helical Magnetic Field in the Orion Molecular Cloud

I've used the GBT to detect magnetic fields that appear to be wrapped around a filament in the Orion molecular cloud (at left). By measuring Zeeman-splitting of the 21-cm neutral hydrogen emission at multiple places above and below the filament, we have shown the field direction is uniformly pointing towards us above the cloud and away from us below. When combined with optical starlight measurements that show the direction of the magnetic field in the plane of the sky in front of the filament, a helical field structure (think about a Slinky®) is suggested.

Image Credit: Tim Robishaw

Polarization Calibration of the Green Bank Telescope

Description coming soon.

Image Credit: Tim Robishaw

GBT Mapping of Filamentary Structure

The Ursa Major loop is a complex of filamentary hydrogen clouds that stretches around the North Celestial Pole (see image at left); some of these clouds have been observed to have molecular components as well. It's a complete mystery what has caused this impressive structure; no supernova or stellar cluster candidates have been detected so this is unlikely a shell. Even more confusing is that a large velocity gradient is measured in the filaments, so what is keeping the structure together? It is our hypothesis that magnetic fields are constraining the filamentary structure of this impressive complex. We therefore went to the GBT and mapped a huge region of over 1000 square degrees in full- Stokes mode. First we can inspect the morphology and kinematics of this data cube in detail. Then we can degrade the resolution of the map to inspect the polarization properties of the region. In the image to the left, each X marks the location of a magnetic field that we have measured via Zeeman splitting of neutral hydrogen using the GBT. It's clear that there are strong magnetic fields (for interstellar gas, that is) in the filamentary structure. The zoomed region shows that the gas filaments appear to wrap around one another in some locations. This is a rich data set so there should be some really interesting results pretty soon.

Image Credit: Tim Robishaw

GBT Mapping of the 11 GHz CCS Transition

Dicarbon sulfide (CCS; also known, apparently, as Thioxoethenylidene) should be one of the best magnetometers for studying the how magnetic fields affect the way in which stars form inside the dense cores of molecular clouds. This molecule should be extra sensitive to magnetic fields because it has a very high Lande g-factor therefore making it susceptible to Zeeman splitting.

In order to have a chance of detecting Zeeman splitting in these clouds, we'd need to first know if the clouds are emitting radio waves brightly at 11 GHz, and then find the place where the emission is brightest. So we went to the GBT to make some maps. But there was a big problem: the GBT doesn't have a receiver that operates at 11 GHz; however, it does have one that works from 8-10 GHz. So we worked with the engineering staff to alter the filters to allow us to probe the 11 GHz transition. Then we mapped a dozen molecular clouds that had previously been shown to emit radio waves at 33 GHz (you could think of this as the molecule spinning around 3 times faster than when it's emitting 11 GHz radio waves). We detected a handful of sources and one of these is shown as a pixelated, low-resolution image to the left.

Image Credit: Tim Robishaw

GBT Mapping of the Galactic Worm W43

Description coming soon.

Image Credit: Dave Jurasevich

Polarization of Anomalous Dust Emission in LDN 1622

The Differential Microwave Radiometer onboard the Cosmic Background Explorer (COBE) spacecraft found microwave emission from dusty regions in space that was far in excess of extrapolations of the thermal emission from dust. Thus, this was named anomalous dust emission. Since the discovery in the mid 1990s, the leading explanation for this emission is that it originates from spinning dust. The theorists predict that the the spinning dust emission should be roughly 1% polarized near 10 GHz. A team led by Doug Finkbeiner found anomalous dust emission from Lynds Dark Nebula (LDN) 1622 (image on left) using the Green Bank 140-foot telescope; this detection was later confirmed by the Wilkinson Microwave Anisotropy Probe (WMAP). We have been trying to carefully measure the polarization of LDN 1622 to a level of half a percent using the GBT. This is work in progress. Stay tuned for results.

Image Credit: Tim Robishaw

The First Dark Galaxy?

We used the Arecibo telescope to make a high-resolution hydrogen map (image on the left) of HVC (high-velocity cloud) 127-41-330. That's the extended cloud with the arms in the center of the image. The red blob is a dwarf galaxy named LGS 3 (which stands for Local Group Satellite 3; it's also known as the Pisces Dwarf) that's orbiting the Andromeda galaxy and is 2.5 million light years away from us. The colors in the image represent how fast the gas is moving towards us. The HVC appears to be rotating, with a total mass that makes it dark matter-dominated for any reasonable distance. This object thus has a dark matter halo, a substantial amount of cool gas, and no stars that we have been able to detect, suggesting that it may be the first dark galaxy. Some other folks claimed they found the first dark galaxy, two years after we did. I guess we didn't make a big enough deal about it; these things happen.

Image Credit: Tim Robishaw

Arecibo and VLA Imaging of HI in the Leo Triplet

The Leo Triplet is a group of nearby strongly interacting galaxies. Josh Simon and Alberto Bolatto made a complete map of the neutral hydrogen emission from this group of galaxies. I reduced the data cube for them and made the nice image on the left. Someday these data might be combined with data that were obtained from the Very Large Array by Fabian Walter. But probably not.

Image Credit: Tim Robishaw

Zeeman Splitting of Carbon Recombination Lines in Photodissociation Regions

Description coming soon.

Image Credit: Art Wolfe

Magnetic Fields in Damped Lyman-alpha Systems

Currently, all high-redshift magnetic field measurements are based on Faraday rotation measurements; the measurement can be affected by the interstellar material bewteen the emitting source and the observer's telescope. However, Zeeman splitting measurements are in situ (which is snooty Latin way of saying what you're measuring is actually determined at the place where the radio waves are being emitted and then there is nothing that can change this measurement as the radio waves travel all the way to the telescope; pretty cool) so any magnetic field measured by this technique could help astronomers understand how strong fields truly are in far-away galaxies and perhaps help them constrain models of how magnetic fields affect the formation of galaxies and stars.

So we went off to Green Bank, and observed absorption lines from the Lyman-alpha forest towards two sources (the quasar 3C286 is shown on the left). We're still reducing the data on these, so stay tuned.