The Multi-Phase ISM in Perseus

Steven Gibson (NAIC)

[No Frames Version]

(This article also appeared in the June 2007 NAIC Newsletter.)

The interstellar medium (ISM) is a turbulent matrix of neutral and ionized gas particles, dust grains, and magnetic fields. To understand the energetics governing the ISM and the role they play in star formation and galaxy evolution, it is critical to image a variety of ISM constituents at high resolution and sensitivity over large areas, using radio wavelengths to avoid dust extinction. The G-ALFA surveys are well placed to address this goal. The Radio Recombination Line group (GALFA-RRL; PI: Y. Terzian) has a major survey of the Galactic plane in the final planning stages. The GALFA Continuum Transit Survey group (GALFACTS; PI: A. R. Taylor) has conducted several test observations and will begin an all-sky full-Stokes continuum survey later this year with the new P-ALFA spectrometer. Finally, the GALFA-H I group has already completed a number of smaller projects and has been running the high-latitude ``Turn On GALFA Survey'' (TOGS; PI: M. Putman) commensally with the ALFALFA and AGES E-ALFA projects since September 2005. Two other large H I surveys will begin soon: an Inner Galaxy low-latitude H I survey awaits the end of telescope painting (I-GALFA; Co-PIs: B.-C. Koo & S. Gibson), and a shallow all-sky H I survey will run commensally with GALFACTS (TOGS2; Co-PIs: C. Heiles & S. Stanimirovic).

Many operational issues for commensal observing of H I and polarized continuum were resolved in the fall of 2006 with an ALFA H I mapping survey of the Perseus Molecular Cloud region (A2174 PI: L. Knee) and a GALFACTS continuum survey observing the same pointings via the WAPP spectrometer (A2294 Co-PIs: A. R. Taylor & C. Salter). The Arecibo maps below are the result of work by Lewis Knee, James Di Francesco, Kevin Douglas, Josh Goldston, and Steven Gibson on the H I side, and Chris Salter, Tapasi Ghosh, Russ Taylor, and Jeff Dever on the continuum side, together with the assistance of Mikael Lerner.

Sample Perseus images of GALFA H I 21cm-line and full-Stokes continuum emission are shown below along with maps of molecular (CO) gas, thermal dust, and ionized gas optical emission for comparison. Together, these observations trace most of the key ISM constituents in this part of the sky. The unparalleled sensitivity of ALFA with the 305-m dish allows much finer H I velocity sampling than comparable synthesis array surveys, which is invaluable for studying the coldest H I features. It also enables full-Stokes spectropolarimetry of ``continuum'' radiation to extract Faraday rotation measures.

Perseus Molecular Cloud Region. Click on the image to obtain a larger version. LEFT PANELS: CfA 12CO J=1-0 line (Dame et al. 2001), GALFA H I 21cm line (A2174), and IRAS 100 micron dust emission; RIGHT PANELS: GALFACTS Stokes I and U continuum (A2294) and VTSS + WHAM H-alpha (Finkbeiner 2003). Only one velocity channel is shown in H I and 12CO. The GALFACTS maps are preliminary and not fully processed. The edge of the higher-resolution VTSS H-alpha coverage in marked in red; H-alpha structure larger than ~ 1o has been removed to highlight narrow filaments. The H I image was produced by Kevin Douglas. The GALFACTS images were produced by Jeff Dever.
The GALFA H I emission appears to envelop the molecular cloud gas traced by CO. The H I shows remarkable fine-scale structure in both space and velocity, including dark pockets of cold H I in several self-absorption features (e.g., near 3h55m  +33.7o). A variety of long, narrow-line emission filaments are also produced by cold gas. Only one velocity channel of H I and CO is shown; the H I data cover +/- 750 km/s with 0.184 km/s per channel to ensure excellent sampling of cold spectral lines. The 3.3' H I beam is very similar to that of IRAS 100 micron emission maps. In many respects, the GALFA-H I survey adds a much-needed velocity dimension to IRAS dust images. At the ~ 250 parsec distance of the Perseus Molecular Cloud, 1o on the map subtends ~ 3.5 pc.

The Stokes I continuum data show a complementary view of the ionized gas, including clear detections of the California Nebula (NGC 1499; 4h  +36o) and IC 348 (3h40m  +32o) H II regions. Most background point sources are active galactic nuclei. At a different velocity than that of the Perseus Molecular Cloud, H I gas appears north of the California Nebula but ends abruptly at an ionization front near 4h  +37o, which also coincides with a bright ridge of IRAS dust emission. IC 348 is probably embedded in the Perseus Molecular Cloud, but the California Nebula lies approximately twice as far away.

The Stokes U polarized continuum data trace the magneto-ionic medium, in which variations in electron density and line-of-sight magnetic field strength cause Faraday rotation of the diffuse polarized Galactic synchrotron background. High electron column densities cause depolarization from Faraday wrapping confusion; this can be seen toward the California Nebula and perhaps IC 348. Elsewhere, combined analysis of the polarized intensity and electron column can reveal magnetic field structures not otherwise visible in the ISM. The radio continuum maps are of limited sensitivity, because they are taken from only one WAPP channel and are not fully processed. However, the reality of the Stokes U Faraday structures is clearly demonstrated by a number of coincident ionized gas filaments traced by optical H-alpha emission.