Imaging galactic diffuse gas: bright, turbulent CO surrounding the line of sight to NRAO150 ^*,**,***

¹ Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, 38406 Saint Martin d'Hères, France e-mail: [pety;lucas]@iram.fr
² Obs. de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA e-mail: hliszt@nrao.edu

Received: 18 March 2008
Accepted: 21 July 2008

Abstract

Aims. To understand the environment and extended structure of the host galactic gas whose molecular absorption line chemistry, we previously observed along the microscopic line of sight to the blazar/radiocontinuum source NRAO150 (aka B0355+508).

Methods. We used the IRAM 30 m Telescope and Plateau de Bure Interferometer to make two series of images of the host gas: i) 22.5″ resolution single-dish maps of ¹²CO and 2-1 emission over a 220″ by 220″ field; ii) a hybrid (interferometer+singledish) aperture synthesis mosaic of ¹²CO  emission at 5.8″ resolution over a 90″-diameter region.

Results. At 22.5″ resolution, the CO  emission toward NRAO150 is 30-100% brighter at some velocities than seen previously with 1′  resolution, and there are some modest systematic velocity gradients over the 220″ field. Of the five CO components seen in the absorption spectra, the weakest ones are absent in emission toward NRAO150 but appear more strongly at the edges of the region mapped in emission. The overall spatial variations in the strongly emitting gas have Poisson statistics with rms fluctuations about equal to the mean emission level in the line wings and much of the line cores. The / line ratios calculated pixel-by-pixel cluster around 0.7. At 6″ resolution, disparity between the absorption and emission profiles of the stronger components has been largely ameliorated. The ¹²CO  emission exhibits i) remarkably bright peaks, = 12-13 K, even as from NRAO150; ii) smaller relative levels of spatial fluctuation in the line cores, but a very broad range of possible intensities at every velocity; and iii) striking kinematics whereby the monotonic velocity shifts and supersonically broadened lines in 22.5″ spectra are decomposed into much stronger velocity gradients and abrupt velocity reversals of intense but narrow, probably subsonic, line cores.

Conclusions. CO components that are observed in absorption at a moderate optical depth (0.5) and are undetected in emission at resolution toward NRAO 150 remain undetected at resolution. This implies that they are not a previously-hidden large-scale molecular component revealed in absorption, but they do highlight the robustness of the chemistry into regions where the density and column density are too low to produce much rotational excitation, even in CO. Bright CO lines around NRAO150 most probably reflect the variation of a chemical process, i.e. the C⁺-CO conversion. However, the ultimate cause of the variations of this chemical process in such a limited field of view remains uncertain.