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Ionized carbon in absorption and emission to better trace the interstellar diffuse gas

26 October 2014

Analyzing observations obtained with the Herschel satellite, an international team led by a researcher at Paris Observatory has obtained new information on the properties of the interstellar matter. The researchers were particularly interested in ionized carbon C+, from both the diffuse gas and also regions of star formation in the background, which can be separated due to the high spectral resolution of the HIFI instrument. On the contrary, in the case of lower spectral resolution, the absorption of diffuse gas on the line of sight completely cancels the emission of the background source. This may explain in part the weakness of the C+ emission observed in ultra-luminous galaxies.

Ionized carbon C + plays a key role in the energy balance of the interstellar gas. Astronomers use a special transition to detect it. Located in the far infrared, at a wavelength of 158 microns, this transition is blocked by the Earth’s atmosphere and thus requires a space telescope to be detected. Observations of this transition made with the high spectral resolution HIFI instrument, on board the Herschel satellite, were used to characterize the pressure of the interstellar medium.

Spectra obtained with the Herschel satellite. Scientists have taken advantage of the observation mode using both the spectra obtained in the direction of regions of massive star formation as W31C (upper Panel) and the emission spectrum obtained in a very similar direction (bottom Panel, "OFF position”). These spectra show both the emission from W31C region (gray zone), and in the velocity range between 10 and 50 km/s, the signal from the interstellar medium located in the foreground (turquoise blue area). The spectra of ionized carbon are drawn in black, those of neutral carbon in red and blue, each color corresponding to a different transition of the atom.

These measurements confirm that the interstellar medium is very porous because the cold gas probed by measurements of satellite Herschel occupies only a small fraction of the length of each line of sight, 2.5% on average. The remainder of the volume is occupied by hot phases, where the gas may be neutral or ionized, and its density very low. The comparison of observations obtained with Herschel with measures of the amount of hydrogen derived from other instruments allowed to derive the relative abundance of carbon in the gas. This is a very important quantity because of the role of the C+ ion in the thermal balance, and because carbon is a major constituent of interstellar dust grains.
In the region probed by the measurements, the determination of the abundance of carbon is in excellent agreement with the values deduced from measurements in the far ultraviolet, and remains almost constant.

Determining the carbon abundance. This figure summarizes the determinations of the abundance of carbon along the entire lines of sight, depending on distance from the center of our Galaxy.

The high spectral resolution allows to identify clearly the phenomenon of absorption of the emission of a background source as W31C by the diffuse interstellar matter located along the line of sight. This phenomenon will recur in all cases where the viewing geometry is similar: a bright source in the background and a dilute environment in the foreground. It is possible that the deficit of C+ emission observed in some very bright far-infrared galaxies is caused in part by this phenomenon.

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