C+ as an Astronomical Tool

Motivation:

The 158µm [CII] fine structure line of ionized carbon, C+, is a tracer for many different conditions in the interstellar medium. It is a major coolant of many phases of the neutral and ionized ISM, and the  observed emission can be attributed to photon-dominated regions (PDRs), dense HII regions, diffuse interstellar clouds, and CO-dark molecular gas (which forms the fractal boundaries of molecular clouds). In these interstellar regions it can be used to measure the gas heating efficiency and temperature, the electron density, i.e. the ionization degree, and the total gas density, and, with ancillary data, may be used to probe such cloud microphysics as the dust grain size distribution and the abundance of PAHs.

Recent observations of the [CII] emission by Herschel and SOFIA reveal an extremely complex spatial and velocity distribution within Galactic sources. When comparing the [CII] line profiles with other typical tracers for PDRs, HII regions, diffuse clouds or molecular gas, we find a much more complex structure in [CII].  On the theoretical side, new models provide a detailed quantitative prediction of the [CII] emission from the different types of interstellar gas but a unified approach is missing so far. The task at hand for observers and modelers is to disentangle the contributions from various phases of the ISM so that CII emission can be used as an effective probe of interstellar gas in galaxies near and far.

Goal:

The goal of the workshop is to review, interpret and understand the use of the brightest sub-mm spectral line emission - that of C+ - as tracer of gas density and star-formation rate in galaxies. This workshop is intended to bring together experts on [CII] observations covering a wide range of astrophysical sources and modelers computing the [CII] emission from PDRs, HII regions, diffuse clouds, and the CO-dark molecular gas.

To answer the global question:

What is the relative contribution of  PDRs, HII regions, diffuse clouds, shocks, and CO-dark molecular gas to the global [CII] emission of a galaxy and how can we retrieve them individually from [CII] observations to trace the star and structure formation in galaxies?

we need to discuss the following topics:

Does [CII] work as a measure of the star-formation rate?

◦     Does the spatial distribution of the [CII] emission reflect the local strength of the UV field?

How large is the contribution from shocked gas to the observed [CII] emission?

◦     What do the observed velocity profile differences between [CII] and other tracers imply and how can they be modelled in a generic way?

◦     Is C+ excited through dissipation of shocks and turbulence?

How do non-LTE excitation and radiative transfer affect the observable [CII] intensity?

◦     Which collision partners are important for excitation of C+ excitation?

◦     Under which conditions can we use [CII] as a tracer for the photoelectric heating efficiency?

How can we use [CII] observations to constrain the diffuse medium chemistry? Are  the mechanisms behind C+ emission in the early universe the same as today?

◦     What effect does metallicity have on the predicted emission from C+?

Invited review talks:

J. Stutzki: Spectrally resolved [CII] observations in PDRs from HIFI GREAT

M. Wolfire: C+ as a primary coolant and tracer of star formation

N. Abel: C+ modelling in HII regions and PDRs

P. Clark: Simulating C+ in the CO-dark molecular gas

E. Sturm: [CII] from nearby galaxies

P. Goldsmith: The excitation of C+

N. Lehner: The UV absorption view of C+

S. Hailey-Dunsheath: [CII] from high-redshift galaxies

J. Pineda: [CII] from the diffuse medium and as tracer of CO-dark molecular gas