%0 Generic %1 narayanan2016physical %A Narayanan, Desika %A Krumholz, Mark %D 2016 %K CII emission fir relation sfr %T A physical model for the CII-FIR deficit in luminous galaxies %U http://arxiv.org/abs/1601.05803 %X Observations of ionised carbon at 158 micron (CII) from luminous star-forming galaxies at z~0 show that their ratios of CII to far infrared (FIR) luminosity are systematically lower than those of more modestly star-forming galaxies. In this paper, we provide a theory for the origin of this so called "CII deficit" in galaxies. Our model treats the interstellar medium as a collection of clouds with radially-stratified chemical and thermal properties, which are dictated by the clouds' volume and surface densities, as well as the interstellar radiation and cosmic ray fields to which they are exposed. CII emission arises from the outer, HI dominated layers of clouds, and from regions where the hydrogen is H2 but the carbon is predominantly C+. In contrast, the most shielded regions of clouds are dominated by CO and produce little CII emission. This provides a natural mechanism to explain the observed CII-star formation relation: galaxies' star formation rates are largely driven by the surface densities of their clouds. As this rises, so does the fraction of gas in the CO-dominated phase that produces little CII emission. Our model further suggests that the apparent offset in the CII-FIR relation for high-z sources compared to those at present epoch may arise from systematically larger gas masses at early times: a galaxy with a large gas mass can sustain a high star formation rate even with relatively modest surface density, allowing copious CII emission to coexist with rapid star formation.

@misc{narayanan2016physical, abstract = {Observations of ionised carbon at 158 micron ([CII]) from luminous star-forming galaxies at z~0 show that their ratios of [CII] to far infrared (FIR) luminosity are systematically lower than those of more modestly star-forming galaxies. In this paper, we provide a theory for the origin of this so called "[CII] deficit" in galaxies. Our model treats the interstellar medium as a collection of clouds with radially-stratified chemical and thermal properties, which are dictated by the clouds' volume and surface densities, as well as the interstellar radiation and cosmic ray fields to which they are exposed. [CII] emission arises from the outer, HI dominated layers of clouds, and from regions where the hydrogen is H2 but the carbon is predominantly C+. In contrast, the most shielded regions of clouds are dominated by CO and produce little [CII] emission. This provides a natural mechanism to explain the observed [CII]-star formation relation: galaxies' star formation rates are largely driven by the surface densities of their clouds. As this rises, so does the fraction of gas in the CO-dominated phase that produces little [CII] emission. Our model further suggests that the apparent offset in the [CII]-FIR relation for high-z sources compared to those at present epoch may arise from systematically larger gas masses at early times: a galaxy with a large gas mass can sustain a high star formation rate even with relatively modest surface density, allowing copious [CII] emission to coexist with rapid star formation.}, added-at = {2016-01-25T10:28:55.000+0100}, author = {Narayanan, Desika and Krumholz, Mark}, biburl = {https://www.bibsonomy.org/bibtex/2bc02e262f6efa6c988557d1eb16b3645/miki}, description = {[1601.05803] A physical model for the [CII]-FIR deficit in luminous galaxies}, interhash = {772fdc9ace537ed900062ef159b0ff00}, intrahash = {bc02e262f6efa6c988557d1eb16b3645}, keywords = {CII emission fir relation sfr}, note = {cite arxiv:1601.05803Comment: 16 pages, 9 figures; Submitted to MNRAS, comments welcome}, timestamp = {2016-01-25T10:28:55.000+0100}, title = {A physical model for the [CII]-FIR deficit in luminous galaxies}, url = {http://arxiv.org/abs/1601.05803}, year = 2016 }