[CI] as a tracer of cold molecular gas in local spiral galaxies



In order to understand how galaxies build up mass through cosmic time and become the objects we see around us in the present-day Universe, we need to accurately probe their cold gas content which reveals their capacity for making stars. As molecular hydrogen itself has no dipole moment, the majority of bulk molecular gas studies have depended on the \(^{12}\)CO lines to obtain molecular masses through the now (in)famous “X-factor” (mapping to H\(_{2}\) column density) or \(\alpha_{\mathrm{CO}}\) (mapping to H\(_{2}\) mass).



The Herschel SPIRE FTS data were obtained over xxx-xxx and reduced as explained in Pellegrini et al., in prep. As a summary, we applied the calibration pipeline up to a certain point. However, corrections in between those of an extended and point source were required for all but three galaxies (NGC 1266, NGC 1377 and NGC 3351, hereafter ‘point-source galaxies’). We made such corrections based upon the spatial distribution of the FIR emission in the SPIRE 250 micron maps, assuming the dust and gas will to follow each other, to first order.

We note that this correction is particularly important when comparing ground-based CO measurements to those obtained from the FTS. Because the correction is necessarily imperfect (CO and 250 micron dust emisison will not be entirely linearly correlated), this introduces a significant systematic error which is releavant whenever ground-base data are direclty compared with those obtained from the FTS such as in CO spectral line energy diagrams, or SLEDs.

[CI] maps and morphologies

For the resolved sample of 18 galaxies (excluding the point-source galaxies and the enuc region of M101), we are able to map the [CI] emission in 16 of them, shown in Fig. \ref{fig:CImaps}. NGC 2976 and NGC 3077 have no detected [CI] emission. The [CI]370 line is more robustly detected than the [CI]609 line in all galaxies. (Why? Actually brighter? Better noise properties in this region of the spectrum?)

The majority of galaxies have centrally-peaked morphologies in [CI]370 (and similarly in [CI]609, when detected). However, NGC3521, NGC 4631 and NGC 7331 have a linear structure due to their edge-on disks while NGC 3627’s linear shape can be attributed to its extremely prominent bar.

The ratio maps are all on the same logarithmic scale so color variations between galaxies may be compared. While the full range of [CI] ratios is only x dex, galaxy to galaxy variations are apparent. For instance, NGC 3521 and NGC 7331 have much lower ratios than the rest of the galaxies. NGC 6946 shows some evidence of a declining radial gradient in the [CI] ratio, while NGC 4631 may show the reverse, with higher values at a larger disk radius.