# CO Spectral Line Energy Distributions in Galactic Sources: Empirical Interpretation of Extragalactic Observations

## TL;DR

This paper analyzes CO spectral line energy distributions in Galactic star-forming regions to understand excitation mechanisms and compares them with extragalactic observations, highlighting differences due to beam filling factors and region sizes.

## Contribution

It provides empirical insights into how Galactic CO SLEDs differ from extragalactic ones, emphasizing the impact of observational beam sizes and the spatial extent of gas.

## Key findings

- Galactic star-forming cores' CO SLEDs differ from extragalactic ones.
- Beam filling factors significantly affect observed CO SLEDs.
- Extragalactic CO SLEDs include cooler gas from larger regions.

## Abstract

The relative populations in rotational transitions of CO can be useful for inferring gas conditions and excitation mechanisms at work in the interstellar medium. We present CO emission lines from rotational transitions observed with Herschel/HIFI in the star-forming cores Orion S, Orion KL, Sgr B2(M), and W49N. Integrated line fluxes from these observations are combined with those from Herschel/PACS observations of the same sources to construct CO spectral line energy distributions (SLEDs) from $5\leq J_u\leq48$. These CO SLEDs are compared to those reported in other galaxies, with the intention of empirically determining which mechanisms dominate excitation in such systems. We find that CO SLEDs in Galactic star-forming cores cannot be used to reproduce those observed in other galaxies, although the discrepancies arise primarily as a result of beam filling factors. The much larger regions sampled by the Herschel beams at distances of several Mpc contain significant amounts of cooler gas which dominate the extragalactic CO SLEDs, in contrast to observations of Galactic star-forming regions which are focused specifically on cores containing primarily hot molecular gas.