Effects of Dust Grain Size Distribution on the Abundances of CO and H2 in Galaxy Evolution

By Keith Cowing
Status Report
April 28, 2023
Filed under , , , ,
Effects of Dust Grain Size Distribution on the Abundances of CO and H2 in Galaxy Evolution
Evolution of grain size distribution. We present the case for silicate, but a similar evolutionary sequence is also obtained for carbonaceous dust. The grain size distribution is multiplied by 푎 4 and divided by 푛H so that the resulting quantity is proportional to the dust mass in logarithmic bins per gas mass. The colour indicates the age as shown in the colour bar. The slope of the MRN grain size distribution is shown by the thin solid line for reference. — astro-ph.GA

We model the effect of grain size distribution in a galaxy on the evolution of CO and H2 abundances. The formation and dissociation of CO and H2 in typical dense clouds are modelled in a manner consistent with the grain size distribution.

The evolution of grain size distribution is calculated based on our previous model, which treats the galaxy as a one-zone object but includes various dust processing mechanisms in the interstellar medium (ISM). We find that typical dense clouds become fully molecular (H2) when the dust surface area increases by shattering while an increase of dust abundance by dust growth in the ISM is necessary for a significant rise of the CO abundance.

Accordingly, the metallicity dependence of the CO-to-H2 conversion factor, XCO, is predominantly driven by dust growth. We also examine the effect of grain size distribution in the galaxy by changing the dense gas fraction, which controls the balance between coagulation and shattering, clarifying that the difference in the grain size distribution significantly affects XCO even if the dust-to-gas ratio is the same.

The star formation time-scale, which controls the speed of metal enrichment also affects the metallicity at which the CO abundance rapidly increases (or XCO drops). We also propose dust-based formulae for XCO, which need further tests for establishing their usefulness.

Hiroyuki Hirashita

Comments: 14 pages, 7 figures, accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2304.13876 [astro-ph.GA] (or arXiv:2304.13876v1 [astro-ph.GA] for this version)
Submission history
From: Hiroyuki Hirashita
[v1] Wed, 26 Apr 2023 23:49:04 UTC (150 KB)
Astrobiology, Astrochemistry

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻