How Low Can Q Go?
Gravitational instability plays a substantial role in the evolution of galaxies. Various schemes to include it in galaxy evolution models exist, generally assuming that the Toomre Q parameter is self-regulated to Q _crit , the critical Q dividing stable from unstable conditions in a linear stability...
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| Language: | English |
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IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/adbf91 |
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| author | John C. Forbes |
| author_facet | John C. Forbes |
| author_sort | John C. Forbes |
| collection | DOAJ |
| description | Gravitational instability plays a substantial role in the evolution of galaxies. Various schemes to include it in galaxy evolution models exist, generally assuming that the Toomre Q parameter is self-regulated to Q _crit , the critical Q dividing stable from unstable conditions in a linear stability analysis. This assumption is in tension with observational estimates of Q that find values far below any plausible value of Q _crit . While the observations are subject to some uncertainty, this tension can more easily be relieved on the theoretical side by relaxing the common assumption that Q ≥ Q _crit . Based on observations of both z ∼ 2 disks and local face-on galaxies, we estimate the effect of gravitational instability necessary to balance out every other physical process that affects Q . In particular, we find that the disk’s response to low Q values can be described by simple functions that depend only on Q . These response functions allow galaxies to maintain Q values below Q _crit in equilibrium over a wide range of parameters. Extremely low values of Q are predicted when the gas surface density is ≳10 ^3 M _⊙ pc ^−2 , the rotation curve provides minimal shear, the orbital time becomes long, and/or when the gas is much more unstable than the stellar component. We suggest that these response functions should be used in place of the Q ≥ Q _crit ansatz. |
| format | Article |
| id | doaj-art-2bd67a14e05c4c2f951a4e72f7764cc9 |
| institution | OA Journals |
| issn | 1538-4357 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal |
| spelling | doaj-art-2bd67a14e05c4c2f951a4e72f7764cc92025-08-20T02:16:07ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-01983211310.3847/1538-4357/adbf91How Low Can Q Go?John C. Forbes0https://orcid.org/0000-0002-1975-4449School of Physical and Chemical Sciences–Te Kura Matū, University of Canterbury , Christchurch 8140, New Zealand ; john.forbes@canterbury.ac.nz; Center for Computational Astrophysics, Flatiron Institute , 162 5th Avenue, New York, NY 10010, USAGravitational instability plays a substantial role in the evolution of galaxies. Various schemes to include it in galaxy evolution models exist, generally assuming that the Toomre Q parameter is self-regulated to Q _crit , the critical Q dividing stable from unstable conditions in a linear stability analysis. This assumption is in tension with observational estimates of Q that find values far below any plausible value of Q _crit . While the observations are subject to some uncertainty, this tension can more easily be relieved on the theoretical side by relaxing the common assumption that Q ≥ Q _crit . Based on observations of both z ∼ 2 disks and local face-on galaxies, we estimate the effect of gravitational instability necessary to balance out every other physical process that affects Q . In particular, we find that the disk’s response to low Q values can be described by simple functions that depend only on Q . These response functions allow galaxies to maintain Q values below Q _crit in equilibrium over a wide range of parameters. Extremely low values of Q are predicted when the gas surface density is ≳10 ^3 M _⊙ pc ^−2 , the rotation curve provides minimal shear, the orbital time becomes long, and/or when the gas is much more unstable than the stellar component. We suggest that these response functions should be used in place of the Q ≥ Q _crit ansatz.https://doi.org/10.3847/1538-4357/adbf91Disk galaxiesGalaxiesGalaxy disksGravitational instability |
| spellingShingle | John C. Forbes How Low Can Q Go? The Astrophysical Journal Disk galaxies Galaxies Galaxy disks Gravitational instability |
| title | How Low Can Q Go? |
| title_full | How Low Can Q Go? |
| title_fullStr | How Low Can Q Go? |
| title_full_unstemmed | How Low Can Q Go? |
| title_short | How Low Can Q Go? |
| title_sort | how low can q go |
| topic | Disk galaxies Galaxies Galaxy disks Gravitational instability |
| url | https://doi.org/10.3847/1538-4357/adbf91 |
| work_keys_str_mv | AT johncforbes howlowcanqgo |