Cloud-scale Gas Properties, Depletion Times, and Star Formation Efficiency per Freefall Time in PHANGS-ALMA
(2025) In Astrophysical Journal 985(1).- Abstract
We compare measurements of star formation efficiency to cloud-scale gas properties across the PHANGS- ALMA sample. Dividing 67 galaxies into 1.5 kpc scale regions, we calculate the molecular gas depletion time τ dep mol = Σ mol / Σ SFR and the star formation efficiency per freefall time ϵ ff mol = τ ff / τ dep mol for each region. Then we test how τ dep mol and ϵ ff mol vary as functions of the regional mass-weighted mean molecular gas properties on cloud scales (60-150 pc): gas surface density, 〈 Σ mol cloud 〉 , velocity dispersion, 〈 σ mol cloud 〉 , virial parameter, 〈 α vir cloud 〉 , and gravitational freefall time, 〈 τ ff cloud 〉 . 〈 τ ff cloud 〉 and τ dep mol correlate positively, consistent with the expectation that gas density... (More)
We compare measurements of star formation efficiency to cloud-scale gas properties across the PHANGS- ALMA sample. Dividing 67 galaxies into 1.5 kpc scale regions, we calculate the molecular gas depletion time τ dep mol = Σ mol / Σ SFR and the star formation efficiency per freefall time ϵ ff mol = τ ff / τ dep mol for each region. Then we test how τ dep mol and ϵ ff mol vary as functions of the regional mass-weighted mean molecular gas properties on cloud scales (60-150 pc): gas surface density, 〈 Σ mol cloud 〉 , velocity dispersion, 〈 σ mol cloud 〉 , virial parameter, 〈 α vir cloud 〉 , and gravitational freefall time, 〈 τ ff cloud 〉 . 〈 τ ff cloud 〉 and τ dep mol correlate positively, consistent with the expectation that gas density plays a key role in setting the rate of star formation. Our fiducial measurements suggest τ dep mol ∝ 〈 τ ff cloud 〉 0.5 and ϵ ff mol ≈ 0.34 % , though the exact numbers depend on the adopted fitting methods. We also observe anticorrelations between τ dep mol and 〈 Σ mol cloud 〉 and between τ dep mol and 〈 σ mol cloud 〉 . All three correlations may reflect the same underlying link between density and star formation efficiency combined with systematic variations in the degree to which self-gravity binds molecular gas in galaxies. We highlight the τ dep mol - 〈 σ mol cloud 〉 relation because of the lower degree of correlation between the axes. Contrary to theoretical expectations, we observe an anticorrelation between τ dep mol and 〈 α vir cloud 〉 and no significant correlation between ϵ ff mol and 〈 α vir cloud 〉 . Our results depend sensitively on the adopted CO-to-H2 conversion factor, with corrections for excitation and emissivity effects in inner galaxies playing an important role. We emphasize that our simple methodology and clean selection allow for easy comparison to numerical simulations and highlight this as a logical next direction.
(Less)
- author
- organization
- publishing date
- 2025-05-20
- type
- Contribution to journal
- publication status
- published
- subject
- in
- Astrophysical Journal
- volume
- 985
- issue
- 1
- article number
- 14
- publisher
- American Astronomical Society
- external identifiers
-
- scopus:105004997073
- ISSN
- 0004-637X
- DOI
- 10.3847/1538-4357/adbcab
- language
- English
- LU publication?
- yes
- additional info
- Publisher Copyright: © 2025. The Author(s). Published by the American Astronomical Society.
- id
- 3cf68f01-356b-41bc-9dad-1f68d180d00c
- date added to LUP
- 2025-08-05 14:17:31
- date last changed
- 2025-08-07 08:55:41
@article{3cf68f01-356b-41bc-9dad-1f68d180d00c,
abstract = {{<p>We compare measurements of star formation efficiency to cloud-scale gas properties across the PHANGS- ALMA sample. Dividing 67 galaxies into 1.5 kpc scale regions, we calculate the molecular gas depletion time τ dep mol = Σ mol / Σ SFR and the star formation efficiency per freefall time ϵ ff mol = τ ff / τ dep mol for each region. Then we test how τ dep mol and ϵ ff mol vary as functions of the regional mass-weighted mean molecular gas properties on cloud scales (60-150 pc): gas surface density, 〈 Σ mol cloud 〉 , velocity dispersion, 〈 σ mol cloud 〉 , virial parameter, 〈 α vir cloud 〉 , and gravitational freefall time, 〈 τ ff cloud 〉 . 〈 τ ff cloud 〉 and τ dep mol correlate positively, consistent with the expectation that gas density plays a key role in setting the rate of star formation. Our fiducial measurements suggest τ dep mol ∝ 〈 τ ff cloud 〉 0.5 and ϵ ff mol ≈ 0.34 % , though the exact numbers depend on the adopted fitting methods. We also observe anticorrelations between τ dep mol and 〈 Σ mol cloud 〉 and between τ dep mol and 〈 σ mol cloud 〉 . All three correlations may reflect the same underlying link between density and star formation efficiency combined with systematic variations in the degree to which self-gravity binds molecular gas in galaxies. We highlight the τ dep mol - 〈 σ mol cloud 〉 relation because of the lower degree of correlation between the axes. Contrary to theoretical expectations, we observe an anticorrelation between τ dep mol and 〈 α vir cloud 〉 and no significant correlation between ϵ ff mol and 〈 α vir cloud 〉 . Our results depend sensitively on the adopted CO-to-H<sub>2</sub> conversion factor, with corrections for excitation and emissivity effects in inner galaxies playing an important role. We emphasize that our simple methodology and clean selection allow for easy comparison to numerical simulations and highlight this as a logical next direction.</p>}},
author = {{Leroy, Adam K. and Sun, Jiayi and Meidt, Sharon and Agertz, Oscar and Chiang, I. Da and Gensior, Jindra and Glover, Simon C.O. and Gnedin, Oleg Y. and Hughes, Annie and Schinnerer, Eva and Barnes, Ashley T. and Bigiel, Frank and Bolatto, Alberto D. and Colombo, Dario and den Brok, Jakob and Chevance, Mélanie and Chown, Ryan and Eibensteiner, Cosima and Gleis, Damian R. and Grasha, Kathryn and Henshaw, Jonathan D. and Klessen, Ralf S. and Koch, Eric W. and Oakes, Elias K. and Pan, Hsi An and Querejeta, Miguel and Rosolowsky, Erik and Saito, Toshiki and Sandstrom, Karin and Sarbadhicary, Sumit K. and Teng, Yu Hsuan and Usero, Antonio and Utomo, Dyas and Williams, Thomas G.}},
issn = {{0004-637X}},
language = {{eng}},
month = {{05}},
number = {{1}},
publisher = {{American Astronomical Society}},
series = {{Astrophysical Journal}},
title = {{Cloud-scale Gas Properties, Depletion Times, and Star Formation Efficiency per Freefall Time in PHANGS-ALMA}},
url = {{http://dx.doi.org/10.3847/1538-4357/adbcab}},
doi = {{10.3847/1538-4357/adbcab}},
volume = {{985}},
year = {{2025}},
}