The First Cloud-by-Cloud Dense gas Map of an External Galaxy – SWAN

Surveying the Whirlpool at Arcseconds with NOEMA (SWAN) is a NOEMA+30m large programm that maps emission from dense gas tracers and CO isotopologues (HCN, HCO+, HNC, N2H+, 13CO, C18O) in the in the iconic nearby grand-design spiral galaxy M51.
The resulting 3” resolution data are combined with data from two other IRAM large programs, namely the cloud-scale 1” CO(1-0) map of M51 by PAWS (Schinnerer et al. 2013) and IRAM 30m dense gas maps by EMPIRE (Bigiel et al. 2016) to bring critical insights for current models of star formation theory.

While highly resolved Milky Way observations link dense gas with immediate sites of star formation, extragalactic works find systematic variations in the dense gas fraction and dense gas star formation efficiency (SFE) on global and kpc-scales. The final cloud-scale dense gas mapping will allow us to test proposed drivers of the dense gas SFE and dense gas fraction, and address how the molecular gas density distribution relates to the local star formation activity, molecular cloud properties and galactic environment of the morphologically richcentral 5 kpc of M51.

Our first results suggest that on our ~100pc resolution observations across the central 5x7kpc of the Whirlpool galaxy, HCN and N2H+ are well but super-linearly correlated!
While HCN is a dense gas tracer commonly used in extragalactic observations, its usage to trace dense gas has been questioned by high-resolution studies in our Milky Way. The molecular ion N2H+ on the other hand is mainly destroyed by the second most common molecule (CO), and does mainly only exist in regions where CO is frozen to dust grains. This makes N2H+ the ideal chemical tracer of cold and dense clumps.
However, being ~100 times fainter than CO and 6 times fainter than HCN, it is very challenging to observe.
Our newly obtained observations for the first time cover emission from both N2H+ and HCN in a large area (5x7kpc) in an extragalactic target at ~100pc resolution.
This allows us to compare these supposed tracer of dense gas which we did in my recent publication (Stuber et al. 2023).
We find them being well correlated with a power in the N2H+-to-HCN-distribution of 1.2, indicating that there is more N2H+ per unit HCN in bright and potentially dense regions.

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The Molecular Gas Morphology of Nearby Galaxies

Resulting bar-type (left panel) and spiral arm (right panel) visual classifications based on the CO distribution in 90 nearby massive main-sequence galaxies. Galaxies without good agreement between the 10 evaluators are marked grey.

Do morphologies look the same in the molecular gas?
To test the predictability of the underlying gravitational potential from gas observations alone, we visually classify the morphology of our PHANGS-ALMA galaxies based solely on their CO distribution by 10 astronomers.

Our classifications agree with 77% of flocculent and grand-design spiral arms from optical/IR-based literature, and with 71% of stongly barred (SB) and unbarred (SA) literature bar classes, which we consider good demonstration of the suitability of the molecular medium as a morphological tracer. Multi-arm spiral arm classes and intermediate bar types show the least good agreement with the literature, which we explain by the clumpy nature of CO and deviating class definitions.
We further find that more curved bar lanes are significantly shorter, have lower molecular to stellar mass ratios than those with straighter geometries and exhibit a weak dependence on bar strengths.

We conclude that CO promises to be a useful tracer for morphological features with advantages in challenging galactic environments.

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Frequency and Nature of Central Molecular Outflows in Nearby Star-Forming Galaxies

frequency of molecular outflows

Nuclear molecular outflows in spiral galaxies are assumed to be able to modulate star formation in their host galaxy by removing gas from the inner region of the disk. Current research suggests that outflows consisting of different gas phases are a common feature of local galaxies, yet, little is known about the frequency and nature of outflows in main-sequence galaxies in the nearby universe.

Therefore, we search for molecular outflows from the central 2kpc in 80 nearby massive (log M* ~ 10) star-forming galaxies in ~1” resolution CO(2-1) data from the PHANGS-ALMA survey.

Using rigorous selection criteria, we reliably identify 16-20 outflow candidates which translates into an outflow frequency of 20-25 ± 2%.

These candidates exhibit a higher AGN (bar) fraction of 50% (90%) compared to the full sample (AGN: 23%, bar: 55%), indicating that these quantities trigger outflow mechanisms.

We further are able to extend previous literature outflow samples to lower stellar masses of log M* = 9.97.

Inferred mass loading factors of unity indicate that these outflows are not efficient in quenching the SFR in our galaxies.

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