WISDOM Project – IX. Giant molecular clouds in the lenticular galaxy NGC 4429: effects of shear and tidal forces on clouds

Abstract

We present high spatial resolution (≈12 pc) Atacama Large Millimeter/sub-millimeter Array 12CO(J = 3 − 2) observations of the nearby lenticular galaxy NGC4429. We identify 217 giant molecular clouds within the 450 pc radius molecular gas disc. The clouds generally have smaller sizes and masses but higher surface densities and observed linewidths than those of Milky Way disc clouds. An unusually steep size – line width relation ($\sigma \propto R_{\rm c}^{0.8}$) and large cloud internal velocity gradients (0.05 – 0.91 km s−1 pc−1) and observed Virial parameters (〈αobs, vir〉 ≈ 4.0) are found, that appear due to internal rotation driven by the background galactic gravitational potential. Removing this rotation, an internal Virial equilibrium appears to be established between the self-gravitational (Usg) and turbulent kinetic (Eturb) energies of each cloud, i.e. $\langle \alpha _{\rm sg,vir}\equiv \frac{2E_{\rm turb}}{\vert U_{\rm sg}\vert }\rangle \approx 1.3$. However, to properly account for both self and external gravity (shear and tidal forces), we formulate a modified Virial theorem and define an effective Virial parameter $\alpha _{\rm eff,vir}\equiv \alpha _{\rm sg,vir}+\frac{E_{\rm ext}}{\vert U_{\rm sg}\vert }$ (and associated effective velocity dispersion). The NGC4429 clouds then appear to be in a critical state in which the self-gravitational energy and the contribution of external gravity to the cloud’s energy budget (Eext) are approximately equal, i.e. $\frac{E_{\rm ext}}{\vert U_{\rm sg}\vert }\approx 1$. As such, 〈αeff, vir〉 ≈ 2.2 and most clouds are not virialised but remain marginally gravitationally bound. We show this is consistent with the clouds having sizes similar to their tidal radii and being generally radially elongated. External gravity is thus as important as self-gravity to regulate the clouds of NGC4429.

Molecular Cloud Populations in the Context of Their Host Galaxy Environments: A Multiwavelength Perspective

“…Adding this term improves the model R 2 by only a small amount (from 0.70 to 0.71; see Figure 6), but it lowers the model BIC by more than 10, which means that our data clearly favor the model with an extra dependence on Σ å . This extra dependence is in line with theoretical models proposing that molecular clouds can be influenced by the external gravitational potential of the host galaxy stellar disk (e.g., Meidt et al 2018Meidt et al , 2020Sun et al 2020b;Liu et al 2021). 7.…”

“…With this definition, a virialized object would have α vir,obj = 1, whereas an object in energy equipartition would have α vir,obj = 2. But we note that the virial parameter estimated in this way might not be a complete description of cloud dynamical states if there are strong magnetic field, surface pressure, or external tidal forces (see discussions in, e.g., Ballesteros-Paredes 2006;Sun et al 2020b;Kim et al 2021b;Liu et al 2021).…”

The cold interstellar medium of galaxies in the Local Universe

“…The PHANGS (Leroy et al 2021) survey has carried out arcsecond CO(2-1) imaging of 70 nearby star-forming galaxies with ALMA, reaching the size of typical GMCs (∼100 pc resolution). These efforts are providing key empirical constraints on the physical link between star formation and gas near the "cloud" scale and the galaxy-scale environment (e.g., Hughes et al 2013a, Sun et al 2018, Schruba et al 2019, Chevance et al 2020, Sun et al 2020, Liu et al 2021. Further progress will require even larger samples, extending the parameter space especially towards low mass galaxies and below the main sequence.…”

WISDOM Project -- X. The morphology of the molecular ISM in galaxy centres and its dependence on galaxy structure

“…Davis et al 2014;Kruijssen et al 2019;Gensior et al 2020), and pull existing molecular clouds apart (see e.g. Liu et al 2021). It should be noted that high shear environments would also be more Toomre-stable, due to the mutual dependence of these quantities on the shape of the rotation curve of the system.…”

“…The first cloud-scale investigations of the ISM of ETGs have revealed some have gas in discrete molecular clouds like spiral galaxies (Utomo et al 2015), while others have very smooth discs (Davis et al 2017), and some show an absence of large molecular clouds (Liu et al 2021). The physical mechanism(s) causing the observed diversity of ISM morphologies in ETGs have not yet been identified.…”