Virial Parameter Research Articles

Breaking beta: a comparison of mass modelling methods for spherical systems

ABSTRACT We apply four different mass modelling methods to a suite of publicly available mock data for spherical stellar systems. We focus on the recovery of the density and velocity anisotropy as a function of radius, either using line-of-sight velocity data only or adding proper motion data. All methods perform well on isotropic and tangentially anisotropic mock data, recovering the density and velocity anisotropy within their 95 per cent confidence intervals over the radial range 0.25 < R/R1/2 < 4, where R1/2 is the half-light radius. However, radially anisotropic mocks are more challenging. For line-of-sight data alone, only methods that use information about the shape of the velocity distribution function are able to break the degeneracy between the density profile and the velocity anisotropy, β, to obtain an unbiased estimate of both. This shape information can be obtained through directly fitting a global phase-space distribution function, by using higher order ‘virial shape parameters’ or by assuming a Gaussian velocity distribution function locally, but projecting it self-consistently along the line of sight. Including proper motion data yields further improvements, and in this case, all methods give a good recovery of both the radial density and velocity anisotropy profiles.

A survey of CH2DOH towards starless and pre-stellar cores in the Taurus molecular cloud

ABSTRACT Recent observations indicate that organic molecules are prevalent towards starless and pre-stellar cores. Deuteration of these molecules has not been well studied during the starless phase. Published observations of singly deuterated methanol, CH2DOH, have only been observed in a couple of well-studied, dense, and evolved pre-stellar cores (e.g. L1544, L183). Since the formation of gas-phase methanol during this cold phase is believed to occur via desorption from the icy grain surfaces, observations of CH2DOH may be useful as a probe of the deuterium fraction in the ice mantles of dust grains. We present a systematic survey of CH2DOH towards 12 starless and pre-stellar cores in the B10 region of the Taurus molecular cloud. Nine of the 12 cores are detected with [CH2DOH]/[CH3OH] ranging from <0.04 to 0.23$^{+0.12}_{-0.06}$ with a median value of 0.11. Sources not detected tend to have larger virial parameters and larger methanol linewidths than detected sources. The results of this survey indicate that deuterium fractionation of organic molecules, such as methanol, during the starless phase may be more easily detectable than previously thought.

The distances of 61 PGCCs in the second galactic quadrant

ABSTRACT Determining the distances to the Planck Galactic cold clumps (PGCCs) is crucial for the measurement of their physical parameters and the study of their Galactic distribution. Based on two large catalogues of stars with robust distances and reddening estimates from the literature, we have estimated accurate distances to 61 PGCCs in the second Galactic quadrant. For this purpose, we have selected stars along the sightlines overlapping with the cores of the sample clumps and fitted the reddening profiles with a simple reddening model. The typical uncertainties of the resultant distances of these PGCCs are less than 8 per cent. The new estimates differ significantly from the kinematic values, well known to suffer from large errors. With the new distances, we have updated the physical properties including the radii, masses, and virial parameters of the cores of the PGCCs.

The elephant in the bathtub: when the physics of star formation regulate the baryon cycle of galaxies

ABSTRACT In simple models of galaxy formation and evolution, star formation is solely regulated by the amount of gas present in the galaxy. However, it has recently been shown that star formation can be suppressed by galactic dynamics in galaxies that contain a dominant spheroidal component and a low gas fraction. This ‘dynamical suppression’ is hypothesized to also contribute to quenching gas-rich galaxies at high redshift, but its impact on the galaxy population at large remains unclear. In this paper, we assess the importance of dynamical suppression in the context of gas regulator models of galaxy evolution through hydrodynamic simulations of isolated galaxies, with gas-to-stellar mass ratios of 0.01–0.20 and a range of galactic gravitational potentials from disc-dominated to spheroidal. Star formation is modelled using a dynamics-dependent efficiency per free-fall time, which depends on the virial parameter of the gas. We find that dynamical suppression becomes more effective at lower gas fractions and quantify its impact on the star formation rate as a function of gas fraction and stellar spheroid mass surface density. We combine the results of our simulations with observed scaling relations that describe the change of galaxy properties across cosmic time, and determine the galaxy mass and redshift range where dynamical suppression may affect the baryon cycle. We predict that the physics of star formation can limit and regulate the baryon cycle at low redshifts (z ≲ 1.4) and high galaxy masses (M* ≳ 3 × 1010 M⊙), where dynamical suppression can drive galaxies off the star formation main sequence.

Highly turbulent gas on GMC scales in NGC 3256, the nearest luminous infrared galaxy

ABSTRACT We present the highest resolution CO (2–1) observations obtained to date (0.25 arcsec) of NGC 3256 and use them to determine the detailed properties of the molecular interstellar medium in the central 6 kpc of this merger. Distributions of physical quantities are reported from pixel-by-pixel measurements at 55 and 120 pc scales and compared to disc galaxies observed by PHANGS-ALMA (Physics at High Angular resolution in Nearby GalaxieS with Atacama Large Millimeter/Submillimeter Array). Mass surface densities range from 8 to 5500 M⊙ pc−2 and velocity dispersions from 10 to 200 km s−1. Peak brightness temperatures as large as 37 K are measured, indicating the gas in NGC 3256 may be hotter than all regions in nearby disc galaxies measured by PHANGS-ALMA. Brightness temperatures even surpass those in the overlap region of NGC 4038/9 at the same scales. The majority of the gas appears unbound with median virial parameters of 7–19, although external pressure may bind some of the gas. High internal turbulent pressures of 105–1010 K cm−3 are found. Given the lack of significant trends in surface density, brightness temperature, and velocity dispersion with physical scale we argue the molecular gas is made up of a smooth medium down to 55 pc scales, unlike the more structured medium found in the PHANGS-ALMA disc galaxies.

Galaxies in Observations and Numerical Models

A comparison of the virial parameters of galaxies and galaxy clusters (radius, density, and entropy function) in a wide range of masses, $${{10}^{6}} \leqslant {{M}_{{{\text{vir}}}}}{\text{/}}{{M}_{ \odot }} \leqslant {{10}^{{14}}}$$ , shows that these parameters are correlated and depend on the virial mass of an object in a regular way. For the observed galaxies and galaxy clusters the available estimates of virial parameters are quite close to each other and the values of reduced virial density Gρ are concentrated in a shallow interval $$0.5 \leqslant {{G}_{\rho }}{\text{/}}\langle {{G}_{\rho }}\rangle \leqslant 2$$ around its mean value. This “virial” paradox sharpen when compared with simulations, in which the number of such objects is very small, and for the majority of dark matter halos these densities are ~50–100 times smaller. Our theoretical analysis shows possible reasons for this discrepancy of observations and simulations, pointing out its connection with the spectrum of cosmological perturbations. After some refinement, the considered data can be used to constrain models of cosmological inflation. A number of accompanying questions is discussed.

Simultaneous Evolution of the Virial Parameter and Star Formation Rate in Molecular Clumps Undergoing Global Hierarchical Collapse

Abstract We compare dense clumps and cores in a numerical simulation of molecular clouds (MCs) undergoing global hierarchical collapse (GHC) to observations in two MCs at different evolutionary stages, the Pipe and the G14.225 clouds, to test the ability of the GHC scenario to follow the early evolution of the energy budget and star formation activity of these structures. In the simulation, we select a region that contains cores of sizes and densities similar to the Pipe cores and find that it evolves through accretion, developing substructure similar to that of the G14.225 cloud after ∼1.6 Myr. Within this region, we follow the evolution of the Larson ratio , where is the velocity dispersion and R is the size; the virial parameter α; and the star formation activity of the cores/clumps. In the simulation, we find that as the region evolves, (i) its clumps have and α values first consistent with those of the Pipe substructures and later with those of G14.225; (ii) the individual cores first exhibit a decrease in α followed by an increase when star formation begins; (iii) collectively, the ensemble of cores/clumps reproduces the observed trend of lower α for higher-mass objects; and (iv) the star formation rate and star formation efficiency increase monotonically. We suggest that this evolution is due to the simultaneous loss of externally driven compressive kinetic energy and increase of the self-gravity-driven motions. We conclude that the GHC scenario provides a realistic description of the evolution of the energy budget of the clouds’ substructure at early times, which occurs simultaneously with an evolution of the star formation activity.

Evolution of the Density PDF in Star-forming Clouds: The Role of Gravity

Abstract We derive an analytical theory of the PDF of density fluctuations in supersonic turbulence in the presence of gravity in star-forming clouds. The theory is based on a rigorous derivation of a combination of the Navier–Stokes continuity equations for the fluid motions and the Poisson equation for the gravity. It extends previous approaches, first by including gravity and second by considering the PDF as a dynamical system, not a stationary one. We derive the transport equations of the density PDF, characterize its evolution, and determine the density threshold above which gravity strongly affects and eventually dominates the dynamics of turbulence. We demonstrate the occurrence of t wo power-law tails in the PDF, with two characteristic exponents, corresponding to two different stages in the balance between turbulence and gravity. Another important result of this study is to provide a procedure to relate the observed column density PDFs to the corresponding volume density PDFs. This allows us to infer, from the observation of column densities, various physical parameters characterizing molecular clouds, notably the virial parameter. Furthermore, the theory offers the possibility to date the clouds in units of t coll, the time since a statistically significant fraction of the cloud started to collapse. The theoretical results and diagnostics reproduce very well numerical simulations and observations of star-forming clouds. The theory provides a sound theoretical foundation and quantitative diagnostics to analyze observations or numerical simulations of star-forming regions and to characterize the evolution of the density PDF in various regions of molecular clouds.

The SEDIGISM survey: molecular clouds in the inner Galaxy

ABSTRACT We use the 13CO (2–1) emission from the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium) high-resolution spectral-line survey of the inner Galaxy, to extract the molecular cloud population with a large dynamic range in spatial scales, using the Spectral Clustering for Interstellar Molecular Emission Segmentation (scimes) algorithm. This work compiles a cloud catalogue with a total of 10 663 molecular clouds, 10 300 of which we were able to assign distances and compute physical properties. We study some of the global properties of clouds using a science sample, consisting of 6664 well-resolved sources and for which the distance estimates are reliable. In particular, we compare the scaling relations retrieved from SEDIGISM to those of other surveys, and we explore the properties of clouds with and without high-mass star formation. Our results suggest that there is no single global property of a cloud that determines its ability to form massive stars, although we find combined trends of increasing mass, size, surface density, and velocity dispersion for the sub-sample of clouds with ongoing high-mass star formation. We then isolate the most extreme clouds in the SEDIGISM sample (i.e. clouds in the tails of the distributions) to look at their overall Galactic distribution, in search for hints of environmental effects. We find that, for most properties, the Galactic distribution of the most extreme clouds is only marginally different to that of the global cloud population. The Galactic distribution of the largest clouds, the turbulent clouds and the high-mass star-forming clouds are those that deviate most significantly from the global cloud population. We also find that the least dynamically active clouds (with low velocity dispersion or low virial parameter) are situated further afield, mostly in the least populated areas. However, we suspect that part of these trends may be affected by some observational biases (such as completeness and survey limitations), and thus require further follow up work in order to be confirmed.

A cold and diffuse giant molecular filament in the region of l = 41°, b = −1°

Data of 12CO/13CO/C18O J = 1 → 0 emission toward the Galactic plane region of l = 35° to 45° and b = –5° to +5° are available with the Milky Way Imaging Scroll Painting (MWISP) project. Using the data, we found a giant molecular filament (GMF) around l ≈ 38° ∼ 42°, b ≈ –3.5° ∼ 0°, VLSR ≈ 27 ∼ 40 km s−1, named the GMF MWISP G041–01. At a distance of 1.7 kpc, the GMF is about 160 pc long. With a median excitation temperature about 7.5 K and a median column density about 1021 cm−2, this GMF is very cold and very diffuse compared to known GMFs. Using the morphology in the data cube, the GMF is divided into four components among which three show filamentary structure. Masses of the components are 103 ∼ 104 M⊙, with a total mass for the whole filament being about 7 × 104 M⊙ from the local thermodynamic equilibrium method. 13CO cores inside each component are searched. Virial parameters are about 2.5 for these cores and have a power-law index of –0.34 against the mass. The mass fraction of dense cores traced by 13CO to the diffuse clouds traced by 12CO is about 7% for all components of the GMF. We found signatures of possible large scale filament-filament collision in the GMF.

Molecular Gas Properties on Cloud Scales across the Local Star-forming Galaxy Population

Abstract Using the PHANGS–ALMA CO(2–1) survey, we characterize molecular gas properties on ∼100 pc scales across 102,778 independent sightlines in 70 nearby galaxies. This yields the best synthetic view of molecular gas properties on cloud scales across the local star-forming galaxy population obtained to date. Consistent with previous studies, we observe a wide range of molecular gas surface densities (3.4 dex), velocity dispersions (1.7 dex), and turbulent pressures (6.5 dex) across the galaxies in our sample. Under simplifying assumptions about subresolution gas structure, the inferred virial parameters suggest that the kinetic energy of the molecular gas typically exceeds its self-gravitational binding energy at ∼100 pc scales by a modest factor (1.3 on average). We find that the cloud-scale surface density, velocity dispersion, and turbulent pressure (1) increase toward the inner parts of galaxies, (2) are exceptionally high in the centers of barred galaxies (where the gas also appears less gravitationally bound), and (3) are moderately higher in spiral arms than in inter-arm regions. The galaxy-wide averages of these gas properties also correlate with the integrated stellar mass, star formation rate, and offset from the star-forming main sequence of the host galaxies. These correlations persist even when we exclude regions with extraordinary gas properties in galaxy centers, which contribute significantly to the inter-galaxy variations. Our results provide key empirical constraints on the physical link between molecular cloud populations and their galactic environment.

Live fast, die young: GMC lifetimes in the FIRE cosmological simulations of Milky Way mass galaxies

ABSTRACT We present the first measurement of the lifetimes of giant molecular clouds (GMCs) in cosmological simulations at z = 0, using the Latte suite of FIRE-2 simulations of Milky Way (MW) mass galaxies. We track GMCs with total gas mass ≳105 M⊙ at high spatial (∼1 pc), mass (7100 M⊙), and temporal (1 Myr) resolution. Our simulated GMCs are consistent with the distribution of masses for massive GMCs in the MW and nearby galaxies. We find GMC lifetimes of 5–7 Myr, or 1–2 freefall times, on average, with less than 2 per cent of clouds living longer than 20 Myr. We find decreasing GMC lifetimes with increasing virial parameter, and weakly increasing GMC lifetimes with galactocentric radius, implying that environment affects the evolutionary cycle of GMCs. However, our GMC lifetimes show no systematic dependence on GMC mass or amount of star formation. These results are broadly consistent with inferences from the literature and provide an initial investigation into ultimately understanding the physical processes that govern GMC lifetimes in a cosmological setting.

Can magnetized turbulence set the mass scale of stars?

ABSTRACT Understanding the evolution of self-gravitating, isothermal, magnetized gas is crucial for star formation, as these physical processes have been postulated to set the initial mass function (IMF). We present a suite of isothermal magnetohydrodynamic (MHD) simulations using the gizmo code that follow the formation of individual stars in giant molecular clouds (GMCs), spanning a range of Mach numbers found in observed GMCs ($\mathcal {M} \sim 10\!-\!50$). As in past works, the mean and median stellar masses are sensitive to numerical resolution, because they are sensitive to low-mass stars that contribute a vanishing fraction of the overall stellar mass. The mass-weighted median stellar mass M50 becomes insensitive to resolution once turbulent fragmentation is well resolved. Without imposing Larson-like scaling laws, our simulations find $M_\mathrm{50} \,\, \buildrel\propto \over \sim \,\,M_\mathrm{0} \mathcal {M}^{-3} \alpha _\mathrm{turb}\, \mathrm{SFE}^{1/3}$ for GMC mass M0, sonic Mach number $\mathcal {M}$, virial parameter αturb, and star formation efficiency SFE = M⋆/M0. This fit agrees well with previous IMF results from the ramses, orion2, and sphng codes. Although M50 has no significant dependence on the magnetic field strength at the cloud scale, MHD is necessary to prevent a fragmentation cascade that results in non-convergent stellar masses. For initial conditions and SFE similar to star-forming GMCs in our Galaxy, we predict M50 to be $\gt 20 \, \mathrm{M}_{\odot }$, an order of magnitude larger than observed ($\sim 2 \, \mathrm{M}_\odot$), together with an excess of brown dwarfs. Moreover, M50 is sensitive to initial cloud properties and evolves strongly in time within a given cloud, predicting much larger IMF variations than are observationally allowed. We conclude that physics beyond MHD turbulence and gravity are necessary ingredients for the IMF.

Cloud Properties and Correlations with Star Formation in Self-consistent Simulations of the Multiphase ISM

Abstract We apply gravity- and density-based methods to identify clouds in self-consistent numerical simulations of the star-forming, multiphase interstellar medium (ISM) and compare their properties and global correlation with the star formation rate (SFR) over time. The gravity-based method identifies bound objects, which have masses at densities , and virial parameters α v ∼ 0.5–5. For clouds defined by a density threshold , the average virial parameter decreases, and the fraction of material that is genuinely bound increases, with increasing . Surprisingly, clouds defined by density thresholds can be unbound even when α v < 2, and high-mass clouds ( ) are generally unbound. This suggests that the traditional α v is at best an approximate measure of boundedness in the ISM. All clouds have internal turbulent motions increasing with size as , similar to observed relations. Bound structures comprise a small fraction of the total simulation mass and have a star formation efficiency per freefall time ∼ 0.4. For , ∼ 0.03–0.3, increasing with density threshold. A temporal correlation analysis between and aggregate mass at varying shows that time delays to star formation are . The correlation between and systematically tightens at higher . Considering moderate-density gas, selecting against high virial parameter clouds improves correlation with the SFR, consistent with previous work. Even at high , the temporal dispersion in is ∼50%, due to the large-amplitude variations and inherent stochasticity of the system.

ALMA Observations of NGC 6334S. I. Forming Massive Stars and Clusters in Subsonic and Transonic Filamentary Clouds

Abstract We present Atacama Large Millimeter/submillimeter Array (ALMA) and Karl G. Jansky Very Large Array (JVLA) observations of the massive infrared dark cloud NGC 6334S (also known as IRDC G350.56+0.44), located at the southwestern end of the NGC 6334 molecular cloud complex. The H13CO+ and NH2D lines covered by the ALMA observations at a ∼3″ angular resolution (∼0.02 pc) reveal that the spatially unresolved nonthermal motions are predominantly subsonic and transonic, a condition analogous to that found in low-mass star-forming molecular clouds. The observed supersonic nonthermal velocity dispersions in massive star-forming regions, often reported in the literature, might be significantly biased by poor spatial resolutions that broaden the observed line widths owing to unresolved motions within the telescope beam. Our 3 mm continuum image resolves 49 dense cores, whose masses range from 0.17 to 14 M ⊙. The majority of them are resolved with multiple velocity components. Our analyses of these gas velocity components find an anticorrelation between the gas mass and the virial parameter. This implies that the more massive structures tend to be more gravitationally unstable. Finally, we find that the external pressure in the NGC 6334S cloud is important in confining these dense structures and may play a role in the formation of dense cores and, subsequently, the embedded young stars.

Magnetic Fields in the Early Stages of Massive Star Formation as Revealed by ALMA

Abstract We present 1.3 mm ALMA dust polarization observations at a resolution of ∼0.02 pc for three massive molecular clumps, MM1, MM4, and MM9, in the infrared dark cloud G28.34+0.06. With these sensitive and high-resolution continuum data, MM1 is resolved into a cluster of condensations. The magnetic field structure in each clump is revealed by the polarized emission. We found a trend of decreasing polarized emission fraction with increasing Stokes I intensities in MM1 and MM4. Using the angular dispersion function method (a modified Davis–Chandrasekhar–Fermi method), the plane-of-sky magnetic field strengths in two massive dense cores, MM1-Core1 and MM4-Core4, are estimated to be ∼1.6 mG and ∼0.32 mG, respectively. The virial parameters in MM1-Core1 and MM4-Core4 are calculated to be ∼0.76 and ∼0.37, respectively, suggesting that massive star formation does not start in equilibrium. Using the polarization-intensity gradient-local gravity method, we found that the local gravity is closely aligned with intensity gradient in the three clumps, and the magnetic field tends to be aligned with the local gravity in MM1 and MM4 except for regions near the emission peak, which suggests that the gravity plays a dominant role in regulating the gas collapse. Half of the outflows in MM4 and MM9 are found to be aligned within 10° of the condensation-scale (<0.05 pc) magnetic field, indicating that the magnetic field could play an important role from condensation to disk scale in the early stage of massive star formation.

Heart of darkness: the influence of galactic dynamics on quenching star formation in galaxy spheroids

ABSTRACT Quenched galaxies are often observed to contain a strong bulge component. The key question is whether this reflects a causal connection – can star formation be quenched dynamically by bulges or the spheroids of early-type galaxies? We systematically investigate the impact of these morphological components on star formation, by performing a suite of hydrodynamical simulations of isolated galaxies containing a spheroid. We vary the bulge mass and scale radius, while the total initial stellar, halo, and gas mass are kept constant, with a gas fraction of 5 per cent. In addition, we consider two different sub-grid star formation prescriptions. The first follows most simulations in the literature by assuming a constant star formation efficiency per free-fall time, whereas in the second model it depends on the gas virial parameter, following high-resolution simulations of turbulent fragmentation. Across all simulations, central spheroids increase the gas velocity dispersion towards the galactic centre. This increases the gravitational stability of the gas disc, suppresses fragmentation and star formation, and results in galaxies hosting extremely smooth and quiescent gas discs that fall below the galaxy main sequence. These effects amplify when using the more sophisticated, dynamics-dependent star formation model. Finally, we discover a pronounced relation between the central stellar surface density and star formation rate (SFR), such that the most bulge-dominated galaxies show the strongest deviation from the main sequence. We conclude that the SFR of galaxies is not only set by the balance between accretion and feedback, but carries a (sometimes dominant) dependence on the gravitational potential.

Dynamical Properties of Molecular-forming Gas Clumps in Galaxies at the Epoch of Reionization

Abstract We study the properties of molecular-forming gas clumps (MGCs) at the epoch of reionization using cosmological zoom-in simulations. We identify MGCs in a prototypical galaxy (“Althæa”) using an H2 density-based clump finder. We compare their mass, size, velocity dispersion, gas surface density, and virial parameter ( ) to observations. In Althæa, the typical MGC mass and size are M ⊙ and pc, which are comparable to those found in nearby spirals and starburst galaxies. MGCs are highly supersonic and supported by turbulence, with rms velocity dispersions of 20–100 km s−1 and pressure of (i.e., with respect to the Milky Way), similar to those found in nearby and z ∼ 2 gas-rich starburst galaxies. In addition, we perform stability analysis to understand the origin and dynamical properties of MGCs. We find that MGCs are globally stable in the main disk of Althæa. Densest regions where star formation is expected to take place in clouds and cores on even smaller scales instead have lower and Toomre Q values. Detailed studies of the star-forming gas dynamics at the epoch of reionization thus require a spatial resolution of ≲40 pc (≃ ), which is within reach with the Atacama Large (sub-)Millimeter Array and the Next Generation Very Large Array.

Star Formation Occurs in Dense Gas, but What Does “Dense” Mean?

Abstract We report results of a project to map HCN and emission toward a sample of molecular clouds in the inner Galaxy, all containing dense clumps that are actively engaged in star formation. We compare these two molecular line tracers with millimeter continuum emission and extinction, as inferred from 13CO, as tracers of dense gas in molecular clouds. The fraction of the line luminosity from each tracer that comes from the dense gas, as measured by mag, varies substantially from cloud to cloud. In all cases, a substantial fraction (in most cases, the majority) of the total luminosity arises in gas below the mag threshold and outside the region of strong millimeter continuum emission. Measurements of toward other galaxies will likely be dominated by such gas at lower surface densities. Substantial, even dominant, contributions to the total line luminosity can arise in gas with densities typical of the cloud as a whole (n ∼ 100 cm−3). Defining the dense clump from the HCN or emission itself, similarly to previous studies, leads to a wide range of clump properties, with some being considerably larger and less dense than in previous studies. HCN and have a similar ability to trace dense gas for the clouds in this sample. For the two clouds with low virial parameters, 13CO is definitely a worse tracer of the dense gas, but for the other four, it is equally good (or bad) at tracing dense gas.

Spinning Bar and a Star-formation Inefficient Repertoire: Turbulence in Hickson Compact Group NGC 7674

Abstract The physics regulating star formation (SF) in Hickson Compact Groups (HCG) has thus far been difficult to describe, due to their unique kinematic properties. In this study, we expand upon previous works to devise a more physically meaningful SF relation able to better encompass the physics of these unique systems. We combine CO(1–0) data from the Combined Array from Research in Millimeter Astronomy to trace the column density of molecular gas and deep Hα imaging taken on the Southern Astrophysical Research Telescope tracing to investigate SF efficiency across face-on HCG, NGC 7674. We find a lack of universality in SF, with two distinct sequences present in the – plane; one for inside and one for outside the nucleus. We devise an SF relation based on the multi-freefall nature of gas and the critical density, which itself is dependent on the virial parameter , the ratio of turbulent to gravitational energy. We find that our modified SF relation fits the data and describes the physics of this system well with the introduction of a virial parameter of about 5–10 across the galaxy. This leads to an order-of-magnitude reduction in SFR compared to systems.