arXiv subjects

J. M. D. Kruijssen

Publications and source records attributed to J. M. D. Kruijssen.

At least 37 records · Page 2Linked to original sources

arXiv2020

Ubiquitous velocity fluctuations throughout the molecular interstellar medium

The density structure of the interstellar medium (ISM) determines where stars form and release energy, momentum, and heavy elements, driving galaxy evolution. Density variations are seeded and amplified by gas motion, but the exact nature of this motion is unknown across spatial scale and galactic environment. Although dense star-forming gas likely emerges from a combination of instabilities, convergent flows, and turbulence, establishing the precise origin is challenging because it requires quantifying gas motion over many orders of magnitude in spatial scale. Here we measure the motion of molecular gas in the Milky Way and in nearby galaxy NGC 4321, assembling observations that span an unprecedented spatial dynamic range ($10^{-1}{-}10^3$ pc). We detect ubiquitous velocity fluctuations across all spatial scales and galactic environments. Statistical analysis of these fluctuations indicates how star-forming gas is assembled. We discover oscillatory gas flows with wavelengths ranging from $0.3{-}400$ pc. These flows are coupled to regularly-spaced density enhancements that likely form via gravitational instabilities. We also identify stochastic and scale-free velocity and density fluctuations, consistent with the structure generated in turbulent flows. Our results demonstrate that ISM structure cannot be considered in isolation. Instead, its formation and evolution is controlled by nested, interdependent flows of matter covering many orders of magnitude in spatial scale.

astro-ph.GA↗

arXiv2019

Mapping metallicity variations across nearby galaxy disks

The distribution of metals within a galaxy traces the baryon cycle and the buildup of galactic disks, but the detailed gas phase metallicity distribution remains poorly sampled. We have determined the gas phase oxygen abundances for 7,138 HII regions across the disks of eight nearby galaxies using VLT/MUSE optical integral field spectroscopy as part of the PHANGS-MUSE survey. After removing the first order radial gradients present in each galaxy, we look at the statistics of the metallicity offset (Delta O/H) and explore azimuthal variations. Across each galaxy, we find low (sigma=0.03-0.05 dex) scatter at any given radius, indicative of efficient mixing. We compare physical parameters for those HII regions that are 1 sigma outliers towards both enhanced and reduced abundances. Regions with enhanced abundances have high ionization parameter, higher Halpha luminosity, lower Halpha velocity dispersion, younger star clusters and associated molecular gas clouds show higher molecular gas densities. This indicates recent star formation has locally enriched the material. Regions with reduced abundances show increased Halpha velocity dispersions, suggestive of mixing introducing more pristine material. We observe subtle azimuthal variations in half of the sample, but can not always cleanly associate this with the spiral pattern. Regions with enhanced and reduced abundances are found distributed throughout the disk, and in half of our galaxies we can identify subsections of spiral arms with clearly associated metallicity gradients. This suggests spiral arms play a role in organizing and mixing the ISM.

astro-ph.GA↗

arXiv2019

Dense gas is not enough: environmental variations in the star formation efficiency of dense molecular gas at 100pc scales in M51

It remains unclear what sets the efficiency with which molecular gas transforms into stars. Here we present a new VLA map of the spiral galaxy M51 in 33GHz radio continuum, an extinction-free tracer of star formation, at 3" scales (~100pc). We combined this map with interferometric PdBI/NOEMA observations of CO(1-0) and HCN(1-0) at matched resolution for three regions in M51 (central molecular ring, northern and southern spiral arm segments). While our measurements roughly fall on the well-known correlation between total infrared and HCN luminosity, bridging the gap between Galactic and extragalactic observations, we find systematic offsets from that relation for different dynamical environments probed in M51, e.g. the southern arm segment is more quiescent due to low star formation efficiency (SFE) of the dense gas, despite having a high dense gas fraction. Combining our results with measurements from the literature at 100pc scales, we find that the SFE of the dense gas and the dense gas fraction anti-correlate and correlate, respectively, with the local stellar mass surface density. This is consistent with previous kpc-scale studies. In addition, we find a significant anti-correlation between the SFE and velocity dispersion of the dense gas. Finally, we confirm that a correlation also holds between star formation rate surface density and the dense gas fraction, but it is not stronger than the correlation with dense gas surface density. Our results are hard to reconcile with models relying on a universal gas density threshold for star formation and suggest that turbulence and galactic dynamics play a major role in setting how efficiently dense gas converts into stars.

astro-ph.GA↗

arXiv2019

Young massive star cluster formation in the Galactic Centre is driven by global gravitational collapse of high-mass molecular clouds

Young massive clusters (YMCs) are the most compact, high-mass stellar systems still forming at the present day. The precursor clouds to such systems are, however, rare due to their large initial gas mass reservoirs and rapid dispersal timescales due to stellar feedback. Nonetheless, unlike their high-z counterparts, these precursors are resolvable down to the sites of individually forming stars, and hence represent the ideal environments in which to test the current theories of star and cluster formation. Using high angular resolution (1$^{\prime\prime}$ / 0.05pc) and sensitivity ALMA observations of two YMC progenitor clouds in the Galactic Centre, we have identified a suite of molecular line transitions -- e.g. c-C$_{3}$H$_{2} $($7-6$) -- that are believed to be optically thin, and reliably trace the gas structure in the highest density gas on star-forming core scales. We conduct a virial analysis of the identified core and proto-cluster regions, and show that half of the cores (5/10) and both proto-clusters are unstable to gravitational collapse. This is the first kinematic evidence of global gravitational collapse in YMC precursor clouds at such an early evolutionary stage. The implications are that if these clouds are to form YMCs, then they likely do so via the "conveyor-belt" mode, whereby stars continually form within dispersed dense gas cores as the cloud undergoes global gravitational collapse. The concurrent contraction of both the cluster-scale gas and embedded (proto)stars ultimately leads to the high (proto)stellar density in YMCs.

astro-ph.GA↗

arXiv2019

Optical IFU spectroscopy of a bipolar microquasar jet in NGC 300

We recently reported the discovery of a candidate jet-driving microquasar (S10) in the nearby spiral galaxy NGC 300. However, in the absence of kinematic information, we could not reliably determine the jet power or the dynamical age of the jet cavity. Here, we present optical MUSE integral field unit (IFU) observations of S10, which reveal a bipolar line-emitting jet structure surrounding a continuum-emitting central source. The optical jet lobes of S10 have a total extent of $\sim$ 40 pc and a shock velocity of $\sim$ 150 km s$^{-1}$. Together with the jet kinematics, we exploit the MUSE coverage of the Balmer H$β$ line to estimate the density of the surrounding matter and therefore compute the jet power to be $P_{jet}\approx$ 6.3 $\times$ 10$^{38}$ erg s$^{-1}$. An optical analysis of a microquasar jet bubble and a consequent robust derivation of the jet power have been possible only in a handful of similar sources. This study therefore adds valuable insight into microquasar jets, and demonstrates the power of optical integral field spectroscopy in identifying and analysing these objects.

astro-ph.HE↗

arXiv2019

The dynamical evolution of molecular clouds near the Galactic Centre - II. Spatial structure and kinematics of simulated clouds

The evolution of molecular clouds in galactic centres is thought to differ from that in galactic discs due to a significant influence of the external gravitational potential. We present a set of numerical simulations of molecular clouds orbiting on the 100-pc stream of the Central Molecular Zone (the central $\sim500$ pc of the Galaxy) and characterise their morphological and kinematic evolution in response to the background potential and eccentric orbital motion. We find that the clouds are shaped by strong shear and torques, by tidal and geometric deformation, and by their passage through the orbital pericentre. Within our simulations, these mechanisms control cloud sizes, aspect ratios, position angles, filamentary structure, column densities, velocity dispersions, line-of-sight velocity gradients, spin angular momenta, and kinematic complexity. By comparing these predictions to observations of clouds on the Galactic Centre 'dust ridge', we find that our simulations naturally reproduce a broad range of key observed morphological and kinematic features, which can be explained in terms of well-understood physical mechanisms. We argue that the accretion of gas clouds onto the central regions of galaxies, where the rotation curve turns over and the tidal field is fully compressive, is accompanied by transformative dynamical changes to the clouds, leading to collapse and star formation. This can generate an evolutionary progression of cloud collapse with a common starting point, which either marks the time of accretion onto the tidally-compressive region or of the most recent pericentre passage. Together, these processes may naturally produce the synchronised starbursts observed in numerous (extra)galactic nuclei.

astro-ph.GA↗

arXiv2019

'The Brick' is not a brick: A comprehensive study of the structure and dynamics of the Central Molecular Zone cloud G0.253+0.016

In this paper we provide a comprehensive description of the internal dynamics of G0.253+0.016 (a.k.a. 'the Brick'); one of the most massive and dense molecular clouds in the Galaxy to lack signatures of widespread star formation. As a potential host to a future generation of high-mass stars, understanding largely quiescent molecular clouds like G0.253+0.016 is of critical importance. In this paper, we reanalyse Atacama Large Millimeter Array cycle 0 HNCO $J=4(0,4)-3(0,3)$ data at 3 mm, using two new pieces of software which we make available to the community. First, scousepy, a Python implementation of the spectral line fitting algorithm scouse. Secondly, acorns (Agglomerative Clustering for ORganising Nested Structures), a hierarchical n-dimensional clustering algorithm designed for use with discrete spectroscopic data. Together, these tools provide an unbiased measurement of the line of sight velocity dispersion in this cloud, $σ_{v_{los}, {\rm 1D}}=4.4\pm2.1$ kms$^{-1}$, which is somewhat larger than predicted by velocity dispersion-size relations for the Central Molecular Zone (CMZ). The dispersion of centroid velocities in the plane of the sky are comparable, yielding $σ_{v_{los}, {\rm 1D}}/σ_{v_{pos}, {\rm 1D}}\sim1.2\pm0.3$. This isotropy may indicate that the line-of-sight extent of the cloud is approximately equivalent to that in the plane of the sky. Combining our kinematic decomposition with radiative transfer modelling we conclude that G0.253+0.016 is not a single, coherent, and centrally-condensed molecular cloud; 'the Brick' is not a \emph{brick}. Instead, G0.253+0.016 is a dynamically complex and hierarchically-structured molecular cloud whose morphology is consistent with the influence of the orbital dynamics and shear in the CMZ.

astro-ph.GA↗

arXiv2018

Feedback from massive stars at low metallicities: MUSE observations of N44 and N180 in the Large Magellanic Cloud

We present MUSE integral field data of two HII region complexes in the Large Magellanic Cloud (LMC), N44 and N180. Both regions consist of a main superbubble and a number of smaller, more compact HII regions that formed on the edge of the superbubble. For a total of 11 HII regions, we systematically analyse the radiative and mechanical feedback from the massive O-type stars on the surrounding gas. We exploit the integral field property of the data and the coverage of the HeII$λ$5412 line to identify and classify the feedback-driving massive stars, and from the estimated spectral types and luminosity classes we determine the stellar radiative output in terms of the ionising photon flux $Q_{0}$. We characterise the HII regions in terms of their sizes, morphologies, ionisation structure, luminosity and kinematics, and derive oxygen abundances via emission line ratios. We analyse the role of different stellar feedback mechanisms for each region by measuring the direct radiation pressure, the pressure of the ionised gas, and the pressure of the shock-heated winds. We find that stellar winds and ionised gas are the main drivers of HII region expansion in our sample, while the direct radiation pressure is up to three orders of magnitude lower than the other terms. We relate the total pressure to the star formation rate per unit area, $Σ_{SFR}$, for each region and find that stellar feedback has a negative effect on star formation, and sets an upper limit to $Σ_{SFR}$ as a function of increasing pressure.

astro-ph.GA↗

arXiv2018

A 50 pc scale view of star formation efficiency across NGC 628

Star formation is a multi-scale process that requires tracing cloud formation and stellar feedback within the local ( 1 Gyr, significantly longer than previously reported for individual star-forming clouds in the Milky Way. Finally, we find no clear trends that relate variations in the depletion time observed on 500 pc scales to physical drivers (metallicity, molecular and stellar mass surface density, molecular gas boundedness) on 50 pc scales.

astro-ph.GA↗

arXiv2018

On the physical mechanisms governing the cloud lifecycle in the Central Molecular Zone of the Milky Way

We apply an analytic theory for environmentally-dependent molecular cloud lifetimes to the Central Molecular Zone of the Milky Way. Within this theory, the cloud lifetime in the Galactic centre is obtained by combining the time-scales for gravitational instability, galactic shear, epicyclic perturbations and cloud-cloud collisions. We find that at galactocentric radii $\sim 45$-$120$ pc, corresponding to the location of the '100-pc stream', cloud evolution is primarily dominated by gravitational collapse, with median cloud lifetimes between 1.4 and 3.9 Myr. At all other galactocentric radii, galactic shear dominates the cloud lifecycle, and we predict that molecular clouds are dispersed on time-scales between 3 and 9 Myr, without a significant degree of star formation. Along the outer edge of the 100-pc stream, between radii of 100 and 120 pc, the time-scales for epicyclic perturbations and gravitational free-fall are similar. This similarity of time-scales lends support to the hypothesis that, depending on the orbital geometry and timing of the orbital phase, cloud collapse and star formation in the 100-pc stream may be triggered by a tidal compression at pericentre. Based on the derived time-scales, this should happen in approximately 20 per cent of all accretion events onto the 100-pc stream.

astro-ph.GA↗

arXiv2018

The young star cluster population of M51 with LEGUS - II. Testing environmental dependencies

It has recently been established that the properties of young star clusters (YSCs) can vary as a function of the galactic environment in which they are found. We use the cluster catalogue produced by the Legacy Extragalactic UV Survey (LEGUS) collaboration to investigate cluster properties in the spiral galaxy M51. We analyse the cluster population as a function of galactocentric distance and in arm and inter-arm regions. The cluster mass function exhibits a similar shape at all radial bins, described by a power law with a slope close to $-2$ and an exponential truncation around $10^5\ \rm{M}_{\odot}$ . While the mass functions of the YSCs in the spiral arm and inter-arm regions have similar truncation masses, the inter-arm region mass function has a significantly steeper slope than the one in the arm region; a trend that is also observed in the giant molecular cloud mass function and predicted by simulations. The age distribution of clusters is dependent on the region considered, and is consistent with rapid disruption only in dense regions, while little disruption is observed at large galactocentric distances and in the inter-arm region. The fraction of stars forming in clusters does not show radial variations, despite the drop in the $H_2$ surface density measured as function of galactocentric distance. We suggest that the higher disruption rate observed in the inner part of the galaxy is likely at the origin of the observed flat cluster formation efficiency radial profile.

astro-ph.GA↗

arXiv2017

Star formation in a high-pressure environment: An SMA view of the Galactic centre dust ridge

The star formation rate in the Central Molecular Zone (CMZ) is an order of magnitude lower than predicted according to star formation relations that have been calibrated in the disc of our own and nearby galaxies. Understanding how and why star formation appears to be different in this region is crucial if we are to understand the environmental dependence of the star formation process. Here, we present the detection of a sample of high-mass cores in the CMZ's "dust ridge" that have been discovered with the Submillimeter Array as part of the CMZoom survey. These cores range in mass from ~ 50 - 2150 Msun within radii of 0.1 - 0.25 pc. All appear to be young (pre-UCHII), meaning that they are prime candidates for representing the initial conditions of high-mass stars and sub-clusters. We report that at least two of these cores ('c1' and 'e1') contain young, high-mass protostars. We compare all of the detected cores with high-mass cores in the Galactic disc and find that they are broadly similar in terms of their masses and sizes, despite being subjected to external pressures that are several orders of magnitude greater - ~ 10^8 K/cm^3, as opposed to ~ 10^5 K/cm^3. The fact that > 80% of these cores do not show any signs of star-forming activity in such a high-pressure environment leads us to conclude that this is further evidence for an increased critical density threshold for star formation in the CMZ due to turbulence.

astro-ph.GA↗

arXiv2017

Star formation rates and efficiencies in the Galactic Centre

The inner few hundred parsecs of the Milky Way harbours gas densities, pressures, velocity dispersions, an interstellar radiation field and a cosmic ray ionisation rate orders of magnitude higher than the disc; akin to the environment found in star-forming galaxies at high-redshift. Previous studies have shown that this region is forming stars at a rate per unit mass of dense gas which is at least an order of magnitude lower than in the disc, potentially violating theoretical predictions. We show that all observational star formation rate diagnostics - both direct counting of young stellar objects and integrated light measurements - are in agreement within a factor two, hence the low star formation rate is not the result of the systematic uncertainties that affect any one method. As these methods trace the star formation over different timescales, from $0.1 - 5$ Myr, we conclude that the star formation rate has been constant to within a factor of a few within this time period. We investigate the progression of star formation within gravitationally bound clouds on $\sim$ parsec scales and find $1 - 4$ per cent of the cloud masses are converted into stars per free-fall time, consistent with a subset of the considered "volumetric" star formation models. However, discriminating between these models is obstructed by the current uncertainties on the input observables and, most importantly and urgently, by their dependence on ill-constrained free parameters. The lack of empirical constraints on these parameters therefore represents a key challenge in the further verification or falsification of current star formation theories.

astro-ph.GA↗

arXiv2017

Simultaneous low- and high-mass star formation in a massive protocluster: ALMA observations of G11.92-0.61

We present 1.05 mm ALMA observations of the deeply embedded high-mass protocluster G11.92-0.61, designed to search for low-mass cores within the accretion reservoir of the massive protostars. Our ALMA mosaic, which covers an extent of ~0.7 pc at sub-arcsecond (~1400 au) resolution, reveals a rich population of 16 new millimetre continuum sources surrounding the three previously-known millimetre cores. Most of the new sources are located in the outer reaches of the accretion reservoir: the median projected separation from the central, massive (proto)star MM1 is ~0.17 pc. The derived physical properties of the new millimetre continuum sources are consistent with those of low-mass prestellar and protostellar cores in nearby star-forming regions: the median mass, radius, and density of the new sources are 1.3 Msun, 1600 au, and n(H2)~10^7 cm^-3. At least three of the low-mass cores in G11.92-0.61 drive molecular outflows, traced by high-velocity 12CO(3-2) (observed with the SMA) and/or by H2CO and CH3OH emission (observed with ALMA). This finding, combined with the known outflow/accretion activity of MM1, indicates that high- and low-mass stars are forming (accreting) simultaneously within this protocluster. Our ALMA results are consistent with the predictions of competitive-accretion-type models in which high-mass stars form along with their surrounding clusters.

astro-ph.GA↗

arXiv2016

A revised planetary nebula luminosity function distance to NGC 628 using MUSE

Distance uncertainties plague our understanding of the physical scales relevant to the physics of star formation in extragalactic studies. The planetary nebulae luminosity function (PNLF) is one of very few techniques that can provide distance estimates to within ~10%, however it requires a planetary nebula (PN) sample that is uncontaminated by other ionizing sources. We employ optical IFU spectroscopy using MUSE on the VLT to measure [OIII] line fluxes for sources unresolved on 50 pc scales within the central star-forming galaxy disk of NGC 628. We use diagnostic line ratios to identify 62 PNe, 30 supernova remnants and 87 HII regions within our fields. Using the 36 brightest PNe we determine a new PNLF distance modulus of 29.91^{+0.08}_{-0.13} mag (9.59^{+0.35}_{-0.57} Mpc), in good agreement with literature values but significantly larger than the previously reported PNLF distance. We are able to explain the discrepancy and recover the previous result when we reintroduce SNR contaminants to our sample. This demonstrates the power of full spectral information over narrowband imaging in isolating PNe. Given our limited spatial coverage within the galaxy, we show that this technique can be used to refine distance estimates even when IFU observations cover only a fraction of a galaxy disk.

astro-ph.GA↗

arXiv2016

Seeding the Galactic Centre gas stream: gravitational instabilities set the initial conditions for the formation of protocluster clouds

Star formation within the Central Molecular Zone (CMZ) may be intimately linked to the orbital dynamics of the gas. Recent models suggest that star formation within the dust ridge molecular clouds (from G0.253+0.016 to Sgr B2) follows an evolutionary time sequence, triggered by tidal compression during their preceding pericentre passage. Given that these clouds are the most likely precursors to a generation of massive stars and extreme star clusters, this scenario would have profound implications for constraining the time-evolution of star formation. In this Letter, we search for the initial conditions of the protocluster clouds, focusing on the kinematics of gas situated upstream from pericentre. We observe a highly-regular corrugated velocity field in $\{l,\,v_{\rm LSR}\}$ space, with amplitude and wavelength $A=3.7\,\pm\,0.1$ kms$^{-1}$ and $λ_{\rm vel, i}=22.5\,\pm\,0.1$ pc, respectively. The extremes in velocity correlate with a series of massive ($\sim10^{4}$M$_{\odot}$) and compact ($R_{\rm eq}\sim2$ pc), quasi-regularly spaced ($\sim8$ pc), molecular clouds. The corrugation wavelength and cloud separation closely agree with the predicted Toomre ($\sim17$ pc) and Jeans ($\sim6$ pc) lengths, respectively. We conclude that gravitational instabilities are driving the condensation of molecular clouds within the Galactic Centre gas stream. Furthermore, we speculate these seeds are the historical analogue of the dust-ridge molecular clouds, representing the initial conditions of star and cluster formation in the CMZ.

astro-ph.GA↗

arXiv2016

The link between solenoidal turbulence and slow star formation in G0.253+0.016

Star formation in the Galactic disc is primarily controlled by gravity, turbulence, and magnetic fields. It is not clear that this also applies to star formation near the Galactic Centre. Here we determine the turbulence and star formation in the CMZ cloud G0.253+0.016. Using maps of 3mm dust emission and HNCO intensity-weighted velocity obtained with ALMA, we measure the volume-density variance $σ_{ρ/ρ_0} = 1.3 \pm 0.5$ and turbulent Mach number $\mathcal{M} = 11 \pm 3$. Combining these with turbulence simulations to constrain the plasma $β= 0.34 \pm 0.35$, we reconstruct the turbulence driving parameter $b = 0.22 \pm 0.12$ in G0.253+0.016. This low value of $b$ indicates solenoidal (divergence-free) driving of the turbulence in G0.253+0.016. By contrast, typical clouds in the Milky Way disc and spiral arms have a significant compressive (curl-free) driving component ($b > 0.4$). We speculate that shear causes the solenoidal driving in G0.253+0.016 and show that this may reduce the star formation rate by a factor of 7 compared to nearby clouds.

astro-ph.SR↗

arXiv2016

The link between turbulence, magnetic fields, filaments, and star formation in the Central Molecular Zone cloud G0.253+0.016

Star formation is primarily controlled by the interplay between gravity, turbulence, and magnetic fields. However, the turbulence and magnetic fields in molecular clouds near the Galactic Center may differ substantially from spiral-arm clouds. Here we determine the physical parameters of the central molecular zone (CMZ) cloud G0.253+0.016, its turbulence, magnetic field and filamentary structure. Using column-density maps based on dust-continuum emission observations with ALMA+Herschel, we identify filaments and show that at least one dense core is located along them. We measure the filament width W_fil=0.17$\pm$0.08pc and the sonic scale λ_sonic=0.15$\pm$0.11pc of the turbulence, and find W_fil~λ_sonic. A strong velocity gradient is seen in the HNCO intensity-weighted velocity maps obtained with ALMA+Mopra, which is likely caused by large-scale shearing of G0.253+0.016, producing a wide double-peaked velocity PDF. After subtracting the gradient to isolate the turbulent motions, we find a nearly Gaussian velocity PDF typical for turbulence. We measure the total and turbulent velocity dispersion, 8.8$\pm$0.2km/s and 3.9$\pm$0.1km/s, respectively. Using magnetohydrodynamical simulations, we find that G0.253+0.016's turbulent magnetic field B_turb=130$\pm$50$μ$G is only ~1/10 of the ordered field component. Combining these measurements, we reconstruct the dominant turbulence driving mode in G0.253+0.016 and find a driving parameter b=0.22$\pm$0.12, indicating solenoidal (divergence-free) driving. We compare this to spiral-arm clouds, which typically have a significant compressive (curl-free) driving component (b>0.4). Motivated by previous reports of strong shearing motions in the CMZ, we speculate that shear causes the solenoidal driving in G0.253+0.016 and show that this reduces the star formation rate (SFR) by a factor of 6.9 compared to typical nearby clouds.

astro-ph.GA↗