Recent Activity
Comparing the pre-SNe feedback and environmental pressures for 6000 HII regions across 19 nearby spiral galaxies
Comparing the pre-SNe feedback and environmental pressures for 6000 HII regions across 19 nearby spiral galaxies
10/2021 - paperAccepted on arXiv soon...The feedback from young stars (i.e. pre-supernova) is thought to play a crucial role in molecular cloud destruction. In this paper, we assess the feedback mechanisms acting within a sample of 5810 HII regions identified from the PHANGS-MUSE survey of 19 nearby (<20 Mpc) star-forming, main sequence spiral galaxies (log(M/Msun)= 9.4 – 11). These optical spectroscopic maps are essential to constrain the physical properties of the HII regions, which we use to investigate their internal pressure terms. We estimate the photoionised gas (Ptherm), direct radiation (Prad), and mechanical wind pressure (Pwind), which we compare to the confining pressure of their host environment (Pde). The HII regions remain unresolved within our 50 to 100 pc resolution observations, so we place upper (Pmax) and lower (Pmin) limits on each of the pressures by using a minimum (i.e. clumpy structure) and maximum (i.e. smooth structure) size, respectively. We find that the Pmax measurements are broadly similar, and for Pmin the Ptherm is mildly dominant.We find that the majority of HII regions are over-pressured, and expanding, yet there is a small sample of compact HII regions that appear under pressured (~1% of the sample). These mostly reside in galaxy centres, or, specifically, environments of high gas surface density. Lastly, we compare to a sample of literature measurements for Ptherm and Prad to investigate how dominant pressure term transitions over around 5 dex in spatial dynamic range and 10 dex in pressure.
ALMA - Cycle 8 success
ALMA - Cycle 8 success
08/2021 - proposalsSome great news that we have our ALMA large program - ACES - now accepted! ACES will provide an unprecedented view of physical/chemical/kinematic conditions (via e.g. HNCO, SiO, H40α, complex molecular line emission) across all star-forming gas within the CMZ, and ranging 4 orders of magnitude in spatial dynamic range: global (100 pc) down to individual star-forming (0.05 pc) scales.
I also had some luck with a 24 hour proposal to observe dense gas lines across a nearby star forming disc galaxy NGC2903. This will provide one of the highest resolution, highest sensitivity complete maps of dense gas across a nearby extragalactic system, which will be extremely interesting to compare to existing datasets of molecular gas as part of the Phangs survey.
I also had some luck with a 24 hour proposal to observe dense gas lines across a nearby star forming disc galaxy NGC2903. This will provide one of the highest resolution, highest sensitivity complete maps of dense gas across a nearby extragalactic system, which will be extremely interesting to compare to existing datasets of molecular gas as part of the Phangs survey.
Hypatia Colloquium
Hypatia Colloquium
02/03/2021 - talkHigh-mass stars inject a large amount of energy and momentum - stellar feedback - into the interstellar medium (ISM) during their relatively short lifetimes. The feedback from these stars can influence the ISM both locally (<1pc) and across their entire host galaxy (~1kpc), and occurs through a variety of feedback processes; e.g. protostellar outflows, stellar winds, ionizing radiation. The most important of these feedback mechanisms for the overall energy and momentum budget of ISM occurs at the end of the stars lifetime, when they explode as supernovae. However, the efficiency with which SNe couple with their environment strongly depends on their local gas density distribution. Hence, the early pre-SNe feedback processes from high-mass stars play a crucial role in setting this environment into which SNe later explode, and, therefore, in effect limit the efficiency of SNe feedback. In this talk, I will discuss our recent efforts in a quantitative study of pre-SNe feedback mechanisms within both the centre Milky Way, and a large sample of nearby extragalactic systems. In these analyses, we focus on the balance of various internal and external pressures within young HII regions. The study of the Galactic Centre represents the first such study in a high- pressure environment, which has important implications for high-redshift environments. The study of extragalactic systems is the first to attempt such a study on a statistically significant sample of HII regions (>2000). Together, these make key advancements in our understanding of young stellar feedback as a function of environment.
Protostars and Planets review
Protostars and Planets review
01/09/2021 - submissionWe recently submitted our chapter to the Protostars and Planets series - more coming soon.....
ALMA-IRDC: Dense gas mass distribution from cloud to core scales
ALMA-IRDC: Dense gas mass distribution from cloud to core scales
03/2021 - paperhttps://arxiv.org/abs/2103.09122Infrared dark clouds (IRDCs) are potential hosts of the elusive early phases of high-mass starformation (HMSF). Here we conduct an in-depth analysis of the fragmentation properties ofa sample of 10 IRDCs, which have been highlighted as some of the best candidates to study HMSF within the Milky Way. To do so, we have obtained a set of large mosaics covering theseIRDCs with ALMA at band 3 (or 3 mm). These observations have a high angular resolution (300; ~0.05pc), and high continuum and spectral line sensitivity (0.15 mJy/beam and 0.2K per 0.1km/s channel at the N2H(1-0) transition). From the dust continuumemission, we identify 96 cores ranging from low- to high-mass that are gravitationally bound and which would require magnetic field strengths of 0.3 - 1mG to be in virial equilibrium. We combine these results with a homogenisedcatalogue of literature cores to recover the hierarchical structure within these clouds overfour orders of magnitude in spatial scale (0.01 pc – 10 pc). Using supplementary observationsat an even higher angular resolution, we find that the smallest fragments (< 0.02 pc) withinthis hierarchy do not currently have the mass and/or the density required to form high-mass stars. Nonetheless, the new ALMA observations presented in this paper have facilitatedthe identification of 19 (6 quiescent and 13 star-forming) cores that retain >16 Mwithoutfurther fragmentation. These high-mass cores contain trans-sonic non-thermal motions, arekinematically sub-virial, and require moderate magnetic field strengths for support againstcollapse. The identification of these potential sites of high-mass star formation representsa key step in allowing us to test the predictions from high-mass star and cluster formation theories.
Which feedback mechanisms dominate in the high-pressure environment of the Central Molecular Zone?
Which feedback mechanisms dominate in the high-pressure environment of the Central Molecular Zone?
09/2020 - paper https://arxiv.org/abs/2009.03901Supernovae (SNe) dominate the energy and momentum budget of stellar feedback, but the efficiency with which they couple to the interstellar medium (ISM) dependsstrongly on how effectively early, pre-SN feedback clears dense gas from star-formingregions. There are observational constraints on the magnitudes and timescales of earlystellar feedback in low ISM pressure environments, yet no such constraints exist formore cosmologically typical high ISM pressure environments. In this paper, we de-termine the mechanisms dominating the expansion of Hiiregions as a function ofsize-scale and evolutionary time within the high-pressure (log(P/kB/K/cm3)~7−8) en-vironment in the inner 100 pc of the Milky Way. We calculate the thermal pressurefrom the warm ionised (PHII; 104K) gas, direct radiation pressure (Pdir), and dust pro-cessed radiation pressure (PIR). We find that (1)Pdirdominates the expansion on smallscales and at early times (0.01-0.1 pc; <0.1 Myr); (2) the expansion is driven byPHIIon large scales at later evolutionary stages (>0.1pc; >1Myr); (3) during the first 1Myr of growth, but not thereafter, eitherPIRor stellar wind pressure likely makea comparable contribution. Despite the high confining pressure of the environment,natal star-forming gas is efficiently cleared to radii of several pc within ∼2 Myr, i.e.before the first SNe explode. This ‘pre-processing’ means that subsequent SNe willexplode into low density gas, so their energy and momentum will efficiently couple tothe ISM. We find the Hiiregions expand to a radius of ∼3pc, at which point they have internal pressures equal with the surrounding external pressure. A comparison with Hii regions in lower pressure environments shows that the maximum size of all Hii regions is set by pressure equilibrium with the ambient ISM.
LEGO – II. A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way disc
LEGO – II. A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way disc
07/2020 - paperhttps://arxiv.org/abs/2007.11005The current generation of (sub)mm-telescopes has allowed molecular line emission to become a major tool for studying the physical, kinematic, and chemical properties of extragalactic systems, yet exploiting these observations requires a detailed understanding of where emission lines originate within the Milky Way. In this paper, we present 60 arcsec (∼3 pc) resolution observations of many 3 mm band molecular lines across a large map of the W49 massive star-forming region (∼100 pc × 100 pc at 11 kpc), which were taken as part of the ‘LEGO’ IRAM-30m large project. We find that the spatial extent or brightness of the molecular line transitions are not well correlated with their critical densities, highlighting abundance and optical depth must be considered when estimating line emission characteristics. We explore how the total emission and emission efficiency (i.e. line brightness per H2 column density) of the line emission vary as a function of molecular hydrogen column density and dust temperature. We find that there is not a single region of this parameter space responsible for the brightest and most efficiently emitting gas for all species. For example, we find that the HCN transition shows high emission efficiency at high column density (1022 cm−2) and moderate temperatures (35 K), whilst e.g. N2H+ emits most efficiently towards lower temperatures (1022 cm−2; <20 K). We determine XCO(1−0) ∼ 0.3 × 1020 cm−2 (K km s−1) −1, and αHCN(1−0) ∼ 30 M (K km s−1 pc2) −1, which both differ significantly from the commonly adopted values. In all, these results suggest caution should be taken when interpreting molecular line emission.
Young massive star cluster formation in the Galactic Centre is driven by global gravitational collapse of high-mass molecular clouds
Young massive star cluster formation in the Galactic Centre is driven by global gravitational collapse of high-mass molecular clouds
03/2019 - paperhttps://arxiv.org/abs/1903.06158Young 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′′ / 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-C3H2 (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.