Does feedback help or hinder star formation? The effect of photoionisation on star formation in Giant Molecular Clouds

We investigated the effect of photoionising feedback inside turbulent star-forming clouds, comparing the resultant star formation in both idealised profiles and more realistic cloud structures drawn from a global galaxy simulation. We performed a series of numerical simulations which compared the effect of star formation alone, photoionisation and photoionisation plus supernovae feedback. In the idealised cloud, photoionisation suppresses gas fragmentation at early times, resulting in the formation of more massive stars and an increase in the star formation efficiency. At later times, the dispersal of the dense gas causes the radiative feedback effect to switch from positive to negative as the star formation efficiency drops. In the cloud extracted from the global simulation, the initial cloud is heavily fragmented prior to the stellar feedback beginning and is largely structurally unaffected by the late injection of radiation energy. The result is a suppression of the star formation. We conclude that the efficiency of feedback is heavily dependent on the gas structure, with negative feedback dominating when the density is high.Comment: Accepted to MNRA

Environmental dependence of star formation induced by cloud collisions in a barred galaxy

Cloud collision have been proposed as a way to link the small-scale star formation process with the observed global relation between the surface star formation rate and gas surface density. We suggest that this model can be improved further by allowing the productivity of such collisions to depend on the relative velocity of the two clouds. Our adjustment implements a simple step function that results in the most successful collisions being at the observed velocities for triggered star formation. By applying this to a high resolution simulation of a barred galaxy, we successfully reproduce the observational result that the star formation efficiency (SFE) in the bar is lower than that in the spiral arms. This is not possible when we use an efficiency dependent on the internal turbulence properties of the clouds. Our results suggest that high velocity collisions driven by the gravitational pull of the clouds are responsible for the low bar SFE.Comment: 6 pages, 4 figures. Accepted for publication in MNRAS Letter

Star Formation in Disk Galaxies. III. Does stellar feedback result in cloud death?

Stellar feedback, star formation and gravitational interactions are major controlling forces in the evolution of Giant Molecular Clouds (GMCs). To explore their relative roles, we examine the properties and evolution of GMCs forming in an isolated galactic disk simulation that includes both localised thermal feedback and photoelectric heating. The results are compared with the three previous simulations in this series which consists of a model with no star formation, star formation but no form of feedback and star formation with photoelectric heating in a set with steadily increasing physical effects. We find that the addition of localised thermal feedback greatly suppresses star formation but does not destroy the surrounding GMC, giving cloud properties closely resembling the run in which no stellar physics is included. The outflows from the feedback reduce the mass of the cloud but do not destroy it, allowing the cloud to survive its stellar children. This suggests that weak thermal feedback such as the lower bound expected for supernova may play a relatively minor role in the galactic structure of quiescent Milky Way-type galaxies, compared to gravitational interactions and disk shear.Comment: 15 pages, 15 figures, accepted for publication in Ap

Formation of an embryonic supermassive star in the first galaxy

We studied the gravitational collapse of a warm (~8000 K) primordial-gas cloud as a candidate progenitor for a supermassive star (SMS; >10^5 Msun) using a three-dimensional hydrodynamical simulation, including all the relevant cooling processes of both H_2 and H, which can potentially induce cloud fragmentation. This is the first simulation of this kind to resolve protostar formation. We find that the cloud undergoes runaway collapse without a major episode of fragmentation. Although the H_2 fraction jumps by a large factor via the three-body reaction at ~10^-13 g/cm^3, its cooling remains inefficient due to the optical thickness, and the temperature remains >3000 K. When the central core of the cloud becomes opaque to continuum radiation at ~10^-8 g/cm^3, a hydrostatic protostar with ~0.2 Msun is formed. The protostar grows to the mass ~1 Msun and the radius ~2 AU within ~1 yr via rapid accretion of dense filamentary flows. With high accretion rate ~2 Msun/yr, the protostar is expected to turn into a SMS within its lifetime, eventually collapsing to a seed for the supermassive black hole observed in the early Universe at z~7.Comment: 6 pages, 6 figures, Accepted for publication in MNRA

Gas and stellar spiral structures in tidally perturbed disc galaxies

Tidal interactions between disc galaxies and low mass companions are an established method for generating galactic spiral features. In this work we present a study of the structure and dynamics of spiral arms driven in interactions between disc galaxies and perturbing companions in 3-D N-body/smoothed hydrodynamical numerical simulations. Our specific aims are to characterize any differences between structures formed in the gas and stars from a purely hydrodynamical and gravitational perspective, and to find a limiting case for spiral structure generation. Through analysis of a number of different interacting cases, we find that there is very little difference between arm morphology, pitch angles and pattern speeds between the two media. The main differences are a minor offset between gas and stellar arms, clear spurring features in gaseous arms, and different radial migration of material in the stronger interacting cases. We investigate the minimum mass of a companion required to drive spiral structure in a galactic disc, finding the limiting spiral generation cases with companion masses of the order $1\times10^9M_\odot$, equivalent to only 4% of the stellar disc mass, or 0.5% of the total galactic mass of a Milky Way analogue.Comment: 20 pages, 23 figures, accepted for publication by MNRA

Do Giant Molecular Clouds Care About the Galactic Structure?

We investigate the impact of galactic environment on the properties of simulated giant molecular clouds formed in a M83-type barred spiral galaxy. Our simulation uses a rotating stellar potential to create the grand design features and resolves down to 1.5 pc. From the comparison of clouds found in the bar, spiral and disc regions, we find that the typical GMC is environment independent, with a mass of 5e+5 Msun and radius 11 pc. However, the fraction of clouds in the property distribution tails varies between regions, with larger, more massive clouds with a higher velocity dispersion being found in greatest proportions in the bar, spiral and then disc. The bar clouds also show a bimodality that is not reflected in the spiral and disc clouds except in the surface density, where all three regions show two distinct peaks. We identify these features as being due to the relative proportion of three cloud types, classified via the mass-radius scaling relation, which we label A, B and C. Type A clouds have the typical values listed above and form the largest fraction in each region. Type B clouds are massive giant molecular associations while Type C clouds are unbound, transient clouds that form in dense filaments and tidal tails. The fraction of each clouds type depends on the cloud-cloud interactions, which cause mergers to build up the GMA Type Bs and tidal features in which the Type C clouds are formed. The number of cloud interactions is greatest in the bar, followed by the spiral, causing a higher fraction of both cloud types compared to the disc. While the cloud types also exist in lower resolution simulations, their identification becomes more challenging as they are not well separated populations on the mass-radius relation or distribution plots. Finally, we compare the results for three star formation models to estimate the star formation rate and efficiency in each region.Comment: 21 pages, 14 figures. Accepted for publication in MNRA