ALBERT

Description: The SILCC (SImulating the Life-Cycle of molecular Clouds) project aims to self-consistently understand the small-scale structure of the interstellar medium (ISM) and its link to galaxy evolution. We simulate the evolution of the multiphase ISM in a (500 pc) 2   x  ±5 kpc region of a galactic disc, with a gas surface density of $\Sigma _{_{\rm GAS}} = 10 \;{\rm M}_{\odot }\,{\rm pc}^{-2}$ . The flash 4 simulations include an external potential, self-gravity, magnetic fields, heating and radiative cooling, time-dependent chemistry of H 2 and CO considering (self-) shielding, and supernova (SN) feedback but omit shear due to galactic rotation. We explore SN explosions at different rates in high-density regions ( peak ), in random locations with a Gaussian distribution in the vertical direction ( random ), in a combination of both ( mixed ), or clustered in space and time ( clus / clus2 ). Only models with self-gravity and a significant fraction of SNe that explode in low-density gas are in agreement with observations. Without self-gravity and in models with peak driving the formation of H 2 is strongly suppressed. For decreasing SN rates, the H 2 mass fraction increases significantly from 〈10 per cent for high SN rates, i.e. 0.5 dex above Kennicutt–Schmidt, to 70–85 per cent for low SN rates, i.e. 0.5 dex below KS. For an intermediate SN rate, clustered driving results in slightly more H 2 than random driving due to the more coherent compression of the gas in larger bubbles. Magnetic fields have little impact on the final disc structure but affect the dense gas ( n   10 cm –3 ) and delay H 2 formation. Most of the volume is filled with hot gas (~80 per cent within ±150 pc). For all but peak driving a vertically expanding warm component of atomic hydrogen indicates a fountain flow. We highlight that individual chemical species populate different ISM phases and cannot be accurately modelled with temperature-/density-based phase cut-offs.

Description: Image cubes of differential column density as a function of dust temperature are constructed for Galactic Centre molecular cloud G0.253+0.016 (‘The Brick’) using the recently described PPMAP procedure. The input data consist of continuum images from the Herschel Space Telescope in the wavelength range 70–500 μm, supplemented by previously published interferometric data at 1.3 mm wavelength. While the bulk of the dust in the molecular cloud is consistent with being heated externally by the local interstellar radiation field, our image cube shows the presence, near one edge of the cloud, of a filamentary structure whose temperature profile suggests internal heating. The structure appears as a cool (~14 K) tadpole-like feature, ~6 pc in length, in which is embedded a thin spine of much hotter (~40–50 K) material. We interpret these findings in terms of a cool filament whose hot central region is undergoing gravitational collapse and fragmentation to form a line of protostars. If confirmed, this would represent the first evidence of widespread star formation having started within this cloud.

Description: It has been shown that the behaviour of primordial gas collapsing in a dark matter minihalo can depend on the adopted choice of three-body H 2 formation rate. The uncertainties in this rate span two orders of magnitude in the current literature, and so it remains a source of uncertainty in our knowledge of Population III star formation. Here, we investigate how the amount of fragmentation in primordial gas depends on the adopted three-body rate. We present the results of calculations that follow the chemical and thermal evolution of primordial gas as it collapses in two dark matter minihaloes. Our results on the effect of three-body rate on the evolution until the first protostar forms agree well with previous studies. However, our modified version of gadget -2 smoothed particle hydrodynamics also includes sink particles, which allows us to follow the initial evolution of the accretion disc that builds up on the centre of each halo, and capture the fragmentation in gas as well as its dependence on the adopted three-body H 2 formation rate. We find that the fragmentation behaviour of the gas is only marginally affected by the choice of three-body rate co-efficient, and that halo-to-halo differences are of equal importance in affecting the final mass distribution of stars.

Description: We use detailed numerical simulations of a turbulent molecular cloud to study the usefulness of the [C i ] 609 and 370 μm fine structure emission lines as tracers of cloud structure. Emission from these lines is observed throughout molecular clouds, and yet they have attracted relatively little theoretical attention. We show that the widespread [C i ] emission results from the fact that the clouds are turbulent. Turbulence creates large density inhomogeneities, allowing radiation to penetrate deeply into the clouds. As a result, [C i ] emitting gas is found throughout the cloud. We examine how well [C i ] emission traces the cloud structure, and show that the 609 μm line traces column density accurately over a wide range of values. For visual extinctions greater than a few, [C i ] and 13 CO both perform well, but [C i ] performs better at A V  ≤ 3. We have also studied the distribution of [C i ] excitation temperatures. We show that these are typically smaller than the kinetic temperature, indicating that the carbon is subthermally excited. We discuss how best to estimate the excitation temperature and the carbon column density, and show that the latter tends to be systematically underestimated. Consequently, estimates of the atomic carbon content of real giant molecular clouds could be wrong by up to a factor of 2.

Description: The SILCC (SImulating the Life-Cycle of molecular Clouds) project aims to self-consistently understand the small-scale structure of the interstellar medium (ISM) and its link to galaxy evolution. We simulate the evolution of the multiphase ISM in a (500 pc) 2   x  ±5 kpc region of a galactic disc, with a gas surface density of $\Sigma _{_{\rm GAS}} = 10 \;{\rm M}_{\odot }\,{\rm pc}^{-2}$ . The flash 4 simulations include an external potential, self-gravity, magnetic fields, heating and radiative cooling, time-dependent chemistry of H 2 and CO considering (self-) shielding, and supernova (SN) feedback but omit shear due to galactic rotation. We explore SN explosions at different rates in high-density regions ( peak ), in random locations with a Gaussian distribution in the vertical direction ( random ), in a combination of both ( mixed ), or clustered in space and time ( clus / clus2 ). Only models with self-gravity and a significant fraction of SNe that explode in low-density gas are in agreement with observations. Without self-gravity and in models with peak driving the formation of H 2 is strongly suppressed. For decreasing SN rates, the H 2 mass fraction increases significantly from 〈10 per cent for high SN rates, i.e. 0.5 dex above Kennicutt–Schmidt, to 70–85 per cent for low SN rates, i.e. 0.5 dex below KS. For an intermediate SN rate, clustered driving results in slightly more H 2 than random driving due to the more coherent compression of the gas in larger bubbles. Magnetic fields have little impact on the final disc structure but affect the dense gas ( n   10 cm –3 ) and delay H 2 formation. Most of the volume is filled with hot gas (~80 per cent within ±150 pc). For all but peak driving a vertically expanding warm component of atomic hydrogen indicates a fountain flow. We highlight that individual chemical species populate different ISM phases and cannot be accurately modelled with temperature-/density-based phase cut-offs.

Description: In prospect of its application in cryogenic rare-event searches, we have investigated the low-temperature scintillation properties of CaWO 4 crystals down to 3.4 K under α and γ excitation. Concerning the scintillation decay times, we observe a long component in the ms range which significantly contributes to the light yield below 40 K. For the first time we have measured the temperature dependence of the α / γ -ratio of the light yield. This parameter, which can be used to discriminate α and γ events in scintillating bolometers, is found to be ∼8%–15% smaller at low temperatures compared to room temperature.

Description: We run numerical simulations of molecular clouds, adopting properties similar to those found in the central molecular zone (CMZ) of the Milky Way. For this, we employ the moving mesh code arepo and perform simulations which account for a simplified treatment of time-dependent chemistry and the non-isothermal nature of gas and dust. We perform simulations using an initial density of n 0 = 10 3 cm –3 and a mass of 1.3 x 10 5 M . Furthermore, we vary the virial parameter, defined as the ratio of kinetic and potential energy, α = E kin /| E pot |, by adjusting the velocity dispersion. We set it to α = 0.5, 2.0 and 8.0, in order to analyse the impact of the kinetic energy on our results. We account for the extreme conditions in the CMZ and increase both the interstellar radiation field (ISRF) and the cosmic ray flux (CRF) by a factor of 1000 compared to the values found in the solar neighbourhood. We use the radiative transfer code radmc -3 d to compute synthetic images in various diagnostic lines. These are [C ii ] at 158 μm, [O i ] (145 μm), [O i ] (63 μm), 12 CO ( J = 1 -〉 0) and 13 CO ( J = 1 -〉 0) at 2600 and 2720 μm, respectively. When α is large, the turbulence disperses much of the gas in the cloud, reducing its mean density and allowing the ISRF to penetrate more deeply into the cloud's interior. This significantly alters the chemical composition of the cloud, leading to the dissociation of a significant amount of the molecular gas. On the other hand, when α is small, the cloud remains compact, allowing more of the molecular gas to survive. We show that in each case the atomic tracers accurately reflect most of the physical properties of both the H 2 and the total gas of the cloud and that they provide a useful alternative to molecular lines when studying the interstellar medium in the CMZ.

Description: The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a more self-consistent understanding of the interstellar medium (ISM) on small scales and its link to galaxy evolution. We present three-dimensional (magneto)hydrodynamic simulations of the ISM in a vertically stratified box including self-gravity, an external potential due to the stellar component of the galactic disc, and stellar feedback in the form of an interstellar radiation field and supernovae (SNe). The cooling of the gas is based on a chemical network that follows the abundances of H + , H, H 2 , C + , and CO and takes shielding into account consistently. We vary the SN feedback by comparing different SN rates, clustering and different positioning, in particular SNe in density peaks and at random positions, which has a major impact on the dynamics. Only for random SN positions the energy is injected in sufficiently low-density environments to reduce energy losses and enhance the effective kinetic coupling of the SNe with the gas. This leads to more realistic velocity dispersions ( $\sigma _\mathrm{H\,{\small {I}}}\approx 0.8\sigma _{300\rm{-}8000\,\mathrm{K}}\sim 10\hbox{-}20\,\mathrm{km}\,\mathrm{s}^{-1}$ , $\sigma _\mathrm{H\,\alpha }\approx 0.6\sigma _{8000-3\times 10^5\,\mathrm{K}}\sim 20\hbox{-}30\,\mathrm{km}\,\mathrm{s}^{-1}$ ), and strong outflows with mass loading factors (ratio of outflow to star formation rate) of up to 10 even for solar neighbourhood conditions. Clustered SNe abet the onset of outflows compared to individual SNe but do not influence the net outflow rate. The outflows do not contain any molecular gas and are mainly composed of atomic hydrogen. The bulk of the outflowing mass is dense ( ~ 10 –25 –10 –24 g cm –3 ) and slow ( v ~ 20–40 km s –1 ) but there is a high-velocity tail of up to v ~ 500 km s –1 with ~ 10 –28 –10 –27 g cm –3 .

Description: Carbon monoxide (CO) is widely used as a tracer of molecular hydrogen (H 2 ) in metal-rich galaxies, but is known to become ineffective in low-metallicity dwarf galaxies. Atomic carbon has been suggested as a superior tracer of H 2 in these metal-poor systems, but its suitability remains unproven. To help us to assess how well atomic carbon traces H 2 at low metallicity, we have performed a series of numerical simulations of turbulent molecular clouds that cover a wide range of different metallicities. Our simulations demonstrate that in star-forming clouds, the conversion factor between [C i ] emission and H 2 mass, X CI , scales approximately as X CI    Z –1 . We recover a similar scaling for the CO-to-H 2 conversion factor, X CO , but find that at this point in the evolution of the clouds, X CO is consistently smaller than X CI , by a factor of a few or more. We have also examined how X CI and X CO evolve with time. We find that X CI does not vary strongly with time, demonstrating that atomic carbon remains a good tracer of H 2 in metal-poor systems even at times significantly before the onset of star formation. On the other hand, X CO varies very strongly with time in metal-poor clouds, showing that CO does not trace H 2 well in starless clouds at low metallicity.