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There is a longstanding discussion about whether low mass stars can form from pristine gas in the early Universe. A particular point of interest is whether we can find surviving pristine stars from the first generation in our local neighbourhood. We present here a simple analytical estimate that puts tighter constraints on the existence of such stars. In the conventional picture, should these stars have formed in significant numbers and have preserved their pristine chemical composition until today, we should have found them already. With the presented method most current predictions for survivor counts larger than zero can be ruled out., Comment: 5 pages, 2 figures, accepted for publication in MNRAS

We present a suite of high resolution radiation hydrodynamic simulations of a small patch ($1 \ {\rm kpc}^2$) of the inter-stellar medium (ISM) performed with Arepo-RT, with the aim to quantify the efficacy of various feedback processes like supernovae explosions (SNe), photoheating and radiation pressure in low gas surface density galaxies ($\Sigma_{\rm gas} \simeq 10 \ {\rm M}_\odot \ {\rm pc}^{-2}$). We show that radiation fields decrease the star formation rate and therefore the total stellar mass formed by a factor of $\sim 2$. This increases the gas depletion timescale and brings the simulated Kennicutt-Schmidt relation closer to the observational estimates. Radiation feedback coupled with SNe is more efficient at driving outflows with the mass and energy loading increasing by a factor of $\sim 10$. This increase is mainly driven by the additional entrainment of medium density ($10^{-2} \leq n< 1 \ {\rm cm}^{-3}$), warm ($300 \ {\rm K}\leq T<8000 \ {\rm K}$) material. Therefore including radiation fields tends to launch colder, denser and higher mass and energy loaded outflows. This is because photoheating of the high density gas around a newly formed star over-pressurises the region, causing it to expand. This reduces the ambient density in which the SNe explode by a factor of $10-100$ which in turn increases their momentum output by a factor of $\sim 1.5-2.5$. Finally, we note that in these low gas surface density environments, radiation fields primarily impact the ISM via photoheating and radiation pressure has only a minimal role in regulating star formation., Comment: 17 pages, 18 figures, Submitted to MNRAS

How, when and where the first stars formed are fundamental questions regarding the epoch of Cosmic Dawn. A second order effect in the fluid equations was recently found to make a significant contribution: an offset velocity between gas and dark matter, the so-called streaming velocity. Previous simulations of a limited number of low-mass dark matter haloes suggest that this streaming velocity can delay the formation of the first stars and decrease halo gas fractions and the halo mass function in the low mass regime. However, a systematic exploration of its effects in a large sample of haloes has been lacking until now. In this paper, we present results from a set of cosmological simulations of regions of the Universe with different streaming velocities performed with the moving mesh code AREPO. Our simulations have very high mass resolution, enabling us to accurately resolve minihaloes as small as $10^5 \: {\rm M_{\odot}}$. We show that in the absence of streaming, the least massive halo that contains cold gas has a mass $M_{\rm halo, min} = 5 \times 10^{5} \: {\rm M_{\odot}}$, but that cooling only becomes efficient in a majority of haloes for halo masses greater than $M_{\rm halo,50\%} = 1.6 \times 10^6 \: {\rm M_{\odot}}$. In regions with non-zero streaming velocities, $M_{\rm halo, min}$ and $M_{\rm halo,50\%}$ both increase significantly, by around a factor of a few for each one sigma increase in the value of the local streaming velocity. As a result, in regions with streaming velocities $v_\mathrm{stream} \ge 3\,\sigma_\mathrm{rms}$, cooling of gas in minihaloes is completely suppressed, implying that the first stars in these regions form within atomic cooling haloes., Comment: 13 pages, 16 figures, resubmitted to MNRAS

We present a [CII] 158um map of the entire M51 (including M51b) grand--design spiral galaxy observed with the FIFI-LS instrument on SOFIA. We compare the [CII] emission with the total far--infrared (TIR) intensity and star formation rate(SFR) surface density maps (derived using H_alpha and 24um emission) to study the relationship between [CII] and the star formation activity in a variety of environments within M51 on scales of 16" corresponding to ~660 pc. We find that [CII] and the SFR surface density are well correlated in the central, spiral arm, and inter-arm regions. The correlation is in good agreement with that found for a larger sample of nearby galaxies at kpc scales. We find that the SFR, and [CII] and TIR luminosities in M51 are dominated by the extended emission in M51's disk. The companion galaxy M51b, however, shows a deficit of [CII] emission compared with the TIR emission and SFR surface density, with [CII] emission detected only in the S-W part of this galaxy. The [CII] deficit is associated with an enhanced dust temperature in this galaxy. We interpret the faint [CII] emission in M51b to be a result of suppressed star formation in this galaxy, while the bright mid- and far-infrared emission, which drive the TIR and SFR values, are powered by other mechanisms. A similar but less pronounced effect is seen at the location of the black hole in M51's center. The observed [CII] deficit in M51b suggests that this galaxy is a valuable laboratory to study the origin of the apparent [CII] deficit observed in ultra-luminous galaxies., Comment: 10 pages, 4 figures, Accepted for publication in Apj Letters

We use ALMA and IRAM 30-m telescope data to investigate the relationship between the spectroscopically-traced dense gas fraction and the cloud-scale (120 pc) molecular gas surface density in five nearby, star-forming galaxies. We estimate the dense gas mass fraction at 650 pc and 2800 pc scales using the ratio of HCN (1-0) to CO (1-0) emission. We then use high resolution (120 pc) CO (2-1) maps to calculate the mass-weighted average molecular gas surface density within 650 pc or 2770 pc beam where the dense gas fraction is estimated. On average, the dense gas fraction correlates with the mass-weighted average molecular gas surface density. Thus, parts of a galaxy with higher mean cloud-scale gas surface density also appear to have a larger fraction of dense gas. The normalization and slope of the correlation do vary from galaxy to galaxy and with the size of the regions studied. This correlation is consistent with a scenario where the large-scale environment sets the gas volume density distribution, and this distribution manifests in both the cloud-scale surface density and the dense gas mass fraction., Comment: 11 pages, 4 figures, accepted for publication in The Astrophysical Journal Letters

In recent years, the discovery of massive quasars at z~7 has provided a striking challenge to our understanding of the origin and growth of supermassive black holes in the early Universe. Mounting observational and theoretical evidence indicates the viability of massive seeds, formed by the collapse of supermassive stars, as a progenitor model for such early, massive accreting black holes. Although considerable progress has been made in our theoretical understanding, many questions remain regarding how (and how often) such objects may form, how they live and die, and how next generation observatories may yield new insight into the origin of these primordial titans. This review focusses on our present understanding of this remarkable formation scenario, based on discussions held at the Monash Prato Centre from November 20--24, 2017, during the workshop "Titans of the Early Universe: The Origin of the First Supermassive Black Holes.", Comment: Solicited review article (accepted) for Publications of the Astronomical Society of Australia. 38 pages, 15 figures, 1 table

Star formation is an inefficient process and in general only a small fraction of the gas in a giant molecular cloud (GMC) is turned into stars. This is partly due to the negative effect of stellar feedback from young massive star clusters. In a recent paper, we introduced a novel 1D numerical treatment of the effects of stellar feedback from young massive clusters on their natal clouds, which we named WARPFIELD. Here, we present version 2 of the WARPFIELD code, containing improved treatments of the thermal evolution of the gas and the fragmentation of the feedback-driven shell. As part of this update, we have produced new cooling and heating tables that account for the combined effects of photoionization and collisional ionization on the cooling rate of the gas, which we now make publically available. We employ our updated version of WARPFIELD to investigate the impact of stellar feedback on GMCs with a broad range of masses and surface densities and a variety of density profiles. We show that the minimum star formation efficiency $\epsilon_{\mathrm{min}}$, i.e. the star formation efficiency above which the cloud is destroyed by feedback and further star formation is shut off, is mainly set by the average cloud surface density. A star formation efficiency of 1-6 % is generally sufficient to destroy a GMC. We also find star formation efficiencies per free-fall time $\epsilon_{\mathrm{ff}} \sim 0.3$ %, in good agreement with recent observations. Our results imply that stellar feedback alone is sufficient to explain the low observed star formation efficiencies of GMCs. Finally, we show that very massive clouds with steep density profiles - possible proxies of the giant clumps observed in galaxies at $z \approx 2$ - are more resilient to feedback than typical GMCs, with $\epsilon_{\mathrm{min}}$ between 1 and 12 %., Comment: 15 pages, 6 figures

Using the FirstLight database of 300 zoom-in cosmological simulations we provide rest-frame UV-optical spectral energy distributions of galaxies with complex star-formation histories that are coupled to the non-uniform gas accretion history of galactic halos during cosmic dawn. The population at any redshift is very diverse ranging from starbursts to quiescent galaxies even at a fixed stellar mass. This drives a redshift-dependent relation between UV luminosity and stellar mass with a large scatter, driven by the specific star formation rate. The UV slope and the production efficiency of Lyman continuum photons have high values, consistent with dust-corrected observations. This indicates young stellar populations with low metallicities. The FirstLight simulations make predictions on the rest-frame UV-optical absolute magnitudes, colors and optical emission lines of galaxies at z=6-12 that will be observed for the first time with JWST and the next generation of telescopes in the coming decade., Comment: 10 pages+appendix , 11 figures. Accepted at MNRAS. The FirstLight database is available at http://www.ita.uni-heidelberg.de/~ceverino/FirstLight/index.html

The spin and shape of galaxies at the present day have been well-studied both observationally and theoretically. At high redshifts, however, we have to rely on numerical simulations. In this study, we investigate the shape and spin of minihaloes with masses of $M \sim 10^5$--$10^7 \, {\rm M_{\odot}}$ which are of particular interest as they are the sites where the first stars in the Universe form. We analyse a large sample of these minihaloes, selected from a high resolution cosmological simulation. The first minihaloes form at $z \simeq 24$ and by the end of the simulation at $z \simeq 14$ our sample includes $\sim 9000$ minihaloes. We find that the spin parameter of the minihaloes follows a log-normal distribution with minimal dependence on redshift. Most minihaloes are prolate, but those formed at the highest redshifts are more prolate than those formed at lower redshifts. On the scale of the virial radius, there is a good correlation between the shape and spin of the gas and that of the dark matter. However, this correlation breaks down in gas which is cooling and undergoing gravitational collapse. We show, contrary to previous assumptions, that although the direction of the spin of the central dense gas correlates well with that of the halo, the magnitude of the spin of the dense gas is uncorrelated with that of the halo. Therefore, measurements of the spin of minihaloes on large scales tell us little about the angular momentum of the gas responsible for forming the first stars., Comment: 13 pages, 16 figures, accepted for publication

Our understanding of how molecular clouds form in the interstellar medium (ISM) would be greatly helped if we had a reliable observational tracer of the gas flows responsible for forming the clouds. Fine structure emission from singly ionised and neutral carbon ([CII], [CI]) and rotational line emission from CO are all observed to be associated with molecular clouds. However, it remains unclear whether any of these tracers can be used to study the inflow of gas onto an assembling cloud, or whether they primarily trace the cloud once it has already assembled. In this paper, we address this issue with the help of high resolution simulations of molecular cloud formation that include a sophisticated treatment of the chemistry and thermal physics of the ISM. Our simulations demonstrate that both [CI] and CO emission trace gas that is predominantly molecular, with a density n ~ 500-1000 cm$^{-3}$, much larger than the density of the inflowing gas. [CII] traces lower density material (n ~ 100 cm$^{-3}$) that is mainly atomic at early times. A large fraction of the [CII] emission traces the same structures as the [CI] or CO emission, but some arises in the inflowing gas. Unfortunately, this emission is very faint and will be difficult to detect with current observational facilities, even for clouds situated in regions with an elevated interstellar radiation field., Comment: 17 pages, 9 figures. Version accepted by MNRAS

Magnetic fields are ubiquitously observed in the interstellar medium (ISM) of present-day star-forming galaxies with dynamically relevant energy densities. Using three-dimensional magneto-hydrodynamic (MHD) simulations of the supernova (SN) driven ISM in the flux-freezing approximation (ideal MHD) we investigate the impact of the magnetic field on the chemical and dynamical evolution of the gas, fragmentation and the formation of molecular clouds. We follow the chemistry with a network of six species (H$^{+}$, H, H$_2$, C$^+$, CO, free electrons) including local shielding effects. We find that magnetic fields thicken the disc by a factor of a few to a scale height of $\sim100\,\mathrm{pc}$, delay the formation of dense (and molecular) gas by $\sim25\,\mathrm{Myr}$ and result in differently shaped gas structures. The magnetised gas fragments into fewer clumps, which are initially at subcritical mass-to-flux ratios, $M/\Phi\approx0.3(M/\Phi)_\mathrm{crit}$, and accrete gas preferentially parallel to the magnetic field lines until supercritial mass-to-flux ratios of up to order 10 are reached. The accretion rates onto molecular clouds scale with $\dot{M}\propto M^{1.5}$. The median of the inter-cloud velocity dispersion is $\sim2-5\,\mathrm{km\,s}^{-1}$ and lower than the internal velocity dispersion in the clouds ($\sim3-7\,\mathrm{km\,s}^{-1}$). However, individual cloud-cloud collisions occur at speeds of a few $10\,\mathrm{km\,s}^{-1}$., Comment: 31 pages, 26 figures, MNRAS accepted

We develop a dynamical theory for the origin of nuclear rings in barred galaxies. In analogy with the standard theory of accretion discs, our theory is based on shear viscous forces among nested annuli of gas. However, the fact that gas follows non circular orbits in an external barred potential has profound consequences: it creates a region of reverse shear in which it is energetically favourable to form a stable ring which does not spread despite dissipation. Our theory allows us to approximately predict the size of the ring given the underlying gravitational potential. The size of the ring is loosely related to the location of the Inner Lindblad Resonance in the epicyclic approximation, but the predicted location is more accurate and is also valid for strongly barred potentials. By comparing analytical predictions with the results of hydrodynamical simulations, we find that our theory provides a viable mechanism for ring formation if the effective sound speed of the gas is low ($\cs\lesssim1\kms$), but that nuclear spirals/shocks created by pressure destroy the ring when the sound speed is high ($\cs\simeq10\kms$). We conclude that whether this mechanism for ring formation is relevant for real galaxies ultimately depends on the effective equation of state of the ISM. Promising confirmation comes from simulations in which the ISM is modelled using state-of-the-art cooling functions coupled to live chemical networks, but more tests are needed regarding the role of turbulence driven by stellar feedback. If the mechanism is relevant in real galaxies, it could provide a powerful tool to constrain the gravitational potential, in particular the bar pattern speed., Comment: Accepted for publication in MNRAS

Sources of X-rays such as active galactic nuclei and X-ray binaries are often variable by orders of magnitude in luminosity over timescales of years. During and after these flares the surrounding gas is out of chemical and thermal equilibrium. We introduce a new implementation of X-ray radiative transfer coupled to a time-dependent chemical network for use in 3D magnetohydrodynamical simulations. A static fractal molecular cloud is irradiated with X-rays of different intensity, and the chemical and thermal evolution of the cloud are studied. For a simulated $10^5$ M$_\odot$ fractal cloud an X-ray flux $<0.01$ erg cm$^{-2}$ s$^{-1}$ allows the cloud to remain molecular, whereas most of the CO and H$_2$ are destroyed for a flux of $>1$ erg cm$^{-2}$ s$^{-1}$. The effects of an X-ray flare, which suddenly increases the X-ray flux by $10^5 \times$ are then studied. A cloud exposed to a bright flare has 99% of its CO destroyed in 10-20 years, whereas it takes $>10^3$ years for 99% of the H$_2$ to be destroyed. CO is primarily destroyed by locally generated far-UV emission from collisions between non-thermal electrons and H$_2$; He$^+$ only becomes an important destruction agent when the CO abundance is already very small. After the flare is over, CO re-forms and approaches its equilibrium abundance after $10^3-10^5$ years. This implies that molecular clouds close to Sgr A$^*$ in the Galactic Centre may still be out of chemical equilibrium, and we predict the existence of clouds near flaring X-ray sources in which CO has been mostly destroyed but H is fully molecular., Comment: Accepted for publication in MNRAS; this version has some additions following the refereeing process

In the FirstLight project, we have used ~300 cosmological, zoom-in simulations to determine the star-formation histories of distinct first galaxies with stellar masses between Ms=10^6 and 3 x 10^9 Msun during cosmic dawn (z=5-15). The evolution of the star formation rate (SFR) in each galaxy is complex and diverse, characterized by bursts of star formation. Overall, first galaxies spend 70% of their time in SF bursts. A sample of 1000 of these bursts indicates that the typical burst at z=6 has a specific SFR (sSFR) maximum of 5-15 Gyr^-1 with an effective width of ~100 Myr, one tenth of the age of the Universe at that redshift. A quarter of the bursts populate a tail with very high sSFR maxima of 20-30 Gyr^-1 and significantly shorter timescales of ~40-80 Myr. This diversity of bursts sets the mean and the mass-dependent scatter of the star-forming main sequence. This scatter is driven by a population of low-mass, Ms < 10^8 Msun, quiescent galaxies. The mean sSFR and the burst maximum at fixed mass increase with redshift, with the later always being a factor ~2 higher than the former. This implies sSFR maxima of ~20-60 Gyr^-1 at z=9-10. The SFR histories are publicly available at the FirstLight website., Comment: 10 pages, 8 figures. Accepted version at MNRAS. The star-formation histories and SEDs of these galaxies are publicly available at the FirstLight database: http://www.ita.uni-heidelberg.de/~ceverino/FirstLight/index.html

Extremely metal-poor (EMP) stars in the Milky Way (MW) allow us to infer the properties of their progenitors by comparing their chemical composition to the metal yields of the first supernovae. This method is most powerful when applied to mono-enriched stars, i.e. stars that formed from gas that was enriched by only one previous supernova. We present a novel diagnostic to identify this subclass of EMP stars. We model the first generations of star formation semi-analytically, based on dark matter halo merger trees that yield MW-like halos at the present day. Radiative and chemical feedback are included self-consistently and we trace all elements up to zinc. Mono-enriched stars account for only $\sim 1\%$ of second generation stars in our fiducial model and we provide an analytical formula for this probability. We also present a novel analytical diagnostic to identify mono-enriched stars, based on the metal yields of the first supernovae. This new diagnostic allows us to derive our main results independently from the specific assumptions made regarding Pop III star formation, and we apply it to a set of observed EMP stars to demonstrate its strengths and limitations. Our results may provide selection criteria for current and future surveys and therefore contribute to a deeper understanding of EMP stars and their progenitors., Comment: 18 pages, 20 figures, published in MNRAS

Synthetic observations are playing an increasingly important role across astrophysics, both for interpreting real observations and also for making meaningful predictions from models. In this review, we provide an overview of methods and tools used for generating, manipulating and analysing synthetic observations and their application to problems involving star formation and the interstellar medium. We also discuss some possible directions for future research using synthetic observations., Comment: 71 Pages, 16 figures. Revised following reviewers comments

In this paper we introduce a new radiative transfer code SPRAI (Simplex Photon Radiation in the Arepo Implementation) based on the SimpleX radiation transfer method. This method, originally used only for post-processing, is now directly integrated into the Arepo code and takes advantage of its adaptive unstructured mesh. Radiated photons are transferred from the sources through the series of Voronoi gas cells within a specific solid angle. From the photon attenuation we derive corresponding photon fluxes and ionization rates and feed them to a primordial chemistry module. This gives us a self-consistent method for studying dynamical and chemical processes caused by ionizing sources in primordial gas. Since the computational cost of the SimpleX method does not scale directly with the number of sources, it is convenient for studying systems such as primordial star-forming halos that may form multiple ionizing sources.

Line emission is strongly dependent on the local environmental conditions in which the emitting tracers reside. In this work, we focus on modelling the CO emission from simulated giant molecular clouds (GMCs), and study the variations in the resulting line ratios arising from the emission from the $J=1-0$, $J=2-1$ and $J=3-2$ transitions. We perform a set of smoothed particle hydrodynamics (SPH) simulations with time-dependent chemistry, in which environmental conditions -- including total cloud mass, density, size, velocity dispersion, metallicity, interstellar radiation field (ISRF) and the cosmic ray ionisation rate (CRIR) -- were systematically varied. The simulations were then post-processed using radiative transfer to produce synthetic emission maps in the 3 transitions quoted above. We find that the cloud-averaged values of the line ratios can vary by up to $\pm 0.3$ dex, triggered by changes in the environmental conditions. Changes in the ISRF and/or in the CRIR have the largest impact on line ratios since they directly affect the abundance, temperature and distribution of CO-rich gas within the clouds. We show that the standard methods used to convert CO emission to H$_2$ column density can underestimate the total H$_2$ molecular gas in GMCs by factors of 2 or 3, depending on the environmental conditions in the clouds., Comment: 15 pages, 14 figures, 1 table

We measure the parsec-scale relationship between integrated CO intensity (I_CO) and visual extinction (A_V) in 24 local molecular clouds using maps of CO emission and dust optical depth from Planck. This relationship informs our understanding of CO emission across environments, but clean Milky Way measurements remain scarce. We find uniform I_CO for a given A_V, with the results bracketed by previous studies of the Pipe and Perseus clouds. Our measured I_CO-A_V relation broadly agrees with the standard Galactic CO-to-H2 conversion factor, the relation found for the Magellanic clouds at coarser resolution, and numerical simulations by Glover & Clark (2016). This supports the idea that CO emission primarily depends on shielding, which protects molecules from dissociating radiation. Evidence for CO saturation at high A_V and a threshold for CO emission at low A_V varies remains uncertain due to insufficient resolution and ambiguities in background subtraction. Resolution of order 0.1 pc may be required to measure these features. We use this I_CO-AV relation to predict how the CO-to-H2 conversion factor (X_CO) would change if the Solar Neighborhood clouds had different dust-to-gas ratio (metallicity). The calculations highlight the need for improved observations of the CO emission threshold and HI shielding layer depth. They are also sensitive to the shape of the column density distribution. Because local clouds collectively show a self-similar distribution, we predict a shallow metallicity dependence for X_CO down to a few tenths of solar metallicity. However, our calculations also imply dramatic variations in cloud-to-cloud X_CO at subsolar metallicity., Comment: Accepted for publication in MNRAS, 16 pages, 9 figures in main text, manuscript also includes a 24 page atlas showing each cloud. Full resolution available here: http://www.astronomy.ohio-state.edu/~leroy.42/MW_Planck_ICO_AV.pdf

The 30 Doradus Nebula in the Large Magellanic Cloud (LMC) contains the massive starburst cluster NGC 2070 with a massive and probably younger stellar sub clump at its center: R136. It is not clear how such a massive inner cluster could form several million years after the older stars in NGC 2070, given that stellar feedback is usually thought to expel gas and inhibit further star formation. Using the recently developed 1D feedback scheme WARPFIELD to scan a large range of cloud and cluster properties, we show that an age offset of several million years between the stellar populations is in fact to be expected given the interplay between feedback and gravity in a giant molecular cloud (GMC) with a density $\gtrsim 500$ cm$^{-3}$ due to re-accretion of gas onto the older stellar population. Neither capture of field stars nor gas retention inside the cluster have to be invoked in order to explain the observed age offset in NGC 2070 as well as the structure of the interstellar medium (ISM) around it., Comment: 5 pages, 3 figures, accepted to MNRAS

It has been known for more than thirty years that the distribution of molecular gas in the innermost 300 parsecs of the Milky Way, the Central Molecular Zone, is strongly asymmetric. Indeed, approximately three quarters of molecular emission comes from positive longitudes, and only one quarter from negative longitudes. However, despite much theoretical effort, the origin of this asymmetry has remained a mystery. Here we show that the asymmetry can be neatly explained by unsteady flow of gas in a barred potential. We use high-resolution 3D hydrodynamical simulations coupled to a state-of-the-art chemical network. Despite the initial conditions and the bar potential being point-symmetric with respect to the Galactic Centre, asymmetries develop spontaneously due to the combination of a hydrodynamical instability known as the "wiggle instability" and the thermal instability. The observed asymmetry must be transient: observations made tens of megayears in the past or in the future would often show an asymmetry in the opposite sense. Fluctuations of amplitude comparable to the observed asymmetry occur for a large fraction of the time in our simulations, and suggest that the present is not an exceptional moment in the life of our Galaxy., Comment: Accepted for publication in MNRAS. Videos of the simulations are available at http://www.ita.uni-heidelberg.de/~mattia/download.html

The search for metal-free stars has so far been unsuccessful, proving that if there are surviving stars from the first generation, they are rare, they have been polluted, or we have been looking in the wrong place. To predict the likely location of Population~III (Pop~III) survivors, we semi-analytically model early star formation in progenitors of Milky Way-like galaxies and their environments. We base our model on merger trees from the high-resolution dark matter only simulation suite \textit{Caterpillar}. Radiative and chemical feedback are taken into account self-consistently, based on the spatial distribution of the haloes. Our results are consistent with the non-detection of Pop III survivors in the Milky Way today. We find that possible surviving Population III stars are more common in Milky Way satellites than in the main Galaxy. In particular, low mass Milky Way satellites contain a much larger fraction of Pop~III stars than the Milky Way. Such nearby, low mass Milky Way satellites are promising targets for future attempts to find Pop~III survivors, especially for high-resolution, high signal-to-noise spectroscopic observations. We provide the probabilities for finding a Pop~III survivor in the red giant branch phase for all known Milky Way satellites to guide future observations., Comment: 17 pages, 12 figures, 1 table, submitted to MNRAS

The ratio of the [CII] 158$\,\mu$m emission line over the total infrared emission (TIR) is often used as a proxy for the photoelectric (PE) heating efficiency ($\epsilon_{\rm PE}$) of the far-ultraviolet (FUV) photons absorbed by dust in the interstellar medium. In the nearby galaxy M31, we measure a strong radial variation of [CII]/TIR that we rule out as being due to an intrinsic variation in $\epsilon_{\rm PE}$. [CII]/TIR fails as a proxy for $\epsilon_{\rm PE}$, because the TIR measures all dust heating, not just the contribution from FUV photons capable of ejecting electrons from dust grains. Using extensive multiwavelength coverage from the FUV to far-infrared (FIR), we infer the attenuated FUV emission ($\rm UV_{att}$), and the total attenuated flux ($\rm TOT_{att}$). We find [CII]/TIR to be strongly correlated with $\rm UV_{att}$/$\rm TOT_{att}$, indicating that, in M31 at least, one of the dominant drivers for [CII]/TIR variation is the relative hardness of the absorbed stellar radiation field. We define $\rm{ \epsilon_{PE}^{UV}}$, [CII]/$\rm{ UV_{att}}$ which should be more closely related to the actual PE efficiency, which we find to be essentially constant ($1.85 \pm 0.8 \%$) in all explored fields in M31. This suggests that part of the observed variation of [CII]/TIR in other galaxies is likely due to a change in the relative hardness of the absorbed stellar radiation field, caused by a combination of variations in the stellar population, dust opacity and galaxy metallicity, although PE efficiency may also vary across a wider range of environments., Comment: 19 pages, 16 figures, accepted for publication in ApJ

We study the influence of a high baryonic streaming velocity on the formation of direct collapse black holes (DCBHs) with the help of cosmological simulations carried out using the moving mesh code {\sc arepo}. We show that a streaming velocity that is as large as three times the root-mean-squared value is effective at suppressing the formation of H$_{2}$-cooled minihaloes, while still allowing larger atomic cooling haloes (ACHs) to form. We find that enough H$_{2}$ forms in the centre of these ACHs to effectively cool the gas, demonstrating that a high streaming velocity by itself cannot produce the conditions required for DCBH formation. However, we argue that high streaming velocity regions do provide an ideal environment for the formation of DCBHs in close pairs of ACHs (the "synchronised halo" model). Due to the absence of star formation in minihaloes, the gas remains chemically pristine until the ACHs form. If two such haloes form with only a small separation in time and space, then the one forming stars earlier can provide enough ultraviolet radiation to suppress H$_{2}$ cooling in the other, allowing it to collapse to form a DCBH. Baryonic streaming may therefore play a crucial role in the formation of the seeds of the highest redshift quasars., Comment: 8 pages, 4 figures, resubmitted to MNRAS

Star clusters interact with the interstellar medium (ISM) in various ways, most importantly in the destruction of molecular star-forming clouds, resulting in inefficient star formation on galactic scales. On cloud scales, ionizing radiation creates \hii regions, while stellar winds and supernovae drive the ISM into thin shells. These shells are accelerated by the combined effect of winds, radiation pressure and supernova explosions, and slowed down by gravity. Since radiative and mechanical feedback is highly interconnected, they must be taken into account in a self-consistent and combined manner, including the coupling of radiation and matter. We present a new semi-analytic one-dimensional feedback model for isolated massive clouds ($\geq 10^5\,M_{\odot}$) to calculate shell dynamics and shell structure simultaneously. It allows us to scan a large range of physical parameters (gas density, star formation efficiency, metallicity) and to estimate escape fractions of ionizing radiation $f_{\rm{esc,i}}$, the minimum star formation efficiency $\epsilon_{\rm{min}}$ required to drive an outflow, and recollapse time scales for clouds that are not destroyed by feedback. Our results show that there is no simple answer to the question of what dominates cloud dynamics, and that each feedback process significantly influences the efficiency of the others. We find that variations in natal cloud density can very easily explain differences between dense-bound and diffuse-open star clusters. We also predict, as a consequence of feedback, a $4-6$ Myr age difference for massive clusters with multiple generations., Comment: 17 pages, 11 figures (additional 4 pages and 33 figures in appendix), Accepted MNRAS June 15, 2017

We introduce the FirstLight project that aims to generate a large database of high-resolution, zoom-in simulations of galaxy formation around the epoch of reionisation ($z\geq6$). The first results of this program agree well with recent observational constraints at z=6-8, including the UV luminosity function and galaxy stellar mass function, as well as the scaling relationships between halo mass, stellar mass, and UV magnitude. The UV luminosity function starts to flatten below MUV>-14 due to stellar feedback in halos with maximum circular velocities of V=30-40 km/s. The power-law slope of the luminosity function evolves rapidly with redshift, reaching a value of alpha=-2.5 at z=10. On the other hand, the galaxy stellar mass function evolves slowly with time between z=8-10, in particular at the low-mass end., Comment: 8 pages, 9 figures, accepted at MNRAS

We present Phantom, a fast, parallel, modular and low-memory smoothed particle hydrodynamics and magnetohydrodynamics code developed over the last decade for astrophysical applications in three dimensions. The code has been developed with a focus on stellar, galactic, planetary and high energy astrophysics and has already been used widely for studies of accretion discs and turbulence, from the birth of planets to how black holes accrete. Here we describe and test the core algorithms as well as modules for magnetohydrodynamics, self-gravity, sink particles, H_2 chemistry, dust-gas mixtures, physical viscosity, external forces including numerous galactic potentials as well as implementations of Lense-Thirring precession, Poynting-Robertson drag and stochastic turbulent driving. Phantom is hereby made publicly available., Comment: 88 pages, 60 figures, accepted to PASA. Code available from https://phantomsph.bitbucket.io/

We present high-resolution hydrodynamical simulations of isolated dwarf galaxies including self-gravity, non-equilibrium cooling and chemistry, interstellar radiation fields (IRSF) and shielding, star formation, and stellar feedback. This includes spatially and temporally varying photoelectric (PE) heating, photoionization, resolved supernova (SN) blast waves and metal enrichment. A new flexible method to sample the stellar initial mass function allows us to follow the contribution to the ISRF, the metal output and the SN delay times of individual massive stars. We find that SNe play the dominant role in regulating the global star formation rate, shaping the multi-phase interstellar medium (ISM) and driving galactic outflows. Outflow rates (with mass-loading factors of a few) and hot gas fractions of the ISM increase with the number of SNe exploding in low-density environments where radiative energy losses are low. While PE heating alone can suppress star formation slightly more (a factor of a few) than SNe alone can do, it is unable to drive outflows and reproduce the multi-phase ISM that emerges naturally when SNe are included. These results are in conflict with recent results of Forbes et al. who concluded that PE heating is the dominant process suppressing star formation in dwarfs, about an order of magnitude more efficient than SNe. Potential origins for this discrepancy are discussed. In the absence of SNe and photoionization (mechanisms to disperse dense clouds), the impact of PE heating is highly overestimated owing to the (unrealistic) proximity of dense gas to the radiation sources. This leads to a substantial boost of the infrared continuum emission from the UV-irradiated dust and a far infrared line-to-continuum ratio too low compared to observations. Though sub-dominant in regulating star formation, the ISRF controls the abundance of molecular hydrogen via photodissociation., Comment: submitted, comments welcome

Direct collapse black holes forming in pristine, atomically-cooling haloes at $z \approx 10-20$ may act as the seeds of supermassive black holes (BH) at high redshifts. In order to create a massive BH seed, the host halo needs to be prevented from forming stars. H$_2$ therefore needs to be irradiated by a large flux of Lyman-Werner (LW) UV photons in order to suppress H$_2$ cooling. A key uncertainty in this scenario is the escape fraction of LW radiation from first galaxies, the dominant source of UV photons at this epoch. To better constrain this escape fraction, we have performed radiation-hydrodynamical simulations of the growth of HII regions and their associated photodissociation regions in the first galaxies using the ZEUS-MP code. We find that the LW escape fraction crucially depends on the propagation of the ionisation front (I-front). For an R-type I-front overrunning the halo, the LW escape fraction is always larger than 95%. If the halo recombines later from the outside--in, due to a softened and weakened spectrum, the LW escape fraction in the rest-frame of the halo (the near-field) drops to zero. A detailed and careful analysis is required to analyse slowly moving, D-type I-fronts, where the escape fraction depends on the microphysics and can be as small as 3% in the near-field and 61% in the far-field or as large as 100% in both the near-field and the far-field., Comment: 14 pages, 10 figures, 7 tables, accepted for publication in MNRAS

The carbon monoxide (CO) rotational transition lines are the most common tracers of molecular gas within giant molecular clouds (MCs). We study the ratio ($R_{2-1/1-0}$) between CO's first two emission lines and examine what information it provides about the physical properties of the cloud. To study $R_{2-1/1-0}$ we perform smooth particle hydrodynamic simulations with time dependent chemistry (using GADGET-2), along with post-process radiative transfer calculations on an adaptive grid (using RADMC-3D) to create synthetic emission maps of a MC. $R_{2-1/1-0}$ has a bimodal distribution that is a consequence of the excitation properties of each line, given that $J=1$ reaches local thermal equilibrium (LTE) while $J=2$ is still sub-thermally excited in the considered clouds. The bimodality of $R_{2-1/1-0}$ serves as a tracer of the physical properties of different regions of the cloud and it helps constrain local temperatures, densities and opacities. Additionally this bimodal structure shows an important portion of the CO emission comes from diffuse regions of the cloud, suggesting that the commonly used conversion factor of $R_{2-1/1-0}\sim 0.7$ between both lines may need to be studied further., Comment: 10 pages, 8 figures, accepted to MNRAS

We present three-dimensional magneto-hydrodynamical simulations of the self-gravitating interstellar medium (ISM) in a periodic (256 pc)$^3$ box with a mean number density of 0.5 cm$^{-3}$. At a fixed supernova rate we investigate the multi-phase ISM structure, H$_{2}$ molecule formation and density-magnetic field scaling for varying initial magnetic field strengths (0, $6\times 10^{-3}$, 0.3, 3 $\mu$G). All magnetic runs saturate at mass weighted field strengths of $\sim$ 1 $-$ 3 $\mu$G but the ISM structure is notably different. With increasing initial field strengths (from $6\times 10^{-3}$ to 3 $\mu$G) the simulations develop an ISM with a more homogeneous density and temperature structure, with increasing mass (from 5% to 85%) and volume filling fractions (from 4% to 85%) of warm (300 K $<$ T $<$ 8000 K) gas, with decreasing volume filling fractions (VFF) from $\sim$ 35% to $\sim$ 12% of hot gas (T $> 10^5$ K) and with a decreasing H$_{2}$ mass fraction (from 70% to $<$ 1%). Meanwhile the mass fraction of gas in which the magnetic pressure dominates over the thermal pressure increases by a factor of 10, from 0.07 for an initial field of $6\times 10^{-3}$ $\mu$G to 0.7 for a 3 $\mu$G initial field. In all but the simulations with the highest initial field strength self-gravity promotes the formation of dense gas and H$_{2}$, but does not change any other trends. We conclude that magnetic fields have a significant impact on the multi-phase, chemical and thermal structure of the ISM and discuss potential implications and limitations of the model., Comment: submitted

We present the Grackle chemistry and cooling library for astrophysical simulations and models. Grackle provides a treatment of non-equilibrium primordial chemistry and cooling for H, D, and He species, including H2 formation on dust grains; tabulated primordial and metal cooling; multiple UV background models; and support for radiation transfer and arbitrary heat sources. The library has an easily implementable interface for simulation codes written in C, C++, and Fortran as well as a Python interface with added convenience functions for semi-analytical models. As an open-source project, Grackle provides a community resource for accessing and disseminating astrochemical data and numerical methods. We present the full details of the core functionality, the simulation and Python interfaces, testing infrastructure, performance, and range of applicability. Grackle is a fully open-source project and new contributions are welcome., Comment: 20 pages, 8 figures, accepted for publication in MNRAS. For more info, visit grackle.readthedocs.io

We present three-dimensional radiation-hydrodynamical simulations of the impact of stellar winds, photoelectric heating, photodissociating and photoionising radiation, and supernovae on the chemical composition and star formation in a stratified disc model. This is followed with a sink-based model for star clusters with populations of individual massive stars. Stellar winds and ionising radiation regulate the star formation rate at a factor of ~10 below the simulation with only supernova feedback due to their immediate impact on the ambient interstellar medium after star formation. Ionising radiation (with winds and supernovae) significantly reduces the ambient densities for most supernova explosions to rho < 10^-25 g cm^-3, compared to 10^-23 g cm^-3 for the model with only winds and supernovae. Radiation from massive stars reduces the amount of molecular hydrogen and increases the neutral hydrogen mass and volume filling fraction. Only this model results in a molecular gas depletion time scale of 2 Gyr and shows the best agreement with observations. In the radiative models, the Halpha emission is dominated by radiative recombination as opposed to collisional excitation (the dominant emission in non-radiative models), which only contributes ~1-10 % to the total Halpha emission. Individual massive stars (M >= 30 M_sun) with short lifetimes are responsible for significant fluctuations in the Halpha luminosities. The corresponding inferred star formation rates can underestimate the true instantaneous star formation rate by factors of ~10., Comment: submitted to MNRAS, movies https://hera.ph1.uni-koeln.de/~silcc/

Dust grains are an important component of the interstellar medium (ISM) of galaxies. We present the first direct measurement of the residence times of interstellar dust in the different ISM phases, and of the transition rates between these phases, in realistic hydrodynamical simulations of the multi-phase ISM. Our simulations include a time-dependent chemical network that follows the abundances of H^+, H, H_2, C^+ and CO and take into account self-shielding by gas and dust using a tree-based radiation transfer method. Supernova explosions are injected either at random locations, at density peaks, or as a mixture of the two. For each simulation, we investigate how matter circulates between the ISM phases and find more sizeable transitions than considered in simple mass exchange schemes in the literature. The derived residence times in the ISM phases are characterised by broad distributions, in particular for the molecular, warm and hot medium. The most realistic simulations with random and mixed driving have median residence times in the molecular, cold, warm and hot phase around 17, 7, 44 and 1 Myr, respectively. The transition rates measured in the random driving run are in good agreement with observations of Ti gas-phase depletion in the warm and cold phases in a simple depletion model, although the depletion in the molecular phase is under-predicted. ISM phase definitions based on chemical abundance rather than temperature cuts are physically more meaningful, but lead to significantly different transition rates and residence times because there is no direct correspondence between the two definitions., Comment: submitted to MNRAS, movies https://hera.ph1.uni-koeln.de/~silcc/

The direct collapse black hole (DCBH) model attempts to explain the observed number density of supermassive black holes in the early Universe by positing that they grew from seed black holes with masses of $10^{4}$-$10^{5} \: {\rm M_{\odot}}$ that formed by the quasi-isothermal collapse of gas in metal-free protogalaxies cooled by atomic hydrogen emission. For this model to work, H$_{2}$ formation must be suppressed in at least some of these systems by a strong extragalactic radiation field. The predicted number density of DCBH seeds is highly sensitive to the minimum value of the ultraviolet (UV) flux required to suppress H$_{2}$ formation, $J_{\rm crit}$. In this paper, we examine how the value of $J_{\rm crit}$ varies as we vary the strength of a hypothetical high-redshift X-ray background. We confirm earlier findings that when the X-ray flux $J_{\rm X}$ is large, the critical UV flux scales as $J_{\rm crit} \propto J_{\rm X}^{1/2}$. We also carefully explore possible sources of uncertainty arising from how the X-rays are modelled. We use a reaction-based reduction technique to analyze the chemistry of H$_{2}$ in the X-ray illuminated gas and identify a critical subset of 35 chemical reactions that must be included in our chemical model in order to predict $J_{\rm crit}$ accurately. We further show that $J_{\rm crit}$ is insensitive to the details of how secondary ionization or He$^{+}$ recombination are modelled, but does depend strongly on the assumptions made regarding the column density of the collapsing gas., Comment: 13 pages, 5 figures. Submitted to MNRAS

Hydrodynamical simulations of star formation often do not possess the dynamic range needed to fully resolve the build-up of individual stars and star clusters, and thus have to resort to subgrid models. A popular way to do this is by introducing Lagrangian sink particles, which replace contracting high density regions at the point where the resolution limit is reached. A common problem then is how to assign fundamental stellar properties to sink particles, such as the distribution of stellar masses. We present a new and simple statistical method to assign stellar contents to sink particles. Once the stellar content is specified, it can be used to determine a sink particle's radiative output, supernovae rate or other feedback parameters that may be required in the calculations. Advantages of our method are (i) it is simple to implement, (ii) it guarantees that the obtained stellar populations are good samples of the initial mass function, (iii) it can easily deal with infalling mass accreted at later times, and (iv) it does not put restrictions on the sink particles' masses in order to be used. The method works very well for sink particles that represent large star clusters and for which the stellar mass function is well sampled, but can also handle the transition to sink particles that represent a small number of stars., Comment: Accepted for publication in MNRAS. The file sink2stars.py included in the source files contains an illustrative python implementation of the method

Metals from Population III (Pop III) supernovae led to the formation of less massive Pop II stars in the early universe, altering the course of evolution of primeval galaxies and cosmological reionization. There are a variety of scenarios in which heavy elements from the first supernovae were taken up into second-generation stars, but cosmological simulations only model them on the largest scales. We present small-scale, high-resolution simulations of the chemical enrichment of a primordial halo by a nearby supernova after partial evaporation by the progenitor star. We find that ejecta from the explosion crash into and mix violently with ablative flows driven off the halo by the star, creating dense, enriched clumps capable of collapsing into Pop II stars. Metals may mix less efficiently with the partially exposed core of the halo, so it might form either Pop III or Pop II stars. Both Pop II and III stars may thus form after the collision if the ejecta do not strip all the gas from the halo. The partial evaporation of the halo prior to the explosion is crucial to its later enrichment by the supernova., Comment: Accepted to ApJ

The critical Lyman--Werner flux required for direct collapse blackholes (DCBH) formation, or J$_{crit}$, depends on the shape of the irradiating spectral energy distribution (SED). The SEDs employed thus far have been representative of {{realistic}} single stellar populations. We study the effect of binary stellar populations on the formation of DCBH, as a result of their contribution to the Lyman--Werner radiation field. Although binary populations with ages $>$ 10 Myr yield a larger LW photon output, we find that the corresponding values of J$_{crit}$ can be up to 100 times higher than single stellar populations. We attribute this to the shape of the binary SEDs as they produce a sub--critical rate of H$^-$ photodetaching 0.76 eV photons as compared to single stellar populations, reaffirming the role that H$^-$ plays in DCBH formation. This further corroborates the idea that DCBH formation is better understood in terms of a critical region in the H$_2$--H$^-$ photo--destruction rate parameter space, rather than a single value of LW flux., Comment: 5 pages, 4 figures. accepted MNRAS

Recent studies seem to suggest that the stellar initial mass function (IMF) in early-type galaxies might be different from a classical Kroupa or Chabrier IMF, i.e. contain a larger fraction of the total mass in low-mass stars. From a theoretical point of view, supersonic turbulence has been the subject of interest in many analytical theories proposing a strong correlation with the characteristic mass of the core mass function (CMF) in star forming regions, and as a consequence with the stellar IMF. Performing two suites of smoothed particles hydrodynamics (SPH) simulations with different mass resolutions, we aim at testing the effects of variations in the turbulent properties of a dense, star forming molecular cloud on the shape of the system mass function in different density regimes. While analytical theories predict a shift of the peak of the CMF towards lower masses with increasing velocity dispersion of the cloud, we observe in the low-density regime the opposite trend, with high Mach numbers giving rise to a top-heavy mass distribution. For the high-density regime we do not find any trend correlating the Mach number with the characteristic mass of the resulting IMF, implying that the dynamics of protostellar accretion discs and fragmentation on small scales is not strongly affected by turbulence driven at the scale of the cloud. Furthermore, we suggest that a significant fraction of dense cores are disrupted by turbulence before stars can be formed in their interior through gravitational collapse. Although this particular study has limitations in its numerical resolution, we suggest that our results, along with those from other studies, cast doubt on the turbulent fragmentation models on the IMF that simply map the CMF to the IMF., Comment: 12 pages, 7 figures, accepted by MNRAS

We investigate the temperature distribution of CO-dark molecular hydrogen (H2) in a series of disk galaxies simulated using the AREPO moving-mesh code. In conditions similar to those in the Milky Way, we find that H2 has a flat temperature distribution ranging from 10 - 100 K. At $T < 30$ K the gas is almost fully molecular and has a high CO content, whereas at $T > 30$ K, the H2 fraction spans a broader range and the CO content is small, allowing us to classify gas in these two regimes as CO-bright and CO-dark, respectively. The mean sound speed in the CO-dark H2 is 0.64 km/s, significantly lower than the value in the cold atomic gas (1.15 km/s), implying that the CO-dark molecular phase is more susceptible to turbulent compression and gravitational collapse than its atomic counterpart. We further show that the temperature of the CO-dark H2 is highly sensitive to the strength of the interstellar radiation field, but that conditions in the CO-bright H2 remain largely unchanged. Finally, we examine the usefulness of the [CII] and [OI] fine structure lines as tracers of the CO-dark gas. We show that in Milky Way-like conditions, diffuse [CII] emission from this gas should be detectable. However, it is a problematic tracer of this gas, as there is only a weak correlation between the brightness of the emission and the H2 surface density. The situation is even worse for the [OI] line, which shows no correlation with the H2 surface density., Comment: 17 pages, 17 figures. Accepted by MNRAS

With new observational facilities becoming available soon, discovering and characterising supernovae from the first stars will open up alternative observational windows to the end of the cosmic dark ages. Based on a semi-analytical merger tree model of early star formation we constrain Population III supernova rates. We find that our method reproduces the Population III supernova rates of large-scale cosmological simulations very well. Our computationally efficient model allows us to survey a large parameter space and to explore a wide range of different scenarios for Population III star formation. Our calculations show that observations of the first supernovae can be used to differentiate between cold and warm dark matter models and to constrain the corresponding particle mass of the latter. Our predictions can also be used to optimize survey strategies with the goal to maximize supernova detection rates., Comment: 12 pages, 6 figures, published in MNRAS

We study the formation of galactic outflows from supernova explosions (SNe) with the moving-mesh code AREPO in a stratified column of gas with a surface density similar to the Milky Way disk at the solar circle. We compare different simulation models for SNe placement and energy feedback, including cosmic rays (CR), and find that models that place SNe in dense gas and account for CR diffusion are able to drive outflows with similar mass loading as obtained from a random placement of SNe with no CRs. Despite this similarity, CR-driven outflows differ in several other key properties including their overall clumpiness and velocity. Moreover, the forces driving these outflows originate in different sources of pressure, with the CR diffusion model relying on non-thermal pressure gradients to create an outflow driven by internal pressure and the random-placement model depending on kinetic pressure gradients to propel a ballistic outflow. CRs therefore appear to be non-negligible physics in the formation of outflows from the interstellar medium., Comment: 8 pages, 4 figures, accepted for publication in ApJL; movie of simulated gas densities can be found here: http://www.h-its.org/tap-images/galactic-outflows/

We present numerical simulations of molecular clouds (MCs) with self-consistent CO gas-phase and isotope chemistry in various environments. The simulations are post-processed with a line radiative transfer code to obtain $^{12}$CO and $^{13}$CO emission maps for the $J=1\rightarrow0$ rotational transition. The emission maps are analysed with commonly used observational methods, i.e. the $^{13}$CO column density measurement, the virial mass estimate and the so-called $X_{\textrm{CO}}$ (also CO-to-H$_2$) conversion factor, and then the inferred quantities (i.e. mass and column density) are compared to the physical values. We generally find that most methods examined here recover the CO-emitting H$_{2}$ gas mass of MCs within a factor of two uncertainty if the metallicity is not too low. The exception is the $^{13}$CO column density method. It is affected by chemical and optical depth issues, and it measures both the true H$_{2}$ column density distribution and the molecular mass poorly. The virial mass estimate seems to work the best in the considered metallicity and radiation field strength range, even when the overall virial parameter of the cloud is above the equilibrium value. This is explained by a systematically lower virial parameter (i.e. closer to equilibrium) in the CO-emitting regions; in CO emission, clouds might seem (sub-)virial, even when, in fact, they are expanding or being dispersed. A single CO-to-H$_{2}$ conversion factor appears to be a robust choice over relatively wide ranges of cloud conditions, unless the metallicity is low. The methods which try to take the metallicity dependence of the conversion factor into account tend to systematically overestimate the true cloud masses., Comment: 24 pages, 11 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

CR7 is the brightest Lyman-$\alpha$ emitter observed at $z>6$, which shows very strong Lyman-$\alpha$ and HeII 1640\AA\ line luminosities, but no metal line emission. Previous studies suggest that CR7 hosts either young primordial stars with a total stellar mass of $\sim 10^7\,\mathrm{M}_\odot$ or a black hole of $\gtrsim 10^6\,\mathrm{M}_\odot$. Here, we explore different formation scenarios for CR7 with a semianalytical model, based on the random sampling of dark matter merger trees. We are unable to reproduce the observational constraints with a primordial stellar source, given our model assumptions, due to the short stellar lifetimes and the early metal enrichment. Black holes that are the remnants of the first stars are either not massive enough, or reside in metal-polluted haloes, ruling out this possible explanation of CR7. Our models instead suggest that direct collapse black holes, which form in metal-free haloes exposed to large Lyman-Werner fluxes, are more likely the origin of CR7. However, this result is derived under optimistic assumptions and future observations are necessary to further constrain the nature of CR7., Comment: 21 pages, 17 figures, 2 tables, MNRAS in press