Cosmological direct-collapse black hole formation sites hostile for their growth

ABSTRACT The direct collapse (DC) is a promising mechanism that provides massive seed black holes (BHs) with ∼105 M⊙ in the early universe. To study a long-term accretion growth of a direct-collapse black hole (DCBH), we perform cosmological radiation-hydrodynamics simulations, extending our previous work where we investigated its formation stage. With a high spatial resolution down below the Bondi radius, we show that the accretion rate on to the BH is far below the Eddington value. Such slow mass growth is partly because of the strong radiative feedback from the accreting BH to the surrounding dense gas. Even after it falls into the first galaxy, the accretion rate is substantially suppressed due to the supernova feedback associated with the intense star formation. Moreover, the BH has a large velocity of ∼100 km s−1 relative to the gas, which further reduces the accretion rate. This large relative velocity stems from the fact that the DCBHs form in metal-free environments typically at ∼1 kpc from the galaxy. The BH accelerates as it approaches the galactic centre due to the gravity. The relative velocity never damps and the BH wanders around the outer galactic region. An analytic estimate predicts that the DCBH formation within ∼100 pc around the galactic centre is necessary to decelerate the BH with dynamical friction before z = 7. Since metal enrichment with Z ∼ 10−5−10−3 Z⊙ is expected there, the formation of DCBHs in the metal-enriched environments is preferable for the subsequent rapid growth.

The nature of giant clumps in high-z discs: a deep-learning comparison of simulations and observations

ABSTRACT We use deep learning to explore the nature of observed giant clumps in high-redshift disc galaxies, based on their identification and classification in cosmological simulations. Simulated clumps are detected using the 3D gas and stellar densities in the VELA zoom-in cosmological simulation suite, with ${\sim}25\ \rm {pc}$ maximum resolution, targeting main-sequence galaxies at 1 < z < 3. The clumps are classified as long-lived clumps (LLCs) or short-lived clumps (SLCs) based on their longevity in the simulations. We then train neural networks to detect and classify the simulated clumps in mock, multicolour, dusty, and noisy HST-like images. The clumps are detected using an encoder–decoder convolutional neural network (CNN), and are classified according to their longevity using a vanilla CNN. Tests using the simulations show our detector and classifier to be ${\sim}80{{\ \rm per\ cent}}$ complete and ${\sim}80{{\ \rm per\ cent}}$ pure for clumps more massive than ∼107.5 M⊙. When applied to observed galaxies in the CANDELS/GOODS S+N fields, we find both types of clumps to appear in similar abundances in the simulations and the observations. LLCs are, on average, more massive than SLCs by ∼0.5 dex, and they dominate the clump population above Mc ≳ 107.6 M⊙. LLCs tend to be found closer to the galactic centre, indicating clump migration to the centre or preferential formation at smaller radii. The LLCs are found to reside in high-mass galaxies, indicating better clump survivability under supernova feedback there, due to clumps being more massive in these galaxies. We find the clump masses and radial positions in the simulations and the observations to agree within a factor of 2.

Galactic 26Al traces metal loss through hot chimneys

ABSTRACT Radioactive 26Al is an excellent tracer for metal ejection in the Milky Way, and can provide a direct constraint on the modelling of supernova feedback in galaxy evolution. Gamma-ray observations of the 26Al decay line have found high velocities and hence require a significant fraction of the Galactic 26Al in the hot component. At the same time, meteoritic data combined with simulation results suggest that a significant amount of 26Al makes its way into stars before decay. We investigated the distribution into hot and cold channels with a simulation of a Milky-Way-like galaxy with massive-star feedback in superbubbles and with ejecta traced by 26Al. About 30–40 per cent of the ejecta remain hot, with typical cooling times of the order Gyr. 26Al traces the footpoints of a chimney-fed outflow that mixes metals turbulently into the halo of the model galaxy on a scale of at least 50 kpc. The rest diffuses into cold gas ≲ 104 K, and may therefore be quickly available for star formation. We discuss the robustness of the result by comparison to a simulation with a different global flow pattern. The branching ratio into hot and cold components is comparable to that of longer term average results from chemical evolution modelling of galaxies, clusters, and the intracluster medium.

Simulations of the Milky Way’s central molecular zone – I. Gas dynamics

ABSTRACT We use hydrodynamical simulations to study the Milky Way’s central molecular zone (CMZ). The simulations include a non-equilibrium chemical network, the gas self-gravity, star formation, and supernova feedback. We resolve the structure of the interstellar medium at sub-parsec resolution while also capturing the interaction between the CMZ and the bar-driven large-scale flow out to $R\sim 5\, {\rm kpc}$. Our main findings are as follows: (1) The distinction between inner (R ≲ 120 pc) and outer (120 ≲ R ≲ 450 pc) CMZ that is sometimes proposed in the literature is unnecessary. Instead, the CMZ is best described as single structure, namely a star-forming ring with outer radius R ≃ 200 pc which includes the 1.3° complex and which is directly interacting with the dust lanes that mediate the bar-driven inflow. (2) This accretion can induce a significant tilt of the CMZ out of the plane. A tilted CMZ might provide an alternative explanation to the ∞-shaped structure identified in Herschel data by Molinari et al. (3) The bar in our simulation efficiently drives an inflow from the Galactic disc (R ≃ 3 kpc) down to the CMZ (R ≃ 200 pc) of the order of $1\rm \, M_\odot \, yr^{-1}$, consistent with observational determinations. (4) Supernova feedback can drive an inflow from the CMZ inwards towards the circumnuclear disc of the order of ${\sim}0.03\, \rm M_\odot \, yr^{-1}$. (5) We give a new interpretation for the 3D placement of the 20 and 50 km s−1 clouds, according to which they are close (R ≲ 30 pc) to the Galactic Centre, but are also connected to the larger scale streams at R ≳ 100 pc.

Starbursting [O iii] emitters and quiescent [C ii] emitters in the reionization era

ABSTRACT Recent observations have successfully detected [O iii] $88.3\, {\rm \mu m}$ and [C ii] $157.6\, {\rm \mu m}$ lines from galaxies in the early Universe with the Atacama Large Millimeter Array. Combining cosmological hydrodynamic simulations and radiative transfer calculations, we present relations between the metal line emission and galaxy evolution at z = 6–15. We find that galaxies during their starburst phases have high [O iii] luminosity of ${\sim}10^{42}~\rm erg~s^{-1}$. Once supernova feedback quenches star formation, [O iii] luminosities rapidly decrease and continue to be zero for ${\sim}100\, {\rm Myr}$. The slope of the relation between $\log {(\rm SFR/\rm M_{\odot }~ yr^{-1})}$ and $\log {(L_{\rm [O\, \small {III}]}/\mathrm{L}_{\odot })}$ at z = 6–9 is 1.03, and 1.43 for $\log {(L_{\rm [C\, \small {II}]}/\mathrm{L}_{\odot })}$. As gas metallicity increases from sub-solar to solar metallicity by metal enrichment from star formation and feedback, the line luminosity ratio $L_{\rm [O\, \small {III}]} / L_{\rm [C\, \small {II}]}$ decreases from ∼10 to ∼1 because the O/C abundance ratio decreases due to carbon-rich winds from AGB stars and the mass ratio of H ii to H i regions decreases due to rapid recombination. Therefore, we suggest that the combination of [O iii] and [C ii] lines is a good probe to investigate the relative distribution of ionized and neutral gas in high-z galaxies. In addition, we show that deep [C ii] observations with a sensitivity of ∼10−2 mJy arcsec−2 can probe the extended neutral gas discs of high-z galaxies.

Inception of a first quasar at cosmic dawn

ABSTRACT Earliest quasars at the cosmic dawn are powered by mass accretion on to supermassive black holes of a billion solar masses. Massive black hole (MBH) seeds forming through the direct collapse mechanism are considered the most promising candidates but how do they grow and coevolve with their host galaxies at early cosmic times remains unknown. We here present results from a cosmological radiation hydrodynamical simulation including self-consistent modelling of both Population III (Pop III) and Population II (Pop II) star formation, their radiative and supernova feedback in the host galaxy along with X-ray feedback from an accreting MBH of $\rm 10^5 \, M_{\odot }$ in a halo of $\rm 2 \times 10^9 \, M_{\odot }$ from z = 26 down to z = 16. Our results show that energy deposition from X-rays in the proximity of MBH suppresses Pop III star formation for about 12 Myr while at the same time these X-rays catalyse $\rm H_2$ formation that leads to the formation of a Pop III star cluster of 500 $\rm M_{\odot }$ in the close vicinity of the MBH. We find that mode of star formation for Pop III is episodic and bursty due to the clumpy accretion, while for Pop II it is continuous. The stellar mass of the host galaxy at z ∼ 16 is $\rm 2 \times 10^7 \, M_{\odot }$ with a star formation rate of ${\sim} 0.1\!-\!1 \, \mathrm{ M}_{\odot }\, \mathrm{ yr}^{-1}$. In total, the MBH accretes $\rm 1.5 \times 10^6\, M_{\odot }$ during 120 Myr with the mean accretion rate of ${\sim} 0.01\, \mathrm{ M}_{\odot }\, \mathrm{ yr}^{-1}$ corresponding to an average Eddington fraction of 50 per cent.

Origin of star-forming rings around massive centres in massive galaxies at z < 4

ABSTRACT Using analytic modelling and simulations, we address the origin of an abundance of star-forming clumpy extended gas rings about massive central bodies in massive galaxies at z &lt; 4. Rings form by high-angular-momentum streams and survive in galaxies of Mstar &gt; 109.5–10 M⊙ where merger-driven spin flips and supernova feedback are ineffective. The rings survive after events of compaction to central nuggets. Ring longevity was unexpected based on inward mass transport driven by torques from violent disc instability. However, evaluating the torques from a tightly wound spiral structure, we find that the time-scale for transport per orbital time is long and $\propto \! \delta _{\rm d}^{-3}$, with δd the cold-to-total mass ratio interior to the ring. A long-lived ring forms when the ring transport is slower than its replenishment by accretion and the interior depletion by star formation rate, both valid for δd &lt; 0.3. The central mass that lowers δd is a compaction-driven bulge and/or dark matter, aided by the lower gas fraction at z &lt; 4, provided that it is not too low. The ring is Toomre unstable for clump and star formation. The high-z dynamic rings are not likely to arise form secular resonances or collisions. Active galactic nucleus feedback is not expected to affect the rings. Mock images of simulated rings through dust indicate qualitative consistency with observed rings about bulges in massive z ∼ 0.5–3 galaxies, in H α and deep HST imaging. ALMA mock images indicate that z ∼ 0.5–1 rings should be detectable. We quote expected observable properties of rings and their central nuggets.