arXiv subjects

Piero Madau

Publications and source records attributed to Piero Madau.

At least 37 records · Page 2Linked to original sources

arXiv2024

Stellar cluster formation in a Milky Way-sized galaxy at z>4 -- II. A hybrid formation scenario for the nuclear star cluster and its connection to the nuclear stellar ring

Nuclear star clusters (NSCs) are massive star clusters found in the innermost region of most galaxies. While recent studies suggest that low-mass NSCs in dwarf galaxies form largely out of the merger of globular clusters and NSCs in massive galaxies accumulate mass primarily through central star formation, the formation channel of the Milky Way's NSC is still uncertain. In this work, we use GigaEris, a high resolution N-body, hydrodynamical, cosmological ``zoom-in'' simulation, to investigate a possible formation path of the NSC in the progenitor of a Milky Way-sized galaxy, as well as its relation to the assembly and evolution of the galactic nuclear region. We study the possibility that bound, young, gas-rich, stellar clusters within a radius of 1.5 kpc of the main galaxy's centre at z>4 are the predecessors of the old, metal-poor stellar population of the Milky Way's NSC. We identify 47 systems which satisfy our criteria, with a total stellar mass of $10^{7.5}$ M$_{\odot}$. We demonstrate that both stellar cluster accretion and in-situ star formation will contribute to the formation of the NSC, providing evidence for a hybrid formation scenario for the first time in an N-body, hydrodynamical, cosmological ``zoom-in'' simulation. Additionally, we find that the gas required for in-situ star formation can originate from two pathways: gas-rich stellar clusters and gas influx driven by large-scale non-axisymmetric structures within the galaxy. This is partly supported by the presence of a stellar ring, resulting from gas dynamics, with properties similar to those of the Milky Way's nuclear stellar disc.

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arXiv2024

The origin of cold gas in the Circumgalactic Medium

The presence of cold ($T \lesssim 10^4$ K) gas in the circumgalactic medium (CGM) of galaxies has been confirmed both in observations and high-resolution simulations, but its origin still represents a puzzle. Possible mechanisms are cold accretion from the intergalactic medium (IGM), clumps embedded in outflows and transported from the disk, gas detaching from the hot CGM phase via thermal instabilities. In this work, we aim at characterizing the history of cold CGM gas, in order to identify the dominant origin channels at different evolutionary stages of the main galaxy. To this goal, we track gas particles in different snapshots of the SPH cosmological zoom-in simulation Eris2k. We perform a backward tracking of cold gas, starting from different redshifts, until we identify one of the followings origins for the particle: cold inflow, ejected from the disk, cooling down in-situ or stripped from a satellite. We also perform a forward tracking of gas in different components of the galaxy (such as the disk and outflows). We find a clear transition between two epochs. For $z>2$, most cold gas (up to 80%) in the CGM comes from cold accretion streams as the galaxy is accreting in the "cold mode" from the IGM. At lower $z$, gas either cools down in-situ after several recycles (with 10-20% of the gas cooling in outflow), or it is ejected directly from the disk (up to 30%). Outflows have a major contribution to the cold CGM gas budget at $z<1$, with almost 50% of hot gas cooling in outflow. Finally, we discuss possible mechanisms for CGM cooling, showing that the thermally unstable gas with $t_{\rm cool}/t_{\rm ff}<1$ (precipitation-regulated feedback) is abundant up to $r\sim 100$ kpc and cooling times are shorter than 50 Myr for densities $n>10^{-2}\,{\rm cm}^{-3}$.

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arXiv2023

Searching for Intermediate Mass Black Holes in Globular Clusters Through Tidal Disruption Events

Intermediate mass black holes (IMBHs) may be the link between stellar mass holes and the supermassive variety in the nuclei of galaxies, and globular clusters (GCs) may be one of the most promising environments for their formation. Here we carry out a pilot study of the observability of tidal disruption events (TDEs) from 10^3 Msun < M_BH < 10^5 Msun IMBHs embedded in stellar cusps at the center of GCs. We model the long super-Eddington accretion phase and ensuing optical flare, and derive the disruption rate of main-sequence stars as a function of black hole mass and GC properties with the help of a 1D Fokker-Planck approach. The photospheric emission of the adiabatically expanding outflow dominates the observable radiation and peaks in the NUV/optical bands, outshining the brightness of the (old) stellar population of GCs in Virgo for a period of months to years. A search for TDE events in a sample of nearly 4,000 GCs observed at multiple epochs by the Next Generation Virgo Cluster Survey (NGVS) yields null results. Given our model predictions, this sample is too small to set stringent constraints on the present-day occupation fraction of GCs hosting IMBHs. Naturally, better simulations of the properties of the cluster central stellar distribution, TDE light curves and rates, together with larger surveys of GCs are all needed to gain deeper insights into the presence of IMBHs in GCs.

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arXiv2023

High resolution modeling of [CII], [CI], [OIII] and CO line emission from the ISM and CGM of a star forming galaxy at z ~ 6.5

The circumgalactic medium (CGM) is a crucial component of galaxy evolution, but thus far its physical properties are highly unconstrained. As of yet, no cosmological simulation has reached convergence when it comes to constraining the cold and dense gas fraction of the CGM. Such components are also challenging to observe, and require sub-millimeter instruments with a high sensitivity to extended, diffuse emission, like the proposed Atacama Large Aperture Sub-millimetre telescope (AtLAST). We present a state-of-the-art theoretical effort at modeling the [CII], [CI](1-0), [CI](2-1), CO(3-2), and [OIII] line emissions of galaxies. We use the high-resolution cosmological zoom-in simulation Ponos, representing a star forming galaxy system at z = 6.5 ($M_*=2\times10^9~M_{\odot}$), undergoing a major merger. We adopt different modeling approaches based on the photoionisation code Cloudy. Our fiducial model uses radiative transfer post-processing with RamsesRT and Krome to create realistic FUV radiation fields, which we compare to sub-grid modeling approaches adopted in the literature. We find significant differences in the luminosity and in the contribution of different gas phases and galaxy components between the different modeling approaches. [CII] is the least model-dependant gas tracer, while [CI](1-0) and CO(3-2) are very model-sensitive. In all models, we find a significant contribution to the emission of [CII] (up to $\sim$10%) and [OIII] (up to $\sim$20%) from the CGM. [CII] and [OIII] trace different regions of the CGM: [CII] arises from an accreting filament and from tidal tails, while [OIII] traces a puffy halo surrounding the main disc, probably linked to SN feedback. We discuss our results in the context of current and future sub-mm observations with ALMA and AtLAST.

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arXiv2023

Beyond the Drake Equation: A Time-Dependent Inventory of Habitable Planets and Life-Bearing Worlds in the Solar Neighborhood

We introduce a mathematical framework for statistical exoplanet population and astrobiology studies that may help directing future observational efforts and experiments. The approach is based on a set of differential equations and provides a time-dependent mapping between star formation, metal enrichment, and the occurrence of exoplanets and potentially life-harboring worlds over the chemo-population history of the solar neighborhood. Our results are summarized as follows: 1) the formation of exoplanets in the solar vicinity was episodic, starting with the emergence of the thick disk about 11 Gyr ago; 2) within 100 pc from the Sun, there are as many as 11,000 (eta/0.24) Earth-size planets in the habitable zone ("temperate terrestrial planets" or TTPs) of K-type stars. The solar system is younger than the median TTP, and was created in a star formation surge that peaked 5.5 Gyr ago and was triggered by an external agent; 3) the metallicity modulation of the giant planet occurrence rate results in a later typical formation time, with TTPs outnumbering giant planets at early times; 4) the closest, life-harboring Earth-like planet would be < 20 pc away if microbial life arose as soon as it did on Earth in > 1 % of the TTPs around K stars. If simple life is abundant (fast abiogenesis), it is also old, as it would have emerged more than 8 Gyr ago in about one third of all life-bearing planets today. Older Earth analogs are more likely to have developed sufficiently complex life capable of altering the environment and producing detectable oxygenic biosignatures.

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arXiv2023

JWST constraints on the UV luminosity density at cosmic dawn: implications for 21-cm cosmology

An unprecedented array of new observational capabilities are starting to yield key constraints on models of the epoch of first light in the Universe. In this Letter we discuss the implications of the UV radiation background at cosmic dawn inferred by recent JWST observations for radio experiments aimed at detecting the redshifted 21-cm hyperfine transition of diffuse neutral hydrogen. Under the basic assumption that the 21-cm signal is activated by the Ly$α$ photon field produced by metal-poor stellar systems, we show that a detection at the low frequencies of the EDGES and SARAS3 experiments may be expected from a simple extrapolation of the declining UV luminosity density inferred at $z\lesssim 14$ from JWST early galaxy data. Accounting for an early radiation excess above the CMB suggests a shallower or flat evolution to simultaneously reproduce low and high-$z$ current UV luminosity density constraints, which cannot be entirely ruled out, given the large uncertainties from cosmic variance and the faint-end slope of the galaxy luminosity function at cosmic dawn. Our findings raise the intriguing possibility that a high star formation efficiency at early times may trigger the onset of intense Ly$α$ emission at redshift $z\lesssim 20$ and produce a cosmic 21-cm absorption signal 200 Myr after the Big Bang.

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arXiv2023

New Constraints on Warm Dark Matter from the Lyman-$α$ Forest Power Spectrum

The forest of Lyman-$α$ absorption lines detected in the spectra of distant quasars encodes information on the nature and properties of dark matter and the thermodynamics of diffuse baryonic material. Its main observable -- the 1D flux power spectrum (FPS) -- should exhibit a suppression on small scales and an enhancement on large scales in warm dark matter (WDM) cosmologies compared to standard $Λ$CDM. Here, we present an unprecedented suite of 1080 high-resolution cosmological hydrodynamical simulations run with the Graphics Processing Unit-accelerated code Cholla to study the evolution of the Lyman-$α$ forest under a wide range of physically-motivated gas thermal histories along with different free-streaming lengths of WDM thermal relics in the early Universe. A statistical comparison of synthetic data with the forest FPS measured down to the smallest velocity scales ever probed at redshifts $4.0\lesssim z\lesssim 5.2$ (Boera et al. 2019) yields a lower limit $m_{\rm WDM}>3.1$ keV (95 percent CL) for the WDM particle mass and constrains the amplitude and spectrum of the photoheating and photoionizing background produced by star-forming galaxies and active galactic nuclei at these redshifts. Interestingly, our Bayesian inference analysis appears to weakly favor WDM models with a peak likelihood value at the thermal relic mass of $m_{\rm WDM}=4.5$ keV. We find that the suppression of the FPS from free-streaming saturates at $k\gtrsim 0.1\,\mathrm{s}\,\mathrm{km}^{-1}$ because of peculiar velocity smearing, and this saturated suppression combined with a slightly lower gas temperature provides a moderately better fit to the observed small-scale FPS for WDM cosmologies.

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arXiv2023

Stellar cluster formation in a Milky Way-sized galaxy at z>4 -- I. The proto-globular cluster population and the imposter amongst us

The formation history of globular clusters (GCs) at redshift $z > 4$ remains an unsolved problem. In this work, we use the cosmological, $N$-body hydrodynamical ``zoom-in'' simulation GigaEris to study the properties and formation of proto-GC candidates in the region surrounding the progenitor of a Milky Way-sized galaxy. The simulation employs a modern implementation of smoothed-particle hydrodynamics, including metal-line cooling and metal and thermal diffusion and allows to resolve systems at the scale of star clusters. We define proto-GC candidate systems as gravitationally bound stellar systems with baryonic mass fraction $F_{\rm b} \geq 0.75$ and stellar velocity dispersion $σ_{\star} < 20$ km s$^{-1}$. At $z=4.4$ we identify 9 systems which satisfy our criteria, all of which form between 10 kpc to 30 kpc from the centre of the main host. Their baryonic masses are in the range $10^5$- $10^7$ M$_{\odot}$. By the end of the simulation, they still have a relatively low stellar mass ($M_{\star} \sim 10^4$--$10^5$ M$_{\odot}$) and a metallicity ($-1.8 \lesssim {\rm [Fe/H]} \lesssim -0.8$) similar to the blue Galactic GCs. All of the identified systems except one appear to be associated with gas filaments accreting onto the main galaxy in the circum-galactic region, and formed at $z=5-4$. The exception is the oldest object, which appears to be a stripped compact dwarf galaxy that has interacted with the main halo between $z = 5.8$ and $z=5.2$ and has lost its entire dark matter content due to tidal mass loss.

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arXiv2022

Modeling Cosmic Reionization

The transformation of cold neutral intergalactic hydrogen into a highly ionized warm plasma marks the end of the cosmic dark ages and the beginning of the age of galaxies. The details of this process reflect the nature of the early sources of radiation and heat, the statistical characteristics of the large-scale structure of the Universe, the thermodynamics and chemistry of cosmic baryons, and the histories of star formation and black hole accretion. A number of massive data sets from new ground- and space-based instruments and facilities over the next decade are poised to revolutionize our understanding of primeval galaxies, the reionization photon budget, the physics of the intergalactic medium (IGM), and the fine-grained properties of hydrogen gas in the "cosmic web". In this review we survey the physics and key aspects of reionization-era modeling and describe the diverse range of computational techniques and tools currently available in this field.

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arXiv2022

Inferring the Thermal History of the Intergalactic Medium from the Properties of the Hydrogen and Helium Lyman-alpha Forest

The filamentary network of intergalactic medium (IGM) gas that gives origin to the Lyman-alpha forest in the spectra of distant quasars encodes information on the physics of structure formation and the early thermodynamics of diffuse baryonic material. Here, we use a massive suite of more than 400 high-resolution cosmological hydrodynamical simulations run with the Graphics Processing Unit-accelerated code Cholla to study the IGM at high spatial resolution maintained over the entire computational volume. The simulations capture a wide range of possible thermal histories of intergalactic gas by varying the amplitude and timing of the photoheating and photoionizing background produced by star-forming galaxies and active galactic nuclei. A statistical comparison of synthetic spectra with the observed 1D flux power spectra of hydrogen in 14 redshift bins over the full range 2.2 <= z <= 5.0 and with the Lyman-alpha opacity of helium in 5 redshift bins over the range 2.4 < z < 2.9 tightly constrains the photoionization and photoheating history of the IGM. By leveraging the constraining power of the available Lyman-alpha forest data to break model degeneracies, we find that the IGM experienced two main reheating events from the non-equilibrium ionization of hydrogen and helium over 1.2 Gyr of cosmic time. For our best-fit model, hydrogen reionization completes by z~6.0 with a first IGM temperature peak T_0 ~ 1.3x10^4 K, and is followed by the reionization of HeII that completes by z~3.0 and yields a second temperature peak of T_0 ~ 1.4x10^4 K. We discuss how our results can be used to obtain information on the timing and the sources of hydrogen and helium reionization.

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arXiv2022

Revealing the Galaxy-Halo Connection Through Machine Learning

Understanding the connections between galaxy stellar mass, star formation rate, and dark matter halo mass represents a key goal of the theory of galaxy formation. Cosmological simulations that include hydrodynamics, physical treatments of star formation, feedback from supernovae, and the radiative transfer of ionizing photons can capture the processes relevant for establishing these connections. The complexity of these physics can prove difficult to disentangle and obfuscate how mass-dependent trends in the galaxy population originate. Here, we train a machine learning method called Explainable Boosting Machines (EBMs) to infer how the stellar mass and star formation rate of nearly 6 million galaxies simulated by the Cosmic Reionization on Computers (CROC) project depend on the physical properties of halo mass, the peak circular velocity of the galaxy during its formation history $v_\mathrm{peak}$, cosmic environment, and redshift. The resulting EBM models reveal the relative importance of these properties in setting galaxy stellar mass and star formation rate, with $v_\mathrm{peak}$ providing the most dominant contribution. Environmental properties provide substantial improvements for modeling the stellar mass and star formation rate in only $\lesssim10\%$ of the simulated galaxies. We also provide alternative formulations of EBM models that enable low-resolution simulations, which cannot track the interior structure of dark matter halos, to predict the stellar mass and star formation rate of galaxies computed by high-resolution simulations with detailed baryonic physics.

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arXiv2022

The Dawn of Disk Formation in a Milky Way-sized Galaxy Halo: Thin Stellar Disks at $z > 4$

We present results from \textsc{GigaEris}, a cosmological, $N$-body hydrodynamical "zoom-in" simulation of the formation of a Milky Way-sized galaxy halo with unprecedented resolution, encompassing of order a billion particles within the refined region. The simulation employs a modern implementation of smoothed-particle hydrodynamics, including metal-line cooling and metal and thermal diffusion. We focus on the early assembly of the galaxy, down to redshift $z=4.4$. The simulated galaxy has properties consistent with extrapolations of the main sequence of star-forming galaxies to higher redshifts and levels off to a star formation rate of $\sim$60$\, M_{\odot}$~yr$^{-1}$ at $z=4.4$. A compact, thin rotating stellar disk with properties analogous to those of low-redshift systems arises already at $z \sim 8$. The galaxy rapidly develops a multi-component structure, and the disk, at least at these early stages, does not grow "upside-down" as often reported in the literature. Rather, at any given time, newly born stars contribute to sustain a thin disk. The kinematics reflect the early, ubiquitous presence of a thin disk, as a stellar disk component with $v_ϕ/σ_R$ larger than unity is already present at $z \sim 9$--10. Our results suggest that high-resolution spectro-photometric observations of very high-redshift galaxies should find thin rotating disks, consistent with the recent discovery of cold rotating gas disks by ALMA. Finally, we present synthetic images for the JWST NIRCam camera, showing how the early disk would be easily detectable already at those early times.

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arXiv2021

Deep Realistic Extragalactic Model (DREaM) Galaxy Catalogs: Predictions for a Roman Ultra-Deep Field

In the next decade, deep galaxy surveys from telescopes such as the James Webb Space Telescope and Roman Space Telescope will provide transformational data sets that will greatly enhance the understanding of galaxy formation during the epoch of reionization (EoR). In this work, we present the Deep Realistic Extragalactic Model (DREaM) for creating synthetic galaxy catalogs. Our model combines dark matter simulations, subhalo abundance matching and empirical models, and includes galaxy positions, morphologies, and spectral energy distributions (SEDs). The resulting synthetic catalog extends to redshifts $z \sim 12 $, and galaxy masses $\log_{10}(M/M_{\odot}) = 5 $ covering an area of $1\, {\rm deg}^2$ on the sky. We use DREaM to explore the science returns of a $1\, {\rm deg}^2$ \emph{Roman} UDF, and to provide a resource for optimizing ultra-deep survey designs. We find that a \emph{Roman} UDF to $\sim 30\,m_{\rm AB}$ will potentially detect more than $10^6$ $M_{\rm UV}<-17$ galaxies, with more than $10^4$ at redshifts $z>7$, offering an unparalleled dataset for constraining galaxy properties during the EoR. Our synthetic catalogs and simulated images are made publicly available to provide the community with a tool to prepare for upcoming data.

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arXiv2021

Effects of Photoionization and Photoheating on Lyman-alpha Forest Properties from Cholla Cosmological Simulations

The density and temperature properties of the intergalactic medium (IGM) reflect the heating and ionization history during cosmological structure formation, and are primarily probed by the Lyman-alpha forest of neutral hydrogen absorption features in the observed spectra of background sources (Gunn & Peterson 1965). We present the methodology and initial results from the Cholla IGM Photoheating Simulation (CHIPS) suite performed with the Graphics Process Unit-accelerated Cholla code to study the IGM at high, uniform spatial resolution maintained over large volumes. In this first paper, we examine the IGM structure in CHIPS cosmological simulations that include IGM uniform photoheating and photoionization models where hydrogen reionization completes early (Haardt & Madau 2012) or by redshift z~6 (Puchwein et al. 2019). Comparing with observations of the large- and small-scale Lyman-alpha transmitted flux power spectra P(k) at redshifts 2 <~ z <~ 5.5, the relative agreement of the models depends on scale, with the self-consistent Puchwein et al. (2019) IGM photoheating and photoionization model in good agreement with the flux P(k) at k >~ 0.01 s/km at redshifts 2 <~ z <~ 3.5. On larger scales the P(k) measurements increase in amplitude from z~4.6 to z~2.2 faster than the models, and lie in between the model predictions at 2.2 <~ z <~ 4.6 for k~= 0.002-0.01 s/km. We argue the models could improve by changing the HeII photoheating rate associated with active galactic nuclei to reduce the IGM temperature at z~3. At higher redshifts z>~4.5 the observed flux P(k) amplitude increases at a rate intermediate between the models, and we argue that for models where hydrogen reionization completes late (z~5.5 - 6) resolving this disagreement will require inhomogeneous or 'patchy' reionization. (Abridged)

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arXiv2020

Stellar and Weak Lensing Profiles of Massive Galaxies in the Hyper-Suprime Cam Survey and in Hydrodynamic Simulations

We perform a consistent comparison of the mass and mass profiles of massive ($M_\star > 10^{11.4}M_{\odot}$) central galaxies at z~0.4 from deep Hyper Suprime-Cam (HSC) observations and from the Illustris, TNG100, and Ponos simulations. Weak lensing measurements from HSC enable measurements at fixed halo mass and provide constraints on the strength and impact of feedback at different halo mass scales. We compare the stellar mass function (SMF) and the Stellar-to-Halo Mass Relation (SHMR) at various radii and show that the radius at which the comparison is performed is important. In general, Illustris and TNG100 display steeper values of $α$ where $M_{\star}\propto M_{\rm vir}^α$. These differences are more pronounced for Illustris than for TNG100 and in the inner rather than outer regions of galaxies. Differences in the inner regions may suggest that TNG100 is too efficient at quenching in-situ star formation at $M_{\rm vir}\simeq10^{13} M_{\odot}$ but not efficient enough at $M_{\rm vir}\simeq10^{14} M_{\odot}$. The outer stellar masses are in excellent agreement with our observations at $M_{\rm vir}\simeq10^{13} M_{\odot}$, but both Illustris and TNG100 display excess outer mass as $M_{\rm vir}\simeq10^{14} M_{\odot}$ (by ~0.25 and ~0.12 dex, respectively). We argue that reducing stellar growth at early times in $M_\star \sim 10^{9-10} M_{\odot}$ galaxies would help to prevent excess ex-situ growth at this mass scale. The Ponos simulations do not implement AGN feedback and display an excess mass of ~0.5 dex at $r<30$ kpc compared to HSC which is indicative of over-cooling and excess star formation in the central regions. Joint comparisons between weak lensing and galaxy stellar profiles are a direct test of whether simulations build and deposit galaxy mass in the correct dark matter halos and thereby provide powerful constraints on the physics of feedback and galaxy growth.

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arXiv2020

The baryon cycle of Seven Dwarfs with superbubble feedback

We present results from a high-resolution, cosmological, $Λ$CDM simulation of a group of field dwarf galaxies with the "superbubble" model for clustered SN feedback, accounting for thermal conduction and cold gas evaporation. The initial conditions and the galaxy formation physics, other than SN feedback, are the same as in Shen et al. (2014). The simulated luminous galaxies have blue colors, low star formation efficiencies and metallicities, and high cold gas content, reproducing the observed scaling relations of dwarfs in the Local Volume. Bursty star formation histories and superbubble-driven outflows lead to the formation of kpc-size DM cores when stellar masses reaches $M_{*} > 10^6$ $M_{\odot}$, similar to previous findings. However, the superbubble model appears more effective in destroying DM cusps than the previously adopted "blastwave" model, reflecting a higher coupling efficiency of SN energy with the ISM. On larger scale, superbubble-driven outflows have a more moderate impact: galaxies have higher gas content, more extended stellar disks, and a smaller metal-enriched region in the CGM. The two halos with $M_{vir} \sim 10^9$ $M_{\odot}$, which formed ultra-faint dwarf galaxies in Shen et al. (2014), remain dark due to the different impact of metal-enriched galactic winds from two nearby luminous galaxies. The column density distributions of H I, Si II, C IV and O VI are in agreement with recent observations of CGM around isolated dwarfs. While H I is ubiquitous with a covering fraction of unity within the CGM, Si II and C IV are less extended. O VI is more extended, but its mass is only 11% of the total CGM oxygen budget, as the diffuse CGM is highly ionised by the UVB. Superbubble feedback produces C IV and O VI an order of magnitude higher column densities than those with blastwave feedback. The CGM and DM cores are most sensitive probes of feedback mechanisms.

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arXiv2020

Global torques and stochasticity as the drivers of massive black hole pairing in the young Universe

The forthcoming Laser Interferometer Space Antenna (LISA) will probe the population of coalescing massive black hole (MBH) binaries up to the onset of structure formation. Here we simulate the galactic-scale pairing of $\sim10^6 M_\odot$ MBHs in a typical, non-clumpy main-sequence galaxy embedded in a cosmological environment at $z = 7-6$. In order to increase our statistical sample, we adopt a strategy that allows us to follow the evolution of six secondary MBHs concomitantly. We find that the magnitude of the dynamical-friction induced torques is significantly smaller than that of the large-scale, stochastic gravitational torques arising from the perturbed and morphologically evolving galactic disc, suggesting that the standard dynamical friction treatment is inadequate for realistic galaxies at high redshift. The dynamical evolution of MBHs is very stochastic, and a variation in the initial orbital phase can lead to a drastically different time-scale for the inspiral. Most remarkably, the development of a galactic bar in the host system either significantly accelerates the inspiral by dragging a secondary MBH into the centre, or ultimately hinders the orbital decay by scattering the MBH in the galaxy outskirts. The latter occurs more rarely, suggesting that galactic bars overall promote MBH inspiral and binary coalescence. The orbital decay time can be an order of magnitude shorter than what would be predicted relying on dynamical friction alone. The stochasticity, and the important role of global torques, have crucial implications for the rates of MBH coalescences in the early Universe: both have to be accounted for when making predictions for the upcoming LISA observatory.

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arXiv2020

The impact of Lyman-$α$ emission line heating and cooling on the cosmic dawn 21-cm signal

Allowing for enhanced Ly$α$ photon line emission from Population III dominated stellar systems in the first forming galaxies, we show the 21-cm cosmic dawn signal at $10<z<30$ may substantially differ from standard scenarios. Energy transfer by Ly$α$ photons emerging from galaxies may heat intergalactic gas if HII regions within galaxies are recombination bound, or cool the gas faster than by adiabatic expansion if reddened by winds internal to the haloes. In some cases, differential 21-cm antenna temperatures near $-500$ mK may be achieved at $15<z<25$, similar to the signature detected by the EDGES 21-cm cosmic dawn experiment.

astro-ph.CO↗