arXiv subjects

Muhammad A. Latif

Publications and source records attributed to Muhammad A. Latif.

At least 37 records · Page 2Linked to original sources

arXiv2022

Unveiling the contribution of Pop III stars in primeval galaxies at redshift $\geq 6$

Detection of the first stars has remained elusive so-far but their presence may soon be unveiled by upcoming JWST observations. Previous studies have not investigated the entire possible range of halo masses and redshifts which may help in their detection. Motivated by the prospects of detecting galaxies up to $z\sim 20$ in JWST early data release, we quantify the contribution of Pop III stars to high-redshift galaxies from $6 \leq z \leq 30$ by employing the semi-analytical model A-SLOTH, which self-consistently models the formation of Pop III and Pop II stars along with their feedback. Our results suggest that the contribution of Pop III stars is the highest in low-mass halos of $\rm 10^7-10^9~M_{\odot}$. While high-mass halos $\rm \geq 10^{10}~M_{\odot}$ contain less than 1\% Pop III stars, they host galaxies with stellar masses of $\rm 10^9~M_{\odot}$ as early as $z \sim 30$. Interestingly, the apparent magnitude of Pop~III populations gets brighter towards higher redshift due to the higher stellar masses, but Pop~III-dominated galaxies are too faint to be directly detected with JWST. Our results predict JWST can detect galaxies up to $z\sim 30$, which may help in constraining the IMF of Pop III stars and will guide observers to discern the contribution of Pop~III stars to high-redshift galaxies.

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arXiv2022

Turbulent Cold Flows Gave Birth to the First Quasars

How quasars powered by supermassive black holes (SMBHs) formed less than a billion years after the Big Bang is still one of the outstanding problems in astrophysics 20 years after their discovery$^{1-4}$. Cosmological simulations suggest that rare cold flows converging on primordial haloes in low-shear environments could have created these quasars if they were 10$^4$ - 10$^5$ M$_{\odot}$ at birth but could not resolve their formation$^{5-8}$. Semianalytical studies of the progenitor halo of a primordial quasar found that it favours the formation of such seeds but could not verify if one actually appeared$^9$. Here we show that a halo at the rare convergence of strong, cold accretion flows creates massive BH seeds without the need for UV backgrounds, supersonic streaming motions, or even atomic cooling. Cold flows drive violent, supersonic turbulence in the halo that prevents star formation until it reaches a mass that triggers sudden, catastrophic baryon collapse that forms 31,000 and 40,000 M$_{\odot}$ stars. This simple, robust process ensures that haloes capable of forming quasars by z $>$ 6 produce massive seeds. The first quasars were thus a natural consequence of structure formation in cold dark matter cosmologies, not exotic, finely-tuned environments as previously thought$^{10-14}$.

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arXiv2022

Radio Power from Direct-Collapse Black Holes

Direct-collapse black holes (DCBHs) forming at $z \sim$ 20 are currently the leading candidates for the seeds of the first quasars, over 200 of which have now been found at $z >$ 6. Recent studies suggest that DCBHs could be detected in the near infrared by the James Webb Space Telescope, Euclid, and the Roman Space Telescope. However, new radio telescopes with unprecedented sensitivities such as the Square Kilometer Array (SKA) and the Next-Generation Very Large Array (ngVLA) may open another window on the properties of DCBHs in the coming decade. Here we estimate the radio flux from DCBHs at birth at $z =$ 8 - 20 with several fundamental planes of black hole accretion. We find that they could be detected at $z \sim$ 8 by the SKA-FIN all-sky survey. Furthermore, SKA and ngVLA could discover 10$^6$ - 10$^7$ $M_{\odot}$ BHs out to $z \sim$ 20, probing the formation pathways of the first quasars in the Universe.

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arXiv2021

The Birth Mass Function of Pop III Stars

Population III (Pop III) stars ended the cosmic Dark Ages and began early cosmological reionization and chemical enrichment. However, in spite of their importance to the evolution of the early Universe, their properties remain uncertain because of limitations to previous numerical simulations and the lack of any observational constraints. Here we investigate Pop III star formation in five primordial halos with 3D radiation-hydrodynamical cosmological simulations. We find that multiple stars form in each minihalo and that their numbers increase over time, with up to 23 stars forming in one of the halos. Radiative feedback from the stars generates strong outflows, deforms the surrounding protostellar disk, and delays star formation for a few thousand years. Star formation rates vary with halo and depend on mass accretion onto the disk, halo spin number, and the fraction of massive stars in the halo. Stellar masses in our models range from 0.1-37 $\rm M_{\odot}$, and of the 55 stars that form in our models twelve are $\rm > 10~ M_{\odot}$ and most of the others are 1-10 $\rm M_{\odot}$. Our simulations thus suggest that Pop III stars have characteristic masses of 1-10 $\rm M_{\odot}$ and a top-heavy IMF with dN/dM $\propto M_*^{-1.18}$. Up to 70\% of the stars are ejected from their disks by three-body interactions which, along with ionizing UV feedback, limits their final masses.

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arXiv2021

Radiation Hydrodynamical Simulations of the Birth of Intermediate-Mass Black Holes in the First Galaxies

The leading contenders for the seeds of $z > 6$ quasars are direct-collapse black holes (DCBHs) forming in atomically-cooled halos at $z \sim$ 20. However, the Lyman-Werner (LW) UV background required to form DCBHs of 10$^5$ \Ms\ are extreme, about 10$^4$ J$_{21}$, and may have been rare in the early universe. Here we investigate the formation of intermediate-mass black holes (IMBHs) under moderate LW backgrounds of 100 and 500 J$_{21}$ that were much more common at early times. These backgrounds allow halos to grow to a few 10$^6$ - 10$^7$ \Ms\ and virial temperatures of nearly 10$^4$ K before collapsing but do not completely sterilize them of H$_2$. Gas collapse then proceeds via Ly$α$ and rapid H$_2$ cooling at rates that are 10 - 50 times those in normal Pop III star-forming haloes but less than those in purely atomically-cooled haloes. Pop III stars accreting at such rates become blue and hot, and we find that their ionizing UV radiation limits their final masses to 1800 - 2800 \Ms\, at which they later collapse to IMBHs. Moderate LW backgrounds thus produced IMBHs in far greater numbers than DCBHs in the early universe.

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arXiv2021

Radio Power from a Direct-Collapse Black Hole in CR7

The leading contenders for the seeds of the first quasars are direct collapse black holes (DCBHs) formed during catastrophic baryon collapse in atomically-cooled halos at $z \sim$ 20. The discovery of the Ly$α$ emitter CR7 at $z =$ 6.6 was initially held to be the first detection of a DCBH, although this interpretation has since been challenged on the grounds of Spitzer IRAC and Very Large Telescope X-Shooter data. Here we determine if radio flux from a DCBH in CR7 could be detected and discriminated from competing sources of radio emission in the halo such as young supernovae and H II regions. We find that a DCBH would emit a flux of 10 - 200 nJy at 1.0 GHz, far greater than the sub-nJy signal expected for young supernovae but on par with continuum emission from star-forming regions. However, radio emission from a DCBH in CR7 could be distinguished from free-free emission from H II regions by its spectral evolution with frequency and could be detected by the Square Kilometer Array in the coming decade.

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arXiv2020

Formation of Pop II star clusters in the aftermath of a pair instability supernova

Pop II stars formed a few hundred million years after the Big Bang were key drivers of cosmic reionization and building blocks of high redshift galaxies. How and when these stars formed is a subject of ongoing research. We conduct cosmological radiation hydrodynamical simulations to investigate the formation of Pop II star clusters in dark matter halos forming at $z=10-25$ in the aftermath of a pair instability supernova (PISN). Our simulations model the formation of Pop III and Pop II stars in a self-consistent manner along with their radiative, chemical and SN feedback in halos of $\rm 5 \times 10^5- 7 \times 10^7~M_{\odot}$. We find that a PISN evacuates the gas from halos $\rm \leq 3 \times 10^{6}~M_{\odot}$ and thereafter shuts off in situ star formation for at-least 30 Myr. Pop II stellar clusters of $\rm 923~M_{\odot}$ and $\rm 6800~M_{\odot}$ form in halos of $\rm 3.5 \times 10^7~M_{\odot}$ and $\rm 7.2 \times 10^7~M_{\odot}$, respectively. The mode of star formation is highly episodic and mainly regulated by Pop II SN feedback. The average star formation rates are $\rm 10^{-5}-10^{-4}~M_{\odot}/yr$ and the star formation efficiency is less than 1\%.

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arXiv2020

Inception of a first quasar at cosmic dawn

Earliest quasars at the cosmic dawn are powered by mass accretion onto supermassive black holes of a billion solar masses. Massive black hole seeds forming through the direct collapse mechanism are considered the most promising candidates but how do they grow and co-evolve with their host galaxies at early cosmic times remains unknown. We here present results from a cosmological radiation hydrodynamical simulation including self-consistent modeling of both Pop III and Pop II star formation, their radiative and supernova feedback in the host galaxy along with X-ray feedback from an accreting massive black hole (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 catalyze $\rm H_2$ formation which 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 $z \sim 16$ is $\rm 2 \times 10^7~M_{\odot}$ with a star formation rate (SFR) of $\rm \sim 0.1-1~M_{\odot}/yr$. In total, the MBH accretes $\rm 1.5 \times 10^6~M_{\odot}$ during 120 Myr with the mean accretion rate of $\rm \sim 0.01~M_{\odot}/yr$ corresponding to an average Eddington fraction of 50\%.

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arXiv2020

The Birth of Binary Direct-Collapse Black Holes

Supermassive primordial stars forming during catastrophic baryon collapse in atomically-cooling halos at $z \sim$ 15 - 20 may be the origin of the first quasars in the universe. However, no simulation to date has followed the evolution of these halos at resolutions that are high enough or for times that are long enough to determine if collapse actually produces SMSs. Here we report new cosmological simulations of baryon collapse in atomically-cooled halos for times that are long enough for SMSs to form and die as direct-collapse black holes (DCBHs). We find that the high infall rates required to build up such stars do persist until the end of their lives and could fuel the rapid growth of their BHs thereafter. Our simulations also demonstrate that binary and even small multiples of SMSs can form in low-spin and high-spin halos, respectively. This discovery raises the exciting prospect of detecting gravitational waves from DCBH mergers with LISA and tidal disruption events in the near infrared with the {\em James Webb Space Telescope} and ground-based telescopes in the coming decade.

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arXiv2019

UV regulated star formation in high-redshift galaxies

The first galaxies forming a few hundred million years after the Big Bang are the key drivers of cosmic evolution and ideal laboratories to study theories of galaxy formation. We here study the role of UV radiation in suppressing star formation in primordial galaxies by destroying molecular hydrogen, the main coolant in primordial gas and provide estimates of cold dense gas at the onset of star formation. To accomplish this goal, we perform three dimensional cosmological simulations of minihalos in different environments forming at $ z\sim 25$ by varying strength of background UV flux below the Lyman limit between 0.01-1000 in units of $\rm J_{21}=10^{-21}~erg/cm^2/s/Hz/sr$. Particularly, we include photo-detachment of $\rm H^-$, the self-shielding of $\rm H_2$ which both were neglected in previous studies and use updated reaction rates. Our results show that depending on the background level $\rm H_2$ formation is suppressed, delaying gravitational collapse until halos reach the atomic cooling limit. We find that the formation of cold dense molecular gas and subsequently star formation gets delayed by 100 to 230 Myr depending on the level of the background radiation and the growth history of the dark matter halos. The fraction of dense self-shielded gas is a strong function of the background flux and exponentially declines with the strength of incident UV flux above $\rm J_{21} \geq 1$. We find that taking into account $\rm H_2$ self-shielding is crucial for accurately estimating the amount of cold dense gas available for star formation.

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arXiv2019

Intermittent fragmentation and statistical variations during gas collapse in magnetised atomic cooling haloes

Observations reveal the presence of supermassive black holes (SMBH) as early as ~700 million years after the Big Bang. Their formation path is still subject to current debate. We explore the influence of magnetic fields, which are strongly amplified via the turbulent small-scale dynamo, on the formation of SMBH seeds within the direct collapse scenario. In this study, we perform for the first time cosmological magnetohydrodynamic large eddy simulations that employ a model for unresolved, compressible MHD turbulence. In total we perform 36 simulations for 9 haloes each with two different initial magnetic field strengths, and with and without employing the unresolved turbulence model. We make use of the adaptive mesh refinement approach to achieve an effective spatial resolution of less than one proper astronomical unit. We consider a regime where cooling is regulated by atomic hydrogen and the molecular hydrogen gets dissociated by a strong radiation field. Our main finding is that the majority of the gas properties in the haloes at the final output are predominantly determined by the run-away gravitational collapse. Turbulence is supersonic and super-Alfvénic in all cases, and magnetic fields are amplified to an approximately dynamically relevant regime. Finally, fragmentation during the collapse is intermittent and mass accretion rates range from 0.2-3 Msun/yr. This suggests that the presence of strongly amplified magnetic fields and turbulence provides additional pressure support on small scales and make the direct collapse a viable scenario for the formation of massive objects under the required ambient conditions.

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arXiv2019

Titans of the Early Universe: The Prato Statement on the Origin of the First Supermassive Black Holes

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."

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arXiv2018

Black hole formation via gas-dynamical processes

Understanding the formation of earliest supermassive black holes is a question of prime astrophysical interest. In this chapter, we focus on the formation of massive black holes via gas dynamical processes. The necessary requirement for this mechanism are large inflow rates of about 0.1 solar mass per year. We discuss how to obtain such inflow rates via an isothermal collapse in the presence of atomic hydrogen cooling, and the outcome of such a collapse from three dimensional cosmological simulations in subsection 2.2. Alternatives to an isothermal direct collapse are discussed in subsection 3 which include trace amounts of metals and/or molecular hydrogen. In the end, we briefly discuss future perspectives and potential detection of massive black hole seeds via upcoming missions.

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arXiv2018

Early growth of typical high redshift black holes seeded by direct collapse

Understanding the growth of high redshift massive black holes (MBHs) is a problem of great astrophysical interest. The most luminous quasars at $z>6$ are frequently observed but they represent only the tip of the iceberg as the majority of the low luminosity AGN population remains undetected. In the present study, we perform a radiation hydrodynamics cosmological simulation to study the growth of "normal" black holes in the high redshift universe. In our simulation we model the formation of Pop III and Pop II stars along with their chemical, mechanical and radiative feedback. We consider both UV and X-ray emission from an accreting BH to simulate its radiative feedback. The selected halo has a mass of $\rm 3 \times 10^{10}~M_{\odot}$ at $z=7.5$ and we turn on radiative feedback from a MBH seed of $\rm 10^5~M_{\odot}$ along with in-situ star formation at $z=12$ when the halo mass reaches well above the atomic cooling limit. We find that the MBH accretes only about 2200 $\rm M_{\odot}$ during 320 Myr and the average mass accretion onto the MBH is a few times $\rm 10^{-6}~M_{\odot}/yr$. Our results suggest that the stunted growth of MBH is a consequence of supernovae in tandem with MBH feedback which drive large outflows and evacuate the gas from MBH vicinity. This may explain why a population of low luminosity AGN has not been detected so-far at $z>6$; the large contrast between the star formation rate and the MBH accretion rate may make then hard to detect even in upcoming deep surveys.

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arXiv2017

The impact of ionizing radiation on the formation of a supermassive star in the early Universe

A massive primordial halo near an intensely star-forming galaxy may collapse into a supermassive star (SMS) and leave a massive black hole seed of about $10^5~M_{\odot}$. To investigate the impact of ionizing radiation on the formation of an SMS from a nearby galaxy, we perform three-dimensional radiation hydrodynamical simulations by selecting a pair of massive dark matter halos forming at $z >10$. We find that rich structures such as clumps and filaments around the source galaxy shield the cloud from ionizing radiation. In fact, in some cases, cloud collapse is accelerated under ionizing radiation. This fact suggests that the ionization of the cloud's surroundings helps its collapse. Only strong radiation at the early stage of structure formation can halt the cloud collapse, but this is much stronger than observationally allowed value. We also explored the effect of ionizing radiation on a sample of 68 halos by employing an analytical model and found that increase in the mean density of the gas between the SMS forming cloud and the source galaxy protects the gas cloud from ionizing radiation as they approach each other. Thus, we conclude that ionizing radiation does not prevent the formation of an SMS in most of the cases.

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arXiv2017

Lyman-Werner Escape Fractions from the First Galaxies

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.

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arXiv2016

Formation of supermassive black hole seeds

The detection of quasars at $z>6$ unveils the presence of supermassive black holes (BHs) of a few billion solar masses. The rapid formation process of these extreme objects remains a fascinating and open issue. Such discovery implies that seed black holes must have formed early on, and grown via either rapid accretion or BH/galaxy mergers. In this theoretical review, we discuss in detail various BH seed formation mechanisms and the physical processes at play during their assembly. We discuss the three most popular BH formation scenarios, involving the (i) core-collapse of massive stars, (ii) dynamical evolution of dense nuclear star clusters, (iii) collapse of a protogalactic metal free gas cloud. This article aims at giving a broad introduction and an overview of the most advanced research in the field.

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arXiv2016

Exploring the nature of the Lyman-$α$ emitter CR7

CR7 is the brightest Lyman-$α$ emitter observed at $z>6$, which shows very strong Lyman-$α$ and HeII 1640Å 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.

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