arXiv subjects

Volker Bromm

Publications and source records attributed to Volker Bromm.

At least 109 records · Page 6Linked to original sources

arXiv2019

Redshift Horizon for Detecting the First Galaxies in Far-Infared Surveys

We explore the possibility of detecting the first galaxies with the next generation of space-based far infrared (FIR) telescopes by applying an analytical model of primordial dust emission. Our results indicate that FIR/sub-mm sources at $z \gtrsim 7$ will experience a strong negative K-correction. Systems of a given virial mass would exhibit larger dust luminosities at higher $z$, as a consequence of the increase in dust temperature driven by the higher temperature floor set by the cosmic microwave background. In addition, high-$z$ systems are more concentrated, which enhances the heating efficiency associated with stellar radiation. By analysing source densities as a function of $z$, and considering survey areas of 0.1 ${\rm deg}^2$ and 10 ${\rm deg}^2$, we find that the redshift horizon for detecting at least one source would be above $z\sim 7$ for instrument sensitivities $\lesssim 0.1-0.5 \ μ{\rm Jy}$ and $\lesssim 0.5-3.0 \ μ{\rm Jy}$, respectively, with the exact values depending on the nature of primordial dust. However, galaxy populations with higher than typical metallicities, star formation efficiencies and/or dust-to-metal ratios could relax such sensitivity requirements. In addition, the redshift horizon shows a significant dependence on the nature of primordial dust. We conclude that future FIR campaigns could play a crucial role in exploring the nature of dust and star formation in the early universe.

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arXiv2019

Constraining the non-gravitational scattering of baryons and dark matter with early cosmic structure formation

We derive new constraints on the non-gravitational baryon-dark-matter scattering (BDMS) by evaluating the mass thresholds of dark matter (DM) haloes in which primordial gas can cool efficiently to form Population III (Pop III) stars, based on the timing of the observed 21-cm absorption signal. We focus on the BDMS model with interaction cross-section $σ=σ_{1}[v/(1\ \mathrm{km\ s^{-1}})]^{-4}$, where $v$ is the relative velocity of the encounter. Our results rule out the region in parameter space with $σ_{1}\gtrsim 10^{-19}\ \mathrm{cm^{2}}$ and DM particle mass $m_χc^{2}\lesssim 3\times 10^{-2}\ \mathrm{GeV}$, where the cosmic number density of Pop III hosts at redshift $z\sim 20$ is at least three orders of magnitude smaller than in the standard Lambda cold DM ($Λ$CDM) case. In these BDMS models, the formation of Pop III stars is significantly suppressed for $z\gtrsim 20$, inconsistent with the timing of the observed global 21-cm absorption signal. For the fiducial BDMS model with $m_χc^{2}=0.3$ GeV and $σ_{1}=8\times 10^{-20}\ \mathrm{cm^{2}}$, capable of accommodating the measured absorption depth, the number density of Pop III hosts is reduced by a factor of $3-10$ at $z\sim 15-20$, when the 21-cm signal is imprinted, compared with the $Λ$CDM model. The confluence of future detailed cosmological simulations with improved 21-cm observations promises to probe the particle-physics nature of DM at the small-scale frontier of early structure formation.

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arXiv2019

Legacy of star formation in the pre-reionization universe

We utilize GIZMO, coupled with newly developed sub-grid models for Population~III (Pop~III) and Population~II (Pop~II), to study the legacy of star formation in the pre-reionization Universe. We find that the Pop~II star formation rate density (SFRD), produced in our simulation ($\sim 10^{-2}\ \Msun{\rm yr^{-1}\, Mpc^{-3}}$ at $z\simeq 10$), matches the total SFRD inferred from observations within a factor of $<2$ at $7\lesssim z \lesssim10$. The Pop~III SFRD, however, reaches a plateau at $\sim10^{-3}\ \Msun{\rm yr^{-1}\, Mpc^{-3}}$ by $z\approx10$, remaining largely unaffected by the presence of Pop~II feedback. At $z$=7.5, $\sim 20\%$ of Pop~III star formation occurs in isolated haloes which have never experienced any Pop~II star formation (i.e. primordial haloes). We predict that Pop~III-only galaxies exist at magnitudes $M_{\rm UV}\gtrsim -11$, beyond the limits for direct detection with the {\it James Webb Space Telescope (JWST)}. We assess that our stellar mass function (SMF) and UV luminosity function (UVLF) agree well with the observed low mass/faint-end behavior at $z=8$ and $10$. However, beyond the current limiting magnitudes, we find that both our SMF and UVLF demonstrate a deviation/turnover from the expected power-law slope ($M_{\rm UV,turn}= -13.4\pm1.1$ at $z$=10). This could impact observational estimates of the true SFRD by a factor of $2 (10)$ when integrating to $M_{\rm UV} = -$12 ($-$8) at $z\sim 10$, depending on integration limits. Our turnover correlates well with the transition from dark matter haloes dominated by molecular cooling to those dominated by atomic cooling, for a mass $M_{\rm halo}\approx 10^{8} \Msun$ at $z\simeq 10$.

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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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arXiv2019

Probing the assembly of dwarf galaxies through cosmic time with damped Lyman-$α$ absorption spectroscopy

We investigate the absorption features associated with a gas-rich dwarf galaxy using cosmological hydrodynamics simulations. Our goal is to explore whether the progenitors of the lowest mass dwarf galaxies known to harbor neutral hydrogen today (M_star~10^6 solar mass, M_halo=4x10^9 solar mass) could possibly be detected as Damped Lyman-alpha Absorbers (DLAs) over cosmic time. We trace the evolution of a single dwarf galaxy, pre-selected to contain DLAs, from the era of the first metal-free, so-called Population~III (Pop~III), stars, down to z=0, thus allowing us to study the metal enrichment history of DLAs associated with the simulated galaxy. We find that the progenitors of the simulated dwarf are expected to be seen for most of their evolution as DLAs that are contaminated by normal, Population~II, stars. The time period during which DLAs are only metal-enriched by Pop~III stars, on the other hand, is likely very brief, confined to high redshifts, z~6. The susceptibility of the dwarfs to the external UV radiation background allows them to preserve neutral gas only at the centre (a few ~100 pc). This results in a small probability that the simulated dwarf would be observed as a DLA. This study suggests that DLAs are unlikely to be hosted in the lowest mass dwarfs that can harbor neutral gas (M_halo~ 4x10^9 solar mass), below which neutral gas is unlikely to exist. However, this study does illustrate that, when detected, absorption lines provide a powerful method for probing ISM conditions inside the smallest dwarf galaxies at intermediate to high redshifts.

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arXiv2019

Constraining First Star Formation with 21cm-Cosmology

Within standard $Λ$CDM cosmology, Population III (Pop III) star formation in minihalos of mass $M_\mathrm{halo}\gtrsim 5\times10^5$ M$_\odot$ provides the first stellar sources of Lyman$α$ (Ly$α$) photons. The Experiment to Detect the Global Epoch of Reionization Signature (EDGES) has measured a strong absorption signal of the redshifted 21 cm radiation from neutral hydrogen at $z\approx 17$, requiring efficient formation of massive stars before then. In this paper, we investigate whether star formation in minihalos plays a significant role in establishing the early Ly$α$ background required to produce the EDGES absorption feature. We find that Pop III stars are important in providing the necessary Ly$α$-flux at high redshifts, and derive a best-fitting average Pop III stellar mass of $\sim$ 750M$_\odot{}$ per minihalo, corresponding to a star formation efficiency of 0.1%. Further, it is important to include baryon-dark matter streaming velocities in the calculation, to limit the efficiency of Pop~III star formation in minihalos. Without this effect, the cosmic dawn coupling between 21 cm spin temperature and that of the gas would occur at redshifts higher than what is implied by EDGES.

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arXiv2019

Supermassive Black Holes in the Early Universe

The emergence of the first black holes during the first billion years of cosmic history marks a key event in cosmology. Their formation is part of the overall process of ending the cosmic dark ages, when the first stars appeared in low-mass dark matter haloes about a few 100 million years after the Big Bang. The first stars, galaxies, and black holes transformed the Universe from its simple initial state into one of ever increasing complexity. We review recent progress on our emerging theoretical picture of how the first black holes appeared on the cosmic scene, and how they impacted the subsequent history of the Universe. Our focus is on supermassive black holes, in particular assessing possible pathways to the formation of the billion-solar-mass black holes inferred to power the luminous quasars at high redshifts. We conclude with a discussion of upcoming empirical probes, such as the James Webb Space Telescope (JWST), and the Laser Interferometer Space Antenna (LISA), further ahead in time.

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arXiv2019

Signature of the first galaxies in JWST deep field observations

We examine the assembly process and the observability of a first galaxy (M_vir~10^9 solar mass at z~8) with cosmological zoom-in, hydrodynamic simulations, including the radiative, mechanical, and chemical feedback exerted by the first generations of stars. To assess the detectability of such dwarf systems with the upcoming James Webb Space Telescope (JWST), we construct the spectral energy distribution for the simulated galaxy in a post-processing fashion. We find that while the non-ionizing UV continuum emitted by the simulated galaxy is expected to be below the JWST detection limit, the galaxy might be detectable using its nebular emission, specifically in the H-alpha recombination line. This requires that the galaxy experiences an active starburst with a star formation rate of Mdot_star > 0.1 Msun/yr at z~9. Due to the bursty nature of star formation in the first galaxies, the time interval for strong nebular emission is short, less than 2-3 Myr. The probability of capturing such primordial dwarf galaxies during the observable part of their duty cycle is thus low, resulting in number densities of order one source in a single pointing with MIRI onboard the JWST, for very deep exposures. Gravitational lensing, however, will boost their observability beyond this conservative baseline. The first sources of light will thus come firmly within our reach.

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arXiv2019

Global radiation signature from early structure formation

We use cosmological hydrodynamic zoom-in simulations to study early structure formation in two dark matter (DM) cosmologies, the standard CDM model, and a thermal warm DM (WDM) model with a particle mass of $m_χc^{2}=3\ \mathrm{keV}$. We focus on DM haloes with virial masses $M\sim 10^{10}\ M_{\odot}$. We find that the first star formation activity is delayed by $\sim 200\ \mathrm{Myr}$ in the WDM model, with similar delays for metal enrichment and the formation of the second generation of stars. However, the differences between the two models in globally-averaged properties, such as star formation rate density and mean metallicity, decrease towards lower redshifts ($z\lesssim 10$). Metal enrichment in the WDM cosmology is restricted to dense environments, while low-density gas can also be significantly enriched in the CDM case. The free-free contribution from early structure formation at redshifts $z>6$ to the cosmic radio background (CRB) is $3_{-1.5}^{+13}\%$ ($8_{-3.5}^{+33}\%$) of the total signal inferred from radio experiments such as ARCADE 2, in the WDM (CDM) model. The direct detection of the $\mathrm{H_{2}}$ emission from early structure formation ($z\gtrsim 7.2$), originating from the low-mass haloes explored here, will be challenging even with the next generation of far-infrared space telescopes, unless the signal is magnified by at least a factor of 10 via gravitational lensing or shocks. However, more massive haloes with $M\gtrsim 10^{12}\ M_{\odot}$ may be observable for $z\gtrsim 10$, even without magnification, provided that our extrapolation from the scale of our simulated haloes is valid.

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arXiv2019

Electromagnetic Window into the Dawn of Black Holes

Massive 10^6-10^10 Msun black holes (BHs) are ubiquitous in local galactic nuclei. They were common by the time the Universe is several Gyr old, and many of them were in place within the first 1~Gyr after the Big Bang. Their quick assembly has been attributed to mechanisms such as the rapid collapse of gas into the nuclei of early protogalaxies, accretion and mergers of stellar-mass BHs accompanying structure formation at early times, and the runaway collapse of early, ultra-dense stellar clusters. The origin of the early massive BHs remains an intriguing and long-standing unsolved puzzle in astrophysics. Here we discuss strategies for discerning between BH seeding models using electromagnetic observations. We argue that the most direct answers will be obtained through detection of BHs with masses M<10^5 Msun at redshifts z>10, where we expect them to first form. Reaching out to these redshifts and down to these masses is crucial, because BHs are expected to lose the memory of their initial assembly by the time they grow well above 10^5 Msun and are incorporated into higher-mass galaxies. The best way to detect 10^4-10^5 Msun BHs at high redshifts is by a sensitive X-ray survey. Critical constraining power is augmented by establishing the properties and the environments of their host galaxies in deep optical/IR imaging surveys. Required OIR data can be obtained with the JWST and WFIRST missions. The required X-ray flux limits (down to 10^{-19} erg/s/cm^2) are accessible only with a next-generation X-ray observatory which has both high (sub-1") angular resolution and high throughput. A combination of deep X-ray and OIR surveys will be capable of probing several generic markers of the BH seed scenarios, and resolving the long-stanging puzzle of their origin. These electromagnetic observations are also highly synergistic with the information from LISA on high-z BH mergers.

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arXiv2019

Astro2020 Science White Paper: Synergies Between Galaxy Surveys and Reionization Measurements

The early phases of galaxy formation constitute one of the most exciting frontiers in astrophysics. It is during this era that the first luminous sources reionize the intergalactic medium - the moment when structure formation affects every baryon in the Universe. Here we argue that we will obtain a complete picture of this era by combining observations of galaxies with direct measurements of the reionization process: the former will provide a detailed understanding of bright sources, while the latter will constrain the (substantial) faint source population. We further describe how optimizing the comparison of these two measurements requires near-infrared galaxy surveys covering large volumes and retaining redshift information and also improvements in 21-cm analysis, moving those experiments into the imaging regime.

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arXiv2019

Astro2020 Science White Paper: Insights Into the Epoch of Reionization with the Highly-Redshifted 21-cm Line

The epoch of reionization, when photons from early galaxies ionized the intergalactic medium about a billion years after the Big Bang, is the last major phase transition in the Universe's history. Measuring the characteristics of the transition is important for understanding early galaxies and the cosmic web and for modeling dwarf galaxies in the later Universe. But such measurements require probes of the intergalactic medium itself. Here we describe how the 21-cm line of neutral hydrogen provides a powerful probe of the reionization process and therefore important constraints on both the galaxies and intergalactic absorbers at that time. While existing experiments will make precise statistical measurements over the next decade, we argue that improved 21-cm analysis techniques - allowing imaging of the neutral gas itself - as well as improved theoretical models, are crucial for testing our understanding of this important era.

astro-ph.CO↗

arXiv2019

Warm H$_2$ as a probe of massive accretion and feedback through shocks and turbulence across cosmic time

Galaxy formation depends on a complex interplay between gravitational collapse, gas accretion, merging, and feedback processes. Yet, after many decades of investigation, these concepts are poorly understood. This paper presents the argument that warm H$_2$ can be used as a tool to unlock some of these mysteries. Turbulence, shocks and outflows, driven by star formation, AGN activity or inflows, may prevent the rapid buildup of star formation in galaxies. Central to our understanding of how gas is converted into stars is the process by which gas can dissipate its mechanical energy through turbulence and shocks in order to cool. H$_2$ lines provide direct quantitative measurements of kinetic energy dissipation in molecular gas in galaxies throughout the Universe. Based on the detection of very powerful H$_2$ lines from z = 2 galaxies and proto-clusters at the detection limits of {\it Spitzer}, we are confident that future far-IR and UV H$_2$ observations will provide a wealth of new information and insight into galaxy evolution to high-z. Finally, at the very earliest epoch of star and galaxy formation, warm H$_2$ may also provide a unique glimpse of molecular gas collapse at 7 $<$ z $<$ 12 in massive dark matter (DM) halos on their way to forming the very first galaxies. Such measurements are beyond the reach of existing and planned observatories.

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arXiv2019

Astro 2020 Science White Paper: Fundamental Cosmology in the Dark Ages with 21-cm Line Fluctuations

The Dark Ages are the period between the last scattering of the cosmic microwave background and the appearance of the first luminous sources, spanning approximately 1100 < z < 30. The only known way to measure fluctuations in this era is through the 21-cm line of neutral hydrogen. Such observations have enormous potential for cosmology, because they span a large volume while the fluctuations remain linear even on small scales. Observations of 21-cm fluctuations during this era can therefore constrain fundamental aspects of our Universe, including inflation and any exotic physics of dark matter. While the observational challenges to these low-frequency 21-cm observations are enormous, especially from the terrestrial environment, they represent an important goal for cosmology.

astro-ph.CO↗

arXiv2019

Astro2020 Science White Paper: First Stars and Black Holes at Cosmic Dawn with Redshifted 21-cm Observations

The "cosmic dawn" refers to the period of the Universe's history when stars and black holes first formed and began heating and ionizing hydrogen in the intergalactic medium (IGM). Though exceedingly difficult to detect directly, the first stars and black holes can be constrained indirectly through measurements of the cosmic 21-cm background, which traces the ionization state and temperature of intergalactic hydrogen gas. In this white paper, we focus on the science case for such observations, in particular those targeting redshifts z $\gtrsim$ 10 when the IGM is expected to be mostly neutral. 21-cm observations provide a unique window into this epoch and are thus critical to advancing first star and black hole science in the next decade.

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arXiv2019

Cosmology with the Highly Redshifted 21cm Line

In addition to being a probe of Cosmic Dawn and Epoch of Reionization astrophysics, the 21cm line at $z>6$ is also a powerful way to constrain cosmology. Its power derives from several unique capabilities. First, the 21cm line is sensitive to energy injections into the intergalactic medium at high redshifts. It also increases the number of measurable modes compared to existing cosmological probes by orders of magnitude. Many of these modes are on smaller scales than are accessible via the CMB, and moreover have the advantage of being firmly in the linear regime (making them easy to model theoretically). Finally, the 21cm line provides access to redshifts prior to the formation of luminous objects. Together, these features of 21cm cosmology at $z>6$ provide multiple pathways toward precise cosmological constraints. These include the "marginalizing out" of astrophysical effects, the utilization of redshift space distortions, the breaking of CMB degeneracies, the identification of signatures of relative velocities between baryons and dark matter, and the discovery of unexpected signs of physics beyond the $Λ$CDM paradigm at high redshifts.

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arXiv2019

Mapping Cosmic Dawn and Reionization: Challenges and Synergies

Cosmic dawn and the Epoch of Reionization (EoR) are among the least explored observational eras in cosmology: a time at which the first galaxies and supermassive black holes formed and reionized the cold, neutral Universe of the post-recombination era. With current instruments, only a handful of the brightest galaxies and quasars from that time are detectable as individual objects, due to their extreme distances. Fortunately, a multitude of multi-wavelength intensity mapping measurements, ranging from the redshifted 21 cm background in the radio to the unresolved X-ray background, contain a plethora of synergistic information about this elusive era. The coming decade will likely see direct detections of inhomogenous reionization with CMB and 21 cm observations, and a slew of other probes covering overlapping areas and complementary physical processes will provide crucial additional information and cross-validation. To maximize scientific discovery and return on investment, coordinated survey planning and joint data analysis should be a high priority, closely coupled to computational models and theoretical predictions.

astro-ph.CO↗

arXiv2019

Cosmic Dawn and Reionization: Astrophysics in the Final Frontier

The cosmic dawn and epoch of reionization mark the time period in the universe when stars, galaxies, and blackhole seeds first formed and the intergalactic medium changed from neutral to an ionized one. Despite substantial progress with multi-wavelength observations, astrophysical process during this time period remain some of the least understood with large uncertainties on our existing models of galaxy, blackhole, and structure formation. This white paper outlines the current state of knowledge and anticipated scientific outcomes with ground and space-based astronomical facilities in the 2020s. We then propose a number of scientific goals and objectives for new facilities in late 2020s to mid 2030s that will lead to definitive measurements of key astrophysical processes in the epoch of reionization and cosmic dawn.

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