Super-Eddington growth of the first black holes

The assembly of the first super massive black holes (SMBHs) at z ≳ 6 is still a subject of intense debate. If black holes (BHs) grow at their Eddington rate, they must start from ≳104 M⊙ seeds formed by the direct collapse of gas. Here, we explore the alternative scenario where ˜100 M⊙ BH remnants of the first stars grow at super-Eddington rate via radiatively inefficient slim accretion discs. We use an improved version of the cosmological, data-constrained semi-analytic model GAMETE/QSODUST, where we follow the evolution of nuclear BHs and gas cooling, disc and bulge formation of their host galaxies. Adopting SDSS J1148+5251 (J1148) at z = 6.4 as a prototype of luminous z ≳ 6 quasars, we find that ˜80 per cent of its SMBH mass is grown by super-Eddington accretion, which can be sustained down to z ˜ 10 in dense, gas-rich environments. The average BH mass at z ˜ 20 is MBH ≳ 104 M⊙, comparable to that of direct collapse BHs. At z = 6.4 the AGN-driven mass outflow rate is consistent with the observations and the BH-to-bulge mass ratio is compatible with the local scaling relation. However, the stellar mass in the central 2.5 kpc is closer to the value inferred from CO observations. Finally, ˜20 per cent of J1148 progenitors at z = 7.1 have BH luminosities and masses comparable to ULAS J1120+0641, suggesting that this quasar may be one of the progenitors of J1148

On the formation of the first quasars

Observations of the most luminous quasars at redshift z>6 reveal the existence of numerous supermasssive black holes (>10^9 Msun) already in place about twelve billion years ago. In addition, the interstellar medium of the galaxies hosting these black holes are observed to be chemically mature systems, with metallicities (Z>Zsun) and dust masses (>10^8 Msun) similar to that of more evolved, local galaxies. The connection between the rapid growth of the first supermassive black holes and the fast chemical evolution of the host galaxy is one of the most puzzling issues for theoretical models. Here we review state-of-the-art theoretical models that focus on this problem with particular emphasis on the conditions that lead to the formation of quasar seeds and their subsequent evolution at z>6

The sustainable growth of the first black holes

Super-Eddington accretion has been suggested as a possible formation pathway of $10^9 \, M_\odot$ supermassive black holes (SMBHs) 800 Myr after the Big Bang. However, stellar feedback from BH seed progenitors and winds from BH accretion disks may decrease BH accretion rates. In this work, we study the impact of these physical processes on the formation of $z \sim 6$ quasar, including new physical prescriptions in the cosmological, data-constrained semi-analytic model GAMETE/QSOdust. We find that the feedback produced by the first stellar progenitors on the surrounding does not play a relevant role in preventing SMBHs formation. In order to grow the $z \gtrsim 6$ SMBHs, the accreted gas must efficiently lose angular momentum. Moreover disk winds, easily originated in super-Eddington accretion regime, can strongly reduce duty cycles. This produces a decrease in the active fraction among the progenitors of $z\sim6$ bright quasars, reducing the probability to observe them

The origin, properties and fate of the first black holes in the Universe

Observations of the Universe’s earliest quasars, less than 1 Gyr after the Big Bang, open the door to many questions. They are found to host supermassive black holes (SMBHs) with MBH = 10^9 - 10^10 Msun (Fan et al., 2001, 2004; Mortlock et al., 2011), and BH formation models need to explain their existence and evolution in such a short time. In the first part of this original work, we introduce the cosmological, semi-analytic code GAMETE/SuperQSOdust, which reconstructs several hierarchical merger histories of high-z bright quasars, following the time evolution of central BHs together with the mass of stars, gas, metals and dust. With this tool, we have studied the relative importance of different accretion regimes for the formation of the first quasars, with particular attention to accretion events occurring over the classical luminosity threshold - the so-called Eddington limit. We find that 80 % of the final SMBH mass is grown by super-Eddington accretion, which can be sustained down to z ~ 10 in dense, gas-rich environments, and the average BH mass at z ~ 20 is MBH ~ 10^4 Msun, comparable to that of direct collapse BHs. However, stellar feedback from BH seed progenitors and winds from BH accretion disks may decrease BH accretion rates. Therefore, we studied the impact of these physical processes on the formation of z ~ 6 quasars, including new physical prescriptions in the model. We find that the feedback produced by the first stellar progenitors on the surrounding environment does not play a relevant role in preventing the SMBH formation. In order to grow the z 6 SMBHs, the accreted gas must efficiently lose angular momentum. Moreover, disk winds, easily originated in the super-Eddington accretion regime, can strongly reduce duty cycles, producing a decrease in the active fraction among the progenitors of z ~ 6 bright quasars and thus reducing the probability to detect them. From an observational point of view, no convincing candidates of faint progenitors of luminous high-z quasars have been selected in X-ray surveys (Treister et al., 2013; Weigel et al., 2015; Cappelluti et al., 2016). In order to interpret this lack of detections, we have modeled the spectral energy distribution of accreting BHs. This modeling has been applied to a sample of simulated z ~ 6 SMBH progenitors, also taking into account the photon trapping effect which plays an important role at high accretion rates. The results show that faint progenitors are still luminous enough to be detected with current X-ray surveys. Even accounting for a maximum obscuration effect, the number of detectable BHs is reduced at most by a factor of 2. In our simulated sample, observations of faint BHs are mainly limited by their very low active fraction (fact ~ 1 percent), which is the result of short, supercritical growth episodes. We suggest that to detect high-z SMBH ancestors, large area surveys with shallower sensitivities, such as COSMOS Legacy and XMM-LSS+XXL, should be preferred with respect to deep surveys probing smaller fields, such as Chandra Deep Field South. An alternative way of constraining the early growth of BHs is to compare theoretical models with observations of massive BHs (MBH ~ 10^5 Msun) in local dwarf galaxies. To this aim, in the last part of this work, we introduced GAMESH, a simulation following the formation of a Milky Way-like halo in a well resolved cosmic volume of (4 cMpc)^3. This model allows to follow the star formation and chemical enrichment histories of all the galaxies in the simulation box. In the near future, we plan to extend the model including a self-consistent evolution of BHs and their feedback onto the host galaxies. This will allow us to compare results obtained by different BH seeding and accretion models with observations of BH masses hosted by the Milky Way and dwarf galaxies. Here, we present a preliminary study, where we have post-processed the simulation output to analyze the mass and redshift distribution of BH seeds formed as remnants of Pop III stars, and the BH occupation fraction at z = 0. Our preliminary results have been obtained under the assumption that gas accretion gives a negligible contribution to BH mass growth and, hence, provide a lower limit to the mass of nuclear BHs found at z = 0. We compare our results with recent studies carried out by means of cosmological hydrodynamical simulations (Marinacci et al. 2014; Bonoli et al. 2016), and - given the quiescent history experienced by the Milky Way-like halo - we conclude that either (i) light BH remnants of Pop III stars are able to rapidly grow their masses soon after their formation, or (ii) that the MilkyWay nuclear BH originates from more massive BH seeds, with masses comparable to the ones that characterize direct collapse BHs. In our future study, we will be able to analyze each of these two possibilities using the detailed treatment of chemical and radiative feedback effects allowed by GAMESH. This thesis is divided into four main parts. In the first part, we introduce some basic theoretical tools for understanding the most important features of the formation of galaxies and black holes: in Chapter 1, we present the CDM Cosmological Model and some fundamental properties of our Universe, including Large Scale Structures and galaxy formation, and in Chapter 2 we briefly describe the main characteristics of black holes and gas accretion disks orbiting around these compact objects. The second part of this work is dedicated to the high-z BHs: Chapter 3 is an extract from the review Valiante R., Agarwal B., Habouzit M., Pezzulli E., 2017, PASA 34, 31. In Chapter 4 we discuss the results obtained in the manuscript Pezzulli E., Valiante R., Schneider R., 2016, MNRAS, 458, 3047, and we also introduce the cosmological, semi-analytic model used for the study on the occurrence of different accretion regimes for the formation of high-z QSOs. In Chapter 5 we discuss on the sustainability of super-Eddington accretion in a cosmological context, including some prescriptions for the two negative feedback mechanisms introduced above. The results have been published in Pezzulli E., Volonteri M., Schneider R., Valiante R., 2017, MNRAS, 471, 589. Possible solutions for the current lack of faint, high-z AGNs observations are reported in Chapter 6, as investigated in Pezzulli E., Valiante R., Orofino M.C., Schneider R., Gallerani S., Sbarrato T., 2017, MNRAS, 466, 2131. In the third part of the thesis, we turn our attention to the Local Universe and to the constraints that can be put on the evolution of nuclear BHs and their hosts from observations of the Milky Way and local dwarf galaxies. In Chapter 7 we present the results of our preliminary study on the mass and redshift distribution of BH seeds and their impact on the z = 0 BH occupation fraction. Finally, in Part IV, we summarize our main conclusions

"Zombie" or active? An alternative explanation to the properties of star-forming galaxies at high redshift

Star-forming galaxies at high redshift show anomalous values of infrared excess, which can be described only by extremizing the existing relations between the shape of their ultraviolet continuum emission and their infrared-to-ultraviolet luminosity ratio, or by constructing \textit{ad-hoc} models of star formation and dust distribution. We present an alternative explanation, based on unveiled AGN activity, to the existence of such galaxies. In fact, the presence of a weak AGN configures as a natural scenario in order to explain the observed spectral properties of such high-$z$ objects in terms of a continuum slope distribution rather than altered infrared excesses, due to the different shape of the AGN continuum emission with respect to quiescent galaxies. To this aim, we directly compare the infrared-to-ultraviolet properties of high-redshift galaxies to those of known categories of AGN (quasars and Seyferts). We also infer the characteristics of their possible X-ray emission. We find a strong similarity between the spectral shapes and luminosity ratios of AGN with the corresponding properties of such galaxies. In addition, we derive expected X-ray fluxes that are compatible with energetics from AGN activity. We conclude that a moderate AGN contribution to the UV emission of such high-$z$ objects is a valid alternative to explain their spectral properties. Even the presence of an active nucleus in each source would not violate the expected quasar statistics. Furthermore, we suggest that the observed similarities between anomalous star-forming galaxies and quasars may provide a benchmark for future theoretical and observational studies on the galaxy population in the early Universe.Comment: 13 pages, 7 figures, 4 tables, accepted for publication in A&

Chasing the observational signatures of seed black holes at z > 7: candidate observability

Observing the light emitted by the first accreting black holes (BHs) would dramatically improve our understanding of the formation of quasars at z > 6, possibly unveiling the nature of their supermassive black hole (SMBH) seeds. In previous works we explored the relative role of the two main competing BH seed formation channels, Population III remnants (low-mass seeds) and direct collapse BHs (high-mass seeds), investigating the properties of their host galaxies in a cosmological context. Building on this analysis, we predict here the spectral energy distribution and observational features of low- and high-mass BH seeds selected among the progenitors of a z~6 SMBH. We derive the processed emission from both accreting BHs and stars by using the photo-ionization code Cloudy, accounting for the evolution of metallicity and dust-to-gas mass ratio in the interstellar medium of the host galaxies, as predicted by the cosmological data- constrained model GAMETE/QSOdust. We show how future missions like JWST and ATHENA will be able to detect the light coming from SMBH progenitors already at z~16. We build upon previous complementary studies and propose a method based on the combined analysis of near infrared (NIR) colors, IR excess (IRX) and UV continuum slopes (i.e. color-color and IRX-Beta diagrams) to distinguish growing seed BH host galaxies from starburst-dominated systems in JWST surveys. Sources selected through this criterion would be the best target for follow-up X-ray observations.Comment: accepted for publicaztion in MNRA

Chasing the observational signatures of seed black holes at z > 7: Candidate statistics

Supermassive black holes (SMBHs) of 109–1010M were already in place ∼13 Gyr ago, at z > 6. Super-Eddington growth of low-mass BH seeds (∼100M) or less extreme accretion on to∼105M seeds have been recently considered as the main viable routes to these SMBHs. Here, we study the statistics of these SMBH progenitors at z ∼ 6. The growth of low- and high-mass seeds and their host galaxies are consistently followed using the cosmological data constrained model GAMETE/QSODUST, which reproduces the observed properties of high-z quasars, like SDSS J1148+5251.We show that both seed formation channels can be in action over a similar redshift range 15 < z < 18 and are found in dark matter haloes with comparable mass, ∼5 × 107M. However, as long as the systems evolve in isolation (i.e. no mergers occur), noticeable differences in their properties emerge: At z ≥ 10 galaxies hosting high-mass seeds have smaller stellar mass and metallicity, the BHs accrete gas at higher rates and star formation proceeds less efficiently than in low-mass seeds hosts. At z < 10 these differences are progressively erased, as the systems experience minor or major mergers and every trace of the BH origin gets lost

Faint progenitors of luminous z ∼ 6 quasars: Why do not we see them?

Observational searches for faint active nuclei at z > 6 have been extremely elusive, with a few candidates whose high-z nature is still to be confirmed. Interpreting this lack of detections is crucial to improve our understanding of high-z supermassive black holes (SMBHs) formation and growth. In this work, we present a model for the emission of accreting black holes (BHs) in the X-ray band, taking into account super-Eddington accretion, which can be very common in gas-rich systems at high-z. We compute the spectral energy distribution for a sample of active galaxies simulated in a cosmological context, which represent the progenitors of a z ˜ 6 SMBH with MBH ˜ 109 M⊙. We find an average Compton-thick fraction of ˜45 per cent and large typical column densities (NH ≳ 1023 cm2). However, faint progenitors are still luminous enough to be detected in the X-ray band of current surveys. Even accounting for a maximum obscuration effect, the number of detectable BHs is reduced at most by a factor of 2. In our simulated sample, observations of faint quasars are mainly limited by their very low active fraction (fact ˜ 1 per cent), which is the result of short, supercritical growth episodes. We suggest that to detect high-z SMBHs progenitors, large area surveys with shallower sensitivities, such as COSMOS Legacy and XMM-LSS+XXL, are to be preferred with respect to deep surveys probing smaller fields, such as Chandra Deep Field South

"Zombie" or active? An alternative explanation to the properties of star-forming galaxies at high redshift

Star-forming galaxies at high redshift show anomalous values of infrared excess, which can be described only by extremizing the existing relations between the shape of their ultraviolet continuum emission and their infrared-to-ultraviolet luminosity ratio, or by constructing \textitad-hoc models of star formation and dust distribution. We present an alternative explanation, based on unveiled AGN activity, to the existence of such galaxies. In fact, the presence of a weak AGN configures as a natural scenario in order to explain the observed spectral properties of such high-$z$ objects in terms of a continuum slope distribution rather than altered infrared excesses, due to the different shape of the AGN continuum emission with respect to quiescent galaxies. To this aim, we directly compare the infrared-to-ultraviolet properties of high-redshift galaxies to those of known categories of AGN (quasars and Seyferts). We also infer the characteristics of their possible X-ray emission. We find a strong similarity between the spectral shapes and luminosity ratios of AGN with the corresponding properties of such galaxies. In addition, we derive expected X-ray fluxes that are compatible with energetics from AGN activity. We conclude that a moderate AGN contribution to the UV emission of such high-$z$ objects is a valid alternative to explain their spectral properties. Even the presence of an active nucleus in each source would not violate the expected quasar statistics. Furthermore, we suggest that the observed similarities between anomalous star-forming galaxies and quasars may provide a benchmark for future theoretical and observational studies on the galaxy population in the early Universe