1,721,035 research outputs found
The dust content of the most metal-poor star-forming galaxies
Full text linkAlthough dust content is usually assumed to depend uniquely on metallicity, recent observations of two extremely metal-poor dwarf galaxies have suggested that this may not always be true. At a similar oxygen abundance of ∼3 per cent Z⊙, the dust-to-gas and dust-to-stellar mass ratios in SBS 0335−052 and I Zw 18 differ by a factor of 40–70 according to including molecular gas or excluding it. Here, we investigate a possible reason for this dramatic difference through models based on a semi-analytical formulation of chemical evolution including dust. Results suggest that the greater dust mass in SBS 0335−052 is due to the more efficient grain growth allowed by the high density in the cold interstellar medium (ISM), observationally inferred to be almost 20 times higher than in I Zw 18. Our models are able to explain the difference in dust masses, suggesting that efficient dust formation and dust content in galaxies, including those with the highest measured redshifts, depend sensitively on the ISM density, rather than only on metallicity
High-redshift quasars host galaxies: is there a stellar mass crisis?
Full text linkWe investigate the evolutionary properties of a sample of quasars (QSOs) at 5 <z <6.4 using the semi-analytical hierarchical model GAMETE/QSODUST. We find that the observed properties of these QSOs are well reproduced by a common formation scenario in which stars form according to a standard initial mass function, via quiescent star formation and efficient merger-driven bursts, while the central black hole (BH) grows via gas accretion and BH-BH mergers. Eventually, a strong active galactic nuclei-driven wind starts to clear up the interstellar medium of dust and gas, damping the star formation and un-obscuring the line of sight towards the QSO. In this scenario, all the QSOs hosts have final stellar masses in the range (4-6) × 1011 M⊙, a factor of 3-30 larger than the upper limits allowed by the observations. We discuss alternative scenarios to alleviate this apparent tension: the most likely explanation resides in the large uncertainties that still affect dynamical mass measurements in these high-z galaxies. In addition, during the transition between the starburst-dominated and the active QSO phase, we predict that ˜40 per cent of the progenitor galaxies can be classified as Submillimetre Galaxies, although their number rapidly decreases with redshift
Quasar feedback in the early Universe: The case of SDSS J1148+5251
Full text linkGalaxy-scale gas outflows triggered by active galactic nuclei have been proposed as a key physical process to regulate the co-evolution of nuclear black holes and their host galaxies. The recent detection of a massive gas outflow in one of the most distant quasars, SDSS J1148+5251 at z = 6.4, presented by Maiolino et al., strongly supports this idea and suggests that strong quasar feedback is already at work at very early times. In a previous work, Valiante et al., we have presented a hierarchical semi-analytical model, GAMETE/QSOdust, for the formation and evolution of high-redshift quasars, and we have applied it to the quasar SDSS J1148+5251, with the aim of investigating the star formation history, the nature of the dominant stellar populations and the origin and properties of the large dust mass observed in the host galaxy. A robust prediction of the model is that the evolution of the nuclear black hole and of the host galaxy are tightly coupled by quasar feedback in the form of strong galaxy-scale winds. In the present Letter, we show that the gas outflow rate predicted by GAMETE/QSOdust is in good agreement with the lower limit of 3500 M circle dot yr(-1) inferred by the observations. According to the model, the observed outflow at z = 6.4 is dominated by quasar feedback, as the outflow rate has already considerably depleted the gas content of the host galaxy, leading to a downturn in the star formation rate at z <78. Hence, we predict that supernova explosions give a negligible contribution to the observed winds at z = 6.4
Super-Eddington growth of the first black holes
Full text linkThe 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
Chasing the observational signatures of seed black holes at z > 7: Candidate statistics
Full text linkSupermassive black holes (SMBHs) of 109–1010 M⊙ were already in place ∼13 Gyr ago, at z > 6. Super-Eddington growth of low-mass BH seeds (∼100 M⊙) or less extreme accretion on to∼105 M⊙ 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 × 107 M⊙. 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
Chasing the observational signatures of seed black holes at z > 7: Candidate statistics
Full text linkSupermassive 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
The low-end of the black hole mass function at cosmic dawn
No full textUnderstanding the formation and growth of supermassive black holes (SMBHs) at high redshift represents a major challenge for theoretical models. In this work we investigate the early evolution of the first SMBHs by constraining their distribution in mass and luminosity at z>4. In particular, we focus on the poorly explored low-mass end of the nuclear black hole (BH) distribution down to z≃4, and explore its connection with the nature of the first BH seeds and the processes governing their mass growth. To this aim, we have developed CAT (Cosmic Archaeology Tool), a new semi-analytic model that describes the formation of the first stars and black holes in a self-consistent way and follows the co-evolution of nuclear BHs and their host galaxies for a representative population at z>4. We find that current observational constraints favour models where the growth of BH seeds is Eddington limited and occurs at the Bondi-Hoyle-Lyttleton rate or where super-Eddington accretion occurs via a slim disk during gas rich galaxy mergers. The main difference between these two model variants lies at the low-end of the predicted mass and luminosity functions at 4≤z≤6, where a clear gap appears in the first model, reflecting the stunted growth of light BH seeds formed as remnants of the first stars. Detecting this signature will be extremely challenging even for the future generation of space observatories, such as JWST, Athena and Lynx
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