A. Ferrara

The X-ray spectra of the first galaxies: 21 cm signatures

The cosmological 21cm signal is a physics-rich probe of the early Universe, encoding information about both the ionization and the thermal history of the intergalactic medium (IGM). The latter is likely governed by X-rays from star-formation processes inside very high redshift (z > 15) galaxies. Due to the strong dependence of the mean free path on the photon energy, the X-ray SED can have a significant impact on the interferometric signal from the cosmic dawn. Recent Chandra observations of nearby, star-forming galaxies show that their SEDs are more complicated than is usually assumed in 21cm studies. In particular, these galaxies have ubiquitous, sub-keV thermal emission from the hot interstellar medium (ISM), which generally dominates the soft X-ray luminosity (with energies < 1 keV, sufficiently low to significantly interact with the IGM). Using illustrative soft and hard SEDs, we show that the IGM temperature fluctuations in the early Universe would be substantially increased if the X-ray spectra of the first galaxies were dominated by the hot ISM, compared with X-ray binaries with harder spectra. The associated large-scale power of the 21cm signal would be higher by roughly a factor of three. More generally, we show that the peak in the redshift evolution of the large-scale (k = 0.2 1/Mpc) 21cm power is a robust probe of the soft-band SED of the first galaxies, and importantly, is not degenerate with their bolometric luminosities. On the other hand, the redshift of the peak constrains the X-ray luminosity and halo masses which host the first galaxies.

Simulating cosmic metal enrichment by the first galaxies

We study cosmic metal enrichment via adaptive mesh refinement hydrodynamical simulations in a (10 Mpc h-1)3 volume following the Population III (PopIII)-PopII transition and for different PopIII initial mass function (IMFs). We have analysed the joint evolution of metal enrichment on galactic and intergalactic scales at z = 6 and z = 4. Galaxies account for ≲9 per cent of the baryonic mass; the remaining gas resides in the diffuse phases: (a) voids, i.e. regions with extremely low density (Δ ≤ 1), (b) the true intergalactic medium (IGM, 1 10^{4.5} K state. Due to these physical conditions, C {IV} absorption line experiments can probe only ≃2 per cent of the total carbon present in the IGM/CGM; however, metal absorption line spectra are very effective tools to study reionization. Finally, the PopIII star formation history is almost insensitive to the chosen PopIII IMF. PopIII stars are preferentially formed in truly pristine (Z = 0) gas pockets, well outside polluted regions created by previous star formation episodes.

The growth efficiency of high-redshift black holes

The observational evidence that Super-Massive Black Holes (M 10 9 10 M ) are already in place less than 1Gyr after the Big Bang poses stringent time constraints on the growth efficiency of their seeds. Among proposed possibilities, the formation of massive ( 10 3 6 M ) seeds and/or the occurrence of super-Eddington ( _ M > _ MEdd) accretion episodes may contribute to the solution of this problem. In this work, using realistic initial conditions, we analytically and numerically investigate the accretion flow onto high-redshift (z 10) black holes to understand the physical requirements favoring rapid and efficient growth. Our model identifies a “feeding-dominated” accretion regime and a “feedback-limited” one, the latter being characterized by intermittent (duty cyclesD 10 5 6 M ) grow very rapidly as they are found in the feeding-dominated regime. In addition to the standard accretion model with a fixed matterenergy conversion factor ( = 0:1), we have also explored slim disk models ( < 0:04), which may ensure a continuous growth with _ M _ MEdd (up to 300 _ MEdd in our simulations). Under these conditions, outflows play a negligible role and a black hole can accrete 80% 100% of the gas mass of the host halo ( 10 7 M ) in 10Myr, while in feedbacklimited systems we predict that black holes can accrete only up to 15% of the available mass.

Shining in the dark: the spectral evolution of the first black holes

Massive Black Hole (MBH) seeds at redshift

Simulating the growth of intermediate mass black holes

z \gtrsim 10

Mining the Galactic halo for very metal‐poor stars

are now thought to be key ingredients to explain the presence of the super-massive (

Can supermassive black hole seeds form in galaxy mergers

10^{9-10} \, \mathrm{M_{\odot}}

Metals and ionizing photons from dwarf galaxies

) black holes in place

Molecular clouds photoevaporation and FIR line emission

< 1 \, \mathrm{Gyr}

The impact of chemistry on the structure of high-z galaxies

after the Big Bang. Once formed, massive seeds grow and emit copious amounts of radiation by accreting the left-over halo gas; their spectrum can then provide crucial information on their evolution. By combining radiation-hydrodynamic and spectral synthesis codes, we simulate the time-evolving spectrum emerging from the host halo of a MBH seed with initial mass