Fabio Pacucci

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.

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.

Focusing on Warm Dark Matter with Lensed High-redshift Galaxies

We propose a novel use of high-redshift galaxies, discovered in deep Hubble Space Telescope (HST) fields around strong lensing clusters. These fields probe small comoving volumes ( 10 3 Mpc 3 ) at high magnification ( > 10), and can detect otherwise inaccessible ultra-faint galaxies. Even a few galaxies found in such small volumes require a very high number density of collapsed dark matter (DM) halos. This implies significant primordial power on small scales, allowing these observations to rule out popular alternatives to standard cold dark matter (CDM) models, such as warm dark matter (WDM). In this work, we analytically compute WDM halo mass functions at z = 10, including the effects of both particle freestreaming and residual velocity dispersion. We show that the two z 10 galaxies already detected by the Cluster Lensing And Supernova survey with Hubble (CLASH) survey are sufficient to constrain the WDM particle mass to mx > 1 (0.9) keV at 68% (95%) confidence limit (for a thermal relic relativistic at decoupling). This limit depends only on the WDM halo mass function and, unlike previous constraints on mx, is independent of any astrophysical modeling. The forthcoming HST Frontier Fields can significantly tighten these constraints.

Essential physics of early galaxy formation

We present a theoretical model embedding the essential physics of early galaxy formation (z ≃ 5-12) based on the single premise that any galaxy can form stars with a maximal limiting efficiency that provides enough energy to expel all the remaining gas, quenching further star formation. This simple idea is implemented into a merger-tree-based semi-analytical model that utilizes two mass and redshift-independent parameters to capture the key physics of supernova feedback in ejecting gas from low- mass haloes, and tracks the resulting impact on the subsequent growth of more massive systems via halo mergers and gas accretion. Our model shows that: (i) the smallest haloes (halo mass Mh ≤ 1010 M☉) build up their gas mass by accretion from the intergalactic medium; (ii) the bulk of the gas powering star formation in larger haloes (Mh ≥ 1011.5 M☉) is brought in by merging progenitors; (iii) the faint-end UV luminosity function slope evolves according to α = -1.75 log z - 0.52. In addition, (iv) the stellar mass-to-light ratio is well fitted by the functional form log M* = -0.38MUV - 0.13 z + 2.4, which we use to build the evolving stellar mass function to compare to observations. We end with a census of the cosmic stellar mass density (SMD) across galaxies with UV magnitudes over the range -23 ≤ MUV ≤ -11 spanning redshifts 5 < z < 12; (v) while currently detected LBGs contain ≈50 per cent (10 per cent) of the total SMD at z = 5 (8), the James Webb Space Telescope will detect up to 25 per cent of the SMD at z ≃ 9.5.

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

Early Galaxy Formation in Warm Dark Matter Cosmologies

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

The nature of the Lyman α emitter CR7: a persisting puzzle

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

Unveiling the first black holes with JWST: multi-wavelength spectral predictions

) black holes in place

The Chandra COSMOS Legacy Survey: Energy Spectrum of the Cosmic X-Ray Background and Constraints on Undetected Populations

< 1 \, \mathrm{Gyr}