Category: Theory

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The first fireworks: A roadmap to Population III stars during the epoch of reionization through pair-instability supernovae

With the launch of JWST and other scheduled missions aimed at probing the distant universe, we are entering a new promising era for high-z astronomy. One of our main goals is the detection of the first population of stars (Population III or Pop III stars), and models suggest that Pop III star formation is allowed well into the Epoch of Reionization (EoR), rendering this an attainable achievement. In this paper, we focus on our chance of detecting massive Pop IIIs at the moment of their death as Pair-Instability Supernovae (PISNe). We estimate the probability of discovering PISNe during the EoR in galaxies with different stellar masses (7.5 ≤ Log(M⋆/M⊙) ≤ 10.5) from six dustyGadget simulations of 50 h-1 cMpc per side. We further assess the expected number of PISNe in surveys with JWST/NIRCam and Roman/WFI. On average, less than one PISN is expected in all examined JWST fields at z ≃ 8 with Δz = 1, and O(1) PISN may be found in a ~1 deg2 Roman field in the best-case scenario, although different assumptions on the Pop III IMF and/or Pop III star formation efficiency can decrease this number substantially. Including the contribution from unresolved low-mass haloes holds the potential for increased discoveries. JWST/NIRCam and Roman/WFI allow the detection of massive-progenitor (~250 M⊙) PISNe throughout all the optimal F200W-F356W, F277W-F444W, and F158-F213 colours. PISNe are also pre-dominantly located at the outskirts of their hosting haloes, facilitating the disentangling of underlying stellar emission thanks to the spatial-resolution capabilities of the instruments.

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    A needle in a haystack? Catching Population III stars in the epoch of reionization: I. Population III star-forming environments

    Despite extensive search efforts, direct observations of the first [Population III (Pop III)] stars have not yet succeeded. Theoretical studies have suggested that late Pop III star formation is still possible in pristine clouds of high-mass galaxies, co-existing with Pop II stars, down to the epoch of reionization. Here, we re-assess this finding by exploring Pop III star formation in six 50 h-1 cMpc simulations performed with the hydrodynamical code dustyGadget. We find that Pop III star formation (~10-3.4-10-3.2 M⊙ yr-1 cMpc-3) is still occurring down to z ~ 6-8, i.e. well within the reach of deep JWST surveys. At these epochs, ≲ 10% of the rare massive galaxies with M⋆ ≳ 3 × 109 M⊙ are found to host Pop III stars, although with a Pop III/Pop II mass fraction ≲ 0.1%. Regardless of their mass, Pop III-hosting galaxies are mainly found on the main sequence, at high star-formation rates, probably induced by accretion of pristine gas. This scenario is also supported by their increasing star-formation histories and their preferential location in high-density regions of the cosmic web. Pop III stars are found both in the outskirts of metal-enriched regions and in isolated, pristine clouds. In the latter case, their signal may be less contaminated by Pop IIs, although its detectability will strongly depend on the specific line of sight to the source, due to the complex morphology of the host galaxy and its highly inhomogeneous dust distribution.

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    Reionization with galaxies and active galactic nuclei

    In this work we investigate the properties of the sources that reionized the intergalactic medium (IGM) in the high-redshift Universe. Using a semi-analytical model aimed at reproducing galaxies and black holes in the first ∼1.5 Gyr of the Universe, we revisit the relative role of star formation and black hole accretion in producing ionizing photons that can escape into the IGM. Both star formation and black hole accretion are regulated by supernova feedback, resulting in black hole accretion being stunted in low-mass haloes. We explore a wide range of combinations for the escape fraction of ionizing photons (redshift-dependent, constant, and scaling with stellar mass) from both star formation ( ⟨fsfesc⟩ ) and AGN ( fbhesc ) to find: (I) the ionizing budget is dominated by stellar radiation from low stellar mass ( M∗<10^9M⊙ ) galaxies at z > 6 with the AGN contribution (driven by Mbh>10^6M⊙ black holes in M∗≳10^9M⊙ galaxies) dominating at lower redshifts; (II) AGN only contribute 10−25 per cent to the cumulative ionizing emissivity by z = 4 for the models that match the observed reionization constraints; (III) if the stellar mass dependence of ⟨fsfesc⟩ is shallower than fbhesc , at z < 7 a transition stellar mass exists above which AGN dominate the escaping ionizing photon production rate; (IV) the transition stellar mass decreases with decreasing redshift. While AGN dominate the escaping emissivity above the knee of the stellar mass function at z ∼ 6.8, they take-over at stellar masses that are a tenth of the knee mass by z = 4.

    https://ui.adsabs.harvard.edu/abs/2020MNRAS.495.3065D/abstract

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    Unveiling early black hole growth with multifrequency gravitational wave observations

    Third-generation ground-based gravitational wave interferometers, like the Einstein Telescope (ET), Cosmic Explorer, and the Laser Interferometer Space Antenna (LISA), will detect coalescing binary black holes over a wide mass spectrum and across all cosmic epochs. We track the cosmological growth of the earliest light and heavy seeds that swiftly transit into the supermassive domain using a semi-analytical model for the formation of quasars at z = 6.4, 2, and 0.2, in which we follow black hole coalescences driven by triple interactions. We find that light-seed binaries of several 102M⊙ are accessible to ET with a signal-to-noise ratio (S/N) of 10-20 at 6 < z < 15. They then enter the LISA domain with larger S/N as they grow to a few 10^4M⊙ . Detecting their gravitational signal would provide first time evidence that light seeds form, grow, and dynamically pair during galaxy mergers. The electromagnetic emission of accreting black holes of similar mass and redshift is too faint to be detected even for the deepest future facilities. ET will be our only chance to discover light seeds forming at cosmic dawn. At 2 < z < 8, we predict a population of ‘starved binaries’, long-lived marginally growing light-seed pairs, to be loud sources in the ET bandwidth (S/N > 20). Mergers involving heavy seeds ( ∼10^5−10^6M⊙ ) would be within reach up to z = 20 in the LISA frequency domain. The lower z model predicts 11.25 (18.7) ET (LISA) events per year, overall.

    https://ui.adsabs.harvard.edu/abs/2021MNRAS.500.4095V/abstract

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    Cosmic archaeology with massive stellar black hole binaries

    The existence of massive stellar black hole binaries (MBHBs), with primary black hole (BH) masses ≥ 31 Mo, was proven by the detection of the gravitational wave (GW) event GW150914 during the first LIGO/Virgo observing run (O1), and successively confirmed by seven additional GW signals discovered in the O1 and 02 data. By adopting the galaxy formation model GAMESH coupled with binary population synthesis (BPS) calculations, here we investigate the origin of these MBHBs by selecting simulated binaries compatible in mass and coalescence redshifts. We find that their cosmic birth rates peak in the redshift range 6.5 < 2 < 10, regardless of the adopted BPS.
    These MBHBs are then old systems forming in low-metallicity (Z~ [0.01-0.1] Zo), low-stellar-mass galaxies, before the end of cosmic reionization, i.e. significantly beyond the peak of cosmic star formation. GW signals generated by coalescing MBHBs open up new possibilities to probe the nature of stellar populations in remote galaxies, at present too faint to be detected by available electromagnetic facilities.

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    The assembly of dusty galaxies at z > 4: statistical properties

    The recent discovery of high redshift dusty galaxies implies a rapid dust enrichment of their interstellar medium (ISM). To interpret these observations, we run a cosmological simulation in a 30h−1 cMpc/size volume down to z ≈ 4. We use the hydrodynamical code dustyGadget, which accounts for the production of dust by stellar populations and its evolution in the ISM. We find that the cosmic dust density parameter is mainly driven by stellar dust at z > 10, so that mass- and metallicity-dependent yields are required to assess the dust content in the first galaxies. At z < 9 the growth of grains in the ISM of evolved systems (Log(M⋆/M⊙) > 8.5) significantly increases their dust mass, in agreement with observations in the redshift range 4 < z < 8. Our simulation shows that the variety of high redshift galaxies observed with ALMA can naturally be accounted for by modeling the grain-growth timescale as a function of the physical conditions in the gas cold phase. In addition, the trends of dust-to-metal (DTM) and dust-to-gas (D) ratios are compatible with the available data. A qualitative investigation of the inhomogeneous dust distribution in a
    representative massive halo at z ≈ 4 shows that dust is found from the central galaxy up to the closest satellites along polluted filaments with Log(D) < −2.4, but sharply declines at distances d > 30 kpc along many lines of sight, where Log(D) < −4.0.