Unravelling obese black holes in the first galaxies (1302.6996v2)

Abstract: We predict the existence and observational signatures of a new class of objects that assembled early, during the first billion years of cosmic time: Obese Black-hole Galaxies (OBGs). OBGs are objects in which the mass of the central black hole initially exceeds that of the stellar component of the host galaxy, and the luminosity from black-hole accretion dominates the starlight. From a cosmological simulation, we demonstrate that there are sites where star formation is initially inhibited and direct-collapse black holes (DCBHs) form due to the photo-dissociating effect of Lyman-Werner radiation on molecular hydrogen. We show that the formation of OBGs is inevitable, because the probability of finding the required extra-galactic environment and the right physical conditions in a halo conducive to DCBH formation is quite high in the early universe. We estimate an OBG number density of 0.009/Mpc^3 at z~8 and 0.03/Mpc^3 at z~6. Extrapolating from our simulation volume, we infer that the most luminous quasars detected at z~6 likely transited through an earlier OBG phase. We find that these primordial galaxies start off with an over-massive BH and acquire their stellar component from subsequent merging as well as in-situ star formation. In doing so, they inevitably go through an OBG phase dominated by the accretion luminosity at the Eddington rate or below, released from the growing BH. The OBG phase is characterised by an ultra-violet (UV) spectrum with slope of beta ~ -2.3 and the absence of a Balmer Break. OBGs should also be spatially unresolved, and are expected to be brighter than the majority of known high-redshift galaxies. OBGs could potentially be revealed via HST follow-up imaging of samples of brighter Lyman-break galaxies provided by wide-area ground-based surveys such as UltraVISTA, and should be easily uncovered and studied with instruments aboard JWST...(abridged)

Unravelling Obese Black Holes in the First Galaxies

The paper "Unravelling obese black holes in the first galaxies" by Agarwal et al. examines the formation and characteristics of what the authors term Obese Black-hole Galaxies (OBGs) in the early universe. These OBGs are hypothesized to exist within the first billion years of cosmic time and are defined by having a central black hole with a mass that initially exceeds the stellar component of the host galaxy. Moreover, the luminosity originating from black-hole accretion in these galaxies is theorized to dominate over starlight.

Key Findings and Methodology

Agarwal et al. diverge from the conventional view that the first galaxies are primarily lit by stars. Instead, they propose the presence of astrophysical conditions allowing the formation of primordial objects where black-hole growth precedes and surpasses stellar assembly in terms of luminosity output. Using cosmological simulations, the paper identifies conditions conducive to the formation of direct-collapse black holes (DCBHs) caused by the inhibitive effect of Lyman-Werner radiation on star formation, which dissociates molecular hydrogen necessary for cooling and subsequent star formation in the early galaxies.

These OBGs, theorized in the paper, are expected to be observable with characteristics such as bright, broad, high-excitation emission lines common to Type-I active galactic nuclei (AGN), though their strength depends on the chemical enrichment of the host galaxy. Agarwal et al. predict an OBG number density of approximately 0.009 Mpc$^{-3}$ at redshift $z \sim 8$ and 0.03 Mpc$^{-3}$ at $z \sim 6$. These findings suggest that many luminous quasars observed at $z \geq 6$ might have evolved through an early OBG phase.

Numerical Results

The paper highlights the inevitability of such formations by calculating the high probability of meeting the necessary extragalactic environments and physical conditions favorable to DCBH creation early in the universe. The simulations show that as these primordial galaxies evolve, they begin with an over-massive black hole and enhance their stellar mass through mergers and in-situ star formation, eventually forming what the authors describe as OBGs.

In the simulations conducted, the OBG phase commences with a UV spectrum characterized by a slope of $\beta \sim -2.3$ and a lack of a Balmer Break. Additionally, the spatially unresolved and brighter nature of OBGs surpasses known high-redshift galaxies.

Observational Implications

This work delivers significant observational predictions. OBGs are expected to be unveield via imaging and spectroscopy with next-generation telescopes such as the James Webb Space Telescope (JWST). Specifically, the absence of Bulmer Breaks and presence of UV slopes could be distinct markers to identify such objects and separate them from high-redshift galaxies.

Conclusion

The paper’s insights into OBGs have substantial implications for understanding the formation mechanisms of the earliest black holes and their influence on the nascent galaxies. By suggesting the potential dominance of black-hole accretion luminosity over stellar light in certain epochs, this work provides a new narrative on the interplay between black-hole and galaxy formation in the young universe.

Future research building on this work could further elucidate the role of OBGs in the context of cosmic evolution and refine the models’ predictions with upcoming observational data. This may offer deeper comprehension of how the earliest massive celestial objects formed and evolved, contributing to the broader understanding of cosmic structure formation.