Quasar Clustering in Cosmological Hydrodynamic Simulations: Evidence for mergers

We examine the clustering properties of a population of quasars drawn from fully hydrodynamic cosmological simulations that directly follow black hole growth. We find that the black hole correlation function is best described by two distinct components: contributions from BH pairs occupying the same dark matter halo ('1-halo term') which dominate at scales below 300 kpc/h, and contributions from BHs occupying separate halos ('2-halo term') which dominate at larger scales. From the 2-halo BH term we find a typical host halo mass for faint-end quasars (those probed in our simulation volumes) ranging from 10^11 to a few 10^12 solar masses from z=5 to z=1 respectively (consistent with the mean halo host mass). The BH correlation function shows a luminosity dependence as a function of redshift, though weak enough to be consistent with observational constraints. At small scales, the high resolution of our simulations allows us to probe the 1-halo clustering in detail, finding that the 1-halo term follows an approximate power law, lacking the characteristic decrease in slope at small scales found in 1-halo terms for galaxies and dark matter. We show that this difference is a direct result of a boost in the small-scale quasar bias caused by galaxies hosting multiple quasars (1-subhalo term) following a merger event, typically between a large central subgroup and a smaller, satellite subgroup hosting a relatively small black hole. We show that our predicted small-scale excess caused by such mergers is in good agreement with both the slope and amplitude indicated by recent small-scale measurements. Finally, we note the excess to be a strong function of halo mass, such that the observed excess is well matched by the multiple black holes of intermediate mass (10^7-10^8 solar masses) found in hosts of 4-8*10^11 solar masses, a range well probed by our simulations.Comment: 12 pages, 10 figures. Submitted to MNRA

Morphological evolution of supermassive black hole merger hosts and multimessenger signatures

With projects such as Laser Interferometer Space Antenna (LISA) and Pulsar Timing Arrays expected to detect gravitational waves from supermassive black hole mergers in the near future, it is key that we understand what we expect those detections to be, and maximize what we can learn from them. To address this, we study the mergers of supermassive black holes in the Illustris simulation, the overall rate of mergers, and the correlation between merging black holes and their host galaxies. We find that these mergers occur in typical galaxies along the $M_{\rm{BH}}-M_*$ relation, and that between LISA and PTAs we expect to probe the full range of galaxy masses. As galaxy mergers can trigger increased star formation, we find that galaxies hosting low-mass black hole mergers tend to show a slight increase in star formation rates compared to a mass-matched sample. However, high-mass merger hosts have typical star formation rates, due to a combination of low gas fractions and powerful AGN feedback. Although minor black hole mergers do not correlate with disturbed morphologies, major mergers (especially at high-masses) tend to show morphological evidence of recent galaxy mergers which survives for ~500 Myr. This is on the same scale as the infall/hardening time of the merging black holes, suggesting that electromagnetic followups to gravitational wave signals may not be able to observe this correlation. We further find that incorporating a realistic timescale delay for the black hole mergers could shift the distribution of merger masses toward higher-masses, decreasing the rate of LISA detections while increasing the rate of PTA detections.Comment: 14 pages, 17 figures. Published in MNRA

The Halo Occupation Distribution of Active Galactic Nuclei

Using a fully cosmological hydrodynamic simulation that self-consistently incorporates the growth and feedback of supermassive black holes and the physics of galaxy formation, we examine the effects of environmental factors (e.g., local gas density, black hole feedback) on the halo occupation distribution of low luminosity active galactic nuclei (AGN). We decompose the mean occupation function into central and satellite contribution and compute the conditional luminosity functions (CLF). The CLF of the central AGN follows a log-normal distribution with the mean increasing and scatter decreasing with increasing redshifts. We analyze the light curves of individual AGN and show that the peak luminosity of the AGN has a tighter correlation with halo mass compared to instantaneous luminosity. We also compute the CLF of satellite AGN at a given central AGN luminosity. We do not see any significant correlation between the number of satellites with the luminosity of the central AGN at a fixed halo mass. We also show that for a sample of AGN with luminosity above 10^42 ergs/s the mean occupation function can be modeled as a softened step function for central AGN and a power law for the satellite population. The radial distribution of AGN inside halos follows a power law at all redshifts with a mean index of -2.33 +/- 0.08. Incorporating the environmental dependence of supermassive black hole accretion and feedback, our formalism provides a theoretical tool for interpreting current and future measurements of AGN clustering.Comment: 14 pages, 11 figures, 2 Tables (Matches the MNRAS accepted version

Massive Black Hole Mergers with Orbital Information: Predictions from the ASTRID Simulation

We examine massive black hole (MBH) mergers and their associated gravitational wave signals from the large-volume cosmological simulation Astrid. Astrid includes galaxy formation and black hole models recently updated with a MBH seed population between $3\times 10^4M_{\odot}/h$ and $3\times 10^5M_{\odot}/h$ and a sub-grid dynamical friction (DF) model to follow the MBH dynamics down to $1.5\;\text{ckpc}/h$. We calculate initial eccentricities of MBH orbits directly from the simulation at kpc-scales, and find orbital eccentricities above $0.7$ for most MBH pairs before the numerical merger. After approximating unresolved evolution on scales below ${\sim 200\,\text{pc}}$, we find that the in-simulation DF on large scales accounts for more than half of the total orbital decay time ($\sim 500\,\text{Myrs}$) due to DF. The binary hardening time is an order of magnitude longer than the DF time, especially for the seed-mass binaries ($M_\text{BH}<2M_\text{seed}$). As a result, only $\lesssim20\%$ of seed MBH pairs merge at $z>3$ after considering both unresolved DF evolution and binary hardening. These $z>3$ seed-mass mergers are hosted in a biased population of galaxies with the highest stellar masses of $>10^9\,M_\odot$. With the higher initial eccentricity prediction from Astrid, we estimate an expected merger rate of $0.3-0.7$ per year from the $z>3$ MBH population. This is a factor of $\sim 7$ higher than the prediction using the circular orbit assumption. The LISA events are expected at a similar rate, and comprise $\gtrsim 60\%$ seed-seed mergers, $\sim 30\%$ involving only one seed-mass MBH, and $\sim 10\%$ mergers of non-seed MBHs.Comment: 17 pages, 13 Figures; comments are welcom

Compaction and Quenching of High-z Galaxies in Cosmological Simulations: Blue and Red Nuggets

We use cosmological simulations to study a characteristic evolution pattern of high redshift galaxies. Early, stream-fed, highly perturbed, gas-rich discs undergo phases of dissipative contraction into compact, star-forming systems (blue nuggets) at z~4-2. The peak of gas compaction marks the onset of central gas depletion and inside-out quenching into compact ellipticals (red nuggets) by z~2. These are sometimes surrounded by gas rings or grow extended dry stellar envelopes. The compaction occurs at a roughly constant specific star-formation rate (SFR), and the quenching occurs at a constant stellar surface density within the inner kpc ($\Sigma_1$). Massive galaxies quench earlier, faster, and at a higher $\Sigma_1$ than lower-mass galaxies, which compactify and attempt to quench more than once. This evolution pattern is consistent with the way galaxies populate the SFR-radius-mass space, and with gradients and scatter across the main sequence. The compaction is triggered by an intense inflow episode, involving (mostly minor) mergers, counter-rotating streams or recycled gas, and is commonly associated with violent disc instability. The contraction is dissipative, with the inflow rate >SFR, and the maximum $\Sigma_1$ anti-correlated with the initial spin parameter, as predicted by Dekel & Burkert (2014). The central quenching is triggered by the high SFR and stellar/supernova feedback (possibly also AGN feedback) due to the high central gas density, while the central inflow weakens as the disc vanishes. Suppression of fresh gas supply by a hot halo allows the long-term maintenance of quenching once above a threshold halo mass, inducing the quenching downsizing.Comment: Resubmitted to MNRAS after responding to referee's comments; Updated and added two figure