ABSTRACT
We estimate the amplitude of the nano-Hz stochastic gravitational wave background (GWB) resulting from an unresolved population of inspiralling massive black hole binaries (MBHBs). To this ...aim, we use the L-Galaxies semi-analytical model applied on top of the Millennium merger trees. The dynamical evolution of MBHBs includes dynamical friction, stellar and gas binary hardening, and gravitational wave (GW) feedback. At the frequencies proved by the Pulsar Timing Array experiments, our model predicts an amplitude of ${\sim }1.2 \times 10^{-15}$ at ${\sim }3 \times 10^{-8}\, \rm Hz$ in agreement with current estimations. The contribution to the background comes primarily from equal-mass binaries with chirp masses above $\rm 10^{8}\, M_{\odot }$. We then consider the recently detected common red noise in NANOGrav, PPTA, and EPTA data, working under the hypothesis that it is indeed a stochastic GWB coming from MBHBs. By boosting the massive black hole growth via gas accretion, we show that our model can produce a signal with an amplitude $A\approx (2\!-\!3) \times 10^{-15}$. There are, however, difficulties in predicting this background level without mismatching key observational constraints such as the quasar bolometric luminosity functions or the local black hole mass function. This highlights how current and forthcoming GW observations can, for the first time, confront galaxy and black hole evolution models.
Abstract
We simulate neutralino dark matter (χDM) haloes from their initial collapse, at ∼ earth mass, up to a few percent solar. Our results confirm that the density profiles of the first haloes are ...described by a ∼r
−1.5 power law. As haloes grow in mass, their density profiles evolve significantly. In the central regions, they become shallower and reach on average ∼r
−1, the asymptotic form of an NFW profile. Using non-cosmological controlled simulations, we observe that temporal variations in the gravitational potential caused by major mergers lead to a shallowing of the inner profile. This transformation is more significant for shallower initial profiles and for a higher number of merging systems. Depending on the merger details, the resulting profiles can be shallower or steeper than NFW in their inner regions. Interestingly, mergers have a much weaker effect when the profile is given by a broken power law with an inner slope of −1 (such as NFW or Hernquist profiles). This offers an explanation for the emergence of NFW-like profiles: after their initial collapse, r
−1.5 χDM haloes suffer copious major mergers, which progressively shallows the profile. Once an NFW-like profile is established, subsequent merging does not change the profile anymore. This suggests that halo profiles are not universal but rather a combination of (1) the physics of the formation of the microhaloes and (2) their early merger history – both set by the properties of the dark matter particle – as well as (3) the resilience of NFW-like profiles to perturbations.
ABSTRACT
We study the mass assembly and spin evolution of supermassive black holes (BHs) across cosmic time as well as the impact of gravitational recoil on the population of nuclear and wandering ...BHs (wBHs) by using the semi-analytical model L-Galaxies run on top of Millennium merger trees. We track spin changes that BHs experience during both coalescence events and gas accretion phases. For the latter, we assume that spin changes are coupled with the bulge assembly. This assumption leads to predictions for the median spin values of z = 0 BHs that depend on whether they are hosted by pseudo-bulges, classical bulges or ellipticals, being $\overline{a} \sim 0.9$, 0.7 and 0.4, respectively. The outcomes of the model display a good consistency with $z \le 4$ quasar luminosity functions and the $z = 0$ BH mass function, spin values, and BH correlation. Regarding the wBHs, we assume that they can originate from both the disruption of satellite galaxies (orphan wBH) and ejections due to gravitational recoils (ejected wBH). The model points to a number density of wBHs that increases with decreasing redshift, although this population is always $\rm {\sim}2\, dex$ smaller than the one of nuclear BHs. At all redshifts, wBHs are typically hosted in $\rm {\it M}_{halo} \gtrsim 10^{13} \, M_{\odot }$ and $\rm {\it M}_{stellar} \gtrsim 10^{10} \, M_{\odot }$, being orphan wBHs the dominant type. Besides, independently of redshift and halo mass, ejected wBHs inhabit the central regions (${\lesssim}\rm 0.3{\it R}_{200}$) of the host DM halo, while orphan wBH linger at larger scales (${\gtrsim}\rm 0.5{\it R}_{200}$). Finally, we find that gravitational recoils cause a progressive depletion of nuclear BHs with decreasing redshift and stellar mass. Moreover, ejection events lead to changes in the predicted local BH–bulge relation, in particular for BHs in pseudo-bulges, for which the relation is flattened at $\rm {\it M}_{bulge} \gt 10^{10.2}\, M_{\odot }$ and the scatter increase up to ${\sim}\rm 3\, dex$.
Abstract
Supermassive black holes (SMBHs) are thought to originate from early universe seed black holes of mass
M
BH
∼ 10
2
–10
5
M
⊙
and grown through cosmic time. Such seeds could be powering the ...active galactic nuclei (AGN) found in today’s dwarf galaxies. However, probing a connection between the early seeds and local SMBHs has not yet been observationally possible. Massive black holes hosted in dwarf galaxies at intermediate redshifts, on the other hand, may represent the evolved counterparts of the seeds formed at very early times. We present a sample of seven broad-line AGN in dwarf galaxies with a spectroscopic redshift ranging from
z
= 0.35 to
z
= 0.93. The sources are drawn from the VIPERS survey as having an Large Magellanic Cloud (LMC) like stellar mass (
M
∗
) derived from spectral energy distribution fitting, and they are all star-forming galaxies. Six of these sources are also X-ray AGN. The AGN are powered by SMBHs of >10
7
M
⊙
, more massive than expected from the
M
BH
–
M
∗
scaling relation of AGN. Based on semianalytical simulations, we find that these objects are likely overmassive with respect to their hosts since early times (
z
> 4), independently of whether they formed as heavy (∼10
5
M
⊙
) or light (∼10
2
M
⊙
) seed black holes. In our simulations, these objects tend to grow faster than their host galaxies, contradicting models of synchronized growth. The host galaxies are found to possibly evolve into massive systems by
z
∼ 0, indicating that local SMBHs in massive galaxies could originate in dwarf galaxies hosting seed black holes at higher
z
.
ABSTRACT We present novel 3D multi-scale smoothed particle hydrodynamics (SPH) simulations of gas-rich galaxy mergers between the most massive galaxies at z ∼ 8-10, designed to scrutinize the direct ...collapse formation scenario for massive black hole seeds proposed in Mayer et al. The simulations achieve a resolution of 0.1 pc, and include both metallicity-dependent optically thin cooling and a model for thermal balance at high optical depth. We consider different formulations of the SPH hydrodynamical equations, including thermal and metal diffusion. When the two merging galaxy cores collide, gas infall produces a compact, optically thick nuclear disk with densities exceeding 10−10 g cm3. The disk rapidly accretes higher angular momentum gas from its surroundings reaching ∼5 pc and a mass of 109 M in only a few 104 years. Outside 2 pc it fragments into massive clumps. Instead, supersonic turbulence prevents fragmentation in the inner parsec region, which remains warm (∼3000-6000 K) and develops strong non-axisymmetric modes that cause prominent radial gas inflows (>104 M yr−1), forming an ultra-dense massive disky core. Angular momentum transport by non-axisymmetric modes should continue below our spatial resolution limit, quickly turning the disky core into a supermassive protostar which can collapse directly into a massive black hole of mass 108-109 M via the relativistic radial instability. Such a "cold direct collapse" explains naturally the early emergence of high-z QSOs. Its telltale signature would be a burst of gravitational waves in the frequency range of 10−4-10−1 Hz, possibly detectable by the planned eLISA interferometer.
ABSTRACT
Laser Interferometer Space Antenna (LISA) will extend the search for gravitational waves (GWs) at $0.1\, {-}\, 100$ mHz where loud signals from coalescing binary black holes of $10^4 \, ...{-}\, 10^7\, \, \rm {M}_{\odot }$ are expected. Depending on their mass and luminosity distance, the uncertainty in the LISA sky-localization decreases from hundreds of deg2 during the inspiral phase to fractions of a deg2 after the merger. By using the semi-analytical model L-Galaxies applied to the Millennium-I merger trees, we generate a simulated universe to identify the hosts of $z\, {\le }\, 3$ coalescing binaries with total mass of $3\, {\times }\, 10^{5}$, $3\, {\times }\, 10^6$, and $3\, {\times }\, 10^7\, \rm {M}_{\odot }$, and varying mass ratio. We find that, even at the time of merger, the number of galaxies around the LISA sources is too large (${\gtrsim }\, 10^2$) to allow direct host identification. However, if an X-ray counterpart is associated to the GW sources at $z\, {< }\, 1$, all LISA fields at merger are populated by ${\lesssim }\, 10$ active galactic nuclei (AGNs) emitting above ${\sim }\, 10^{-17} \, \rm erg\, cm^{-2}\, s^{-1}$. For sources at higher redshifts, the poorer sky-localization causes this number to increase up to ${\sim }\, 10^3$. Archival data from eRosita will allow discarding ${\sim }\, 10{{\ \rm per\ cent}}$ of these AGNs, being too shallow to detect the dim X-ray luminosity of the GW sources. Inspiralling binaries in an active phase with masses ${\lesssim }\, 10^6\, \rm {M}_{\odot }$ at $z\, {\le }\, 0.3$ can be detected, as early as 10 h before the merger, by future X-ray observatories in less than a few minutes. For these systems, ${\lesssim }\, 10$ AGNs are within the LISA sky-localization area. Finally, the LISA-Taiji network would guarantee the identification of an X-ray counterpart 10 h before merger for all binaries at $z\, {\lesssim }\, 1$.
Abstract
Bars are a key factor in the long-term evolution of spiral galaxies, in their unique role in redistributing angular momentum and transporting gas and stars on large scales. The Eris-suite ...simulations are cosmological zoom-in, N-body, smoothed-particle hydrodynamic simulations built to follow the formation and evolution of a Milky-Way-sized galaxy across the build-up of the large-scale structure. Here we analyse and describe the outcome of two particular simulations taken from the Eris suite – ErisBH and Eris2k – which mainly differ in the prescriptions employed for gas cooling, star formation, and feedback from supernovae and black holes. Our study shows that the enhanced effective feedback in Eris2k, due to the collective effect of the different micro-physics implementations, results in a galaxy that is less massive than its ErisBH counterpart till z ∼ 1. However, when the stellar content is large enough so that global dynamical instabilities can be triggered, the galaxy in Eris2k develops a stronger and more extended bar with respect to ErisBH. We demonstrate that the structural properties and time evolution of the two bars are very different. Our results highlight the importance of accurate sub-grid prescriptions in cosmological zoom-in simulations of the process of galaxy formation and evolution, and the possible use of a statistical sample of barred galaxies to assess the strength of the stellar feedback.
We present a new zoom-in hydrodynamical simulation, ‘ErisBH’, which features the same initial conditions, resolution, and sub-grid physics as the close Milky Way-analogue ‘Eris’ (Guedes et al. 2011), ...but it also includes prescriptions for the formation, growth and feedback of supermassive black holes. This enables a detailed study of black hole evolution and the impact of active galactic nuclei (AGN) feedback in a late-type galaxy. At z = 0, the main galaxy of ErisBH hosts a central black hole of 2.6 × 106 M⊙, which correlates to the bulge mass and the galaxy's central velocity dispersion similarly to what is observed in the Milky Way and in pseudobulges. During its evolution, the black hole grows mostly through mergers with black holes brought in by accreted satellite galaxies and very little by gas accretion (due to the modest amount of gas that reaches the central regions). AGN feedback is weak and it affects only the central
$1\text{--}2 \,\rm {kpc}$
. Yet, it limits the growth of the bulge, which results in a rotation curve that, in the inner ∼ 10 kpc, is flatter than that of Eris. We find that ErisBH is more prone to instabilities than Eris, due to its smaller bulge and larger disc. At z ∼ 0.3, an initially small bar grows to be of a few disc scalelengths in size. The formation of the bar causes a small burst of star formation in the inner few hundred pc, provides new gas to the central black hole and causes the bulge to have a boxy/peanut morphology by z = 0.
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