We search for an isotropic stochastic gravitational-wave background (GWB) in the newly released 11-year dataset from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). While we find no evidence for a GWB, we place constraints on a population of inspiraling supermassive black hole (SMBH) binaries, a network of decaying cosmic strings, and a primordial GWB. For the first time, we find that the GWB constraints are sensitive to the Solar System ephemeris (SSE) model used, and that SSE errors arXiv:1801.02617v2 [astro-ph.HE] 7 Jun 2018 2 THE NANOGRAV COLLABORATION can mimic a GWB signal. We developed an approach that bridges systematic SSE differences, producing the first PTA constraints that are robust against SSE errors. We thus place a 95% upper limit on the GW strain amplitude of A GWB < 1.45 × 10 −15 at a frequency of f = 1-yr −1 for a fiducial f −2/3 power-law spectrum, and with inter-pulsar correlations modeled. This is a factor of ∼ 2 improvement over the NANOGrav 9-year limit, calculated using the same procedure. Previous PTA upper limits on the GWB (as well as their astrophysical and cosmological interpretations) will need revision in light of SSE systematic errors. We use our constraints to characterize the combined influence on the GWB of the stellar mass-density in galactic cores, the eccentricity of SMBH binaries, and SMBH-galactic-bulge scaling relationships. We constrain cosmic-string tension using recent simulations, yielding an SSE-marginalized 95% upper limit of Gµ < 5.3 × 10 −11 -a factor of ∼ 2 better than the published NANOGrav 9-year constraints. Our SSE-marginalized 95% upper limit on the energy density of a primordial GWB (for a radiation-dominated post-inflation Universe) is Ω GWB ( f )h 2 < 3.4 × 10 −10 .

Pulsar timing array (PTA) collaborations in North America, Australia, and Europe, have been exploiting the exquisite timing precision of millisecond pulsars over decades of observations to search for correlated timing deviations induced by gravitational waves (GWs). PTAs are sensitive to the frequency band ranging just below 1 nanohertz to a few tens of microhertz. The discovery space of this band is potentially rich with populations of inspiraling supermassive black-holes binaries, decaying cosmic string networks, relic postinflation GWs, and even non-GW imprints of axionic dark matter.This article aims to provide an understanding of the exciting open science questions in cosmology, galaxy evolution, and fundamental physics that will be addressed by the detection and study of GWs through PTAs. The focus of the article is on providing an understanding of the mechanisms by which PTAs can address specific questions in these fields, and to outline some of the subtleties and difficulties in each case. The material included is weighted most heavily towards the questions which we expect will be answered in the near-term with PTAs; however, we have made efforts to include most currently anticipated applications of nanohertz GWs.PACS numbers: § Note, a more precise remark is that the Earth and pulsar terms are not correlated as long as two pulsars are separated by many gravitational wavelengths, that is to say that f L 1, where f is the GW frequency and L is distance to the pulsar. This assumption is called the short-wavelength approximation (e. g. Mingarelli & Mingarelli 2018).

The recent discovery of super-massive black holes (SMBHs) in high-mass ultra-compact dwarf galaxies (UCDs) suggests that at least some UCDs are the nuclear star clusters of stripped galaxies. In this paper we present a new method to estimate how many UCDs host an SMBH and thus are stripped galaxy nuclei. We revisit the dynamical mass measurements that suggest many UCDs have more mass than expected from stellar population estimates, which observations have shown is due to the presence of an SMBH. We revise the stellar population mass estimates using a new empirical relation between the mass-to-light ratio (M/L) and metallicity to predict which UCDs most likely host an SMBH. We calculate the fraction of UCDs that host SMBHs across their entire luminosity range for the first time. We then apply the SMBH occupation fraction to the observed luminosity function of UCDs and estimate that in the Fornax and Virgo cluster alone there should be 69 +32 −25 stripped nuclei with SMBHs. This analysis shows that stripped nuclei are almost as common in clusters as present-day galaxy nuclei. We estimate the SMBH number density caused by stripped nuclei to be 2 − 8 × 10 −3 M pc −3 , which represents a significant fraction (8-32%) of the SMBH density in the local Universe. These SMBHs hidden in stripped nuclei increase expected event rates for tidal disruption events and SMBH-SMBH and SMBH-BH mergers. The existence of numerous stripped nuclei with SMBHs are a direct consequence of hierarchical galaxy formation, but until now their impact on the SMBH density had not been quantified.

Interaction of a binary supermassive black hole with stars in a galactic nucleus can result in changes to all the elements of the binary's orbit, including the angles that define its orientation. If the nucleus is rotating, the orientation changes can be large, causing large changes in the binary's orbital eccentricity as well. We present a general treatment of this problem based on the Fokker-Planck equation for f , defined as the probability distribution for the binary's orbital elements. First-and second-order diffusion coefficients are derived for the orbital elements of the binary using numerical scattering experiments, and analytic approximations are presented for some of these coefficients. Solutions of the Fokker-Planck equation are then derived under various assumptions about the initial rotational state of the nucleus and the binary hardening rate. We find that the evolution of the orbital elements can become qualitatively different when we introduce nuclear rotation: 1) the orientation of the binary's orbit evolves toward alignment with the plane of rotation of the nucleus; 2) binary orbital eccentricity decreases for aligned binaries and increases for counter-aligned ones. We find that the diffusive (random-walk) component of a binary's evolution is small in nuclei with non-negligible rotation, and we derive the time-evolution equations for the semimajor axis, eccentricity and inclination in that approximation. The aforementioned effects could influence gravitational wave production as well as the relative orientation of host galaxies and radio jets. arXiv:1610.08555v3 [astro-ph.GA] 4 Apr 2017 Fokker-Planck equationThe binary is assumed to interact with stars, causing changes in its orbital elements. 1 In the simplest representation, the binary's orbit would evolve smoothly and deterministically with respect to time. We consider a slightly more complex model, in which a random, or diffusive, component to the binary's evolution is allowed as well.Accordingly, define f (E, L, t) dE dL to be the probability that the binary's energy E and angular momentum L lie in the intervals E to E + dE and L to L + dL, respectively, at time t. Let ∆t denote an interval of time that is short compared with the time over which the orbit of the binary changes due to encounters with stars, but still long enough that many encounters occur. Define the transition probability Ψ(E, L; ∆E, ∆L) that the energy and angular momentum of the binary change by ∆E and ∆L, respectively, in time ∆t. Then

We consider a black hole (BH) density cusp in a nuclear star cluster (NSC) hosting a supermassive back hole (SMBH) at its center. Assuming the stars and BHs inside the SMBH sphere of influence are mass-segregated, we calculate the number of BHs that sink into this region under the influence of dynamical friction. We find that the total number of BHs increases significantly in this region due to this process for lower mass SMBHs by up to a factor of 5, but there is no increase in the vicinity of the highest mass SMBHs. Due to the high BH number density in the NSC, BH-BH binaries form during close approaches due to GW emission. We update the previous estimate of O'Leary et al. for the rate of such GW capture events by estimating the n 2 / n 2 parameter where n is the number density. We find a BH merger rate for this channel to be in the range ∼ 0.002 − 0.04 Gpc −3 yr −1 . The total merger rate is dominated by the smallest galaxies hosting SMBHs, and the number of heaviest BHs in the NSC. It is also exponentially sensitive to the radial number density profile exponent, reaching > 100 Gpc −3 yr −1 when the BH mass function is m −2.3 or shallower and the heaviest BH radial number density is close to r −3 . Even if the rate is much lower than the range constrained by the current LIGO detections, the GW captures around SMBHs can be distinguished by their high eccentricity in the LIGO band.