© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Max Planck Institute for Radio Astronomy, Bonn, Germany.
TAPIR, MC 350-17, California Institute of Technology, Pasadena, CA, USA.
Laboratory, California Institute of Technology, Pasadena, CA, USA.
School of Physics and Astronomy and Institute of Gravitational Wave Astronomy,
University of Birmingham, Birmingham, UK.
Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA.
Department of Astronomy,
University of California Berkeley, Berkeley, CA, USA.
Department of Physics and Astronomy, West Virginia University, Morgantown, WV, USA.
Gravitational Waves and Cosmology, West Virginia University, Morgantown, WV, USA. Present address:
Center for Computational Astrophysics, Flatiron
Institute, New York, NY, USA. *e-mail: email@example.com
upermassive black holes (SMBHs) are widely held to exist at the
heart of massive galaxies
. Galaxy mergers should form super-
massive black hole binary (SMBHB) systems, which eventually
emit gravitational waves and merge
. Galaxy mergers are a funda-
mental part of hierarchical assembly scenarios, forming the back-
bone of current structure formation models. Thus, the detection of
gravitational waves from merging SMBHs would be of far-reaching
importance in cosmology, galaxy evolution and fundamental phys
ics, providing information not accessible by any other means.
Pulsar timing arrays (PTAs) can detect nanohertz gravitational
waves by monitoring radio pulses between millisecond pulsars,
which are highly stable clocks. Gravitational waves change the
proper distance between the pulsars and the Earth, thus inducing a
delay or advance of the pulse arrival times. The difference between
the expected and actual arrival times of the pulses—the timing
residuals—carries information about the gravitational waves that is
extracted by cross-correlating the pulsar residuals
. Current PTAs
include European PTA
(EPTA), the North American Nanohertz
Observatory for Gravitational Waves (NANOGrav)
, the Parkes
and the International PTA (IPTA)
, the latter being the union
of the former three.
Here we introduce a bottom-up approach to constructing both
realistic gravitational-wave skies and future IPTA projections: we
use IPTA pulsars with their real noise properties, and galaxies
from the 2 Micron All Sky Survey (2MASS)
, together with galaxy
merger rates from the Illustris cosmological simulation project
to form multiple probabilistic realizations of the local gravitational-
wave Universe. In each realization, we search for SMBHB systems
that emit continuous gravitational waves (CGWs) in the PTA band,
and also compute their contribution to the nanohertz gravitational-
wave background (GWB) and its anisotropy
. We report on the
physical properties of the most frequently selected SMBHBs and
their host galaxies, and estimate their time to detection.
SMBHB merger timescales can be of the order of 10
years after the
galaxy merger, and therefore morphological merger signatures can be
difficult to identify. Our focus here is on massive early-type galaxies, as
these are likely to have formed from major mergers and would there
fore host SMBHBs with approximate mass ratios, q, of 0.25 ≤ q ≤ 1.
To approximate a mass selection, we select in the K-band using the
Extended Source Catalog
, following the procedure out-
lined in detail elsewhere
, but to a distance of 225 Mpc and over the
full sky. We do not excise the galactic plane, but there are fewer reliable
sources there. We convert from the 2MASS K-band luminosity (M
to the stellar mass (M
=.−. +MMlog ()10 58 044( 23)
appropriate for early-type galaxies
, and apply a K-band cut M
≤ − 25
to select galaxies with stellar mass of
, as these are
likely to host SMBHBs
. At distances > 225 Mpc, 2MASS itself
begins to become incomplete at
. Although our sample is
not formally volume-complete within our chosen mass and distance
cuts, it includes the majority of massive, nearby galaxies, which are
expected to dominate the signal for PTAs.
This process creates a galaxy catalogue with 5,110 early-type gal
axies. In earlier work
, it was found that 33 of these galaxies contain
dynamically measured SMBHs (Fig. 1a). Moreover, we manually
add a further nine galaxies from 2MASS: NGC 4889, NGC 4486a,
NGC 1277, NGC 1332, NGC 3115, NGC 1550, NGC 1600, NGC
7436 and A1836 BCG, which did not make the luminosity cut, but
which also host dynamically measured SMBHs.
The SMBHB total mass, M = M
, is estimated by taking
the stellar mass to be the bulge mass, and applying the M
The local nanohertz gravitational-wave landscape
from supermassive black hole binaries
Chiara M. F. Mingarelli
*, T. Joseph W. Lazio
, Alberto Sesana
, Jenny E. Greene
, Justin A. Ellis
, Steve Croft
, Sarah Burke-Spolaor
and Stephen R. Taylor
Supermassive black hole binary systems form in galaxy mergers and reside in galactic nuclei with large and poorly constrained
concentrations of gas and stars. These systems emit nanohertz gravitational waves that will be detectable by pulsar timing
arrays. Here we estimate the properties of the local nanohertz gravitational-wave landscape that includes individual supermas-
sive black hole binaries emitting continuous gravitational waves and the gravitational-wave background that they generate.
Using the 2 Micron All-Sky Survey, together with galaxy merger rates from the Illustris simulation project, we find that there are
on average 91 ± 7 continuous nanohertz gravitational-wave sources, and 7 ± 2 binaries that will never merge, within 225 Mpc.
These local unresolved gravitational-wave sources can generate a departure from an isotropic gravitational-wave background at
a level of about 20 per cent, and if the cosmic gravitational-wave background can be successfully isolated, gravitational waves
from at least one local supermassive black hole binary could be detected in 10 years with pulsar timing arrays.
NATURE ASTRONOMY | VOL 1 | DECEMBER 2017 | 886–892 | www.nature.com/natureastronomy