title: Gravitational-wave limits from pulsar timing constrain supermassive black hole evolution.
abstract: The formation and growth processes of supermassive black holes (SMBHs) are not well constrained. SMBH population models, however, provide specific predictions for the properties of the gravitational-wave background (GWB) from binary SMBHs in merging galaxies throughout the universe. Using observations from the Parkes Pulsar Timing Array, we constrain the fractional GWB energy density (Ω(GW)) with 95% confidence to be Ω(GW)(H0/73 kilometers per second per megaparsec)(2) < 1.3 × 10(-9) (where H0 is the Hubble constant) at a frequency of 2.8 nanohertz, which is approximately a factor of 6 more stringent than previous limits. We compare our limit to models of the SMBH population and find inconsistencies at confidence levels between 46 and 91%. For example, the standard galaxy formation model implemented in the Millennium Simulation Project is inconsistent with our limit with 50% probability.
authors.affiliation: Commonwealth Scientific and Industrial Research Organisation (CSIRO) Astronomy and Space Science, Australia Telescope National Facility, Post Office Box 76, Epping, New South Wales 1710, Australia.
authors.forename: R M
authors.initials: RM
authors.lastname: Shannon
authors.forename: V
authors.initials: V
authors.lastname: Ravi
authors.forename: W A
authors.initials: WA
authors.lastname: Coles
authors.forename: G
authors.initials: G
authors.lastname: Hobbs
authors.forename: M J
authors.initials: MJ
authors.lastname: Keith
authors.forename: R N
authors.initials: RN
authors.lastname: Manchester
authors.forename: J S B
authors.initials: JS
authors.lastname: Wyithe
authors.forename: M
authors.initials: M
authors.lastname: Bailes
authors.forename: N D R
authors.initials: ND
authors.lastname: Bhat
authors.forename: S
authors.initials: S
authors.lastname: Burke-Spolaor
authors.forename: J
authors.initials: J
authors.lastname: Khoo
authors.forename: Y
authors.initials: Y
authors.lastname: Levin
authors.forename: S
authors.initials: S
authors.lastname: Osłowski
authors.forename: J M
authors.initials: JM
authors.lastname: Sarkissian
authors.forename: W
authors.initials: W
authors.lastname: van Straten
authors.forename: J P W
authors.initials: JP
authors.lastname: Verbiest
authors.forename: J-B
authors.initials: JB
authors.lastname: Wang