Glimmers in the Cosmic Dawn: A Census of the Youngest Supermassive Black Holes by Photometric Variability
arxiv(2024)
摘要
We report first results from a deep near infrared campaign with the Hubble
Space Telescope to obtain late-epoch images of the Hubble Ultra-Deep Field
(HUDF), 10-15 years after the first epoch data were obtained. The main
objectives are to search for faint active galactic nuclei (AGN) at high
redshifts by virtue of their photometric variability, and measure (or
constrain) the comoving number density of supermassive black holes (SMBHs),
n_SMBH, at early times. In this Letter we present a brief overview of the
program and preliminary results regarding eight objects. Three variables are
supernovae, two of which are apparently hostless with indeterminable redshifts,
although one has previously been recorded at a z≈6 galaxy. Two further
objects are clear AGN candidates at z=2.0 and 3.2, based on morphology and/or
spectroscopy, in particular infrared spectroscopy from JWST. Three variable
targets are identified at z=6-7, which are also likely AGN candidates. These
sources provide a first measure of n_SMBH in the reionization epoch by
photometric variability, which places a firm lower limit of 3x10^-4
cMpc^-3. After accounting for variability and luminosity incompleteness, we
estimate n_SMBH ≳8x10^-3 cMpc^-3, which is the largest value so far
reported at these redshifts. This SMBH abundance is also strikingly similar to
estimates of n_SMBH in the local Universe. We discuss how these results test
various theories for SMBH formation.
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