Paper: astro-ph/0601202

Date: Tue, 10 Jan 2006 11:27:42 GMT (542kb)

Title: Did Galaxy Assembly and Supermassive Black-Hole Growth go hand-in-hand?

Authors: R.A. Windhorst, S.H. Cohen, A.N. Straughn, R.E. Ryan Jr., N.P. Hathi,
R.A. Jansen (ASU), A.M. Koekemoer, N. Pirzkal, C. Xu, B. Mobasher, S.
Malhotra, L. Strolger & J.E. Rhoads (STScI)

Comments: 9 pages, Latex2e requires 'elsart' and 'elsart3' (included), 10
postscript figures. To appear in the Proceedings of the Leiden Workshop on
"QSO Host Galaxies: Evolution and Environment", eds. P.D. Barthel & D.B.
Sanders (New Astron. Rev., 2006)
\\
In this paper, we address whether the growth of supermassive black-holes has
kept pace with the process of galaxy assembly. For this purpose, we first
searched the Hubble Ultra Deep Field (HUDF) for "tadpole galaxies", which have
a knot at one end and an extended tail. They appear dynamically unrelaxed --
presumably early-stage mergers -- and make up ~6% of the field galaxy
population. Their redshift distribution follows that of field galaxies,
indicating that -- if tadpole galaxies are indeed dynamically young -- the
process of galaxy assembly generally kept up with the reservoir of field
galaxies as a function of epoch. Next, we present a search for HUDF objects
with point-source components that are optically variable (at the >~3.0 sigma
level) on timescales of weeks--months. Among 4644 objects to i_AB=28.0 mag (10
sigma), 45 have variable point-like components, which are likely weak AGN.
About 1% of all field objects show variability for 0.1 < z < 4.5, and their
redshift distribution is similar to that of field galaxies. Hence supermassive
black-hole growth in weak AGN likely also kept up with the process of galaxy
assembly. However, the faint AGN sample has almost no overlap with the tadpole
sample, which was predicted by recent hydrodynamical numerical simulations.
This suggests that tadpole galaxies are early-stage mergers, which likely
preceded the ``turn-on'' of the AGN component and the onset of visible
point-source variability by >~1 Gyr.

\\ ( http://arXiv.org/abs/astro-ph/0601202 , 542kb)