Bob Abraham

IOA, Cambridge

Physical Morphology at High Redshifts

In this talk I will show how morphological ideas shape our understanding
of galaxy evolution, and present an overview of the remarkable progress
made in recent years in understanding how the local morphological
composition has transformed into that seen the distant Universe.  I will
highlight work from the LDSS+CFRS HST imaging survey and the Hubble Deep
Field (HDF), which shows show quite clearly that the early-late axis
Hubble system does not provide a useful framework for describing the
appearance of galaxies at redshifts z > 1. I will then describe how recent
data from the Southern HDF shows that the "orthogonal" axis of Hubble's
tuning fork, namely the bifurcation into barred and regular systems,
breaks down at even lower redshifts. Barred spirals are essentially absent
in the distant Universe at z > 0.5.  I argue that as a result of this work
the Hubble system needs to be replaced by a system that is more objective,
more physically meaningful, and which is applicable across a wider range
of redshifts.

In view of this observational evidence for changing morphology (and
corresponding theoretical predictions from hierarchical clustering
models), I will argue that we need a better tool for probing morphological
evolution at high redshifts. The observational focus of morphological work
should be explicitly linked to star-formation history, and assume that
morphology itself is a transient property. I will present a new technique
that attempts to achieve this, based on matching *spatially resolved*
four-band HST colour data to the predictions of evolutionary synthesis
models. Given some simplifying assumptions I demonstrate how our technique
is capable of probing the evolutionary history of galaxy morphology. I
will quantify the relative age, dispersion in age, ongoing star-formation
rate, and star-formation history of distinct galaxy components, and
explicitly test for the presence of dust and quantify its effect on our
conclusions. I will then address the following questions: (1) Are
ellipticals in the field old at high redshifts? (2) What came first,
bulges or disks? (3) Does the Hubble Sequence contain its own ground
state, ie. are there classes of galaxies at high redshifts with no local
counterparts? (4) Does the starlight inventory obtained by resolved colour
fitting add up in a consistent way in order to account for the
star-formation history of the Universe measured by other techniques?