The Morphological Evolution of Galaxies
Roberto G. Abraham,1*
Sidney van den Bergh2
Many galaxies have taken on their familiar appearance relatively
recently. In the distant Universe, galaxy morphology deviates significantly (and systematically) from that of nearby galaxies at
redshifts (z) as low as 0.3. This corresponds to a time
~3.5 × 109 years in the past, which is only ~25%
of the present age of the Universe. Beyond z = 0.5 (5 × 109 years in the past), spiral arms are less
well developed and more chaotic, and barred spiral galaxies may become
rarer. At z = 1, around 30% of the galaxy population
is sufficiently peculiar that classification on Hubble's traditional
"tuning fork" system is meaningless. On the other hand, some
characteristics of galaxies have not changed much over time. The space
density of luminous disk galaxies has not changed significantly since
z = 1, indicating that although the general appearance
of these galaxies has continuously changed over time, their overall
numbers have been conserved.
1 Department of Astronomy and Astrophysics,
University of Toronto, 60 St. George Street, Toronto, Ontario M5S 3H8,
Canada.
2 Dominion Astrophysical
Observatory, Herzberg Institute of Astrophysics, National
Research Council of Canada, Victoria, British Columbia V9E 2E7, Canada.
*
To whom correspondence should be addressed. E-mail:
abraham{at}astro.utoronto.ca
