Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt.........5p&link_type=abstract
Thesis (PHD). ARIZONA STATE UNIVERSITY , Source DAI-B 58/06, p. 3091, Dec 1997, 152 pages.
Astronomy and Astrophysics
Astronomy
1
Cosmology, Extragalactic Astronomy
Scientific paper
Galaxies are the building blocks of the universe, but how and when they formed is largely still a mystery. Only recently, with the advent of very large ground-based telescopes and the Hubble Space Telescope (HST), have astronomers been able to see far enough away, and therefore far enough back in time, to begin to be able to tackle this problem. As it currently stands, the idea that galaxies may have been built up from many smaller pieces very early in the history of the universe has been receiving the most attention. The discovery of numerous compact star-forming objects at z≃ 2.4 with HST represents the first direct observation of what may be the building blocks of present-day luminous galaxies. Two such objects were initially found from ground-based imaging and later confirmed spectroscopically to be at z≃ 2.4, the same redshift as the weak radio galaxy 53W002 in the same field. The Wide Field Planetary Camera 2 onboard HST imaged this field, and photometry revealed 14 additional z≃ 2.4 candidates that were far too faint and compact to have been detected from the ground. Thus far, seven of the original 17 candidates from this field have spectroscopic confirmations. Surprisingly, velocity measurements from the redshifts suggest that six of those seven objects are gravitationally bound at z≃ 2.39, and therefore are destined to merge together. All the candidates are extremely compact, with sizes slightly smaller than the bulge of a present-day spiral galaxy like the Milky Way. To test the possibility that these 'sub-galactic clumps' are actually a widespread population at z≃ 2.4, two random observations of the sky were obtained with HST. These images yielded 14 additional candidates (three in one field and 11 in the other), and suggest that indeed they are not peculiar to the 53W002 field. The significant difference in the number of candidates found in the two fields hints at the existence of clustering or other structure at high-redshift as suggested from the 53W002 field data. It is therefore possible that such a widespread population of sub-galactic clumps provided a reservoir of building blocks for the formation of the present-epoch elliptical and early- to mid-type spiral galaxies through the process of repeated hierarchical merging (with a merger rate that was higher in the past). The finding of additional compact/ z≃ 2.4 candidates in random fields with HST implies that these objects are indeed ubiquitous, and, coupled with other recent reports of similarly high-redshift star-forming clumps,. are a significant part of the general high-redshift (z = 1-4) faint blue galaxy population.
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