Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aipc..470..202w&link_type=abstract
The 9th astrophysics conference: After the dark ages, when galaxies were young (the Universe at 2<Z<5). AIP Conference Proceedi
Astronomy and Astrophysics
Astrophysics
1
Observational Cosmology, Origin, Formation, Evolution, Age, And Star Formation, Galaxy Mergers, Collisions, And Tidal Interactions
Scientific paper
We review recent clues from deep HST images on the formation and evolution of galaxies, and the role that mergers have played in this process. First, we first review the evidence from deep ground-based and HST studies for an epoch dependent merger rate, which was higher in the past by ~(1+z)m with m~=1.5-3 out to z<~1-2. Next, we review clues from deep HST broad-band images to the role of (minor) mergers. The galaxy counts as a function of restframe type show that E/SO's and Sabc's are only marginally evolving, but show that Sd/Irr's are the dominant population for B<~27 mag, or 1<~z<~2. The scale-lengths of all galaxy types decrease steadily towards the faintest flux limits. A significant fraction of the faint blue irregulars have close companions and/or tidal features indicating merging at z>~1.5-2, implying an epoch of merger-induced star-formation for z~1.5-2, which also coincides with the peak in the star-formation rate [44]. This suggests a Universe at z>~1.5 dominated by hierarchical merging of star-forming (irregular) ``sub-galactic'' sized objects. We then summarize the clues that HST medium-band searches have played in finding these ``sub-galactic'' building blocks, as well as searches with similar techniques from the ground. Next, we review high resolution PC images, as well as other data, on the faint radio galaxy 53W002 at z=2.390, and suggest how and when merging would have taken place in this object to produce its most likely counterpart today-a giant elliptical galaxy. Last, we present recent HST/NICMOS images of microJansky radio sources without clear optical counterparts in the HDF/Flanking Fields, and suggest how the process of (early) major mergers could have resulted in a non-negligible fraction of the radio-selected star-formation rate to be optically enshrouded by dust.
Cohen Seth H.
Waddington Ian
Windhorst Rogier A.
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