Published in "The Hubble Deep Field", eds. M. Livio, S.M. Fall and P. Madau 1998

GALAXY COUNTS VS. TYPE FOR 19^m B 29^m, AND GALAXY FORMATION FROM SUB-GALACTIC CLUMPS

R. Windhorst ¹, S. Pascarelle ¹, S. Odewahn ¹, S. Cohen ¹, C. Burg ¹, W. Keel² and S. Driver³

¹ Dept. of Physics & Astronomy, Arizona State University, Tempe, AZ 85287

² Dept. of Physics & Astronomy, University of Alabama, Tuscaloosa, AL 35487

³ School of Physics, University of New South Wales, Sydney, NSW 2052, Australia

Abstract. We review the results from recent parallel and other deep HST surveys. Deep HST/WFPC2 images in U, B, V, I were analyzed using artificial neural network (ANN) classifiers, which are based on galaxy surface brightness and light profiles, and which distinguish quite well between E/S0's, Sabc's, and Sd/Irr for B ltapprox 27 mag. We discuss effects from the cosmological SB-dimming and from the redshifted UV-morphology on the classifications, and correct for the latter. The median scale-length at B appeq 27 mag is r_hl appeq 0".25-0".3 (1-2 kpc at z appeq 1-2). Early and late-type galaxies are fairly well-separated in BVI color-magnitude diagrams for B ltapprox 27 mag, with E/S0's being the reddest and Sd/Irr's generally blue. We present the B-band galaxy counts for ~ 36 WFPC2 fields as a function of morphological type for B 27 mag. E/S0's are only marginally above the no-evolution predictions, and Sabc's are at most 0.5 dex above the non-evolving models for B gtapprox 24 mag. The faint blue galaxy counts in the B-band are dominated by Sd/Irr's, and are explained by a combination of a moderately steep local luminosity function undergoing strong luminosity evolution plus low-luminosity lower-redshift dwarf galaxies.

Deep WFPC2 images in the medium-band filter F410M (Ly alpha at z appeq 2.4) yielded 18 faint, compact objects surrounding the radio galaxy 53W002 at z appeq 2.390. These objects appear to be star-forming spheroids smaller than the bulge of a spiral galaxy. They are much smaller (r_hl appeq 0".1 appeq 0.5-1 kpc) and fainter (M_V (z = 0) = -21 -> -17) than typical galaxies today, and may the building blocks from which many of the luminous nearby galaxies were formed through repeated hierarchical mergers. Parallel F410M images of two other random fields yielded 15 more z appeq 2.4 candidates - as faint and compact as the z appeq 2.4 candidates in the 53W002 field - confirming that there may exist a widespread population of compact, faint Ly alpha emitting galaxy building blocks at z appeq 2.4, and presumably anywhere in the range z ~ 1-4. The difference in the number of objects from each F410M parallel field can be explained if these subgalactic clumps exhibit some level of clustering at z appeq 2.4 on Mpc scales, and suggests that the group around 53W002 is no more than a ~ 2 sigma fluctuation in the density distribution. Deep KPNO 4m F410M imaging and spectroscopy shows that the 53W002 cluster stretches over 7'(~ 5 Mpc), and may be part of some larger-scale structure at z appeq 2.39.

Deep HST/PC images at ~ 0".06 FWHM resolution in BVI - as well as in redshifted Ly alpha - of the weak radio source 53W002 show several morphological components for this compact narrow-line galaxy at z = 2.390: (1) a blue point source (central AGN, ltapprox 500 pc) with 20-25% of the total continuum light; (2) a small, concentrated and somewhat redder inner core; (3) an r^1/4-like light distribution with colors indicating an overall stellar population age ~ 0.4 Gyr and no large color or age gradient out to r ~ 9 kpc; and (4) two small blue clouds roughly aligned with the radio axis and the main stellar population, and separated by a red linear feature, possibly a "dust" lane. The size and shape of the Ly alpha clouds suggests reflected AGN continuum-light shining through a cone (plus re-radiated Ly alpha in emission). A recent OVRO interferometric CO-detection on both sides of 53W002 - and in the same direction as the continuum clouds and the radio jet - suggest a star-bursting region induced by the radio jet. Hence, both mechanisms likely play a role in explaining the "alignment effect." We discuss the formation and evolution scenario for 53W002 in context of its surrounding sub-galactic objects.