The [ITAL]Hubble Space Telescope[/ITAL] WFPC2 [ITAL]B[/ITAL]-Band Parallel Survey: A Study of Galaxy Morphology for Magnitudes 18 ≤ [ITAL]B[/ITAL] ≤ 27

Abstract

We present the results of the Hubble Space Telescope B-Band Parallel Survey (BBPS). It covers 0.0370 deg2 and consists of 31 shallow (four- to six-orbit), randomly selected high-latitude HST WFPC2 parallel fields with images taken in both the B (F450W) and I (F814W) filters. The goal of this survey is to morphologically classify the galaxies in a homogeneous manner and study galaxy properties as a function of type and B-band magnitude for 18 mag ≲ bJ ≲ 23.5 mag. The full sample contains 1800 galaxies, 370 of which are brighter than the formal statistical completeness limit of bJ ≲ 23.5 mag. The galaxies are selected from the B-band images and classified using an artificial neural network (ANN) galaxy classifier on the higher signal-to-noise ratio I-band images. These provide (more) reliable types for I ≲ 24 mag (or bJ ≲ 26 mag), since these I-band classifications are less subject to the uncertain redshifted rest-frame UV morphology. The ANN classification depends on the shape of the surface brightness profile, but not on color. These results are combined with similar (deeper) studies in the Hubble Deep Field and the deep WFPC2 field surrounding the radio galaxy 53W002, for which galaxies have been classified to bJ ≲ 27 mag. The galaxy counts for the combined B-band–selected samples show adequate statistics for a range 19 mag ≲ bJ ≲ 27 mag and are in good agreement with other studies in the flux range where they overlap, while showing improved statistics at the bright end. The galaxies are subdivided into three morphological classes: early types (E/S0), mid types (Sabc), and late types (Sd/Irr), and the B-band counts are presented for each class, as well as the total counts. The faint end of the counts is dominated by the irregular galaxies, which have a steep count slope of d log N/dm ≈ 0.4. These type-dependent counts are compared with models based on local luminosity functions that include the effects of the cosmological constant, ΩΛ. The whole BBPS sample, along with the two deeper fields, is used to delineate the general trends of effective radius and B-I color as function of both morphological type and apparent magnitude for 18 mag ≲ bJ ≲ 27 mag. These properties are discussed in the context of recent redshift surveys. A possible explanation for the combined results is given in terms of the effects of ΩΛ on the evolution of the merger rate in a hierarchical scenario.