STScI Preprint #1248

High Redshift Galaxies in the Hubble Deep Field: II. Colours and Number Counts

We discuss the deep galaxy counts from the Hubble Deep Field (HDF) imaging survey. At faint magnitudes, the slope of the differential number-magnitude relation is flatter than 0.2 in all four HDF bandpasses. In the ultraviolet, a fluctuation analysis shows that the flattening observed below U₃₀₀ 26 mag is not due to incompleteness and is more pronounced than in the other bands, consistent with the idea that a redshift limit has been reached in the galaxy distribution. A reddening trend of 0.5 magnitude is observed at faint fluxes in the colour-magnitude diagram, (U₃₀₀ - V₆₀₆)_eff versus V₆₀₆. We interpret these results as the effect of intergalactic attenuation on distant galaxies. At flux levels of AB 27 mag and in agreement with the fluctuation analysis and the colour-magnitude relation, about 7% of the sources in U₃₀₀, 30% in B₄₅₀ and 35% in V₆₀₆ are Lyman-break "dropouts", i.e. candidate star-forming galaxies at z>2. By integrating the number counts to the limits of the HDF survey we find that the mean surface brightness of the extragalactic sky is dominated by galaxies that are relatively bright and are known to have <z>~0.6. To AB 29 mag, the integrated light from resolved galaxies in the I-band is 2.1 ^+0.4_-0.3 × 10^-20erg  cm^-2 s^-1 Hz^-1 sr^-1 and its spectrum is well described by a broken power-law (I  ² from 2000 to 8000 Å and I   from 8000 to 22000 Å). We discuss the predictions for the counts, colours, and luminosity densities from standard low-q₀ pure-luminosity-evolution models without dust obscuration, and find that they are unable to reproduce all the observed properties of faint field galaxies.

Status:

Appeared in: MNRAS, 298:1133-1144, 1998

Affiliations:

¹)Dipartimento di Astronomia, Università di Bologna,via Zamboni 33, I-40126 Bologna, Italy
²)Osservatorio Astronomico di Bologna,via Zamboni 33, I-40126 Bologna, Italy
³)Space Telescope Science Institute, 3700 San Martin Drive,Baltimore, MD 21218, USA
⁴)Istituto di Radioastronomia del CNR, via Gobetti 101, I-40129 Bologna, Italy
⁵)Centro de Investigationes de Astronomía, A.P. 264, Mérida 5101-A, Venezuela