Morphological Number-counts from Ultradeep Hst Images

We discuss the observations leading to the Faint Blue Galaxy problem and the uncertainties upon which faint galaxy models are based. Using deep Hubble Space Telescope (HST) imaging with the Wide Field Planetary Camera (WFPC2), we show how morphological information has been used to shed new light on the problem. Initial results indicate that the giant galaxies (ellipticals and early-type spirals), are well t by no-evolution standard models down to m I 24:5 (z 0:8). The data also show that the faint blue galaxies have late-type/irregular morphologies and cannot be adequately modelled until better constraints are placed on the local space density for this class of galaxies. One of the most basic astronomical observations is to simply count the number of galaxies in a given direction as a function of apparent magnitude and use these observations to help understand the nature of our Universe. The advantages of such a simple observation, is that good statistical data can be obtained over a large magnitude range. Figure 1 shows some of the number-count data, obtained by many groups over a wide range of magnitude. The original contention 21], was that data such as these would allow a direct measurement of the cosmological parameters ((, q o ,) | the idea being that the departure of the number-counts from a purely Euclidean slope of 0.6 is attributable to the geometry of the Universe. The lines shown in Figure 1, reeect the predicted number-counts based on the best non-evolving standard models and it is clear that the models and the data strongly disagree at progressively fainter magnitudes. This discrepancy gives rise to the fairly long standing Faint Blue Galaxy (FBG) problem 23], 2], 24], discussed in the next section. In this article, we brieey outline the current state of the observations, the uncertainties that go into generating a faint galaxy model, and how the latest HST images, obtained with the WFPC2, now allow us to use morphological information to simplify the problem.