5 Investigations on redshifts of formation

Given the photometric redshift and age found with Z-PEG for any observed galaxy, a corresponding formation redshift $z_{{\rm for}}$ , defined as the epoch of the first stars generation, may be derived. The result, constrained by the age of the universe (age $_{\textrm{\tiny U}}$), depends on cosmology; as previously mentioned, standard cosmological parameters are hereafter adopted. Figure 8 shows iso- $z_{{\rm for}}$ curves. The galaxy sample is also plotted and compared to curves. For $z_{{\rm for}}<5$, our formation redshift determination is reliable, if we keep in mind that we constrained the ages of galaxies and thus the formation redshift. Unfortunately, the precision of redshift determination from photometry is quite poor (about 0.1 for z < 1.5 with our sample). Above $z_{{\rm for}}= 5$, the iso- $z_{{\rm for}}$ lines are so close to each other that we cannot rely on this formation redshift determination.

The histogram of the formation redshifts is shown in Fig. 9. Note that this histogram is in agreement with Lanzetta et al. (2001) figures of the star formation rate as a function of redshift, deduced from FSLY's photometry, and taking into account the effects of cosmological surface brightness.

Figure 8: Galaxies of the HDF-N sample in the redshift-age diagram, after Z-PEG fits. The dotted lines show the z-age relations for various $z_{{\rm for}}$(iso- $z_{{\rm for}}$ lines) and the standard cosmology, previously defined. For example, the dotted line crossing the y-axis ( ${\rm age}=0$) at z=2 corresponds to $z_{{\rm for}}=2$. The solid line shows the age of the universe at a given redshift (formally, this line is equivalent to $z_{{\rm for}}=\infty$). As a result, most galaxies form at z>3 (we can also check this result in Fig. 9).

Figure 9: Histogram of $z_{{\rm for}}$ from our sample. For $z_{{\rm for}}> 5$, due to the lack of precision on the determination of (age, $z_{\rm phot}$), we cannot conclude on $z_{{\rm for}}$ real values. $z_{{\rm for}}= 5$ is then a lower value.