here - Institute for Astronomy Umleitung

Title: Science Case
Reference: MUSE-MEM-SCI-052
Issue: 1.3
Date: 04/02/2004
Page: 15/100
observations for the local and z=3 universe. They correspond to a flat or decreasing cosmic
SFR at z>4, for which very little data is available. Their predictions should be considered as
quite conservative lower values since it is pretty possible that the cosmic SFR strongly
increases at z>4, maybe by a factor 10 as suggested by the analysis of Lanzetta et al. (2002).
Ultimately, the solution to clarify this point is to obtain the data with MUSE.
We assume that Ly α photons are produced by ionized regions (case B recombination). In such
models, the description of the Ly α line is particularly delicate, because of the existence of
stellar absorption after a starburst (due to A stars), and of resonant scattering. Only a small
fraction of the emitted Ly α photons should escape from a homogeneous dusty medium.
Nevertheless, Ly α emission is observed in local objects (such as Blue Compact Dwarfs) and
high-redshift galaxies (Ly-break galaxies and Ly α galaxies found in wide-field, narrow-band
surveys). It is thought that the existence of an expanding medium prevents Ly α photons from
resonant scattering, and produces characteristic P-Cygni profiles. There is no simple
modelling of such a process. At the current stage, we make predictions for two different
models: (i) a fixed escape fraction f esc which would correspond to «holes» in the gas and dust
distribution, and that is normalised in order to reproduce number counts obtained by current
surveys; (ii) absorption in a dusty
medium without resonant scattering
(let’s say, if the velocity of the
expanding medium is high enough to
completely hamper resonant scattering).
A more realistic model is clearly needed.
It is foreseen that the interpretation of
current Ly α surveys and the preparation
of MUSE will foster theoretical activity
on this point.
There is very little observational
constraint on the value of the Ly α escape
fraction. Steidel et al. (2001) give f esc ≥
0.07–0.1 from Ly-break galaxy spectra,
but it is very likely that stellar
populations at higher redshifts are
younger, less chemically evolved and
less dusty. We choose to fix f esc in order
to reproduce current Ly α surveys. At
z=3.4, the observed surface density of
reasonable confirmed objects at the
2.10 -17 erg.s -1 .cm -2 flux level is 12,000
deg -2 (∆z) –1 , that is, 3.3 arcmin -2 (∆z) –1 .
This is obtained with f esc =0.15. This
number appears reasonable in the light
of more specific studies that take into
account plausible values for the covering
factor and metallicity (e.g. Haiman &
Spaans 1999). The decrease of
Figure 2-2: Predictions of faint galaxy counts in
three redshift bins with the GalICS model. The
escape fraction for Ly α photons is fixed in order to
reproduce the current counts at a flux limit of
2.10 -17 erg.s -1 .cm -2 in a redshift slice at z=3.4. Solid
dots show the results of the recent surveys taken
from Table 2-1 (error bars are only Poissonian).
The effect of clustering on the cosmic variance
appears from the scatter of the points. The model
seems to scale fairly with redshift. The open
triangle shows an estimate based on a single lensed
source at a redshift z=5.7. The MUSE Deep Field
flux limit is also shown