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\title{
Galaxies in the reionization epoch: the first science case for MOSAIC
}

   \subtitle{}

\author{
L. \,Pentericci\inst{} 
          }

  \offprints{L. Pentericci}
 

\institute{
Istituto Nazionale di Astrofisica --
Osservatorio Astronomico di Roma, Via di Frascati 33,
I-00078,Monte Porzio Catone Italy
\email{laura.pentericci@oa-roma.inaf.it}
}

\authorrunning{Pentericci }


\titlerunning{MOSAIC}


\abstract{



One of the principal science goals for the E-ELT is to find the very first
galaxies, that probably reionized the universe and  probe their  physical properties, 
which  may well be very different 
from those of their lower redshift counterparts, as they are forming from pristine gas, with
very low  metal abundances. Current results show that  the Ly$\alpha$ line
coming from young,  dust free
stellar populations is not only a powerful  redshift indicator but its
demographics can also be used to constrain the neutral hydrogen fraction at the
various epochs,  and thus trace the reionization history.


MOSAIC, the Multi-Object spectrograph  planned for E-ELT 
will be able to  observe the Ly$\alpha$ emission line in star forming 
galaxies,  up to the very earliest epochs (z$\sim$10). 
 The spectral resolution and high sensitivity of MOSAIC   will allow us to probe the warm and hot ionized gas 
in the interstellar medium of these extremely distant galaxies via studies of
the rest-frame UV emission and absorption lines.

 MOSAIC  will have the combination of capabilities  i.e. the required
sensitivity, spectral coverage, wide field and multiplex  to allow us to
 reproduce at z $\sim 7-10$, what we can now routinely do  at z$\sim  3-5$  with
8-10 m class telescopes.  



\keywords{Stars: abundances --
Galaxies: atmospheres -- Stars: Population II -- Galaxy: globular clusters -- 
Galaxy: abundances -- Cosmology: observations }
}
\maketitle{}

\section{Introduction: the first galaxies and the reionization epoch}
The reionization epoch was a  key period of the history of the Universe which
marked the transition from a neutral  intergalactic medium  (IGM)  to a completely ionized one:
understanding  when and how this transition occurred  and what were the sources
that emitted the ultraviolet radiation necessary to ionize the IGM,  are
amongst the  greatest challenges in modern astronomy.

The so called epoch of reionization also saw the birth of the first 
galaxies  which are often  identified as 
sources responsible  for the reionizatiion.
Observational constraints on the properties of galaxies at z $\geq$ 7 are still  scarce, with
only a handful of galaxies confirmed at this epoch (e.g. Vanzella et al. 2011,
Finkelstein et al. 2013, Pentericci et al. 2014). 
Due to their extremely faint magnitudes, recent efforts have focused on redshift
confirmation, using the rest-frame Lyman-$\alpha$  line 
and/or the  detection of the Lyman break in the stellar continuum caused by
absorption by neutral hydrogen in IGM.  
Little is known about basic properties of the objects detected, such as their
stellar  masses, their stellar populations or
the chemical composition of their interstellar gas (e.g. Jiang et al. 2013).



\section{Ly$\alpha$ emission in LBGs as a powerful probe of reionization}

The use of Ly$\alpha$ transmission by the intergalactic medium as a probe of
its ionization state during the reionization epoch was proposed many years ago
(Miralda-Escude \& Rees 1998). The  Ly$\alpha$ emission which 
 is present in many distant galaxies, is sensitive to even small quantities of
 neutral hydrogen in the IGM, and it is easily suppressed (Zheng et al. 2010). 
We thus expect the observed properties of Ly$\alpha$-emitting galaxies to
change at higher redshifts, when the IGM becomes more neutral. A recent
approach to study  the reionization history of the Universe is to measure the redshift evolution of the
Ly$\alpha$ fraction in Lyman-break galaxies (LBGs), i.e., the percentage of LBGs that
have an appreciable Ly$\alpha$ emission line (e.g., Stark et al. 2010;
Pentericci et al. 2011, LP11;  Pentericci et
al. 2014, LP14): this
fraction is supposed to increase as we move to higher redshift because
galaxies are increasingly young  and almost
dust-free (Bouwens  et al. 2014), which facilitates the escape of Ly$\alpha$
photons. On the other hand, this fraction must fall off as we
approach the epoch when the IGM becomes significantly neutral. 

To study this problem in a systematic and statistically solid way we have
designed CANDELSz7, an ESO Large Program to observe 200 galaxies at z$\sim5.8$ to  7.3
with FORS2, with integration times that makes us reach very faint EW limits
(15 \AA\ for J=25.5 galaxies and 25 \AA\ for J$=$26.8 galaxies) or a Ly$\alpha$
flux of about 0.5 $\times 10^{-17} erg sec^{-1} cm^{-2}$.
Galaxies are selected in the three CANDELS fields visible from Paranal,
namely GOODS-South, UDS and COSMOS.  The availability of the CANDELS
multiwavelength data (e.g. Koekemoer et al. 2011), both  from HST and at other facilities  means that we can determine the physical properties of the
galaxies, such as  their  stellar masses, dust content, stellar ages and star
formation rates, with great accuracy. We can thus disentangle any possible
evolution of the intrinsic physical  properties from the evolution of the
presence of Ly$\alpha$ in the spectra. 


Although the program is not completed yet, we have already secured the 
redshifts of more than 45  galaxies between 5.5 and 7.2. Most of the confirmed
galaxies show a clear Ly$\alpha$ line in emission but thanks to our very deep
spectra we are able to confirm the redshift of a sizable number of z$\sim$6-6.4
galaxies which show a clear Lyman break with  very faint of even absent Ly$\alpha$  (e.g. Figure 1 for some examples of
new confirmed targets).

With this much enlarged sample we can place solid constraints on the declining
fraction of Ly$\alpha$ emission in z$\sim$7 Lyman-break galaxies compared to z
$\sim$6 both for bright and faint galaxies. The decline is very strong
especially for faint ($M_{UV}< -21.25$) galaxies. Applying simple models 
(Dijkstra et al 2014), assuming that   the decline is only due to the
evolution of the IGM while all other galaxy properties remain unchanged in this redshift interval,
and assuming that at z=6 the Universe is completely ionized,   a very large change in the fraction  of neutral hydrogen is needed to explain the
 observations ($\sim 0.5$). 
Obviously we cannot rule out that an evolution of other properties, and in
particular the  escape fraction of the Lyman continuum emission  come into
play and contribute at least partially   to the Ly$\alpha$ quenching (e.g. Mesinger et al. 2015).

Recently, the search for primeval galaxies  was pushed to even  higher redshift
using current near-IR MOS facilities:  
in particular MOSFIRE was able to confirm galaxies showing Ly$\alpha$ out to z
greater than 8 (e.g. Zitrin et al. 2015) out of several tens observed.  
As expected the trend of decreasing Ly$\alpha$ (and thus  increasing
neutral hydrogen) continues to higher redshift.

Alternative emission lines in the spectra of star forming galaxies of such as
CII] at 1908 \AA\ are probing too faint to be considered a valid alternative
(e.g. Stark et al. 2015). 
This means that current 8-10 meter telescope even if used at full power will
not  be able to secure the redshift of sizable samples of galaxies at z$>7.5$
but only a few of the brights ones, or those rare objects  showing 
extreme Ly$\alpha$ emission.
To gather large samples (which are also  needed e.g. to validate the UV  
Luminosity function) will have to wait for JWST and E-ELT. 

\section{MOSAIC a high multiplexing MOS to observe the first galaxies}

Since  multi-object spectrographs are currently the workhorse instruments of the 8-10 meter class
observatories, it is obvious  the need for an  optical and infrared  multi
object spectrograph also for the planned 40 meter telescopes. 
Given that the European ELT 39m telescope, the E-ELT, will have a typical $\sim$40
$arcmin^2$ patrol FoV, it  will provide hundreds of faint targets to be studied spectroscopically.
Exploiting these densities efficiently also  calls for a MOS relatively 
early in the E-ELT instrument suite.

The recent  ELT-MOS White Paper (Evans et al. 2013 and its revised version
Evans et al. 2015) led the MOSAIC consortium to define two
main observing modes, i.e., the HighMultiplexMode (HMM) \`a la OPTIMOS-EVE,
with several hundreds  mono-aperture fibers with Ground Layer Adaptive Optics
(GLAO)/seeing resolution
and a High Definition Mode \`a la EAGLE, with 10 MOAO-fed IFUs with 40-80
 mas sampling. 

The HMM mode will be  dedicated
to the study of the integrated (or coarsely resolved with GLAO) light emitted
by the most compact sources such as   the  very
first galaxies in the reionization epoch;   the HDM will be  necessary to
spatially-resolve the properties of, e.g., distant star forming galaxies with
high enough signal to noise.   

Obviously one of the initial  science cases for MOSAIC will be the search and
study of the primeval galaxies via Ly$\alpha$ emission, Lyman break in the
continuum and UV absorption lines as discussed extensively in Evans et al. 2015.

\begin{figure*}[]
\resizebox{\hsize}{!}{\includegraphics[clip=true]{Fig_LP.eps}}
\caption{
\footnotesize
The spectra of some of the new galaxies confirmed by our ESO Large Program
CANDELSz7 (Pentericci et al. in preparation). Most confirmed
galaxies show a bright Ly$\alpha$ in emission as the only signature in the
spectrum. When the signal to noise is sufficiently high, the line shows the
typical asymmetry.
}
\label{spectra}
\end{figure*}


\subsection{Simulation of MOSAIC observations of Ly$\alpha$ and UV absorption in
high z galaxies} 

We simulated the detection of UV interstellar lines at z = 7 and the detection
of the Lyman $\alpha$ line and the Lyman break at z = 9, both assuming 
 MOAO-assisted IFUs and GLAO-fed fibers. 
The  simulations are performed with a  scientific simulator that was developed
in  the context  of the E-ELT phase A studies
(M. Puech et al. 2010). The properties of the galaxies are modeled as  those
found mostly from the CANDELS survey: the typical sizes (half light radii)
are scaled  with their absolute magnitude  as found for example by Grazian et al (2012). The fluxes and
shapes of the Ly$\alpha$ emission  line are also scaled according to what is
observed at high redshift: 
in particular we assume a truncated Gaussian profile for the Ly$\alpha$ line
with a  a FWHM of 150-500 km/s   and  a correlation of the Ly$\alpha$  EW
with magnitude in the sense that faint galaxies have on average larger EWs as
routinely observed at z=3-7 (e.g. Stark et al. 2011).

Using the above parameters simulations are run with varying pixel size and
the signal to noise achieved is recorded. The most constraining 
case is the detection of UV interstellar lines, since in this case we need to
achieve a very high signal to noise in the continuum: the optimal pixel size is
found to be ~80 mas, which allows detecting UV lines up to $J_{AB}\sim 27$ in
40 hours of integration time. 
Lyman Alpha Emitters and Lyman Break Galaxies are detected respectively up to 
$J_{AB} \sim 30$ and $J_{AB}\sim 28$ with a 80 mas/pixel IFU and within only 10 hours of
integration time. 

Detection limits are typically 0.5-1 mag fainter using MOAO-fed IFUs than
using GLAO-fed fibers, but the multiplex is one magnitude larger in the mode
using GLAO-fed fibers.
For full details of the simulation see Disseau et al.2014, and Puech et al. 2013.




\section{Conclusions}
The search for primeval galaxies and the exploration of the reionization epoch
are the current key issues for extragalactic astronomy.
The space density of these faint targets  requires the next 
generation of telescope to be equipped with a multi objects spectrograph 
facility . MOSAIC will offer both a high  multiplexing mode with a 
large number of mono apertures fibers to observe large  numbers of galaxies  
and a high definition model with 10 MOAO IFUS for detailed studies of the
brightest objects,  thus  providing the most complete view of the
evolution of galaxies in the early universe. 




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\end{thebibliography}


\end{document}

