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%                  VISTA SCIENCE COMMITTEE  
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%                  Pro-forma for Key Programme Proposal
%                  December 1999  Andy Lawrence (VSC chair)
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{KEY PROGRAMME PROPOSAL : High redshift quasars}
{KEY PROGRAMME PROPOSAL : High redshift quasars}
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{\large\it VISTA SCIENCE COMMITTEE  : KEY PROGRAMME PROPOSAL \\ 

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\LARGE\bf Very high redshift quasars with VISTA\rm \\ 
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\large
{\bf Contributors} : R.G. McMahon(IoA), 
S.J. Warren (ICSTM), P.C. Hewett (IoA), M.J. Irwin(IoA), R.G. Sharp(IoA)
\\
{\bf Date } : 2000 Jan 27th

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{\bf SUMMARY}

We propose to carry out a series of nested multi-colour surveys 
over 1000 to  10000 deg$^2$  to search for quasars with $5.5<z<7$.
The primary goals are:  

\begin{itemize}
\item to establish the space density and form of the
quasar luminosity function at the earliest epoch possible.
\item to identify the most luminous quasars in the high redshift
Universe in order to facilitate detailed absorption-line studies of
the metallicity, ionization state, dynamics and clustering properties
of the baryonic content of the Universe.
\end{itemize}

At this stage there are significant sources of uncertainty in
designing the survey. The situation will improve to some extent before
observations begin, but a flexible survey campaign is desirable,
allowing modification on the basis of the results as the survey
progresses.


{\bf Except where explicitly mentioned all quoted magnitudes
are on a Vega based normalisation.}

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\section{Science Case}
The most distant quasar currently
known is at redshift $z\sim5$ and was discovered
by the Sloan team. Quasars at larger redshifts still will
provide vital clues to the processes involved in the formation of the
first bound objects. We propose to use the power of VISTA to survey
for the first quasars, opening up the redshift range $5.5<z<7$. The
principal scientific goals are the following

\begin{itemize}

\item to determine the epoch of reionization of the Universe
via the Gunn-Peterson test.

\item to establish the contribution of AGN to the integrated extragalactic
background light(EBL) and in particular the meta-galactic UV background 
at z$>$5, and hence determine the major source of ionization of the high 
redshift Universe. 


\item to measure the chemical, ionization and dynamical state of
the intergalactic medium, where most of the baryonic content of the
Universe will reside, at the earliest epoch possible and to compare
this with numerical simulations.


\item to establish unambiguously the total UV luminosity density in the
high redshift Universe due to quasars 
via the explicit detection of the
characteristic break or knee in the quasar luminosity function out 
at z$>$6


\item to identify a sample of z$>$5 quasars suitable for the detection
  of damped Lyman-$\alpha$ absorbers to measure the cosmic density of
neutral gas in the high redshift Universe.


\end{itemize}

While there is little doubt that the quasar population reaches a peak
in the redshift range 1.5 to 3.0 and that the total space density of
optically selected quasars at higher redshifts is declining (Warren,
Hewett\& Osmer 1994; Schmidt, Schneider and Gunn 1995) the high
redshift (z$>$4) luminosity function is basically unknown, as
illustrated in Figure 1. The lack of knowledge is particularly acute at
fainter magnitudes and at z$>$5. Over the next few years we can 
expect considerable
progress and for $4<z<5$ the quasar luminosity function will become
quite well established as the result of work based on the SDSS, INT
WFS(McMahon, Sharp etal), CFHT 12K(Crampton, Hook, McMahon, Schade)
and a number of other deeper multi-colour surveys that are currently
in progress.

In other respects the return from current surveys will be limited. For
example many of the luminous $z>4$ in the Sloan survey area have
already been discovered using Schmidt plates. Similarly it is unlikely
that SDSS will find many (if any) quasars above z$>$5.5 since SDSS is
a shallow survey t=55secs at a mediocre site(1-1.5'' seeing) and the
reddest band is the Gunn z band. To make a significant step forward
requires IR observations. For VISTA we assume the minimum redshift 
of interest in the context of high redshift quasars is $z=5.5$. The
combination of depth, and extension to the near-infrared, will allow
VISTA to survey for quasars over the redshift range $5.5<z<7$.


%By the time VISTA starts operating at the end of 2004, UKIRT WFCAM
%will have been operational for a year and one would expect that
%WFCAM will be have surveyed the 100-200deg$^2$ that has been surveyed 
%with the INT WFS.


\section{Technical Case}

\subsection*{Wavebands required}

To make a significant step forward we assume the minimum redshift of
interest for VISTA is $z=5.5$.  The multicolour technique works best 
at high
redshift using up to four bands: i) a blue band lying immediately blueward
of the Lyman limit($\lambda_{rest}$=912\AA), ii) two bands that straddle blueward and redward of
the Ly$\alpha$ line($\lambda_{rest}$=1216\AA),
iii) a red band at as long a wavelength as possible, but
still blueward of the peak of the black-body curve for the red stars
against which the quasars are being discriminated. 
J is preferred because of faster survey speed. 

%At higher redshift
%this strategy is modified by the fact that absorption in the Lyman$\alpha$
%forest is so strong, allowing a redder passband to be used for the
%short passband, which has the consequence this passband does not have
%to reach so deep. ie ideally we would use the g band but to be useful
%these observation need to be prohibitively deep. We therefore propose
%to use r as the bluest bands.

The above considerations indicate the use of the combinations $r$,
$i$, J for $5.5<z<6.0$, and $i$, $z$, J for $6.0<z<7.0$. 
In reality one would use multi-dimensional analysis using all
4 bands simultaneously. 
%It is also, possible that a redder band
%such as H or K may be required in the long term.

Simulation
of a two-colour diagrams indicating quasar selection for these redshift
ranges are shown in Figure 3. These simulations require an extrapolation
beyond z$\sim$4.5 of our knowledge of the properties of intervening 
absorbers. As more
quasars of $z\sim5$ our discovered this uncertainty will be reduced.

On this basis for a survey magnitude limit (S/N=10) in J, the required survey
depths in the other bands are as follows: 
{\bf $r=J+4.0$, $i=J+3.0$, $z=J+1.0$}.
%H or K may be used in instead of J, but since quasars are bluer than
%the sky background, J is more efficient although a zJH survey for
%z$>$7 quasars might be considered fruitful in the long term.

Note that the simulations in Figure 3, take no account of
contamination of the colour-colour space by misclassified
red galaxies. In particular we need to include the evolved
elliptical galaxies and EROs(Extremely Red Objects). It is
important also to realise the success of high redshift
quasar surveys depends not just on the number of candidate
quasars but on the ratio of candidates to true quasars. This
ratio is currently unknown since neither the numerator or
denominator is known.



\subsubsection*{Expected surface densities and magnitude limits at z$>$6}

To satisfy some of the science goals of this program we do not
necessarily need large numbers of objects in the first stage. At the
highest redshifts even one object would be enough.  However to
guarantee success one should aim to detect 10 objects in the case of
the optimistic scenario and then proceed to larger area or fainter
flux limits in the next phase.

Also, the current uncertainty in the evolution of the quasar
luminosity makes it pointless to try to design a survey that will
answer everything. We therefore propose a series of surveys that are
incremental with branch points. To maximise the final return it will
be important to execute the first stage as early as possible.

To determine a statistically significant(3$\sigma$) space density one
requires $\sim$10 objects ie $N/\sqrt{N} \sim 3$. This logically leads
one towards surface densities of 10$^{-2}$ deg$^2$ if you expect to
get significant estimate in 1000deg$^2$. Also one has the potential of
still doing a survey which is 10-15 times higher ie all the high
latitude sky visible to VISTA, should one want to expand the survey
significantly at a later date.

Figure 3 shows the predicted number of high redshift(5$<$z$<$7)
quasars per square degree for two parameterizations of the quasar
luminosity function. Since for work at z$>$6 we shall need at least
one IR band we quote our surface densities in the J band.

From Figure 3a based on WHO we see that one needs to reach J=22.5 to 
reach a surface density of 1 quasar at z$>$6 in 100deg$^2$.
From Figure
3b based on SSG, we see that one need to just get to J=19 to reach a favourable
surface density. A reasonable best bet first phase would be to survey
1000deg$^2$ to J=20.5 as soon as possible and then design a second
phase on the basis of this. The second phase could be either deeper over
the same area of to the same depth or shallower over a larger area.


\newpage

\section{Summary of Requirements, and key tradeoffs}


\begin{itemize}

\item {\bf Area:}    
\begin{itemize}
\item 1000deg$^2$ for survey  A (should be done in 2DF + XMM XID survey regions)
\item 1000deg$^2$ for survey  B (should be done in 2DF + XMM XID survey regions)
\item 5--10,000deg$^2$ for survey  C (ie almost all $|b|$ sky visble for N and S hemispheres) 
\end{itemize}

\item {\bf Wavebands:} 
\begin{itemize}
\item {\bf optical bands} r, i and z   [g would be useful for 4$<$z$<$5.5]
\item {\bf IR bands} J
\end{itemize}


\item {\bf Any specialised filters required:} None

\item {\bf Depth in each band:} 
\begin{itemize}
\item J=20.5(10sigma) for survey  A; r=24.5; i=23.5; z=21.5(10sigma)
\item J=21.5 for survey  B; 1 mag fainter than A
\item J=20.5 for survey  C; as A 
\end{itemize}
A total S/N of 20 is required to facilitate star galaxy separation especially
since at these magnitudes galaxies outnumber z$>$5.5 quasars by $>$10$^4$.


\item {\bf Spatial resolution:} FWHM$<$1'' is essential for efficient star 
galaxy.

\item {\bf Astrometric accuracy:} 0.5''

\item {\bf Time sampling pattern:} None but IR and optical data should
be quasi simultaneous to avoid galactic variables or SN contaminating 
the  quasar sample. Long term variability could maybe be used to 
screen candidates. Lets say we take two I band images 5 years apart.

\item {\bf Spatial sampling pattern:} None


\item {\bf Photometric accuracy:} S/N of 10 in redder 2-3 bands.
If survey is contiguous MAYBE non-photometric conditions could be used.

\item {\bf Reliability and completeness required:} Upper
limits for magnitudes will be required when objects are detected in
a redder bands but not detected in the bluer bands; 99.9\% reliability 
is required otherwise
we will be swamped by spurious quasar candidates, since at the
flux limits we need to attain, stars and galaxies outnumber quasars
10$^{4-5}$ to 1.


\item {\bf Timescale on which the programme would be of interest:}
\begin{itemize}
\item survey  A needed ASAP so that choice between B and C can be
taken as early as possible.
\end{itemize}


\item {\bf Estimated time to completion.}
\begin{itemize}
\item 1 year for survey     A (20nights per band)
\item 5 years for survey  B (100nights per band)
\item 5 years for survey  C (100nights per band)
\end{itemize}


\item {\bf Other issues:} J needs only 100secs per pointings which
may require a special observing strategy  to minimise the
overheads and dither strategy. Note that even with 6pt dithering and 1.3
sec per read out, 2MASS claim 85\% observing efficiency. 

\end{itemize}





\begin{table}[h]
\begin{center}
%\centering
%\small
\caption
[Survey limits]{Nominal photometric S/N=5 survey limits in AB and Vega magnitudes; as an independent check on VISTA predictions we includ comparisons
with SDSS throughput }
\label{table_sddd_nominal_limits}
\begin{tabular}{|c|c|c|c||c|c||c|c||c|c||c|c|}
\hline
\multicolumn{1}{|c|}{}  
&\multicolumn{1}{|c|}{} 
&\multicolumn{2}{|c||}{} 
&\multicolumn{2}{|c||}{} 
&\multicolumn{2}{|c||}{} 
&\multicolumn{2}{|c|}{} 
&\multicolumn{2}{|c|}{Proposal} 
\\
\multicolumn{1}{|c|}{}  
&\multicolumn{1}{|c|}{} 
&\multicolumn{2}{|c||}{SDSS} 
&\multicolumn{2}{|c||}{WFS$^1$} 
&\multicolumn{2}{|c||}{WFS$^2$} 
&\multicolumn{2}{|c|}{VISTA$^3$} 
&\multicolumn{2}{|c|}{VISTA$^4$} 
\\
\multicolumn{1}{|c|}{}  
&\multicolumn{1}{|c|}{} 
&\multicolumn{2}{|c||}{2.5m + 55s} 
&\multicolumn{2}{|c||}{2.5m + 600s}
&\multicolumn{2}{|c||}{2.5m + 600s}
&\multicolumn{2}{|c|}{4.2m + 900s}
&\multicolumn{2}{|c|}{4.2m + 900s}
 
\\
\hline
\multicolumn{1}{|c|}{}  
&\multicolumn{1}{|c|}{AB mag$^5$} 
&\multicolumn{1}{|c|}{} 
&\multicolumn{1}{|c||}{} 
&\multicolumn{1}{|c|}{} 
&\multicolumn{1}{|c||}{} 
&\multicolumn{1}{|c|}{} 
&\multicolumn{1}{|c||}{} 
&\multicolumn{1}{|c|}{} 
&\multicolumn{1}{|c||}{} 
&\multicolumn{1}{|c|}{} 
&\multicolumn{1}{|c|}{} 

\\
\multicolumn{1}{|c|}{Waveband}
&\multicolumn{1}{|c|}{of Vega}   
&\multicolumn{1}{|c|}{AB} 
&\multicolumn{1}{|c||}{Vega}
&\multicolumn{1}{|c|}{AB} 
&\multicolumn{1}{|c||}{Vega}
&\multicolumn{1}{|c|}{AB} 
&\multicolumn{1}{|c||}{Vega}
&\multicolumn{1}{|c|}{AB} 
&\multicolumn{1}{|c||}{Vega}
&\multicolumn{1}{|c|}{AB} 
&\multicolumn{1}{|c|}{Vega}

\\
\multicolumn{1}{|c|}{}  
&\multicolumn{1}{|c|}{} 
&\multicolumn{1}{|c|}{} 
&\multicolumn{1}{|c||}{}
&\multicolumn{1}{|c|}{} 
&\multicolumn{1}{|c||}{}
&\multicolumn{1}{|c|}{}
&\multicolumn{1}{|c||}{}
&\multicolumn{1}{|c|}{}
&\multicolumn{1}{|c||}{}
&\multicolumn{1}{|c|}{}
&\multicolumn{1}{|c|}{}
\\
\hline
%                            SDSS        WFS       WFS2            VISTA(1)  VISTA(2)
     u'      &$+$0.98    &22.1&21.1 &23.4&22.4 &24.6$^6$ &23.6$^6$ &24.4 &23.4  &26.3 &25.6(U)\\
     g'      &$-$0.09    &23.3&23.4 &24.6&24.7 &25.0$^6$ &25.1$^6$ &25.6 &25.7  &26.5 &26.5(V) \\
     r'      &$+$0.17    &23.0 &22.8  &24.3 &24.1 &24.3 &24.1  &25.3 &25.1  &25.5 &25.3(R) \\
     i'      &$+$0.40    &22.4 &22.0  &23.7 &23.3 &23.7 &23.3 &24.7 &24.3  &25.1$^8$ &24.7(I)$^8$ \\
     z'      &$+$0.57    &20.8 &20.2  &22.1 &21.5 &22.2 &21.6  &23.1 &22.5  &24.8$^8$ &24.2(z)$^8$ \\
\hline
     J      &$+$0.90 &&&&&& & &  &24.3 &23.4 \\
     H      &$+$1.37 &&&&&& & &  &23.3 &21.9 \\
     K      &$+$1.88 &&&&&& & &  &22.8 &20.9 \\
\hline 
\end{tabular}
\end{center}
{\bf Notes:}\\ 
(1) INT WFS limiting magnitudes are scaled from the SDSS nominal limits which
assume seeing of 1''[see www.sdss.org]. \\
(2) Actual WFS survey limits in 1'' seeing
(3) VISTA limits scaled from SDSS
(4) From VISTA proposal and values circulated in KP pro forma.\\
(5) Vega normalisation for Sloan AB system from Fukugita et al(1996), AJ, 111, 1748 \\
(6) WFS U and g' are better than SDSS due to more modern blue sensitive
CCDs. Note also that the INT U throughput is down by 30\% due to 
corrector recoating delays; ie should be 0.15mag better  \\
(7) For S/N=5 add 0.75 magnitudes. SDSS to INT=+1.30mags;
INT to VISTA=+0.80 magnitudes; 600secs to 900secs add 0.22magnitudes \\
(8) proposal assumed Z would use IR detector; if VISTA uses deep depletion
devices i and z should be a factor of 2 better than SDSS/INT chips
(9) JHK zeropints from Bessel \& Brett(1988), AJ, 100, 1134.
(11) All results are for stars with gaussian profiles. For resolved
galaxies limits are around 0.5mag worse. 
\end{table}





\begin{figure}
\begin{center}
\hspace*{-1.0cm}\psfig{file=lf_fig_z_4.0.ps,width=15cm}
\hspace*{-1.0cm}\psfig{file=lf_fig_z_5.0.ps,width=15cm}
\end{center}
\caption{For illustration we shown two recent parameterisations of the quasar
  luminosity function (dashed - Warren, Hewett \& Osmer 1994 ,
  dot-dash - Schmidt et al. 1995) are shown computed at redshifts
  4 and 5.  Whilst there is good agreement at z=4 at M$_B$$\sim$$-$26,
  which is close to the regime encompassed by the input observations,
 by a redshift
  of 5 the extrapolated luminosity functions are quite different.
  The vertical line indicates the
  absolute magnitude of a quasar at a effective survey limit of
  z'=21.0(Vega normalisation) which is the regime that is being
 probed by current surveys}
\end{figure}



\begin{figure}
\hspace*{0.0cm}\psfig{file=qso_spec_6_0.ps,width=15cm}
\caption{
Synthetic quasar spectrum based on
a power law with spectral index ($S_\nu\propto \nu^{-0.7}$)
with an emission line spectrum based on the Francis et al.(1991)
composite spectrum.
Absorption due to the intervening Lyman-$\alpha$ forest and metal
line systems has been monte-carlo simulated on the basis of an absorption
line population described by the parameters derived
by Williger et al.(1994) for the forest and by Storrie-Lombardi et al.(1994)
for the optically thick lyman-limit absorption systems}
\end{figure}



\begin{figure}
\centerline{\hspace*{-2cm}\psfig{file=rzj.ps,width=15cm}} 
\caption{
Theoretical r-z, z-J colour-colour diagrams.  
The crosses are colours for stars calculated using the Gunn \& Stryker(1991) stellar atlas with the
Persson extension into the IR(from SYNPHOT in STSDAS). The filled
circles are quasar
colours that have been simulated using the monte-carlo approach
described above for two different continuum spectral indices
-0.50 and -0.75. The solid wobbly line is quasar colours calculated using
the analytic IGM model proposed by
Madau 1995. The differences between the two approaches are beyond
the scope of this study. The solid line near the top shows the effect
of underlying spectral index on colours. Yes, we could also add a reddening
vector.}
\end{figure}



\begin{figure}
\hspace*{1.0cm}{\psfig{file=lf_J_4-7_ssg.ps,width=12cm}} 
\hspace*{1.0cm}{\psfig{file=lf_J_4-7_who.ps,width=12cm}}
\caption{
The predicted number of quasars per square degree as a function of apparent 
J magnitude is shown for the two luminosity functions in figure 1.  
The horizontal line indicates a fiducial surface density of 1 quasar
per 100 deg$^2$. To convert to a K band limit assume J$-$K=1.0; ie J=20 is
equivalent to K=19.}

\end{figure}




\end{document}






