%\documentclass[referee]{aa}
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\begin{document}
\thesaurus{11(11.05.1; 11.05.02; 11.06.1; 13.09.1; 12.03.3;)}
\authorrunning{M. Stiavelli et al.\ }
\titlerunning{A candidate high-z elliptical galaxy}
%\singlespace
\title{VLT and HST observations of a candidate high redshift elliptical
galaxy in the Hubble Deep Field South
\thanks{Based on observations collected at the European
Southern Observatory, Paranal, Chile (VLT-UT1 Science Verification
Program) and with the NASA/ESA HST, obtained at the STScI, which is
operated by AURA, under NASA contract NAS5-26555. }}

\author{M. Stiavelli \inst{1,2,6}
\and T. Treu \inst{1,2} 
\and C.~M. Carollo \inst{3}\thanks{Hubble Fellow}
\and P. Rosati \inst{4}
\and R. Viezzer \inst{4}
\and S. Casertano \inst{1,6}
\and M. Dickinson \inst{1}
\and H. Ferguson \inst{1}
\and A. Fruchter \inst{1}
\and P. Madau \inst{1}
\and C. Martin \inst{1}
\and H. Teplitz \inst{5}
}


\offprints{M. Stiavelli}
\mail{M. Stiavelli} 

\institute{Space Telescope Science Institute, 3700 San Martin Dr.,
21218 MD, U.S.A.; mstiavel@stsci.edu, treu@stsci.edu \and Scuola
Normale Superiore, Piazza dei Cavalieri 7, I56126, Pisa, Italy;
mstiavel@astro.sns.it, treu@cibs.sns.it \and Johns Hopkins University,
3701 San Martin Dr., 21218 MD, U.S.A.; marci@pha.jhu.edu 
\and European Southern Observatory, Karl-Schwarzschild Str. 2, D85748,
Garching; prosati@eso.org 
\and Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.;
hit@binary.gsfc.nasa.gov
\and On assignment from the Space Science Department of the European
Space Agency} \date{Received / Accepted}

\maketitle

\begin{abstract}
The combined use of the ESO Very Large Telescope (VLT) UT1 Science
Verification (SV) images and of the Hubble Space Telescope (HST)
Hubble Deep Field South observations allows us to strengthen the
identification as a candidate elliptical galaxy of the Extremely Red
Object HDFS 223251-603910 previously identified by us on the basis of
NICMOS and Cerro Tololo Interamerican Observatory imaging.  The
photometry presented here includes VLT data in U, B, V, R, I, a STIS
unfiltered image, NICMOS J, H, and K band data, thus combining the
16.5 hours of VLT SV exposures with 101 hours of HST observing.
The object is detected in all images except the
VLT U band and is one of the reddest known with B-K$=9.7\pm0.5$.  We
consider a wide range of models with different ages, metallicities,
star formation histories and dust content, and conclude that the
observed spectral energy distribution agrees best with that of an old
elliptical galaxy at redshift just below 2. Alternative possibilities
are discussed in light of their likelihood and of the perspective of
spectroscopic confirmation.

\keywords{Galaxies: elliptical and lenticular, cD--Infrared:
galaxies--Galaxies: formation--cosmology: observations--early universe}

\end{abstract}

\section{Introduction}

\label{sec:intro}

The formation process of elliptical galaxies is still unknown. Direct
observations of elliptical galaxies show that ellipticals exist up to
a redshift of one, but the statistics are too poor to draw firm
conclusions concerning the evolution of the population as a whole
(see, e.g., Kauffmann et al., 1996; Im et al. 1996; and Im and
Casertano 1998).  Indirect evidence based on the evolution with
redshift of the Fundamental Plane of elliptical galaxies (van Dokkum
et al. 1998), on the colors of $z \simeq 0.9$ cluster ellipticals
(Stanford et al. 1998), and on the global correlation of metallicity
with velocity dispersion (Ziegler \& Bender et al. 1997), as well as
direct measurement of Balmer line strengths (see, e.g., Bressan et
al. 1996), indicate that the stellar populations of ellipticals are
old, with ages in excess of 10 Gyrs. Unfortunately, the uncertainties
in the basic cosmological parameters do not allow us to firmly convert
age estimates into a formation redshift. In particular, if the
geometry of our Universe is dominated by a cosmological constant,
stars in ellipticals could have formed at a redshift of 1.65 and still
be older than 10 Gyrs at the present time (with $\Omega=0.35$,
$\Omega_\Lambda=0.65$, $H_0=65$ km s$^{-1}$ Mpc$^{-1}$).
Stellar-gaseous mergers of disk systems at moderate redshifts appear
to reproduce successfully the observed properties of elliptical
galaxies (see, e.g., Kauffmann and Charlot 1998 and references
therein); in such models, most stars form before the galaxy itself is
finally assembled.  However, the predictive power of these models is
limited by our modest knowledge on star formation processes which are
usually treated according to simple empirical relations.  On the basis
of entirely different theoretical arguments including a detailed
description of ISM properties but no cosmology nor dynamical
evolution, one could instead argue that spheroidal systems are
preferentially formed at high redshift (Spaans \& Carollo 1997,
Carollo et al. 1997a). Thus, it appears that the present stalemate
between different theories and low-to-moderate redshift observations
awaits an observational answer.

Important clues may indeed be provided by the discovery of a
population of Extremely Red Objects (EROs): relatively bright in the
NIR (K$\sim 19$--$20$) and very faint in the optical ((R$-$K)$>5$
typically \footnote{The definition of EROs is not uniform in the
literature. Other values are, for example, (R$-$K)$>4.5$ (Elston et
al.\, 1991) and (R$-$K)$>6$ (Graham \& Dey, 1996).}). Such a red color
can be produced by an ``old'' stellar population at high redshift
(e. g., LBDS 53W091 at z=1.55, \cite{SP}) or by a younger stellar
population enshrouded by dust lanes (e. g., HR10 at z=1.44, \cite{GD},
with one detected emission line). Besides the two prototypical (and
particularly bright) cases of LBDS 53W091 and HR10 where it was
possible to determine the redshift using the Keck 10-m telescope, the
``dust-age'' degeneracy in the broad band colors is generally coupled
with the redshift and metallicity uncertainties. In other words, the
extremely red R-K observed in the EROs can be explained by different
combinations of star formation history, redshift, dust content and
chemical composition. The only way to photometrically reduce this
degeneracy is to obtain multiple broad band colors, over the widest
possible spectral range, and to measure morphological properties.

In this Letter we present photometric information of unprecedented
completeness about the extremely red $R^{1/4}$ galaxy identified by
Treu et al. (1998a) in the Test Image of the Hubble Deep Field
South. The new VLT and HDFS data strengthen the early identification
as an optimal candidate high redshift elliptical galaxy. If
spectroscopically confirmed, this identification would imply that some
ellipticals can be in place already at redshift 2 and that their stars
have formed at even higher redshifts.  In Section 2 we describe the
observations and the photometric measurements. In Section 3 we discuss
the identification of the object.  In this Letter the Hubble constant
is assumed to be 65 km/s/Mpc. 

\section{Observations}

The HDFS NICMOS field was observed with the Test Camera on VLT-UT1
during the VLT Science Verification (Renzini, 1998) and with NICMOS on
board the Hubble Space Telescope during the HDFS (Williams et
al. 1999) campaign. The field was observed for an additional 9 orbits
with the Space Telescope Imaging Spectrograph (STIS) to obtain a
broad, high resolution, visible image, and, as one of the flanking
fields, with the Wide Field and Planetary Camera 2
(WFPC2). Unfortunately, we were not able to include the WFPC2 images
in our analysis as, at the time of this writing, the final reduced
frame had not been released. We refer to Fruchter et al.  (1999) for
the description of the observations and the data reduction.

In the following discussion we shall indicate the Hubble Space
Telescope filters in the following way: C = STIS 50CCD (clear);
J$_{110}$= NICMOS F110W; H$_{160}$= NICMOS F160W; K$_{222}$= NICMOS
F222M.

\subsection{Morphology}

\label{sec:morfe}

The galaxy is clearly resolved in the NICMOS and STIS images, as shown
in Figure~\ref{fig:IM}. The light profiles, derived as described by
Treu et al. (1998a), are shown in Figure 2. The shaded areas in the
Figure represent the data and the errorbars including the formal error
in isophotal fitting and the ``sky'' subtraction error. The ``bumps''
present in the observed profiles are mostly due to structures in the
PSF (particularly for the NICMOS data). To strenghten the visual
classification as an early type galaxy, we fitted the luminosity
profile with an $R^{1/4}$ model and an exponential model. We performed
the fit independently on each STIS and NICMOS image using two
different techniques: the two-dimensional fit code described in Treu
et al. (1998b) and the luminosity profile code described in Carollo et
al.\ (1997b). With the latter technique we fitted also a bulge+disk
model. We used in each case two different stars as PSFs, in order to
check the sensitivity of our results to a particular choice of PSF. We
confirm the Treu et al.\ (1998a) result that only an $R^{1/4}$ profile
provides an acceptable fit.  The high resolution STIS observations
confirm the size we determined from the earlier NICMOS observations;
note that the derived half-light radius is larger than both the STIS
pixel size and PSF FWHM, making our result more robust. The
uncertainties listed in Table \ref{tab:data} include the error on the
sky subtraction and the error due to PSF estimates.  We cannot exclude
the presence of a faint, extended disk, but we have verified that our
results are not significantly affected by the presence of such disk
components by carrying out a variety of bulge+disk fitting experiments
(e.g., we find that the ``bulge'' magnitude does not change by more
than $\sim$ 0.1 mag).

\begin{figure}
\resizebox{\hsize}{!}{\includegraphics{gC.ps}\includegraphics{gJ.ps}
\includegraphics{gH.ps}\includegraphics{gK.ps}}
\resizebox{\hsize}{!}{\includegraphics{imas.ps}}
\caption{HST and VLT images of HDFS 223251-603910 {\bf C.} STIS clear
.  {\bf J.} NICMOS J$_{110}$ {\bf H.} NICMOS H$_{160}$ {\bf K.} NICMOS
K$_{222}$.  {\bf B.} VLT B {\bf V.} VLT V {\bf R.} VLT R {\bf I.} VLT
I.  The images are $3\farcs75$ on a side. A faint blue source very
near to the center of the galaxy is visible in the STIS and NICMOS J
image.  }
\label{fig:IM}
\end{figure}

\begin{figure}
\resizebox{\hsize}{!}{\includegraphics{newprofiles.ps}}
\caption{Luminosity profile of the galaxy (shaded areas) in the STIS
and NICMOS passbands. The size of the shaded area is given by the
formal error bar. The PSF-convolved best-fit $R^{1/4}\,$ profile is
shown for comparison (solid line). }
\label{fig:prof}
\end{figure}

\subsection{Colors}
\label{ssec:col}

The multiband colors of HDFS 223251-603910 are summarized in Table
\ref{tab:data}. The STIS and NICMOS magnitudes are derived by fitting
$R^{1/4}$ luminosity profiles as described in the previous section but
were also checked with aperture measurements and found to be
consistent to within $\sim$0.1 mags. For the VLT data we give aperture
magnitudes corrected for PSF losses. We estimate the systematic error
in the correction to be in the range $0.05-0.1$ mags. The
uncertainties listed ($\delta$m) do not include systematic calibration
errors, which might be as high as 0.1 mags for the NICMOS magnitudes
and as high as 0.05 mags for the VLT magnitudes. We adopt here the
NICMOS Pipeline zeropoints. Particularly the H$_{160}$ zeropoint used
here is 0.08 mags larger than the one used in Treu et al. (1998a). The
VLT zero points in the final coadded images, as released on the WEB
site http://www.hq.eso.org/paranal/sv/, were calculated by matching
the photometry in the final stacks with that of a single photometric
night.  The overall consistency of the zeropoints was checked by
comparing the VLT data with CTIO data and the main WFPC2 HDFS
field. The colors listed in Table 1 are corrected for galactic
extinction, using the value E(B-V)=0.026 reported by Schlegel et
al. (1998).

\section{Discussion of the Spectral Energy Distribution}

Before starting the automated best fitting procedure described in the
following section we have tried to explore qualitatively the
implications of the observed SED by considering models as young as 1
Myrs and a variety of star formation histories and dust content. Since
the SED is relatively flat in both the optical bands (B through I
band) and in the near-IR, it is very hard to reproduce it with a
combination of a young population and dust obscuration. A combination
of an old population with a young population could be tuned to
reproduce the observed SED by having the old population providing the
near-IR flux and the young population providing the optical flux. This
would require the old component to be significantly older than 1 Gyr and
thus lead to an even stronger case. Adding dust reddening to such
multi-population models would not significantly alter the conclusions
on the age of the oldest component. As expected, we find that at fixed
redshift, age, metallicity and dust content the reddest models are the
single burst ones, which are also the ones better motivated by the
observed morphology.  As a further check we also compared the colors
of HDFS 223251-603910 with the colors of highly reddended starbust
galaxies (\cite{D}, and references therein) and we did not find any
combination that could reproduce the measured Spectral Energy
Distribution.

\section{Identification and conclusions}

Using nine broad band magnitudes derived from VLT and HST data, we are
now able to constrain the SED of ERO HDFS 223251-603910 very well. In
order to provide a firm identification, we computed the colors of a
large grid of models, with different redshift, age, metallicity and
dust content, using the population synthesis code by Bruzual \&
Charlot (1993, GISSEL96 version).  On the basis of the qualitative
investigation described in the previous section we have focussed most
of our efforts on single burst stellar populations.  Specifically we
considered models with metallicity Z/Z$_{\odot}$=1,0.4,0.2 and dust
reddening A$_V$=0,0.5,1 (using the extinction law from Cardelli et
al. 1989).  The IMF is a Salpeter IMF between 0.1 and 125 M$_\odot$.
Here, we considered stellar population ages down to 250 Myrs, with a
roughly uniform spacing in age.  We computed the $\chi^2$ of each model
fitted to the observed SED. In Figure~\ref{fig:chi2} the
contour levels of probability 68, 80 and 95 \% are shown.  In the nine
panel the metallicity decreases from left to right, while the dust
reddening increases from top to bottom. As can be seen, the redshift
is relatively well constrained, but the age of the object is much more
uncertain. This is a consequence of the so-called age-metallicity
degeneracy (e.g., Worthey, 1994), i.e., that broad band colors cannot
distinguish the effects of age and metallicity. Only spectroscopy
could constrain both the metal content and the age of the object.  The
dotted and dashed lines on the upper left corner of the plots are the
age of Universe as a function of redshift (h=0.65; dotted :
$\Omega=1,\Omega_{\Lambda}=0$, dashed :
$\Omega=0.3,\Omega_{\Lambda}=0.7$). As the galaxy cannot be older than
the Universe, only the region below the curves is allowed.  The best
fitting models of our grid in the cases A$_v=0,0.5$ and
Z/Z$_{\odot}$=1,0.4 are shown in Figure \ref{fig:spectra}, together
with the photometric points. The $\chi^2$ and the model
characteristics are also shown in the figure.


\begin{figure}
\resizebox{\hsize}{!}{\includegraphics{isochinew.eps}}
\caption{Contour plots of the $\chi^2$. The colors of a grid of models
(see text for details) have been computed and compared with the
observed ones. The age is given in Gyrs.
The metallicity of the stellar populations is
Z/Z$_{\odot}$=1,0.4,0.2 from left to right, the dust reddening is
A$_V$=0,0.5,1 from top to bottom. The contours correspond to a
probability of 68, 80 and 95 \% from the inside to the outside.  The
two curves in the upper left corner represent the age of the Universe
as a function of redshift for two different cosmologies (see
text). The object must be younger than the Universe and therefore only
the region below the curves is allowed. Its redshift is better
constrained than its age. }

\label{fig:chi2}
\end{figure}

\begin{figure}
\resizebox{\hsize}{!}{\includegraphics{speccnew.eps}}
\caption{The best fitting spectra for the cases with
Z=1,0.4Z$_{\odot}$ and $A_{V}=0,0.5$ are shown, together with the
observed photometric measurements and the values of the
$\chi^2$.  The filled square is the U band
VLT upper limit. The error bars on the wavelength axis represent the
filter bandwidths. The main contribuition to STIS CCD clear filter
comes from the reddest part of the spectrum. The $\chi^2$ with seven degrees of freedom is shown in the upper left corner.}
\label{fig:spectra}
\end{figure}

As already noticed in Treu et al. (1998a), the luminosity of this
galaxy is consistent with its identification as an elliptical galaxy
at $z\simeq 1.8$, e.g., for $\Omega=0.35$ and $\Omega_\Lambda=0.65$,
the galaxy would have, after passive dimming down to z=0, M$_V \simeq
-21.4$ (M$_V \simeq -24$ without passive dimming) and an effective
radius of about 4 kpc.

Thus, all the photometric data collected so far are consistent with
the identification of this object as a high redshift elliptical. This
renders HDFS 223251-603910 a prime target for spectroscopic followup
(see Benitez et al. 1998, Fontana et al. 1999 
for additional candidates in the same field).
This is within reach with IR spectrograph on 8m class telescopes, such
as ISAAC on the VLT. If confirmed, its identification will demonstrate
that ellipticals can already be assembled at redshift almost
2. Furthermore, by measuring its absorption line strengths we will
determine in great detail its physical properties, such as age, metal
content and star formation history, thus improving our knowledge of
the early stages of galaxy formation.


\begin{table}
\caption{Photometric properties. For every passband total magnitudes
are listed with their errors, except for U and in which case the 95\%
CL limits is shown. The errors on the magnitudes do not include
systematic calibration errors (see Section \ref{ssec:col}). The
effective radius, as measured by fitting the $R^{1/4}$ profile to the
HST images is also listed. The exposure times (texp), the zero points
(zp) and colour terms (ct), referred to the ESO and STScI released
data, are shown for reference. }

\label{tab:data}
{\scriptsize
\begin{tabular}{c c c c c c}
Band & m $\pm \delta$ m & $r_{\textrm{e}}$ $\pm \delta$$r_{\textrm{e}}$ &
texp (s) & zp & ct \\
\hline
U               & $<26.5$      &  -             &  17788  & 31.39 & 0.239(U-B)\\    
B               & 29$\pm$0.5   &  -            &  10200   & 33.26 & 0.128(B-V)\\    
V               & 26.7$\pm$0.3  &  -            &  14400  & 33.87 & -0.066(B-V)\\
R               & 25.81$\pm$0.15 &  -           &  7200   & 34.13&  -0.061(V-R)\\
C               & 25.38$\pm$0.15 & 0$\farcs26$$\pm$0$\farcs$06 &  25900 & 26.15& -\\
I               & 24.65$\pm$0.15 &  -  &  10158           & 33.55 & 0.023(V-I)\\
J$_{110}$       & 22.01$\pm$0.10 & 0$\farcs17$$\pm$0$\farcs$04     &  108539 & 22.16 & -\\
H$_{160}$       & 20.47$\pm$0.10 & 0$\farcs16$$\pm$0$\farcs$04     &  128441 & 21.52 & -\\
K$_{222}$       & 19.33$\pm$0.15 & 0$\farcs20$$\pm$0$\farcs$05   &  103163   & 20.00 & -\\
\hline  
\end{tabular}}
\end{table}

\begin{acknowledgements}

The authors would like to thank Bob Williams, Alvio Renzini, the
VLT-UT1 Science Verification Team, and the Hubble Deep Field South
Team, for having provided the astronomical community with this
exceptional data set and R. Pello for useful comments on a previous 
version of the paper.
\label{sec:ak}

\end{acknowledgements}

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