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\title{
Exploring the stellar populations of nearby and high
redshift galaxies with ELTs
}
   \subtitle{}
\author{
M. \. Gullieuszik\inst{1} 
\and R. \, Falomo\inst{1}
\and L. \, Greggio\inst{1}
\and M. \, Uslenghi\inst{2}
\and D. \, Fantinel\inst{1}
}

  \offprints{M. Gullieuszik}
 

\institute{
  INAF OaPD, 
  Vicolo dell'Osservatorio 5, I-35122 Padova, Italy
  \and
  INAF IASF, 
  Via Bassini 15, I-20133 Milano, Italy 
}

\authorrunning{Gullieuszik}


\titlerunning{Stellar populations with ELTs}


\abstract{ The high sensitivity and spatial resolution of future ELTs
  facilities will offer the unique opportunity to probe directly the
  stellar populations of the very inner regions of galaxies in the
  local Universe and to derive morphological and photometric information for high redshift galaxies.  We present our project aimed
  at assessing the expected capabilities of ELTs in the study of nearby
  and high-redshift stellar populations.  To this end, we simulated
  imaging observations of different stellar populations in the local
  Universe and in high-redhshift galaxies with the MICADO camera at the E-ELT.
  Detailed photometric analyses of these images were used to
  probe the feasibility of science cases dealing with
  photometry of resolved stars in crowded fields, and with surface
  photometry of distant galaxies.  We find that the future facilities
  will allow us to greatly improve our knowledge of the stellar
  populations in galaxies, especially in the innermost and most crowded
  regions. Accurate photometry of turn-off stars in nuclear star
  clusters of intermediate age will be possible up to distances of $\sim 3$
  Mpc. The
  exquisite spacial resolution will also drive great progress in
  unresolved stellar populations studies, enabling the detailed measurement of
  structural parameters, colour profiles, and the detection of
  signature of star formation sub-structures in galaxies at redshifts
  up to z=3.

\keywords{instrumentation: adaptive optics --
  instrumentation: high angular resolution --
  galaxies: clusters: general -- 
  galaxies: fundamental parameters --
  infrared: stars} } \maketitle{}

\section{Introduction}

The advent of next-generation ground-based extremely large telescopes (ELTs)
will offer completely new scientific opportunities to study fundamental issues of distant galaxies like their stellar content in their centres and the morphological structures at early cosmic epochs. 
The combination of  extraordinary sensitivity and spatial resolution
will in fact allow us to characterise the photometry and structural parameters of stellar system with unprecedented
precision. The E-ELT will be the largest operating telescope and the Multi-AO Imaging Camera
for Deep Observations \citep[MICADO][]{davi+2010} that will be mounted on it will be the best instrument to achieve a wide range of scientific goals requiring extremely deep and high resolution photometry.
To explore the new challenges that we could face using this unique instrument, we started to study the feasibility of a number of science cases for MICADO.
For each of them,
we simulated  imaging observations and performed a detailed photometric analysis on the images.
The results were used to evaluate the  photometric performances of the instrument and then asses the 
reliability of the scientific results. 
The first results of our project were presented by \citet{greg+2012} and \citet{schr+2014} where we demonstrated the feasibility of the reconstruction of the star formation
history in galaxies 
the inner parts of disk galaxies in the Sculptor group, as well as the measurement of the photometric metallicity of red giant branch stars in the central regions of Ellipticals in the Virgo cluster. Recently we further developed our project by investigating the performances of MICADO and other imagers to be mounted on ELTs and the JWST
on the study of extremely crowded stellar systems.
Here we present two new science cases, focusing on resolved and unresolved stellar populations, respectively.

\section{Simulated images}

Simulated images were produced using the Advanced Exposure
Time Calculator (AETC) tool \citep{falo+2011}. We
adopted as baseline the parameters from the phase A study of MICADO, the instrument designed for the 39 aperture E-ELT to provide diffraction limited imaging over a 
 $\sim1^\prime$ field of view. 
We used the  point spread functions (PSFs) of the multi conjugate
adaptive optics post focal relay (MAORY)
for the E-ELT, calculated for a $0\farcs6$ seeing at the centre of the
corrected field of view. The final simulated images were
obtained as the stack of 180 images with 60s exposures (total exposure time of 3h). Further details are given in \citet{gull+2014}.

\section{Resolved stellar populations in Nuclear star clusters}\label{sec:nsc}

The study of the nuclei of galaxies are extremely important to understand the processes
that drive the formation and the evolution of galaxies. An important point that is not still completely understood
is the formation and evolution mechanism of the nuclear star clusters (NSC) and the possible links between NSCs and the 
central super-massive black-holes (SMBHs).
 A detailed study of NSC stellar populations is a key tool to understand the main physical processes
 involved in the formation and evolution of these stellar systems.
 Even for the closest NSCs, current instrumentation allows us to study their properties only from integrated light data.
 The extremely high stellar density in NSCs is
 an ideal test-bed to explore the photometric capabilities of MICADO at the E-ELT in severe crowding conditions.

Our aim  is to explore under which conditions MICADO would provide photometry
with an accuracy suitable to reconstruct the star formation history of NSCs. The most reliable age indicator
for a stellar population is the magnitude and colour of its main-sequence turn off (MSTO). We therefore
aim at understanding for which NSCs MICADO would be able to measure the magnitude of the MSTO.
The feasibility of this measurement and its accuracy will clearly depend on the crowding conditions and on the age of the NSC stellar populations,
as the MSTO luminosity increase at decreasing stellar ages. In our study we thus produced a grid of six 
synthetic single stellar populations, representative of six NSCs located at two different distances ( 2 and 4 Mpc)
with three ages (1, 4, and 10 Gyr). Stars were spatially distributed following a King profile model 
with the same structural parameters as the NSC in NGC~300, a typical nearby NSC.
For each simulations we added a stellar component to include the contribution of the host galaxy stellar population \citep[for details, see][]{gull+2014}.
We simulated observations in the $I$, $J$, $H$, and $K$ bands of the six stellar populations using AETC as described in the previous section.

\begin{figure}[t!]
\centering
\resizebox{\hsize}{!}{\includegraphics[clip=true]{gullieuszik_fig1.eps}}\hfill
\caption{\footnotesize
Age of the oldest MSTO detectable in a nuclear star clusters at different distances. 
We show the results obtained adopting a detection magnitude limit of 30.0 and 29.5 in the $J$-band. These are the magnitudes corresponding to the 50\% and 80\% photometric completeness level.
}
\label{fig:nsc}
\end{figure}
We performed PSF photometry on each simulated image using  Starfinder \citep{diol+2000}, a package
specifically developed for images obtained with high resolution AO images.
The resulting photometric catalogues and
colour-magnitude diagrams were analysed and compared with the lists of input magnitudes to asses the accuracy of our photometry.
We estimated the photometric errors and completeness
for each simulated image. The results depends on the NSC age and distance, but on average we found that 
the 50\% completeness level is found at $I\simeq 30.5$ mag and $J\simeq30.0$ mag.
These results apply to the region beyond a few ($\sim 3$) effective radii from the cluster centre, because stellar blends strongly increase the photometric errors and 
reduce the photometric depth in the crowded central regions of the NSCs. 
Figure 1 summarises our results on the reliability of age estimates of NSCs as 
function of their age and distance.
Under the above conditions, E-ELT photometry could be used to age-date NSCs up to 10 Gyr in
galaxies at 2 Mpc. 
For the oldest stellar populations the MSTO at this distance fall close to the detection limit, so that the corresponding age estimate will 
be affected by some uncertainties.
For the case at 4 Mpc, we found that the MSTO is below the detection limit for the 10 Gyr NSC and at the detection limit for the 4 Gyr one.
Therefore in the latter case 
the age of the NSC stellar population can be estimated with accuracy only for young clusters.
We extrapolated our results to galaxies at different distances. By using our estimates of the photometric completeness
and the theoretical relations between the MSTO magnitude and the cluster age, we calculated the maximum age
of a stellar population whose MSTO would result to be brighter than the 50\% ad 80\% completeness
level of typical MICADO photometry of a NSC. Results are over-plotted in Fig. 1.
We also explored the feasibility of this science case with the 
Thirty Meter Telescope and we found that, for this specific case, the performances of TMT and E-ELT will be
comparable. A detailed discussion of this result can be found in \citet{gull+2014}.

\section{Structural parameters in high-redshift galaxies} \label{sec:highz}


\begin{figure}[t!]
\centering
\resizebox{\hsize}{!}{\includegraphics[clip=true]{gullieuszik_fig2.eps}}
\caption{\footnotesize
Uncertainties in the measurements of the $R_e$, Sersic index, and 
magnitudes for $z~2$ galaxies of different masses expected to be derived from 
MICADO at the E-ELT observations.
}
\label{fig:highz_res}
\end{figure}
%

\begin{figure}[t!]
\centering
\resizebox{\hsize}{!}{\includegraphics[clip=true]{gullieuszik_fig3.eps}}\hfill
\caption{\footnotesize
Uncertainties in the colour gradient in high-redshift galaxies at redshift 2 ({\it upper panel}) and 3 ({\it lower panel}) measured from MICADO observations.
The three series of dots, connected by lines shows results for galaxies with Sersic index od 1.0, 2.5, and 4.0 (see the top-legend).
}
\label{fig:grad}
\end{figure}

To extend our project, we explored the possibilities offered by the unique spatial resolution of the E-ELT in the study of unresolved stellar population.
Galaxies at high redshift are extremely small, with half-light ratios of a few Kpc at $z\sim2$.
They therefore have sizes of the order of just a few times the resolution of HST.
As a result, to date, the measurements of the structural properties of galaxies with intermediate and small masses 
are strongly affected by relevant uncertainties.
To provide a detailed analysis of the impact of MICADO photometry on this research field,
we simulated two large sets of observations of galaxies at redshift 2 and 3
with the goal of understanding the capabilities of the E-ELT for the study of compact galaxies at high $z$.

We defined a grid of template galaxies with masses in the range $10^9$ to $10^{11}$ $M_\odot$, and Sersic index of 1.0, 2.5, and 4.0.
To study the dependences of the results on the size and morphological type.
The structural parameters of the simulated galaxies were chosen by extrapolating the 
scaling relations obtained from observed  galaxies \citep{vand+2014}. The effective radii adopted for the late-type galaxies
range between 1.5 and 4.2 Kpc  (corresponding to 170 and 500 mas), and
between 0.04 and 1.5 Kpc (5 and 170 mas) in the case of early-type galaxies.
To evaluate the effect of statistical noise on the results, we performed 50 simulations for each template galaxy.

We used the GALFIT program to fit Sersic structural parameters to the galaxies in the simulated images.
The accuracy of the results were derived by comparing the measured values of the effective radii, Sersic index and total magnitude
with the input parameters are shown in Fig. 2.
We conclude that the size of galaxies could be measured with errors smaller than 10\% in all cases. 
The error on the Sersic index is around 5\% for most galaxies. Only for the smallest ones, with mass smaller than $10^{9.5}$ $M_\odot$ 
the uncertainties result to be $\sim 10\%$. The very good quality of the reconstruction of the surface brightness profiles
allowed us also to reliably measure the colour gradients. Results are shown in Fig. 3.
At $z=2$ errors in the retrieved colour gradients are smaller than 0.15 mag even for the smallest galaxies.
At $z=3$ the uncertainties are somewhat larger, in particular for the compact early-type galaxies. In any case,
for galaxies more massive than $10^{9.5}$ $M_\odot$ colour gradients will be measured with an accuracy of $\sim0.15$ mag.
We calculated that with this accuracy it will be possible to detect radial variations of $\sim 25\%$ in age and/or 0.3 dex in metallicity,
that will provide extremely accurate insight in the spatial distribution of the stellar populations in high-redshift galaxies.
This will in turn provide extremely clear clues to understand the details of the mechanisms driving galaxy formation and evolution.


\begin{acknowledgements}
We acknowledge support of INAF and MIUR through the grant Progetto premiale T-REX.
\end{acknowledgements}

\bibliographystyle{aa}
%\bibliography{gullieuszik}

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\end{document}

