---
abstract: 'We present some preliminary results related to a project aimed at studying the evolution of the galaxy population in rich environments by means of the Color-Magnitude relation and of the Fundamental Plane. We derive the NIR and optical structural parameters for a sample of galaxies in the cluster AC118 at z=0.31. We prove that reliable structural parameters of galaxies at z$\sim$0.3 can be still derived from ground–based observations. The NIR effective radii, measured for the first time at this redshift, turn out to be significantly smaller than those derived from the optical data, providing new insight into the evolution of colour gradients in galaxies.[^1]'
author:
- 'G. Busarello, P. Merluzzi, M. Massarotti'
- 'F. La Barbera, M. Capaccioli'
- 'G. Theureau'
title: 'NIR and Optical Structural Parameters of Galaxies in the Cluster AC118 at z=0.31'
---

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Introduction
============

Numerous photometric surveys are currently done on wide areas of the sky with ground-based telescopes. They are providing an enormous output of data that, in particular, are quickly improving our knowledge of distant galaxies. In addition to integrated quantities like magnitudes and colours, the possibility of extracting galaxy structural information would significantly increase their scientific outcome.

Structural parameters have been recently derived for distant galaxies from HST photometry in optical wave bands (Kelson et al. 2000, and references therein). Jørgensen et al. (1999) proved that structural parameters can be still derived from ground–based data up to z=0.18. We now extend the redshift range and present the first measurement of NIR effective radii at z=0.31.

The Data
========

We obtained photometry in the V, R, I, K bands for the cluster of galaxies AC118 with the ESO NTT telescope (EMMI and SOFI) during four observing runs (October 1998 – September 2000). Here we will make use of the optical and K-band images, and HST (WFPC2-F702W) archive data to derive surface photometry for a sample of cluster members.

The data reduction and photometric calibration were performed following the standard procedures and are described elsewhere. The sample of cluster members were selected via photometric redshifts.

Structural Parameters
=====================

Galaxy sizes at intermediate redshifts are comparable with the typical seeing of ground-based images ($\sim 1''$). To obtain reliable estimates of the galaxy structural parameters, it is therefore crucial to take into account the effects of the Point Spread Function (PSF).

This is usually performed 1) by a one dimensional fitting approach (see Saglia et al. 1997) and 2) by two–dimensional fitting methods (see e.g. van Dokkum & Franx 1996). In case 1) the integrated light curve or the galaxy intensity profile are constructed by an elliptical fit of the galaxy isophotes and fitted with a proper model. Due to the small galaxy size in ground-based images, the isophotal fit has to be done with few pixels and the brightness profiles consist of very few data points. In the two–dimensional approach, a direct fit of the galaxy brightness distribution is performed without any intermediate step. Because of these reasons, we adopted the two-dimensional approach (see Figure 1).

Optical and NIR Effective Radii
===============================

Structural parameters were derived in the optical wave bands for galaxies brighter than R=21 mag. The effective radii derived from the HST (F702W) and from the NTT (R) images are compared in Figure 2 (left panel). The fitted parameters are fully consistent, with a dispersion of $\sim$40%, that is the typical uncertainty on effective radii measurements (see Kelson et al. 2000). This result shows that reliable structural parameters of galaxies can be still derived from ground–based observations at z$\sim$0.3.

The right panel of Figure 2 shows the ratios of the NIR to the optical effective radii. The K–band (rest-frame H) effective radii turn out to be $\sim$50% smaller than those measured in the R–band (rest-frame V). This implies the presence of strong internal (optical-NIR) colour gradients in galaxies at z=0.31. Following Sparks & Jørgensen (1993, eq. 21) we obtain $\Delta$(R-K)/$\Delta$log(r)$\sim$-0.28 at z=0.31. When compared with the local (z$\sim$0) value $\Delta$(V-H)/$\Delta$log(r)$\sim$-0.18 (Scodeggio et al. 1998, see also Peletier et al. 1990), this result implies that the colour gradients of galaxies decreased by a factor $\sim$1.5 since z=0.31 (or $\sim$4.5 Gyr). In a forthcoming paper we will make a more reliable comparison between optical and NIR structural parameters by extending the sample to $\sim$100 cluster galaxies.

Jørgensen, I., Franx, M., Hjorth, J., van Dokkum, P.G. 1999, , 308, 833 Kelson, D.D., Illingworth, G.D., van Dokkum, P.G., Franx, M. 2000, , 531, 137 Peletier, R. F., Valentijn, E. A., Jameson, R. F. 1990, , 233, 62 Saglia, R.P., et al. 1997, , 109, 79 Scodeggio, M., et al. 1998, , 301, 1001 Sparks, W.B., & Jørgensen, I. 1993, , 105, 1753 van Dokkum, P.G., & Franx, M. 1996, , 281, 985

[^1]: Based on observations at ESO NTT (OAC guaranteed time) and HST Data Archive.
