1 Introduction

Figure 1: Completeness diagrams for four major surveys covering the LMC. Each plot gives the number of sources per magnitude bin. The bin size is 0.1 mag. Note that: 2MASS observations (upper right) are deeper than DENIS observations (upper left) in the $K_{\rm s}$ band; the GSC2.2 magnitudes (bottom left) show a sharp cut-off; the UCAC1 magnitudes (bottom right) are only indicative, since the UCAC1 is not a photometric catalogue and furthermore this is only a preliminary catalogue, which means some improvements are expected in future releases.

The Magellanic Clouds (MCs) are among the best suitable places to study the stellar evolution outside the Milky Way, because of their proximity and their various stellar populations. Near-infrared surveys provide useful data for this kind of study because of their insensitivity to interstellar reddening. The Magellanic Clouds have been recently fully observed by two major infrared surveys: the DEep Near-Infrared Survey of the Southern Sky - DENIS (Epchtein et al. 1997) and the Two Micron All Sky Survey - 2MASS (Skrutskie et al. 1997). A Near-Infrared Point Source Catalogue towards the Magellanic Clouds, based on DENIS data, has been published (Cioni et al. 2000a; DCMC). The part of this catalogue devoted to the Large Magellanic Cloud (LMC) covers an area of $19.87 \times 16$ square degrees centered on ( $5^{\rm h}27^{\rm m}20^{\rm s}$,  $-69^{\circ}00\hbox{$^\prime$ }00\hbox{$^{\prime\prime}$ }$). To compile this catalogue, the objects were required to be detected in at least two of the three DENIS bands $I ({\rm Gunn-}i, 0.79~\mu {\rm m})$, $J (1.22~\mu {\rm m})$, $K_{\rm s} (2.15~\mu {\rm m})$. The 2MASS project observed the whole Magellanic Clouds in three photometric bands: $J (1.23~\mu {\rm m})$, $H (1.63~\mu {\rm m})$ and $K_{\rm s} (2.15~\mu {\rm m})$. For this work we used only the data available from the 2nd Incremental Release PSC, which do not cover two rectangular regions crossing the bar of the Large Magellanic Cloud and some cross-like gaps around bright stars ( $4^{{\rm h}}00^{{\rm m}}00^{{\rm s}}$ < RA < $7^{{\rm h}}00^{{\rm m}}00^{{\rm s}}$; $-78^{\circ}01\hbox{$^\prime$ }37\hbox{$^{\prime\prime}$ }$ < Dec < $-60^{\circ}48\hbox{$^\prime$ }00\hbox{$^{\prime\prime}$ }$).

The number of sources from both surveys are recorded in Table 1. Because of different sensitivity limits, DENIS sources detected only in the I and J bands are often detected in H and $K_{\rm s}$ by 2MASS. 2MASS observations are more than one magnitude deeper than DENIS in the $K_{\rm s}$ channel (due to a better thermalization), while they are roughly equivalent in the J channel (Fig. 1). Thus it appeared very interesting to cross-match the two catalogues to complete the spectral range of the DCMC IJ-sources with the H and $K_{\rm s}$ bands coming from 2MASS, though observations are not simultaneous.

More generally, cross-matching catalogues is highly relevant for completing the spectral or spatial coverage when there are missing or unpublished data. It is also a powerful tool to cross-validate the catalogues and search for discrepancies.

Cross-matching infrared (IR) with optical catalogues, such as DCMC/2MASS with the Guide Star Catalog II (GSC2.2), helps on producing new colour-magnitude and colour-colour diagrams, thus offering multispectral views of the LMC. In the cross-matching procedure we also included the proper motions from the USNO CCD Astrograph Catalogue (UCAC1), in order to discriminate MC members from foreground stars. The resulting MC2 catalogue provides an unprecedented basis for the study of stellar populations in the Magellanic Clouds and for further cross-identifications with catalogues at other wavelengths.

Section 2 gives an overview of each survey towards the LMC. Section 3 deals with the strategy developed to cross-match the infrared DENIS and 2MASS catalogues. Following in Sect. 4 is a comparison of the DENIS and 2MASS photometric systems. In Sect. 5 we add the optical GSC2.2 and UCAC1 catalogue to the cross-matching procedure. In Sect. 6 we present a few multispectral views of the stellar populations of the Clouds, based on the MC2 data.

  

 

Table 1: Number of sources as a function of detected wavebands in the DCMC and 2MASS catalogues.

LMC
DCMC		2MASS
$IJK_{\rm s}$	297 031	$JHK_{\rm s}$	1 996 382
IJ	1 151 789	$JK_{\rm s}$	66
$IK_{\rm s}$	8 724	JH	-
$JK_{\rm s}$	1 897	$HK_{\rm s}$	4
		J	11
		H	-
		$K_{\rm s}$	23
		Saturated	259
Total	1 459 441	Total	1 996 745