1 The accurate determination of the distance to Local Group galaxies is critical for the establishment of a reliable extra-galactic distance scale. In particular, local late-type galaxies as the Large Magellanic Cloud, M 31 and M 33 are the sites of choice for the calibration of several secondary distance indicators.
The Sc II-III spiral M 33 (NGC 598 or Triangulum galaxy) is the third-brightest member of the Local Group (see Van den Bergh, 2000, for a review). In spite of that, while Cepheid variables were discovered as early as 1920 in this galaxy (Hubble, 1926), reliable distance estimates have become available only after 1980 (Sandage & Carlson, 1983; Christian & Schommer, 1987) and differences of the order of ~ 0.3 mag can be found even among the most recent estimates of the M 33 distance modulus (see McConnachie et al., 2004, and references therein).
Concerning the stellar populations of M 33, most of the studies have centered their attention on the massive (young) stars populating the disc (see, for example Van den Bergh, 2000; Urbaneja et al., 2002; Van den Bergh, 1991, and references therein), and/or on the innermost regions of the galaxy (Mighell & Rich, 1995; Stephens & Frogel, 2002). On the other hand, only a few authors have provided some insight into the outer regions and the (possible) halo population (see Mould & Kristian (1986), Wilson et al. (1990), Davidge (2003) and the HST-WFPC2 study by Kim et al. (2002)).
Here we present V and I photometry (reaching V ~ 24.5) of two fields located at ~ 15' and ~ 28' from the center of M 33, to the North-West of the main body of the galaxy, approximately in the direction of the minor axis (see Fig. 1). The Tip of the Red Giant Branch (TRGB) is cleanly detected with our data and we use it to derive a robust estimate of the distance modulus of M 33 adopting the RR Lyrae-independent calibration provided by Bellazzini, Ferraro & Pancino (2001), Bellazzini et al. (2004). This is the main aim of the present study which is part of a large programme devoted to the determination of homogeneous distances for most of the galaxies of the Local Group (see Bellazzini et al., 2002). The metallicity distribution of the RGB stars in the considered fields is also obtained from photometric estimates, following the method by Bellazzini et al. (2003). Shortly after this paper was submitted, a preprint was posted (Tiede et al., 2004, hereafter T04) presenting the analysis of a 6.8' × 6.8' field located in the South-Eastern region of M 33 at ~ 20' from its center. The analysis is very similar to that performed here and the results are in excellent agreement, as we will show below.
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Figure 1. The position of the observed fields over-plotted on a DSS-II image of M33. The dimension of the image is 40' × 33'. |
The structure of the paper is as follows: in Sect. 2 we describe the observational material, the data reduction process and the photometric calibration; in Sect. 3 we present the color-magnitude diagrams (CDMs), our TRGB estimate of the distance modulus, and the (photometric) metallicity distributions. The stellar content of the considered fields is also briefly discussed. A brief summary and the main conclusions are reported in Sect. 4.
1 Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Centro Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Back.