Astron. Astrophys. 354, 823-835 (2000)

1. Introduction

Since the work of Hodge and Kennicutt in the 1970's and 1980's (e.g. Hodge 1976, 1987; Kennicutt 1984, 1989; Kennicutt, Edgar & Hodge 1989) it has become increasingly clear that the set of HII regions in a spiral, or irregular galaxy, is worth studying as a population, i.e. for its statistical properties as well as for the detailed physical properties of the individual HII regions. As well as being of key importance for the energy balance of the interstellar medium as a whole, the HII region population reflects, in a manner readily observable, the parameters of the massive exciting stars within the HII regions, and is therefore of interest in the continuing study of the star formation rate (SFR) and the initial mass function (IMF) at the massive end of the stellar mass range (Kennicutt 1992). The number of complete studies of this type both before, and since the beginning of the CCD era, is relatively restricted. We ourselves have contributed a series of studies of the statistical properties of the HII region in NGC 4321 (Cepa & Beckman 1990), NGC 3992 (Cepa & Beckman 1989), NGC 6814 (Knapen et al. 1993), NGC 157, NGC 3631, NGC 6764 and NGC 6951 (Rozas, Beckman & Knapen 1996a; Rozas, Knapen & Beckman 1996b) and of NGC 7479 (Rozas et al. 1999b). In these papers we concentrated mainly on the HII region luminosity functions (LF's) in H and their global properties, as well as the disc-wide geometrical distribution of emission, although in Rozas et al. (1996b), as well as in Rozas, Castañeda & Beckman (1998) we derived the internal physical parameters: emission measure, electron density, and total mass of representative samples of, especially, the most luminous HII regions. Other studies based on statistical properties of HII regions in spiral galaxies have been presented by Rand (1992), Ryder & Dopita (1993), Hodge & Miller (1995), Feinstein (1997), Knapen (1998).

In the present study we offer a complete study of the HII regions in NGC 3359 combined with broad-band optical and NIR imaging.

In Sect. 2 we explain the observational and data reduction procedures, in Sect. 3 we describe the construction of the HII region catalogue. Sect. 4 shows the luminosity function and the diameter distribution, and in Sect. 5 we analyze the distribution of the ionized gas. In Sect. 6 we derive some of the physical properties of the HII regions. In Sect. 7 we compare the measured diffuse H flux in the disc of NGC 3359 with the computed ionizing flux escaping from the density bounded HII regions, and in Sect. 8 we set out our conclusions.

1.1. NGC 3359

NGC 3359 is a strongly barred galaxy, classified as SBc(s) 1.8 (Sandage & Tammann 1981) or SBc(rs) by de Vaucouleurs et al. (1991). From its radial velocity of 1008 km s^-1, using Ho = 75 km s^-1 Mpc^-1, we obtain a distance of 13.4 Mpc. NGC 3359 was first observed in H by Hodge (1969), in the near IR by Elmegreen (1981) and in CO by Stark, Elmegreen & Chaude (1987), and by Braine & Combes (1992), but the most detailed studies of this galaxy have been undertaken using HI . The structure and kinematics of the neutral atomic hydrogen were analyzed in detail by Gottesman (1982) and by Ball (1986), with best resolutions of 42"31" and 24.5 km s^-1, and 18" and 25 km s^-1 respectively. The most important results of these papers were the distribution of the HI in clumps, and the low surface density within the annular zone of strong star formation, which can be explained as due to the effect of the bar in sweeping up gas as it rotates. Striking non-circular velocities in HI , of up to 45 km s^-1 were found around the bar.

Ball (1992) produced a hydrodynamical analysis of the galaxy, reproducing the gas kinematics with models based on the stellar density in the bar, though he concluded that the spiral structure of the galaxy could not be directly reproduced from calculation of the non-axisymmetric potential obtained from surface photometry of the stellar bar, since the force thus obtained falls off too quickly at large radii.

The global properties of the galaxy are set out in Table 1.

Table 1. NGC 3359: basic parameters

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
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