1 Introduction

Figure 1: H I total intensity map of NGC3175, with a resolution ( FWHM) of $121''\times44\hbox{$.\!\!^{\prime\prime}$ }5$ (see beam profile in the lower left), overlaid on a Digital Sky Survey image. The contour levels are 0.42, 0.6, 0.9, 1.2, 1.5, ... 3.6 Jy beam-1 km s-1

A large number of "typical'' global properties of galaxies of various Hubble types are presented and discussed by Roberts & Haynes (1994; in the following RH94). We will use these as standard reference values, with respect to which we will discuss our results from H I observations of the southern edge-on spiral galaxy NGC3175. RH94, as well as e.g. Rhee & van Albada (1996), determined that typical L_* spiral galaxies, i.e. galaxies near the "knee'' of the galaxy luminosity function, with an absolute B magnitude of $M_{\rm B}\ \simeq\ -20$, normally have H I gas masses on the order of $10^9-10^{10}\ M_\odot$. This usually represents a few percent of their total mass (RH94). The spatial distribution of H I gas is usually more extended than that of the stellar disk (Bosma 1981).

With respect to its global properties the edge-on spiral galaxy NGC3175 is quite peculiar. Its Hubble type is uncertain; the Third Reference Catalogue of Bright Galaxies (RC3; de Vaucouleurs et al. 1991) classifies it as SAB(s)a?, while it is listed as Sc in the Revised Shapley-Ames Catalog (RSA; Sandage & Tammann 1981). In the Uppsala General Catalog of Galaxies (UGC), Nilson (1973) claims that de Vaucouleurs' classification (which had been taken over from the RC2) is incorrect and that NGC3175 is "probably a late-type galaxy''. Because of this confusion, we will compare its global properties with those of galaxies ranging from type Sa to Sc.

Although at a redshift of only 1098 km s^-1 (and thus at a distance of only 15.9 Mpc), this galaxy was, until a few years ago, not detected in H I emission (Mathewson et al. 1992). Recently, Mathewson & Ford (1996) and Theureau et al. (1998) found weak H I emission from NGC3175. Despite this difficulty in tracing its H I gas, it has been readily detected in H$\alpha$ (Ryder & Dopita 1993, 1994), radio continuum (Condon et al. 1996) and CO emission (Elfhag et al. 1996), which are tracers of star forming regions. The Infra-Red Astronomical Satellite (IRAS) measured 60 $\mu$m and 100 $\mu$m fluxes of 13.1 and 28.2 Jy, respectively. With an f₆₀/f₁₀₀ flux ratio of 0.46, tracing warm dust, and in the absence of a luminous AGN, this galaxy is obviously forming stars that heat the interstellar medium (ISM) and dust in its disk. The detection of CO emission in the central part of NGC3175 proves that there is molecular gas from which stars can form. On the other hand, with only barely detectable H I emission, it is not clear where NGC3715 has its gas reservoir from which to form stars in the future. We investigate this riddle here based on new H I observations conducted with the Australia Telescope Compact Array (ATCA).