3 Results

3.1 HI distribution

Figure 1 displays our ATCA image of the total H I emission from NGC3175, superposed as contours on a Digital Sky Survey (DSS) image. The angular resolution (FWHM) of the H I image is $121''\times44\hbox{$.\!\!^{\prime\prime}$ }5$, as indicated in the lower left. The emission is marginally resolved along the disk plane of the galaxy in what appears to be a double-peaked distribution.

A cut through the H I emission distribution along the galaxy's major axis, for which we adopt a position angle of $\rm PA = 51^\circ$ (Dahlem et al. 2001), is displayed in Fig. 2. In this figure the data are represented by a solid line.

Figure 2: Cut through the H I line emission distribution of NGC3175 along its major axis. The width of each Gaussian (dotted lines) is that of the angular resolution in the direction of the major axis, $FWHM = 67\hbox{$.\!\!^{\prime\prime}$ }7$. Also displayed are the sum of the three Gaussians (dotted line) and the residual after subtracting this sum from the data (bold dashed line). Measured values are listed in Table 1

  

Table 1: Positions and intensities of emission components along the major axis of NGC3175

$$\begin{tabular}{lcccccc} \noalign{\hrule\smallskip } Compone... ... & $0.34\pm 0.03$ \\ \noalign{\smallskip\hrule } \end{tabular}$$

Notes to Table 1:
a) Our data.
b) Condon et al. (1996).
c) The intensity of the central peak was normalised to unity.

It became clear very soon during our spatial analysis of the emission distribution that a two-component approximation is not adequate. Thus, a three-Gaussian model was computed and graphically displayed. The individual Gaussians and the sum of the three are represented by dotted lines. The remaining residual is shown as a bold dashed line. The width of $67\hbox{$.\!\!^{\prime\prime}$ }7$ of all three Gaussians is identical; it represents the resolution of our data in the direction of the major axis over the width of the cut of 35''. The fact that the three Gaussians leave virtually no residuals indicates that most of the H I line emission arises from three maxima, with almost no emission from further out in the disk. The positions of the three Gaussian components along the major axis and their relative intensities, normalising that of the central peak to unity, are tabulated in Table 1. The zero point of the radial axis is the position of the radio continuum maximum, which we assume to be associated with the galaxy centre. A justification for using a three-Gaussian approximation lies in the structure of the 1.49 GHz radio continuum emission distribution, where three emission maxima are visible (Condon et al. 1996). A cut through the 1.49 GHz map (with an angular resolution of 15'') is displayed in Fig. 3. The corresponding offset and intensity values were measured by us and are also listed in Table 1. A direct comparison of the radial offsets and flux densities shows that, although both emission distributions can be approximated by the same model, neither the positions nor the relative intensities of the peaks coincide. The central H I component is located, within the measuring accuracy, at the position of the central radio continuum peak (and thus the centre of the galaxy). The two outer H I components are located further out along the major axis than the extranuclear radio continuum peaks. Star formation thus takes place within the H I gas layer in the central disk, as might be expected.

Figure 3: Cut through the 1.49 GHz radio continuum distribution of NGC3175 along its major axis. The angular resolution is 15'' (Condon et al. 1996). Both the orientation and the range of this cut are identical to that presented above in Fig. 2. The same three-Gaussian approximation is displayed as above. Measured values are listed in Table 1

The secondary H I emission feature near the southwestern edge of the galaxy disk might be a small companion or a weak remnant of a tidal spur or arm. Its recession velocity of about 1050 km s^-1 does not match up with the velocities on that side of the galaxy disk closest to it (see below).

3.2 HI kinematics

Figure 4: The H I velocity field of NGC3175. The grey scale ranges from minimum (920 km s-1; light grey) to maximum (1270 km s-1; dark grey). The contours shown are 1025 km s-1, 1100 km s-1 (approximately the systemicvelocity) and 1175 km s-1

The velocity field (Fig. 4) shows that the lowest velocities are observed in the northeastern part of NGC3175 and the highest accordingly in the southwestern half. The dust lane is located on the south-east side of NGC3175, tracing its "near side''. This determines the sense of the galaxy's rotation uniquely. The velocity of the secondary emission blob to the southwest is clearly similar to that of the opposite side of the disk.

Figure 5: Position-velocity cut through the H I line emission distribution of NGC3175 along its major axis. The angular resolution in the direction of the major axis is $67\hbox{$.\!\!^{\prime\prime}$ }7$, the velocity resolution is 6.6 km s-1

A position-velocity (pv) diagram along the major axis of NGC3175 (along ${\rm PA} = 231^\circ$) is displayed in Fig. 5. One can discern solid body rotation out to a radius of about $\pm 45''$ from the centre of the galaxy. A comparison with Fig. 3 by Mathewson et al. (1992) reveals that the velocity gradient is not so low because of beam smearing effects, but the same gradient is observed in H$\alpha$, with much higher angular resolution. However, it is unusual for spirals of types earlier than Scd not to have any H I gas beyond the turnover radius in the rotation curve.

The secondary emission blob is not visible here because of its low signal-to-noise ratio. It arises from several marginally positive signals in the channels in the range 1000-1050 km s^-1 (at a radial offset of about 120''-200'').

3.3 Total HI line flux and gas mass

Figure 6: Integral H I line spectrum of NGC3175 measured with the ATCA. The velocity resolution of the displayed data is 6.6 kms-1. The velocities on the x-axis are heliocentric

The total H I line spectrum of NGC3175 is displayed in Fig. 6. The integral H I line spans a velocity range of 350 km s^-1, from 920 to 1270 km s^-1 (cf. also Fig. 5), with an approximate width at 20% of the peak of W₂₀ = 330 kms^-1. From the integral spectrum we derive a heliocentric systemic velocity of $v_{\rm hel}\ = 1095\,\pm\, 10$ kms^-1, which is compatible with earlier measurements. The integral line flux from NGC3175 is 11.0 Jy km s^-1, with an estimated uncertainty of about 20%, which is roughly compatible with the non-detection reported earlier, with an upper limit of 8.3 Jy kms^-1 (Mathewson et al. 1992). Our H I flux measurement is only slightly lower than the values obtained by Mathewson & Ford (1996) of 14.64 Jy km s^-1 and Theureau et al. (1998) of $12.9\pm1.2$ Jy km s^-1. One can estimate the amount of flux that might possibly have been missed by our interferometer observations by taking the flux values from our ATCA data and the one from the Parkes data by Mathewson & Ford (1996) at face value. Our measurement reflects the amount of H I gas in the disk of NGC3175, while the value by Mathewson and Ford might be considered as the possible "full flux'', including low surface brightness or intergalactic H I. One can then argue that if any extended flux had been missed by the interferometer, it would not exceed the difference between both flux measurements, i.e. about one third of the flux displayed in Fig. 1.

The 1.344 GHz continuum flux density of NGC3175 from our ATCA run of $71.5\pm5$ mJy is in very good agreement with the value from the VLA data at 1.425 GHz (Condon et al. 1996) of 71.8 mJy. Thus, the relative calibration between our data and the VLA is good. Because of the weak continuum emission, one can assume that the measured total H I line flux is not measurably influenced by intrinsic self-absorption. Our continuum map is not displayed here, because it does not add anything new to what is already known from the one by Condon et al. (1996).

Using the relation by Roberts (1975) in Eq. (1) and assuming optically thin emission, we can calculate the H I gas mass in NGC3175 from the integral H I line flux as follows:

$$M_{\rm H\,I} = 2.356\,10^5\ D^2\ f_{\rm H\,I}\ [M_\odot] ,$$ (1)

where D is the distance in units Mpc and $f_{\rm H\,I}$ is the measured integral H I line flux. $f_{\rm H\,I}$ = 14.64 Jy km s^-1 (from the Parkes data by Mathewson & Ford 1996) then translates into a total H I gas mass of NGC3175 of $7.8\,10^8$ $M_{\odot}$, which is quite low for an Sa-Sc type spiral. Other late-type spirals have H I gas masses of order 10⁹-10¹⁰ $M_{\odot}$, typically (e.g. Rhee & van Albada 1996).

The H I line flux of $f_{\rm H\,I}$ = 11.0 Jy km s^-1 measured from our data corresponds to an H I gas mass of $5.8\,10^8$ $M_{\odot}$. This implies that there are, if any, only small amounts ($2\,10^8$ $M_{\odot}$) of intergalactic H I gas in the vicinity of NGC3175 that might have such a low surface brightness as to be missed by the ATCA.

Thus, our present observations prove the presence of small amounts of H I gas in the inner disk of NGC3175, an area where other tracers of star formation (SF) processes had been detected previously. Elfhag et al. (1996) report the detection of CO emission from the central part of NGC3175. It appears that the gas from which stars can be formed has finally been detected in emission.

One can use the CO(1-0) line flux derived by Elfhag et al. (1996) of $f_{\rm CO}\ = 19.3\pm1.4$ K km s^-1 (on the $T_{\rm mb}$ main beam temperature scale) to calculate an estimate of the total molecular gas mass of NGC3175. This estimate is a lower limit, because only one beam area ( FWHM = 44''; Elfhag et al. 1996) was observed. Assuming that the radial CO emission distribution is similar to that of the radio continuum, as observed in many galaxies (see for example García-Burillo et al. 1992), the CO emission of NGC3175 should arise from within the central 90''. At the high inclination angle of NGC3175, the apparent thickness of the disk will probably be only a few arcseconds, thus contained within the SEST beam. Based on the fact that the CO spectrum is peaked near the systemic velocity of NGC3175 it is also likely that the CO gas distribution is centrally peaked. Therefore, we estimate that the SEST has gathered at least 50% of the total CO(1-0) line flux from this object.

We calculate the molecular gas mass, $M_{\rm H_2}$, following the relation

$$M_{\rm H_2} = I_{\rm CO}\ N({\rm H_2})/I_{\rm CO}\ D^2\ \theta\ m({\rm H_2})\ [M_\odot],$$ (2)

where $I_{\rm CO}\ = 19.3$ K km s^-1 is the observed surface brightness of CO line emission, $N({\rm H_2})/I_{\rm CO}$ is the CO-to-H₂ conversion factor ("X'' factor) between H₂ column density and observed CO line surface brightness, for which we adopt the "standard'' value of $2\,10^{20}$ cm^-2/K km s^-1, D is the distance of 15.9 Mpc in units cm, $\theta$ is the area over which emission is observed (in units sterad; here the beam FWHM of the SEST) and $m({\rm H_2})$ is the mass of an H₂ molecule of 1.6810^-57 $M_{\odot}$.

Filling in these quantities, we determine that $M_{\rm H_2}\ \geq\ 5.8\,10^{8}$ $M_{\odot}$. This infers a ratio of $M_{\rm H_2}/M_{\rm HI} \geq\ 0.74$.

In the context of the results by Young & Knezek (1989), this makes it likely that NGC3175 is an Sc type spiral. If up to 50% of the total CO line flux should have been missed, the corrected ratio is still consistent with an Sb-Sbc classification. This ratio of 0.74 would be unusual for a galaxy of type earlier than Sb.