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Astron. Astrophys. 360, 439-446 (2000)

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3. The morphology of NGC 4672

NGC 4672 (ESO 322-G73) is a highly-inclined early-type disk galaxy classified as Sa(s) pec sp by de Vaucouleurs et al. (1991). An overview of the properties of NGC 4672 is given in Table 2.


Table 2. Optical and radio properties of NGC 4672.
a) from Corwin et al. (1985).
b) from de Vaucouleurs et al. (1991). The apparent isophotal diameters are measured at a surface brightness level of [FORMULA] [FORMULA].
c) from this paper. The distance is derived as [FORMULA] with [FORMULA] the velocity relative to the centroid of the Local Group obtained from the heliocentric systemic velocity as in Sandage & Tammann (1981) and [FORMULA] [FORMULA] Mpc-1. The inclination i is derived as [FORMULA], where the observed axial ratio corresponds to [FORMULA] (de Vaucouleurs et al. 1991) and an intrinsic flattening of [FORMULA] has been assumed following Guthrie (1992).
d) from de Grijs (1998).
e) from Aaronson et al. (1989).
f) from Richter et al. (1994) for the adopted distance.
g) derived following Young et al. (1989) from the IRAS flux densities of NGC 4672 at 60 and 100 µm (Moshir et al. 1990).

The galaxy is characterized by an intricate dust pattern made of patches rather than lanes, which pervade the disk and cross the galaxy near the center of the bulge (Fig. 1). NGC 4672 is a dust-rich system as one can also infer by a comparison of its dust-to-blue luminosity ratio with the mean values found by Bregman et al. (1992) for the early-type disk galaxies studied by Roberts et al. (1991). The dust pattern is so relevant in determining the shape of NGC 4672 that Arp & Madore (1987) included it (with the name of AM 1243-412) in the section of the Catalogue of Southern Peculiar Galaxies devoted to the galaxies with prominent or unusual dust absorption. Corwin et al. (1985) pointed out a resemblance between the overall structure of NGC 4672 and that of NGC 5128 in their Southern Galaxy Catalogue. In spite of this similarity with a dust-lane elliptical they classified NGC 4672 a S0/a: pec sp galaxy since they recognized in the disk the presence of spiral arms. Recently, NGC 4672 has been included by de Grijs (1998) in a large sample of edge-on disk galaxies which have been observed in different optical and near-infrared passbands to study the global properties of galactic disks.

[FIGURE] Fig. 1. NGC 4672 as it appears in Fig. 3i of the Polar-Ring Galaxy Catalog by Whitmore et al. (1990). The orientation of the image is north to right and east bottom.

Like in the case of NGC 4698 (Bertola et al. 1999), the bulge of NGC 4672 appears elongated in an orthogonal way with respect to the disk at a simple visual inspection of galaxy images (Fig. 1). This remarkable feature is confirmed by the shape of the inner ([FORMULA]) [FORMULA]band isophotes, as shown in the isophotal map of NGC 4672 in Fig. 2.

[FIGURE] Fig. 2. [FORMULA]band isophotes of NGC 4672 (boxcar smoothed over [FORMULA] pixels). The inner isophotes are given in steps of 0.25 (uncalibrated) mag between -3.5 and -0.5 mag (see Fig. 3), while the outermost isophote corresponds to a level of 1.0 mag.

For this peculiarity NGC 4672 has been included by Whitmore et al. (1990) in the Polar-Ring Galaxy Catalog as a possible candidate polar-ring galaxy (PRC C-42). They stated that even if long exposures show a fairly normal Sa galaxy, shorter exposures reveal that the bulge component of the galaxy is slightly elongated in a direction perpendicular to the disk. They considered this feature as a signature of the structure of a polar-ring galaxy, since the observed bulge and disk components of NGC 4672 could be interpreted respectively as an almost face-on disk and a nearly edge-on ring due to the orientation of the galaxy.

However there is a number of reasons to reject the polar-ring classification and to consider the edge-on component of NGC 4672 a disk rather than a polar ring, and the face-on component a bulge rather than a low-inclined disk:

  • (i) From the uncalibrated [FORMULA]band acquisition image we extracted the luminosity profile at [FORMULA] along the major axis of the almost face-on component (i.e. , along the minor axis of the edge-on component). It turns out that this profile follows an [FORMULA] law with an effective radius [FORMULA] (Fig. 3). This indicates that we are looking at a stellar spheroid (i.e. , the bulge of an early-type disk galaxy or an elliptical galaxy) rather than a face-on disk.

  • (ii) The edge-on component contributes a large fraction of light to the total luminosity of the galaxy. In fact the disk-to-total luminosity ratio we derived from de Grijs (1998) is [FORMULA] in the [FORMULA]band, and [FORMULA] in the [FORMULA]band. The [FORMULA]band [FORMULA] is a typical value for an intermediate-type spiral galaxy (Kent 1985; Simien & de Vaucouleurs 1986).

  • (iii) The ionized-gas rotation curve measured along the major axis of the edge-on component ([FORMULA]) exhibits the differential rotation (Fig. 5) typical of a disk component. The absence of the characteristic linear rise of the gas rotation velocity due to the so-called `rim of the wheel' effect (e.g. , Vega Beltrán et al. 1997) is an indication that we are not facing an edge-on ring.

[FIGURE] Fig. 3. Uncalibrated luminosity profile of NGC 4672 extracted along the major axis of the bulge component (i.e. , the galaxy minor axis at [FORMULA]). Filled squares and open circles represent the data obtained along the SE and the NW (dust affected) sides, respectively. The folding has been performed to obtain the best match of the luminosity profile on the two sides in the outer, dust-free region. In the inset the data are plotted adopting a [FORMULA]-radial scale and the dashed-line represents the best-fitting [FORMULA] profile to the SE side data ([FORMULA]).

Summarizing, NGC 4672 edge-on component can not be a ring-like structure rotating around a low-inclined disk galaxy. Therefore the possibility that NGC 4672 could be a polar-ring galaxy dressed up as a spiral is ruled out. As a consequence, we assume as major-axis position angle of NGC 4672 that of the disk component ([FORMULA]) instead of the value of [FORMULA] given by de Vaucouleurs et al. (1991) and corresponding to the apparent major axis of the bulge.

NGC 4672 was observed in the 21-cm neutral hydrogen line by Aaronson et al. (1989) and by Richter et al. (1994). The large H I linewidth suggests that most of this gas is associated with the edge-on disk.

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Online publication: August 17, 2000