3 Results

3.1 Integrated photometry

The total magnitude ( $B_{\rm T}=15.0 \pm 0.1$) found by us from the multiaperture photometry is somewhat brighter than that of NED (15.3) and of PRC (15.5). Our R magnitude 14.1$\pm$0.1 is in agreement with the PRC value m_R=14.07.

Some results of color measurements are presented in Arnaboldi et al. (1994). According to those authors, the B-R color index in the galaxy center is +2, while the outer regions are rather bluer, with $B-R \approx 1$. Our values (see Figs. 3 and 4) are in good agreement with those results.

3.2 Optical morphology

In order to enhance possible internal structures of this galaxy, some tests have been carried out following the experiments performed by Faúndez-Abans and de Oliveira-Abans (1998) and employing some IRAF tasks. For illustrative purposes, Fig. 1a displays the residual image of the subtraction of a $60 \times 60$-pixel median-filtered kernel from the original R frame. An inner elongated bright stellar component, around another yet smaller round off-center component have been enhanced. The dark dust lane (ring?) probably immersed in the warped material external to the disk/ring, and some faint clumps to the east have also been enhanced.

Figure 1: a) Inverse gray scale residual image of the subtraction of the $60 \times 60$-pixel median-filtered R frame from the original R image. North is to the top and east is on the left. The image size is $224\hbox {$^{\prime \prime }$ }\times 200\hbox {$^{\prime \prime }$ }$. b) The same as a) for a $10\times 10$-pixel kernel. c) Enhancement by transform processing and high-pass filtering of the R image. The mark-lines are on purpose faint and poor so as not to cover the image.

The use of a median $30\times 30$-pixel kernel filter has enhanced the fuzzy material aligned with the apparent major axis of the bulge. Interestingly enough, after the application of a $10\times 10$-pixel kernel median filter, this fuzzy material turned out to be filaments (see Fig. 1b). This result is in agreement with Arnaboldi et al. (1995). The "five to ten" filaments so-found seem to lie perpendicular to an almost edge-on exponential disk. This is indeed an interesting PR dusty galaxy.

Other features have also been revealed through a high-pass filtering transform processing of the original R frame: clumps (satellites?) in the bulge and within the polar ring, a faint and underlying smooth component (in the central part of the bulge), and an off-center component inside the central structure. The most prominent clumps have been indicated in Fig. 1c by thin lines. Careful visual inspection of the image display (SAO's ds9) shows that the filaments tails point towards the West, superimposed on faint smooth regions that we call lobes.

3.3 Global photometric structure

Figure 2: Contour maps of ESO 603-G21 in the B, V and R passbands. The faintest contours in the B, V, and R bands are 25.2, 25.3, and 25.4 magarcsec-2, respectively. The isophotes are separated by 0.75 mag. The large tickmarks are 20 $\hbox {$^{\prime \prime }$ }$ apart. North is to the top and east is on the left.

Figure 3: Photometric profiles for ESO 603-G21: a), b) along the major axis ( ${\rm PA}=114^{\rm o}$); c), d) along the minor axis ( ${\rm PA}=24^{\rm o}$). Solid lines in a) and c) represent the distributions in the B passband, dotted lines in V, and dashed ones in the R. Solid lines in b) and  d) show the distribution of the B-V color, dashed in the B-R color.

Contour maps of ESO 603-G21 are presented in Fig. 2. The main features of the galaxy are clearly visible: a main body with approximately round isophotes and an almost edge-on warped structure (polar ring?) crossing the central object and strongly distorting the surface brightness distribution. The galaxy is surrounded by a faint halo whose major axis is aligned with the major axis of the possible ring.

Figures 3a,c display the surface brightness profiles of ESO 603-G21 along the major and minor axes. In the major axis profile the possible disk/ring is seen as two symmetrical "bumps'' at $r \approx \pm 20''$. At the SW part of the profile along the minor axis at $r\approx -2''$, a depression due to absorption in the ring/disk projected here onto the central part of the galaxy is seen.

Excluding the regions of the "bumps'' ( $\mid\! r\!\mid=10\hbox{$^{\prime\prime}$ }{-}25\hbox{$^{\prime\prime}$ }$), the surface brightness distribution along the major axis may be approximated by that of an exponential disk (see also Arnaboldi et al. 1995). In the R passband the disk characteristics are: $\mu_0^0(R)=19.55$ (corrected for galactic absorption) and h=1.22kpc. The disk of the galaxy is thus relatively bright and compact. The minor axis profile in the R filter is also approximated by an exponential one with $h=3\hbox{$.\!\!^{\prime\prime}$ }5=0.73$ kpc (Fig. 3). The total magnitude of the central exponential object is R=14.9 (assuming an apparent axis ratio b/a=1.0). Therefore, the ratio of luminosities of the central round object (bulge?) to the disk/ring is $\sim$1 in the R passband.

Figures 3b,d display the behavior of the observed color indices along the major and minor axes. Both profiles show very strong color gradients: the central parts of the galaxy are red ( $B-V \approx +0.8-1.0, B-R \approx +1.5-2.0$), while the outer ones are blue ( $B-V \approx +0.2-0.5, B-R \approx +0.5-1.0$). The galaxy disk/ring is very blue: $B-V \approx +0.2-0.3$ and $B-R \approx +0.5$ at $r \approx \pm 20''$along the major axis (where the two "bumps'' are visible). In Fig. 3d, the region of the disk/ring projection exibits a local color minimum, thus supporting our conclusion about the blueness of the ring.

Figure 4 gives the 3D distribution of the observed color index B-R within the central region of the galaxy. In this figure, the disk/ring is the notably blue path (narrow "valley") crossing the central region. A prominent color gradient is evident in the figure.

In order to study the galaxy structure in the near-infrared (NIR) spectral region, we have extracted the J, H and K images of ESO 603-G21 from the second incremental data release of the Two Micron All Sky Survey (Skrutskie et al. 1997; see http://www.ipac.caltech.edu/2mass). The NIR colors of the galaxy (Table 2) is usual for spiral galaxies (see Fig. 9 in Iodice et al. 2001). We found that to a first approximation the galaxy structure can be described as a thick double exponential disk with strong color gradients along the major and minor axes. In Table 3 we present the scalelength ratios in different color bands, both along the major and minor axes. The large observed ratios are typical for dusty late-type spiral galaxies (e.g. de Grijs 1998).

   

Table 2: General properties of ESO 603-G21.

Parameter	Value	Ref.
Morphological type	Sbc	NED
Heliocentric systemic velocity	3150 kms-1	PRC
Distance	42.9 Mpc
	(1''=208 pc)
Redshift	0.01042	NED
PA	114$^{\rm o}$
Major axis, D25 ($\mu_B=25$)	55'' (11.4 kpc)
Axial ratio, $b/a~(\mu_B=25)$	0.7
Inclination, i	80$^{\rm o}$:
Total apparent
magnitudes and colors:
$B_{\rm T}$	$15.0\pm0.1$
$(B-V)_{\rm T}$	$+0.30\pm0.05$
$(V-R)_{\rm T}$	$+0.60\pm0.05$
$(J-H)_{\rm 2MASS}$	+0.86	[1]
$(H-K)_{\rm 2MASS}$	+0.33	[1]
Galactic absorption (B-band)	0.14	[2]
Internal absorption (B-band)	1.0
Absolute magnitude, MB0(0)	-19.3
Exponential disk:
major axis:
$\mu_0^0(R)$	19.55
h	5 $.\!\!^{\prime\prime}$9 (1.22 kpc)
minor axis:
h	3 $.\!\!^{\prime\prime}$5 (0.73 kpc)
Exponential central object:
$R_{\rm T}$	14.9
$L_R^{\rm central ~object}$ / $L_R^{\rm disk/ring}$	$\sim$1
M(HI)	$6.2\times 10^9~M_\odot$	[3,4]
M(H2)	$1.1\times 10^9~M_\odot$	[5]
M(HI)/LB0(0)	0.76 $M_\odot$/ $L_{\odot,B}$
M(H2)/M(HI)	0.18
HI linewidth, W20	286 kms-1	[4]
HI linewidth, W50	251 kms-1	[4]
Far-infrared luminosity, $L_{\rm FIR}$	$4.85\times 10^9~L_{\odot}$	NED, [3]
Far-infrared color, f60/f100	0.50	NED
Mass of dust, $M_{\rm d}$	$1.5\times 10^6~M_\odot$	[6]
SFR $_{\rm FIR}$	2.5 $M_\odot$/yr	[7]
SFE (= $L_{\rm FIR}$/M(H2))	4.4 $L_{\odot}$/$M_\odot$

[1] - Skrutskie et al. (1997), [2] - Schlegel et al. (1998),
[3] - Richter et al. (1994), [4] - van Driel et al. (2000),
[5] - Galletta et al. (1997), [6] - Young et al. (1989),
[7] - Hunter et al. (1986).

   

Table 3: Exponential scalelength ratios.

Gradient	Major axis	Minor axis
hR/hK	$1.9\pm0.3$	$1.8\pm0.2$
hJ/hK	$1.3\pm0.2$	$1.7\pm0.2$
hH/hK	$1.4\pm0.3$	$1.4\pm0.3$

Table 2 summarizes the main characteristics of ESO 603-G21, both found in this work and collected from the literature. The last column provides the corresponding references, where the absence of a reference indicates that the given value has been determined in this work.

3.4 Dust and internal absorption

The mass of warm ( $T_{\rm d} \approx 35$K) dust found from the 100 $\mu$m IRAS flux is $1.5\times 10^6~M_\odot$ (Table 2). Assuming that only $\sim$$10{-}20\%$ of the dust mass in disk galaxies is warm enough to radiate in the IRAS bands (Devereux & Young 1990), we can estimate the total dust mass in the galaxy as $\sim$ $10^7~M_\odot$.

What is the total internal absorption in ESO 603-G21? The standard empirical description of the extinction as a function of galactic inclination is

$$A(i)=C\log(a/b),$$

where the extinction parameter C depends on the morphological type, total luminosity, and on the maximum rotaton velocity of a galaxy (e.g. de Vaucouleurs et al. 1991; Tully et al. 1998). For the purposes of this discussion, we replace the axial ratio a/b with 1/seci and take the extinction parameter in the B passband from Tully et al. (1998):

$$C_B=1.57+2.75[\log(2V_{\rm max}){-}2.5]$$

Adopting $V_{\rm max}=126$ kms^-1 (see item 3.5) and $i=80^{\rm o} \pm 5^{\rm o}$, we obtain $A_B = 0\hbox{$.\!\!^{\rm m}$ }99^{+0.39}_{-0.22}$. Therefore, the total extinction-corrected B-band absolute magnitude of ESO 603-G21 is M_B⁰(0)=-19.3^+0.4_-0.2 or $L_B^0(0)=8.2\times 10^9~L_{\odot,B}$. On the other hand, the Tully-Fisher relation predicts that the extinction-corrected luminosity of a galaxy with $V_{\rm max}=126$ kms^-1 is also M_B⁰(0)=-19.3 (Tully et al. 1998). Thus, one can conclude that $A_B=1\hbox{$.\!\!^{\rm m}$ }0$ is a reasonable estimate of the internal absorption in the galaxy and that ESO 603-G21 probably satisfies the Tully-Fisher relation for normal spirals.

The relatively high degree of symmetry of this object and its high inclination angle make it suitable for a study of the dust lane. In order to estimate the extinction law in the dust lane, we compare the surface brightness of regions which are equidistant from the nucleus on either side along the minor axis (see e.g. Knapen et al. 1991). Locating the exact center of the galaxy is very important for the asymmetry study. We adopt the center position as determined from the I-band image of ESO 603-G21. In Fig. 5 we display the selective asymmetry (which is the difference between the unobscured NE part of the minor axis profile and the SW part in one passband versus the same difference in another passband) at $r=3\hbox{$^{\prime\prime}$ }{-}5\hbox{$^{\prime\prime}$ }$ from the galaxy center (dust obscuration is probably present in the center). The mean extinction relations for the dust lane are:

$$A_B=(1.17\pm 0.13)\,A_V,$$

$$A_R=(0.48\pm 0.18)\,A_V,$$

$$A_R=(0.42\pm0.17)\,A_B.$$

The slopes of the selective asymmetry relations indicate that galactic extinction law (given as solid straight lines in Fig. 5) is valid, at least as a first approximation, for the dust in ESO 603-G21.

Figure 4: A 3-dimensional distribution of the B-R color index within the central ($\pm 10''$) part of ESO 603-G21. The orientation is such that the line of sight coincides with the major axis of the galaxy.

3.5 Rotation curve

The emission-line rotation curve for ESO 603-G21 along the apparent major axis (PA=114$^{\rm o}$) has been published in PRC. In Fig. 6 we show the observed rotation curve of the galaxy and our fit by an exponential disk with h=1.22 kpc (Table 2) and intrinsic axial ratio =0.1. It is evident that the exponential disk approximation gives a good description of the observed rotation curve within 20''from the nucleus.

To obtain the global maximum rotation velocity, we have used the following usual definition:

$${V}_{\rm max} = W_{50}/2~(1+z)^{-1}({\rm sin}\,i)^{-1},$$

where W₅₀ is the HI line width at 50% of the peak, z is the galaxy redshift, and i is the inclination angle (see Table 2). We have obtained $V_{\rm max}=126$ kms^-1 and a total mass within the optical radius ( R₂₅=5.7 kpc) of $2.1\times 10^{10}~M_{\odot}$ (assuming a spherical mass distribution). Thus, the mass-to-luminosity ratio is $M/L_B^0(0)=2.6~M_{\odot}/L_{\odot,B}$, a value that is usual for disk galaxies.

Figure 5: Selective asymmetry for ESO 603-G21 at $3\hbox {$^{\prime \prime }$ }{-}5\hbox {$^{\prime \prime }$ }$ from the nucleus (circles). The solid line represents the galactic standard extinction law.

Figure 6: Emission-line rotation curve along the major axis of ESO 603-G21 according to PRC. The circles are from H$\alpha$measurements, the triangles are from [NII] measurements. The solid line shows the rotation curve of an exponential disk with scalelength of 1.22 kpc.